Proceedings of the American Society for Composites: Thirty-First Technical Conference
Proceedings of the American Society for Composites: Thirty-First Technical Conference - DEStech
Fused Deposition Modeling of Fiber-Reinforced Thermoplastic Polymers: Past Progress and Future Needs
Fused Deposition Modeling (FDM) is one of the most common Additive Manufacturing (AM) techniques for polymeric materials. To improve mechanical properties of the printed parts and improve their dimensional stability, often fibers are added as a reinforcement. This paper provides a thorough review of past work on the FDM process with fiberreinforced polymers. Mechanical properties published in the previous work are summarized. Here, it is differentiated between data of discontinuous fiber-reinforced materials both parallel to the bead direction and transverse to it. Furthermore, available data parallel to the print direction for continuous fiber-reinforced material systems are shown. Next, past modeling work of the FDM method and its related physical phenomena are reviewed and discussed, yet with a special emphasis on FDM with fiber-reinforced polymers. However, due to a lack of published work, promising studies modeling neat polymer behavior are reviewed as well as they can act as initial points for future work. The findings are structured and reported based on these four different research areas of FDM: material flow and fiber orientation, interface formation, thermomechanical analysis of the deposited material and mechanical characterization of printed parts. Finally, this paper provides a detailed review of the FDM method with fiberreinforced materials and it explains relevant physical phenomena involved in the process as well as their couplings. Based on the review, the current status of the research is evaluated, future needs are formulated and suggestions for ongoing research are given.
ANTHONY FAVALORO, EDUARDO BAROCIO, NICHOLAS DENARDO, VLASTIMIL KUNC and R. BYRON PIPES
Nano-scaled Structures through Charged Particle Interactions
The challenge of additive nanomanufacturing is to create and assemble literally quadrillions of nano-scaled structures. That cannot be done with typical 1-nozzle / 1- trace techniques. Instead there must be a utilization of macro-scale (e.g. easily controlled) forces to assemble large numbers of structures in parallel. For example, physical and chemical vapor deposition use momentum, condensation, and chemical reactions to deposit coatings, grow nanotubes, and form nanostructures from gas-phase precursors in a vacuum. An intriguing challenge is to utilize similar techniques, but move from traditional gas-phase materials in a vacuum to nanoparticles at atmospheric conditions. That transition would enable 1) larger area coating and cheaper processes, 2) an important meso-scale of structures that are on the order of 100Ã¢â‚¬â„¢s of nanometers to micrometers, which is the scale for wetting, adhesion, and other surface interactions, and 3) a straightforward path to polymer-particle nanocomposite structures by depositing multiple materials either in parallel or in sequence. We are investigating additive nanomanufacturing using nanoparticles formed by electrospray ionization (ESI). One key challenge is to determine the interplay between particle charge/momentum and the electric fields at the substrate to control assembly. Relevant forces include attraction to the grounded substrate, repulsion from likecharged particles in the spray, and repulsion from previously deposited particles or electrode patterning on the substrate. We examine three different cases: an electrically nonconductive substrate (Parylene), a nonconductive substrate with 1 mm gold bands about 1mm apart, and a nonconductive substrate with 1 mm gold bands 3 mm apart. These three cases give vital insight into the length-scales at which structures begin to develop in large-scale nanoparticle deposition systems. We show that the nature of the substrate did not appear to significantly affect the bulk spray, but deposition was strongly focused onto conductive, grounded bands. We also showed a gradient in the deposition with a length scale on the order of 1-100 ÃŽÂ¼m.
Additive Manufacturing for Bonded Composite Joints
Interface design is very critical for bonded joints in fiber reinforced polymer composites (FRCPs) and a smart designing technique should be developed to minimize the damage and failure incurred by weak bond interfaces. It is hypothesized that improvement in strength and toughness can be achieved by imparting structural texture at the adhesively bonded joint interfaces. Towards that, a novel technology using additive manufacturing at the bond interfaces has been explored in this paper. Various computational models were developed to simulate stress distribution along the overlap region of single lap shear joints. The models that performed best were chosen for manufacturing and testing. Pure adhesive joints were manufactured first, followed by two 3D-printed adhesive (3D-PA) joints. Peak loads, shear stresses, and failure types were compared between each design. It was observed from this study that pure adhesive joints failed predominantly adhesively, resulting in low peak loads and shear strength (average 1502.06 KPa). According to 3D-PA results, model 1 and 2 registered an average shear strength of 6716.45 KPa and 10050.24 KPa, respectively exhibiting mainly cohesive type failure. Overall, the experimental results corroborated the expected behavior from the computational predictions, suggesting a major improvement imparted by the additively manufactured bonded joint reinforcements.
Techniques for Relating Stresses and Strains in Fabrics and Fiber-Reinforced Composites between Various Hierarchical Scales
Fiber-reinforced composites and fabrics in armor systems and other structures subjected to high-rate impacts resist perforation through multiple deformation and failure mechanisms, which individually affect the constitutive stress-strain behavior of these materials. The complex behavior of these materials requires either a complex mesh geometry, as is characteristic of mesoscale models, or a complex material model, as is characteristic of continuum models. However, many purported continuum material models for composites in the literature require separate elements for different unidirectional lamina and only consider the effects of stresses and strains on the material behavior at the lamina scale, i.e. the material model has little knowledge about the fibers or matrix constituents that make up the composite. A full understanding of the average stresses and strains at the element, lamina, and constituent scales requires methods to relate stresses or strains at one scale to those at another scale. Techniques used to bridge these scales are briefly presented here along with initial results comparing results using the modeling approach to ballistic impact data found in the literature. The simulations shown here are of dry fabric layups, which are conceptually and computationally simpler, but some initial discussion of how the techniques apply to full composites is also presented.
SIDNEY CHOCRON and CHARLES ANDERSON, JR.
Ballistic Strength of Kevlar KM2 Fabric Resisting FSP Projectiles
A micro-level numerical tool, based on an explicit digital element approach (DEA), is developed to simulate impact and penetration of textiles using projectiles of arbitrary shapes. In this approach, the projectile surface is discretized into surface triangle elements and the fabric is discretized into digital elements. A special algorithm is employed to search contacts between digital elements of the fabric and surface elements of the projectile of arbitrary shape. In each simulation step, forces applied to projectile surface elements and digital elements are calculated. Then, projectile motions, such as velocity and acceleration, can be calculated based on the resultant force applied to projectile surface elements and the fabric deformation can be calculated based upon the force applied to digital elements. In the previous DEA analysis, the projectile was simplified as a rigid body particle. As such, only spherical and cylindrical projectiles could be modeled. The new Algorithm proposed in this paper expands the capability of the DEA to simulate ballistic impact using projectiles with arbitrary shapes. In this paper, the contact search algorithm and the contact force calculation between the solid body projectile and the fibrous fabric are explained in detail. Firstly, the surfaces of spherical and RCC projectiles are discretized into triangle meshes. Ballistic strength V50 derived from the discretized surface mesh are compared to results derived from the previous approach in order to verify the accuracy of the method. Then, the new algorithm is applied to simulate the ballistic impact using FSP the projectile. The numerical results are compared to real scale standard ballistic tests using FSP.
YOUQI WANG, MARIO DIPPOLITO, CHIAN-FONG YEN, JAMES ZHENG and VIRGINIA HALLS
Low Velocity Impact Damage and Response of Stringer Stiffened Composite Panel
Engineering applications with complex composite structures like hat stringer stiffened flat panels when subject to impact result in a varying degree of damage, damage behavior, and loading paths compared to traditional composite panels. These differences are investigated in a co-cured stiffened T800/3900-2 carbon/epoxy laminate by subjecting it to low velocity impacts with varying impactor tip radii (12.7 and 76.2 mm), impact location (edge and center of stringer flange), and impact type (metal versus rubber protected). For all cases, the damage response is quantified (force time history and delamination area) and the energy is held constant at nominally 90 J. Impact of the 5 kg pendulum head occurs at 5 m/s. It is shown that the onset of delamination and the delamination area are independent of the impactor radius in the stringer stiffened panels. Similarly, the force time histories at a given site are independent of the tip radii (e.g. both 12.7 and 76.2 mm striking the center of the stringer/panel interface had similar force response). The force time history results show that the contact force at the center of the stringer is higher than that at the edge due to the added stiffness of the stringer and panel interface. The impactors fitted with rubber tips reduces the contact force levels by up to 15% and 8% at the center and edge of the stinger, respectively. Finite element simulation of the 12.7 mm radius tip at the edge of the stringer/panel interface were validated with the experimental data using ABAQUS/Explicit.
Auxetic and Hybrid Honeycomb Structures for Energy Absorption Applications: Design and In-plane Dynamic Crushing Behaviors
Honeycomb structures have been used in various impact energy absorbing applications because of high performance and low density properties. Recent studies have shown that auxetic honeycombs (with negative angle ÃŽÂ¸) display lateral expansion when stretched and densification when compressed because of their negative PoissonÃ¢â‚¬â„¢s ratio (NPR). These structures have been extensively studied due to unique mechanical properties and excellent energy absorption ability that can be implemented in various engineering applications such as aeronautics, biomedical engineering, automobile industry, military affairs etc. In this study we present a new design of auxetic-strut structure and hybrid of auxetic-strut and honeycomb structure for inplane performance improvement. A comparative study of, design and in-plane dynamic crushing behavior of regular honeycomb, re-entrant auxetic, auxetic-strut and two hybrid structures of combining regular honeycomb and auxetic-strut structures was carried out. Finite element modelling is carried out to compare the in-plane dynamic crushing behavior of these structures at different impact velocities. Deformation mechanism of these structures was studied, that gave an insight on how to control the deformation of a structure and tailor the properties. The honeycomb and re-entrant auxetic structures deform and collapse side-ways, and the auxetic-strut structure along with hybrid structures collapse within it-self making them denser as they deform. The amount of energy absorbed by all the structures was found out. The energy absorbed by auxetic- strut and hybrid structures was half when compared with honeycomb and re-entrant auxetic structures at lower strain levels. But at higher strain levels it was twice as that of the later. These results are valuable to provide new fundamental understanding of structure-property relationship for new auxetic-strut and their hybrid honeycomb core structures.
Numerical Investigation of the Damage in Composite Materials Under Dynamic Loads Using a Combination of Intralaminaire and Interlaminaire Model
Numerical finite element method using solid elements discretisation is increasingly used to simulate the impact behaviour of composite materials. In order to describe the intralaminar damage of composite materials, a three dimensional damage model is investigated. The purpose of this work is to explore the effect of the failure criteria on the dynamic failure properties of carbone fibre composite laminate. The intralaminar damage model is performed with different failure criteria including the twodimensional Hashin criteria extended to the three-dimensional case and the threedimensional Hashin criteria for the fibres coupled with the Puck criteria for the matrix. The two criteria were than incorporated in a VUMAT subroutine implemented in the explicit finite element code Abaqus. Furthermore, the delamination is simulated by the bilinear cohesive zone model (CZM) available in the finite element code Abaqus. To validate the material model implemented in the VUMAT subroutine and the effect of its association with the CZM, the numerical impact computations performed for various incident velocities were compared to the experimental measurements available in the literature. The results show that the impact response in term of residual velocity presents some difference due essentially to the effect of the criteria used in the intralaminar model. The Hashin criteria for the fibres associated with the Puck criteria for the matrix presents good agreement with the experimental data. The association of the intralaminar model with the CZM leads to a slightly larger value of the residual velocity. This research work outlines the importance of the use of appropriate failure criteria at high velocity impact analysis.
Development of a New Finite Element Simulation Strategy for Prediction of Thermal and Resin Shrinkage Deformations of Composite Parts During Cure
In order to achieve first-time-right in production of aircraft parts and to reduce overall costs due to time consuming manual rework, it is necessary to come up with efficient and precise simulation strategies to predict manufacturing induced deformations. These undesired deformations cause deviations from the nominal shape leading to increased scrap rate and difficulties during the assembly process. In composites the spring-back or spring-forward is generally induced by the resin shrinkage and layupÃ¢â‚¬â„¢s anisotropic thermal properties. In this paper a new finite element simulation strategy for prediction of thermal and resin shrinkage deformations of composite parts during cure is presented. The effect of the layup and the thermal and shrinkage fraction of the spring-back or spring-forward deformation is analytically quantified. The developed approach is applied to a single and double curvature part and numerically validated.
3D Permeability of Thick-section Off-axis Glass Fabric Vinyl ester Composite by VARIM Processing
The vacuum-assisted resin infusion molding (VARIM) method has been increasingly used in manufacturing large composite structures, such as MW-scale wind turbine blades, due to structural size, cost-effectiveness and out-of-autoclave curing requirements. Among various material and processing parameters, 3D permeability of composites with thick section laminate construction is critical in controlling resin flow in an efficient manufacturing process development and in accurate numerical modeling and simulations. Resin flow in a mold with complex geometries, such as a wind turbine blade with a perforated foam core and multiple fabric orientations, is affected by its through-thickness, 3D permeability. The objectives of this research are to: (1) establish analytical models to include 3D flow and off-axis flow front tracking, and (2) determine orientation and principal components of permeability tensor for two types of E-glass fabrics in both homogeneous and symmetric, hybrid lay-ups, such as those found in a spar cap. The number of fabric layers is varied to investigate scaling and compressibility. A noninvasive point infusion method with vinyl ester resin as the infusing medium was used. In experiments, the flow front was tracked by a video camera, and both the laminate thickness and the mass flow rate were measured. The measurement method required an isotropic transform solution for the Laplace equation to relate the flow rate through the inlet tube, DarcyÃ¢â‚¬â„¢s Law, and an expanding hemisphere. The orientation of the principal axes was calculated by comparing the in-plane flow front along the 0Ã‚Â°, 45Ã‚Â° and 90Ã‚Â° axes. The analytical models provide a method to describe the permeability tensor with the flow front position, laminate thickness and fabric and resin properties as the only inputs. The results provided reasonable values for the components of the permeability tensor and demonstrated that the analytical models behaved well as laminate thickness and the flow front radius increased.
Composite Cure Process Modeling and Simulations using COMPROÂ® and Validation of Residual Strains using Fiber Optics Sensors
Composite cure process induced residual strains and warping deformations in composite components present significant challenges in the manufacturing of advanced composite structure. As a part of the Manufacturing Process and Simulation initiative of the NASA Advanced Composite Project (ACP), research is being conducted on the composite cure process by developing an understanding of the fundamental mechanisms by which the process induced factors influence the residual responses. In this regard, analytical studies have been conducted on the cure process modeling of composite structural parts with varied physical, thermal, and resin flow process characteristics. The cure process simulation results were analyzed to interpret the cure response predictions based on the underlying physics incorporated into the modeling tool. In the cure-kinetic analysis, the model predictions on the degree of cure, resin viscosity and modulus were interpreted with reference to the temperature distribution in the composite panel part and tool setup during autoclave or hot-press curing cycles. In the fiber-bed compaction simulation, the pore pressure and resin flow velocity in the porous media models, and the compaction strain responses under applied pressure were studied to interpret the fiber volume fraction distribution predictions. In the structural simulation, the effect of temperature on the resin and ply modulus, and thermal coefficient changes during curing on predicted mechanical strains and chemical cure shrinkage strains were studied to understand the residual strains and stress response predictions. In addition to computational analysis, experimental studies were conducted to measure strains during the curing of laminated panels by means of optical fiber Bragg grating sensors (FBGs) embedded in the resin impregnated panels. The residual strain measurements from laboratory tests were then compared with the analytical model predictions. The paper describes the cure process procedures and residual strain predications, and discusses pertinent experimental results from the validation studies.
TYLER HUDSON, TAN-HUNG HOU and BRIAN GRIMSLEY
Homogenization of Linearly Elastic Materials with Pores of Irregular Shapes via Direct FEA and Single Pore Approaches
We compare two approaches to homogenization of linearly elastic solids with irregularly shaped pores: direct finite element analysis (FEA) of periodic representative volume elements (RVEs) and micromechanical modeling using the elasticity solution for a single pore. To generate periodic RVEs for the direct FEA approach, a simplified algorithm of collective rearrangement type is utilized. Homogeneity and isotropy of the generated RVEs are confirmed using two-point statistics (also known as covariograms). The Ã¢â‚¬Å“single poreÃ¢â‚¬Â micromechanical modeling approach is based on the cavity compliance contribution tensor (H-tensor), which is calculated for a pore in a large reference volume by an automated FEA procedure. The resulting H-tensors are then used in non-interaction, Mori-Tanaka, self-consistent and Maxwell homogenization schemes. Results are obtained for microstructures containing spherical, prolate spheroidal, cubical, and a specific case of irregular pore shape. They show good correspondence between direct FEA simulations of periodic RVEs and analytical micromechanical predictions of Mori-Tanaka and Maxwell schemes while self-consistent scheme significantly underpredicts the effective stiffness of porous materials.
BORYS DRACH, ANTON TROFIMOV and KOSTIANTYN VASYLEVSKYI
Modeling Aperiodic Dimensionally Reducible Structures Using Mechanics of Structure Genome
This work systematically constructs multiscale constitutive models for aperiodic dimensionally reducible structures (beams/plates) using Mechanics of Structure Genome (MSG). MSG simplifies the original complex analysis into a dimensionally reduced structural analysis and a constitutive modeling over the Structure Genome (SG). The approach starts with expressing the kinematics including both displacements and strains of the original heterogeneous structures in terms of those of the dimensionally reduced structures and fluctuating functions. To ensure the kinematic equivalency of the original heterogeneous structures and the dimensionally reduced structures, integral constraints on the fluctuating functions are developed. Then based on the principle of minimum information loss, the variational statement governing the SG can be formulated and solved. As this theory does not require boundary conditions, one is free to choose the analysis domain of arbitrary shape and they need not to be volumes with periodic boundaries. This theory can also handle periodic microstructures by enforcing the periodicity of the fluctuating functions. As a partial validation of this new theory, the results using the current approach are shown to converge to those of periodic heterogeneous beam/plate structures obtained using periodic boundary conditions.
A Micromechanics Based Processing Model for the Curing Response of a Unidirectional Fiber Reinforced Composite
A micromechanics based thermal model was developed to predict the temperature and degree of cure (DOC) distribution inside a unidirectional fiber reinforced composite during the curing process. Using the concentric cylinder assemblage (CCA) model as a representation of the composite microstructure, the effective thermodynamic properties of the composite were computed based on the fiber and matrix properties through closed-form expressions. By incorporating the cure kinetics of epoxy as the heat generation term in the thermal analysis, the temperature field of the homogenized composite model was solved based on the prescribed cure cycle at the boundary. Meanwhile, this micromechanics model was implemented at the integration point of the homogenized model to predict the local temperature and DOC fields at the microscopic (fiberÃ¢â‚¬â€œmatrix) level. Since the heat flux at the axial direction is negligible, this axisymmetric CCA model can be further reduced to 1D heat transfer problem, which can be efficiently solved through the finite difference method. The proposed homogenization scheme and the 1D heat transfer model were validated through a full 3D finite element model. This study suggests that the curing response is influenced by the size of the composite under the same cure cycle.
Polymer Nanocomposite in Flexible Electronics Packaging
Commercial electronic products are not specifically designed to perform in extremely transient high impact scenarios. Future electronic device packagings to perform in high impact scenarios need to exhibit and meet electrical performance along with mechanical flexibility. The new materials in this study consist of an elastomeric matrix with nano constituents forming a percolated electrically conductive network and therefore exhibit very high electrical conductivity, flexibility, and shock absorption. For electrically conductive and strain-resilient materials, Ag-CB (silver-carbon black)/thermosetting epoxy polymer and Ag- CB/thermoplastic polyurethane composites were developed. Excellent electrical conductivity could be achieved for these nanocomposites. The morphology of the nanocomposite was investigated by transmission electron microscopy (TEM), indicating that the Ag-coated carbon blacks and/or silver nanoparticles were wellconnected to form the network; resulting in excellent electrical conductivity of the nanocomposite. Split Hopkinson Pressure Bar (SHPB) test of the Ag-CB/epoxy nanocomposite showed that this material exhibited excellent durability and small changes in electrical conductivity, indicating this material could be promising to be used as the solder material in flexible electronics. These nanocomposite materials were also successfully used to print flexible circuits using a 3D-printing. The electrical resistance change under tensile strain for the Ag-CB/TPU nanocomposite printing as a trace on polyethylene terephthalate (PET) showed a small increase of ÃŽâ€R/R0 until 40% strain. Simultaneously, there are almost no changes of the electrical resistance after the bending cycles (fatigue test) for Ag-CB/TPU on PET. So it is promising for these highly conductive polymer nanocomposites to be used as an alternative solution for electronic materials needed to perform in extremely transient high impact scenarios.
SABYASACHI GANGULI, AMANDA SCHRAND and AJIT ROY
Carbon Nanotubes Influence on Natural/Synthetic Hybrid Composites Mechanical Properties
This investigation focuses on hybrid composites nano-modified by carbon nanotubes. Flax fibers were intercalated by E-glass fibers. The entire system is consolidated by a nanocomposite Ã¢â‚¬â€œ multi-walled carbon nanotube (MWNT) dispersed into the epoxy resin. A failure mechanism change from brittle to ductile was observed into the fibreless specimens with MWNT during tension. The flax/epoxy specimens under tension presented a non-linear behavior. The stressstrain behavior returns to linear when the hybridization takes place. The hybrid composites failure mechanism, however, seems to be much more complex than the ones with E-glass or carbon fiber/epoxy. At the initial tensile stage the flax fibers waviness leads to an increase on stiffness, which can be observed at the initial part of the stress-strain curve. The decrease on stiffness for the second part of the non-linear curve could be explained by the inter-yarn interactions due to the friction between the now straight flax fibers, which can lead to a fiber/yarn debonding. The fiber/yarn debonding brings as consequence the failure at interface flax/epoxy. In the case of this research, the inter-ply interaction cannot be neglected, as it seems to be the final stage of the damage mechanism.
FERNANDA LIMA, CAMILA SILVA, SUCHILLA LEAO, GUILHERME ARANTES, MARINA MARTINS and ANTONIO
Carbon Nanotubes to Improve Short Glass Fiber Composites
This paper discusses the potential use of multi-walled carbon nanotubes (MWCNTs) to improve the mechanical properties of short glass fiber composites. Two methods to incorporate MWCNTs in short glass fiber composite fabrication were examined. First, Carboxylic-functionalized MWCNTs were dispersed in a silane fiber sizing material and then were allowed to bond to the surface of glass fibers. Second, MWCNTs were grown on the surface of glass fibers using the chemical vapor deposition (CVD) method. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used to examine the significance of MWCNTs on fiber surface. Fourier Transform Infrared (FTIR) analysis was also used to identify chemical reactions between MWCNTs, the glass fiber surface, and the sizing material. Short MWCNTsmodified glass fiber reinforced epoxy composites were also fabricated. Mechanical properties of the MWCNTs-modified composites were compared with short glass fiber composites produced with neat fibers using dynamic mechanical analysis (DMA). The experimental investigation showed that dispersing MWCNTs in a silane sizing agent is more efficient than growing MWCNTs on the surface of glass fibers. A significant improvement in the storage elastic modulus was observed when MWCNTs were dispersed in silane. FTIR analysis confirmed the formation of chemical bonds between both COOHÃ¢â‚¬â€œMWCNTs and the glass fiber surface. No significant difference in the loss modulus (i.e. time dependent behavior) of the glass fiber composites was found with the different methods of incorporating MWCNTs during composite fabrication.
ELISA BOROWSKI, USAMA KANDIL, AHMED AWADALLAH, ATEYYA ABOUL-ENEIN and MAHMOUD TAHA
Local Mean Fiber Orientation via Computer Assisted Tomography Analysis for Long Discontinuous Fiber Composites
Platelet-based long discontinuous fiber (LDF) composites have a unique microstructure with fiber collimation within platelets, discontinuity at platelet boundaries, and a large degree of heterogeneity at the macro-scale. This extreme macro-heterogeneity requires microstructural characterization of all regions of a part, as compared to the assumption that a few sample regions sufficiently represent all present microstructures. In this work, nondestructive computed tomography (CT) scanning with a meso-scale, 53ÃŽÂ¼m resolution (voxel edge length) from a North Star Imaging X-50 system is analyzed. This resolution will not allow for the distinct viewing of individual carbon fibers (5-7ÃŽÂ¼m diameter) or even distinct platelets (~100ÃŽÂ¼m thick), but is the highest resolution possible on the equipment used for the 65x65x65mm part studied. Two principles are used to extract micro-scale details from the meso-scale CT scan resolution: 1) There is fiber alignment locally within each platelet as evidenced in micrographs, and 2) The direction of least density change within a stack of aligned platelets is aligned with the fiber direction. Using Eigen analysis of the density gradients, the locally averaged fiber orientation at each voxel within the CT scan is determined. Optical microscopy of selected regions within the scanned part allow for comparison of the fiber orientation vector components in the global X, Y, and Z directions. The relatively low resolution CT scan does not provide exact characterization of distinct platelet boundaries, but still yields a mean local fiber orientation. This improved mapping of the entire part microstructure may be used to better understand the manufacturing process, compare with manufacturing simulations, or implement in a finite element model for simulations where discrete platelet boundaries are not required.
Assessing Progressive Failure of Large-Scale Composite Structures using a Damage-Based Multi-Scale Model
A damage-based multi-scale model was developed for predicting progressive failure of large-scale laminated composite structures subjected to practical multiaxial loading. Computational micromechanics was coupled with a continuum damage mechanics constitutive law in order to predict stiffness degradation of multidirectional laminates containing ply cracks in multiple orientations. An energy based technique based on linear elastic fracture mechanics concepts was used to simulate the evolution of ply cracks. The model was then implemented within commercial finite element software through a user-defined subroutine for evaluating subcritical damage evolution and stiffness degradation of wind turbine blades. The study is the first step in developing an accurate prediction model that accounts for the multi-scale nature of damage, which can be used for predicting progressive failure of large-scale composite structures. The results for a long wind turbine blade illustrated the capabilities of the developed simulation model to predict subcritical damage evolution. This is an important contribution since the developed model accounts for the early stages of progressive structural failure, which is vital for increasing the accuracy of damage tolerance analysis of structures and for structural health monitoring.
Multiscale Modeling for Prediction of Initial Matrix Crack in Laminated Composites
A multiscale approach based on the mesh superposition method is applied to unidirectional CFRP laminates to evaluate the influence of the singular deformation field at the free-edge region on crack formation. In our multiscale approach, two different scale analyses are employed. First one is a local analysis utilizing a model composed of carbon fibers with micron-scale diameters and matrix resin. Two failure criteria are applied to the matrix resin to predict the matrix cracks which occur under elastic deformation and plastic deformation. Second one is a global analysis which employs a homogenized model assumed to be an anisotropic elasto-plastic body. The global analysis is conducted to evaluate the macroscopic deformation behavior of laminates. The local model is superimposed on the global model, maintaining the continuity of the displacement field between global and local domains. The local analysis is then performed to predict crack initiation and crack propagation, using the displacement field obtained from the global analysis. The global analysis and the local analysis are conducted iteratively to obtain the convergence results considering the interaction between macroscopic deformation behavior and heterogeneity of the material. The multiscale approach is applied to the tensile tests of the unidirectional CFRP laminates under off-axis loading to predict the failure strain. Our simulated results reveal that the initial crack occurring on the free-edge region doesn't affect the failure strain.
A Biaxial-Bending Test to Observe the Growth of Interacting Delaminations in a Composite Laminate Plate
It is not easy to isolate the damage mechanisms associated with low-velocity impact in composites using traditional experiments. In this work, a new experiment is presented with the goal of generating data representative of progressive damage processes caused by low-velocity impact in composite materials. Carbon fiber reinforced polymer test specimens were indented quasi-statically such that a biaxialbending state of deformation was achieved. As a result, a three-dimensional damage process, involving delamination and delamination-migration, was observed and documented using ultrasonic and x-ray computed tomography. Results from two different layups are presented in this paper. Delaminations occurred at up to three different interfaces and interacted with one another via transverse matrix cracks. Although this damage pattern is much less complex than that of low-velocity impact on a plate, it is more complex than that of a standard delamination coupon test and provides a way to generate delamination, matrix cracking, and delamination-migration in a controlled manner. By limiting the damage process in the experiment to three delaminations, the same damage mechanisms seen during impact could be observed but in a simplified manner. This type of data is useful in stages of model development and validation when the model is capable of simulating simple tests, but not yet capable of simulating more complex and realistic damage scenarios.
Distributed Optical Sensing in Composite Laminate End-Notched Flexure Tests
The end-notched flexure test (ENF) evaluates the mode-II interlaminar fracture toughness (GIIc) for unidirectional fiber-reinforced polymer matrix composite laminates. To obtain correct GIIc values, care must be taken to locate the crack tip and verify that the growing crack front is straight. This is often done by performing an ultrasonic scan between each crack growth which is very time consuming. This paper presents a method for locating the growing crack front by observing the changing stain field through embedded distributed optical fiber sensors. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process are investigated. Specimens with and without embedded fibers are tested to verify that the embedded fibers do not affect the resulting GIIc values. True crack growth values are compared to the optical fiber data to validate the sensorsÃ¢â‚¬â„¢ ability to map the growing crack front shape.
RANI SULLIVAN, VIPUL RANATUNGA, KEITH VEHORN, KEVIN BROWN and STEVE OLSON
Development of a Novel In-plane Tension-Tension Biaxial Cruciform Specimen
This study presents a combined experimental and numerical investigation of an inplane tension-tension (IPTT) biaxial test geometry. The IPTT specimen was based on an existing design which was previously optimized to minimize stress concentrations in the corners of the cruciform geometry. To facilitate ex situ 3D Xray computed tomography (CT) imaging of the gage region, the overall dimensions of the specimen were scaled down. In parallel with the experimental effort, a numerical simulation using AFRLÃ¢â‚¬â„¢s BSAM code was performed to assess the feasibility of using high-fidelity progressive damage tools for design and optimization of biaxial test geometries. The single experiment performed in this study revealed that the IPTT test is highly sensitive to local geometric imperfections, which had an unexpected effect on initiation and evolution of transverse cracking and delamination in the gage region. This was concluded from post-mortem X-ray CT and optical imaging of the specimen, which revealed an intricate network of intralaminar cracks and delaminations that emanated from a machining flaw on the surface of the circular gage region. The numerical simulation of the idealized IPTT test geometry showed good promise in predicting the overall pattern of inter- and intralaminar damage. Overall, this study provided a wealth of experimental and numerical data that will be used as a starting point for further development of the IPTT test geometry.
DANIEL RAPKING, DAVID MOLLENHAUER and MICHAEL CZABAJ
Highly Stretchable Strain Sensor based on Polyurethane-modified Carbon Nanotube Buckypaper
This work focuses on the development of flexible strain sensors based on nanocomposites of thermoplastic polyurethane (TPU) and carbon nanotube (CNT) buckypaper (BP). A one-step filtration process is used to fabricate the TPU-CNT BP sensors, providing tailorability of CNT:TPU ratio. The developed sensors retain the porous morphology of buckypaper even at relatively high TPU content. Characterization of morphology, electrical conductivity, and mechanical properties is reported. In addition, electromechanical response was investigated for both quasistatic loads (strains up to 125%) and cyclic loads (1% and 5% strains). Cyclic tests show a repeatable response that becomes predictable after a few cycles. The combination of mechanical strength, electrical conductivity, and constant and high gauge factor makes this material promising as a free-standing sensing material.
KURTIS LAQUA, YADIENKA MARTINEZ-RUBI, MICHAEL JAKUBINEK, BENOIT SIMARD, DAESUN PARK and K
Design of Carbon Nanotube Sheet Embedded Fiber Composites with In Situ Structural Health Monitoring Capability
Damage accumulation in fiber composites is complex and incipient damage is difficult to detect using conventional non-destructive evaluation techniques. Different damage modes ranging from micro-level to macro-level (fiber breakage, micro and transverse matrix cracks development, fiber-matrix de-bonding, delamination, etc.), will affect the structural performance and durability of the materials. There is an emerging need for smart composite materials with self-damage sensing capabilities to increase the reliability of composite materials. Recently, advances in macroscopic forms of carbon nanotube materials provide the potential for structural health monitoring using carbon nanotube sheets integrated in the interlaminar areas of advanced fiber composites. This research explores the fabrication of fiber composites with laminated carbon nanotube sheets embedded for strain sensing and damage detection based on the spatial electrical property changes of the carbon nanotube sheets. The mechanical-electrical response behavior is recorded under multiple loading conditions, including quasi-static and impact loading, and the data are analyzed to determine the sensing ability. The manufacturing process of integrating the carbon nanotube sheets into the system strongly affects the sensitivity and repeatability of the integrated multifunctional sensors. The ultimate goal of this research is to develop reliable structural health monitoring methods capable of obtaining information on the damage state in real-time.
KAN LIU, GAO CHEN, DIRK HEIDER and ERIK THOSTENSON
A Biocompatible Pressure Sensor Based on Styrene-Isobutylene-Styrene (SIBS) and Carbon Black
The effect of carbon black (CB) loading percentage on the pressure-sensing capability of a thermoplastic elastomer (SIBS) composite was investigated. SIBS is well-known for its excellent biocompatibility and lack of foreign body reaction during in vivo use. As such, the goal of this effort is the exploitation of this characteristic to develop implantable and/or external (stick-to-skin) pressure sensors. This study represents an initial analysis of sensing performance as a function of carbon black content; a necessary step toward probing the interaction between the competing design goals of retained biocompatibility, suitable mechanical strength, flexibility, and sufficient carbon black content to meet or exceed the percolation threshold as required for sensor functionality. The SIBS/CB composites were manufactured through solution casting in toluene and a combination of high-shear mixing and ultrasonication. The dependence on carbon black content and optimum carbon loading for pressure sensing applications was determined for these composites by measuring the change in voltage (5V max) across the bulk film as a function of pressure via a custom Arduino/Matlab voltmeter. The highest sensitivity was recorded for samples with 20% CB by weight, which exhibited a voltage drop of 1.9 V at the maximum loading of 4861 Pa and a maximum rate of 0.380 mV/Pa. The optimum carbon black loading for maximum sensitivity, 20% by weight, is considerably higher than typical loadings intended to improve mechanical performance. Accordingly, design of pressure-sensing composites based on SIBS will likely require careful tradeoff studies specific to the requirements of each application. Fortunately, SIBS-based composites are uniquely suited to this type of application-specific tailoring due to the ability to adjust molecular weight, styrene content, and carbon black loading to achieve a wide range of material performance and functionality.
MAURO FITTIPALDI, ALEXANDRA DAMLEY-STRNAD and LANDON GRACE
Modeling Fatigue Damage Onset and Progression in Composites Using an Element-Based Virtual Crack Closure Technique Combined with the Floating Node Method
A new methodology is proposed to model the onset and propagation of matrix cracks and delaminations in carbon-epoxy composites subject to fatigue loading. An extended interface element, based on the Floating Node Method, is developed to represent delaminations and matrix cracks explicitly in a mesh independent fashion. Crack propagation is determined using an element-based Virtual Crack Closure Technique approach to determine mixed-mode energy release rates, and the Paris-Law relationship to obtain crack growth rate. Crack onset is determined using a stressbased onset criterion coupled with a stress vs. cycle curve and Palmgren-Miner rule to account for fatigue damage accumulation. The approach is implemented in Abaqus/StandardÃ‚Â® via the user subroutine functionality. Verification exercises are performed to assess the accuracy and correct implementation of the approach. Finally, it was demonstrated that this approach captured the differences in failure morphology in fatigue for two laminates of identical stiffness, but with layups containing ÃŽÂ¸Ã‚Â° plies that were either stacked inÃ¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â single Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â Ã¯â‚¬Â distributed through the laminate thickness.
Prediction of Delamination Migration at a 0Â°/Î¸ Ply Interface in Composite Tape Laminates
Delamination in composite laminates can migrate through the thickness, from one interface to another. The Ã¢â‚¬Å“Delamination MigrationÃ¢â‚¬Â test method allows a detailed observation of migration, and the evaluation of the parameters affecting it. Delamination migration tests conducted on specimens containing a 0Ã‚Â°/ÃŽÂ¸ interface demonstrated that migration is governed by the sign of the component of interlaminar shear stress perpendicular to the ÃŽÂ¸ fiber direction, Ãâ€ž23. At a 0Ã‚Â°/ÃŽÂ¸ interface, when the shear stress sign is favorable for migration, delamination tends to propagate along the ÃŽÂ¸ fiber direction, before migrating through the ÃŽÂ¸-oriented ply. The conditions at the 0Ã‚Â°/ÃŽÂ¸ interface, which determine either delamination growth along the ÃŽÂ¸ fiber direction or migration, were evaluated using the Virtual Crack Closure Technique (VCCT). The strain energy release rates in the direction of the fibers, Gf, and perpendicular to the fibers, Gm, were calculated, along with the total strain energy release rate, GT. The components Gf and Gm were then used with the shear stress sign, to account for the effect of both parameters on delamination growth and migration. Correlation with experimental results for 0Ã‚Â°/60Ã‚Â° delamination migration specimens showed that if the sign of Ãâ€ž23 is favorable for migration but Gf/GT is greater than Gm/GT, delamination tends to propagate along the ÃŽÂ¸ fiber direction and migration will not occur. Delamination can migrate only if Gf/GT is less than Gm/GT. This method improves the prediction of migration in the delamination migration specimen, compared to the analysis using the shear stress sign only.
In Search of a Time Efficient Approach to Crack and Delamination Growth Predictions in Composites
Analysis benchmarking was used to assess the accuracy and time efficiency of algorithms suitable for automated delamination growth analysis. First, the Floating Node Method (FNM) was introduced and its combination with a simple exponential growth law (Paris Law) and Virtual Crack Closure technique (VCCT) was discussed. Implementation of the method into a user element (UEL) in Abaqus/StandardÃ‚Â® was also presented. For the assessment of growth prediction capabilities, an existing benchmark case based on the Double Cantilever Beam (DCB) specimen was briefly summarized. Additionally, the development of new benchmark cases based on the Mixed-Mode Bending (MMB) specimen to assess the growth prediction capabilities under mixed-mode I/II conditions was discussed in detail. A comparison was presented, in which the benchmark cases were used to assess the existing low-cycle fatigue analysis tool in Abaqus/StandardÃ‚Â® in comparison to the FNM-VCCT fatigue growth analysis implementation. The low-cycle fatigue analysis tool in Abaqus/StandardÃ‚Â® was able to yield results that were in good agreement with the DCB benchmark example. Results for the MMB benchmark cases, however, only captured the trend correctly. The user element (FNM-VCCT) always yielded results that were in excellent agreement with all benchmark cases, at a fraction of the analysis time. The ability to assess the implementation of two methods in one finite element code illustrated the value of establishing benchmark solutions.
Parametric Homogenization Based Continuum Damage Mechanics Model for Composites
Micromechanical analysis of S-glass fiber reinforced epoxy composites is performed. The effect of strain-rate is modeled adopting a rate-dependent constitutive model for the composite system. It is computationally formidable to simulate any composite structure accounting for the microstructural details entirely. In the present research we develop a parametrically homogenization based continuum damage mechanics (PHCDM) model for composite failure. A composite plate under pressure wave is analyzed in ABAQUS with PHCDM model incorporated into user-subroutine.
ZHIYE LI, SOMNATH GHOSH and DANIEL Oâ€™BRIEN
Characterizing the Energy Absorption of Rigid Polymeric Foams under Compressive Direct Impact Loading
The use of rigid polymeric foams has been well-established to result in a significant improvement in the energy absorption and safety of the vehicles, owing to the excellent energy absorption characteristics of these materials. As an optimum energy-absorbing material system, polymeric foams need to dissipate the kinetic energy of the impact, while maintaining the impact force transferred to the protected object at a low level. Therefore, it is crucial to accurately characterize the load bearing and energy dissipation performance of foams at high strain rate loading conditions. However, there are challenges render to accurately measure the deformation response of foams due to their low mechanical impedance properties. Recently a nonparametric method was proposed and successfully implemented to enable the accurate measurement of the compressive constitutive response of rigid polymeric foams subjected to impact loading conditions. The method is based on stereovision high speed photography in conjunction with 3D digital image correlation and allows for accurate evaluation of inertia stresses developed within the specimen during deformation time. The inertia stresses are then superimposed with the boundary measured impact force to facilitate the measurement of full-field stresses in the specimen. Full-field distributions of stress, strain and strain rate are then used to extract the local constitutive response of the material at any given location along the specimen length. Accordingly, the local energy absorption curves are extracted and then averaged to give the effective energy absorption response of the examined foam. Finally, results obtained from the proposed non-parametric analysis are compared with the data obtained from the conventional test procedures.
Human Tissue Simulants for Study of Traumatic Brain Injury (TBI)
Blast exposure or head impacts in accidents or contact sports may cause Traumatic Brain Injury (TBI), which is one of most crucial and poorly understood areas of research in the 21st century. To date, human tissues especially the skin and brain tissues (white and grey matter especially) are difficult to obtain and work in a lab setting due to ethical and biosafety issues. This has not allowed for much experimental research into the study of the mechanics of brain tissues under impact or dynamic loading, which is the key to TBI investigation. In the current work, tissue surrogates were developed to precisely mimic the linear and non-linear mechanical behavior of the skin, white and grey matter tissues using a low cost, castable (to any shape or size), multi-part silicone based material system. The fabrication methodology involves the iterative mixing of the two or four parts of silicone at certain mix ratios (by weight) to generate a biomechanical behavior similar to the specific tissues at different strain rates. The non-linear stress versus stretch responses of the tissue surrogates were characterized using the YeohÃ¢â‚¬â„¢s hyperelastic constitutive model. Besides TBI studies, these novel tissue surrogates have no biosafety or handling issues, and could be indispensable for replacing cadavers and human tissues for clinical surgical trainings.
Investigation of High Velocity Impact Responses of a Glass/Epoxy Composite with a Gas Gun
The single stage gas gun testing methodology was investigated. To provide data for computational model development, quantitative measurements of force and displacement were attempted. The transmitted impact force was measured with four load cells installed behind the target. The out-of-plane deformation was measured with a projection grating profilometry method using a high speed camera. The velocity of the projectile was determined using images from the high speed footage. The force and deflection measurements were successful for tests conducted below the ballistic limit. The improved gas gun experiment was used to investigate the high velocity impact behavior of a S2-glass/SC15 epoxy composite. The ballistic limit was found to be 329 and 381 m/s for the 3.8mm (6-ply) and 6.2mm (10-ply) composite panels, respectively. The failure modes of the composite panels were inspected with backlight. The area of damage and failure was found first to increase and then become localized with increasing velocity. As a result, the panels absorbed the same or even a slightly higher amount of energy beyond the ballistic limit, until damage localization. Finally, a new method to compare the ballistic protection capability of different materials is presented.
ANDY STAMM, MATTHEW AUVENSHINE, ERYI HU, JAMES DORER and XINRAN XIAO
Experimental and Modeling Study on Ballistic Impact Behavior of a Woven Carbon/Epoxy Composite
Textile composites are becoming more and more popular in armor application due to their lightweight and excellent impact resistance properties. In this paper, we investigate the ballistic impact behavior of a plain-weave carbon/epoxy composite experimentally and numerically. The impact test is conducted using a gas gun system with a cylindrical projectile. The damage phenomena and impact tolerance of the composite plates are examined under various impact velocities (220~360 m/s). An analytical approach is developed based on energy equilibrium and information of fracture pattern. It predicts the impact threshold velocity in a reasonable accuracy and the profile of residual impact velocity after penetration. On the other hand, finite element model is developed to model the damage behavior of the composite plate. The results described the progressive failure process and predicted the fracture area in both the front-side and backside of the composite panel. Overall, the presented analytical and numerical models correlate well with the experimental results. They are useful for further investigation on the quantitatively analyzing the energy absorption and failure mechanisms.
GUANGMENG YANG, CHAO ZHANG and WENZHI WANG
Damage Behavior of Stitched CFRP Laminate on High-Velocity Rigid Body Impact
Stitched carbon/epoxy laminated composites have excellent mechanical properties such as higher delamination toughness, better impact resistance under low-velocity impact than those of conventional CFRP laminates. The stitching technique is expected that it can also improve high-velocity impact resistance. In this study, highvelocity impact test against the stitched material was performed and its delamination and absorbed energy was acquired. The damage process of the stitched laminate on high-velocity impact was simulated and compared with the experimental results. The high-velocity impact tests were performed with an air gun. Steel balls are used as projectiles. Two stitching parameters, density and thread thickness were considered. Non-stitched specimens were also experimented for comparison. Impacted specimens were investigated by ultrasonic inspection. The results suggest that stitched specimens have smaller delamination area than those not stitched. The thicker the stitching threads are and the denser the stitches are, the delamination area become smaller. A finite element model for the impact test was constructed to reveal the damage mechanisms of the stitched material. In spite of the analyses underestimated the delamination area, energy absorbed by specimen during impact load was calculated with accuracy. Damage propagation process of the stitched material during the highvelocity impact load was discussed.
Temperature Dependent Flexural Rigidities and Thickness Investigation
This paper discusses the characterization of temperature-dependent flexural rigidities of DyneemaÃ‚Â® HB80, a cross-ply thermoplastic lamina. The properties are then used in the modeling a thermoforming process. A vertical-cantilever experimental setup is presented to characterize the bending behavior at elevatedtemperature conditions. The material properties derived from the test data are implemented in a finite element model of the cross-ply lamina. Thickening of the lamina resulting from shear deformation is investigated and incorporated in the model. The finite element model uses a hybrid discrete mesoscopic approach, and deep draw forming of the material is simulated to investigate its formability to a hemispherical geometry. Simulation results are compared with an experimental forming trial to demonstrate the capabilities of the model to predict the development of out-of-plane waves during preform manufacturing and to inform the design of multiple-ply thermoforming with thickness variation data.
Multi-Die, Multi-Stage Pultrusion Process for Hybrid Composites: Degree of Cure and Temperature Profiles
Fiber reinforced polymer (FRP) composites have been rivaling conventional structural materials for several decades and are now pervasive in innumerable applications due to many attractive properties. Even though FRPs can be manufactured by many methods, pultrusion process is the most cost-effective and common technique for manufacturing constant cross-section composites. In this work, a thermo-chemical modeling study on an untapped and enhanced pultrusion concept known as Multi-Die, Multi-Stage (MDMS) hybrid pultrusion process based on US patent No. 6007655 is presented. This study is unique and novel, and has never been presented in the literature. Two dimensional (2-D), steady state, finite difference models (FDM) are developed for modeling a conventional Single- Die, Single-Stage (SDSS) process and the MDMS process. Since the purpose of this study is to understand the thermochemical modeling results of the MDMS concept, only steady state approach is used. Different pultruded thicknesses with varying pull speeds are considered. Fiberglass/epoxy composite is used as pultruded part for the first stage, while carbon fiber/vinyl ester composite is used for the second stage. Steel is used as a die material for both stages. The Gauss Seidel iterative scheme is implemented in MATLAB R2013b to solve the energy and species equations for the pultrusion process model. Pultrusion process for a rectangular flat plate is considered as a validation case. Due to the symmetry, only a half of pultrusion domain is modeled. The proper boundary conditions are applied to match the actual manufacturing process. Multiple pull speeds (200 mm/min, 400 mm/min, and 500 mm/min) with different composite thicknesses (20 mm and 25 mm) are considered in the present analysis to investigate the effects of composite thickness and pull speed on degree of cure. It is clear from modeling results that the short and long SDSS fail to achieve even moderate degree of cure. However, the MDMS achieves a high and uniform degree of cure. The developed model can further be used as a design tool for developing hybrid composites employing MDMS pultrusion process.
Specialized Elastomeric Tooling for Resin Infusion (SETRI) Applied to Bio-based Composites
The capabilities of Specialized Elastomeric Tooling for Resin Infusion (SETRI), a low-cost and -energy autoclaving alternative for consolidating and curing thermoset composite parts, are expanded to bio-based composites through a demonstration project. Specifically, multiple flat laminate parts (consisting of a 10-ply stack of woven cellulose fiber with a high bio-content, recyclable epoxy resin matrix) are infused with resin, consolidated between a temperature-controlled rigid tool and matching rubber-faced tool specially engineered to provide uniform pressure under load, and then thermally cured in place. As expected, parts made using this process are thinner, have higher stiffness and strength, and have fewer surface voids as consolidation pressure is increased. Experimental results reported include infusion observations, part thickness, surface roughness, tensile strength/modulus, and flexural strength/modulus. Following consolidation and curing, the recyclability of the cellulose textile is assessed by dissolving the commercially available bioresin in a dilute acetic acid bath to create a recyclable thermoplastic. Future work includes demonstration for complex 3D shapes with laminate and sandwich construction, recycling and testing of different natural fiber materials (e.g., jute, flax), recycling of the dissolved thermoplastic resin for future use and reuse of the recycled textiles in other biocomposite components.
Influence of Fiber Distribution on the Responses of Fiber Tows for Textile Composites
This research is focused on homogenized responses of different representative unit cells in fiber tows of textile composites. The fiber in a representative unit cell of the fiber tow locally is simulated as a unidirectional transversely isotropic fiber. Each representative unit cell has the same fiber volume fraction of fibers but different microstructural topology. The simulation results show that, under uniaxial homogenized compression in the fiber direction and under uniaxial homogenized compression in the transverse direction, microstructure topology has little influence on the maximum failure strength and homogenized tangential stiffness. Under in-plane shear homogenized deformation, microstructure topology has influence on the maximum failure strength but has little influence on homogenized tangential stiffness.
Micro-Mechanical Modelling of Fiber Tows Prior to Infiltration
One of the main drivers of the limited use of carbon fiber composites is the raw material cost. To bring this cost down, tows with higher fiber counts (~50k fibers) are becoming more popular. These so-called Ã¢â‚¬Å“heavy towsÃ¢â‚¬Â carry with them several challenges in manufacturing and a decrease in performance. Several aspects that affect the performance and manufacturability of heavy tows are the degree of fiber entanglement and amount of sizing. Highly entangled tows may increase the manufacturability because it binds the tow together, but non-aligned fibers result in decreased stiffness in the tow axial direction in the final part. Additionally, sizing is added to coat the fibers and adhere them together, allowing tow to stay together during processing. The present research introduces a model which will be used to look at the effects of such fiber-scale parameters on the mechanical response of a dry tow under different loading conditions, and results for tows with sizing will be presented. Methods for including these aspects will be highlighted, and conclusions will be drawn about the necessity of including fiber-scale parameters in predictive models to predict the behavior of dry carbon fiber tows during manufacturing.
Micromechanical Modeling of Oxidation Induced Stresses in SiC/SiC Composites
Silicon Carbide fiber reinforced composites are used in high temperature aerospace and propulsion applications due to their light weight, increased toughness and high thermal stability. However, their durability is limited due to environmental degradation. This paper focuses on the oxidation of SiC fibers and SiC/SiC composites at high temperature. The constrained volume expansion accompanying oxidation leads to development of large residual volumetric growth stresses. A micromechanical model was used to study the effect of oxidation induced stresses in a SiC fiber and in a unitcell of a SiC/SiC lamina. The oxidation induced strains are estimated by using Pilling- Bedworth expansion ratio and applied to the oxide layer as volumetric strains. The evolution of the oxide layer and stresses during isothermal aging was simulated using finite element methods. Two studies have been conducted considering with intact PyC and eroded PyC interphase regions. Paper describes the stresses developing during oxide growth with linear elastic and visco-elastic oxide layer behaviors. Simulations results are compared with experimental data and predictions from other models obtained from published literature.
Simulation of the Electromechanical Properties of Carbon Nanotube Polymer Nanocomposites for Strain Sensing
We present a Finite Element (FE) approach to model the conductivity and strain-sensing capabilities of Carbon Nanotube (CNT) reinforced polymer composites. A periodic Representative Volume Element (RVE) is constructed based on measurable statistical descriptors of the microstructure, and its response is simulated by concurrent electrical and mechanical FE analyses; the scatter of the predictions and their sensitivity to RVE size are explored. The model captures the nanoscale tunneling effect and its sensitivity to the imposed strain field. Predictions are found in good agreement with previously published data.
Molecular Dynamics Modeling of Carbon Nanotube Composite Fracture using ReaxFF
Carbon nanotubes (CNTs) are well known to have exceptionally high mechanical properties when measured individually. Recently, CNT fiber composites have been enabled by the production of high-tex yarns in quantities on the order of kilometers. These high-tex CNT yarns have recently become comparable in specific stiffness and specific strength to carbon fiber. Despite these advancements, CNT yarns still have mechanical properties substantially lower than their CNT constituents. Closing this gap requires understanding load transfer between CNTs and the role of matrix binders such as amorphous carbon at the nanoscale. This work uses reactive molecular dynamics simulations to gain a nanoscale understanding of the key factors of CNT nanocomposite mechanical performance and to place more realistic upper bounds on the target properties. While molecular dynamics simulations using conventional force fields can predict elastic properties, the ReaxFF reactive force field can also model fracture behavior because of its ability to accurately describe bond breaking and formation during a simulation. The upper and lower bounds of CNT composite properties are investigated by comparing systems composed of CNTs continuously connected across the periodic boundary with systems composed of finite length CNTs. These lengths, effectively infinite for the continuous tubes and an aspect ratio of 13 for the finite length case, result from simulation limitations. Experimentally measured aspect ratios are typically on the order of 100,000, so the calculated results should represent upper and lower limits on experimental mechanical properties. Finally, the effect of various degrees of crosslinking to the amorphous carbon matrix is considered in an attempt to identify the amount of CNT-matrix covalent bonding that maximizes overall composite properties.
Predicting Thermo-Mechanical Properties of PEEK using Reactive Molecular Dynamics
Polyether ether ketone (PEEK) is a high-performance thermoplastic that is used in a wide range of engineering applications, including some structural components of aircraft. The design of new PEEK-based materials requires a precise understanding of the molecular-level structure and behavior of PEEK composites. Molecular dynamics (MD) modeling can efficiently provide such physical insight and predict bulk-scale thermo-mechanical properties. In this project, MD modeling techniques for amorphous PEEK have been developed and used to predict the elastic modulus, glass transition temperature, and initial yield point. The predictions are shown to be in agreement with literature and experimentally-obtained values. These results indicate that the reactive force field (ReaxFF) can be successfully used to predict thermo-mechanical properties of PEEK.
MATTHEW RADUE, SORAYOT CHINKANJANAROT, DANIELLE KLIMEK-MCDONALD, JULIE TOMASI, JULIA KING
Damage Analysis of Various CNT Architectures in Nanocomposites Using a Multiscale Approach
A multiscale modeling framework is developed to study damage evolution and failure in carbon nanotube (CNT)-reinforced nanocomposites with different CNT configurations. The framework models the constituents of the nanocomposite explicitly, combines it in a single representative unit cell (RUC), and describes the evolution of damage in these systems under loading. The damage evolution methodology integrates information from atomistic simulations pertaining to polymer chain sliding and bond dissociation using continuum damage mechanics. Two different CNT architectures - randomly dispersed CNTs and radially grown CNTs are investigated and damage initiation sites and damage evolution trends are observed under quasi-static loading conditions. Results indicate that the presence of CNTs cause a unique stress state at the sub micro scale which can lead to accelerated damage progression and can be mitigated by architectural reconfiguration of the CNTs.
A Multiscale Approach for Progressive Fatigue Failure Modeling of a Woven Composite RVE
Use of fiber-reinforced polymer (FRP) composites is becoming ubiquitous in highperformance aerospace structures due to their superior mechanical properties. Fatigue is an important design factor for use in these applications and due to the anisotropic nature of FRPs, it is difficult to predict their fatigue. A multiscale physics-based methodology is required to replace the phenomenological fatigue theories for FRP composites, which work for only specific scenarios and require extensive characterization data. Such a theory based on the kinetic theory of fracture (KTF) was used in this study to model fatigue in a plain woven fabric composite representative volume element (RVE). Fatigue damage initiation, propagation path and the damage amount in the constituents of the woven RVE as a function of load cycles were investigated. The S-N curves for the tension-tension and shear load cases were also determined.
Effect of Notch-Induced Strain Gradients on the Applicability of Multiscale Approaches for Woven Composites: Combined Experimental and Computational Investigation
Woven composites are widely used in the aerospace and wind energy structures due to their high strength to weight ratio and their ability to conform to irregular shapes better than unidirectional composites. These structures frequently contain stress risers such as notches, fasteners, and pre-existing damage, all of which can act as failure nucleation points. The complexities associated with accurately modeling damage initiation and progression in woven composites have led to the development of multiscale models for this class of materials. However, the intrinsic size of the tows in a woven composite is often of the same order as the size of defect features in these structures, which can invalidate many multiscale assumptions that are typically used and have been shown to work well in unidirectional composites. For woven composites, the problem is identified for 12k RVE size by comparing computationally calculated and experimentally determined OHT strengths. In our previous work we utilized a multiscale modeling approach to quantify a critical regime around a hole in an open-hole tension (OHT) test [1]. In the current study we conduct an experimental investigation to compare with finite element analysis. For each OHT test, [0]n and [0/45]ns layup is manufactured. OHT tests are performed for two ratios of hole-size to RVE size. Strains calculated from Digital correlation analysis are plotted across the hole perpendicular to loading directions. These strains and strain gradients are compared with quantities calculated with homogenized model and discrepancies are discussed.
Microscale Investigation of the Compressive Behavior in Unidirectional PMCs Through In-situ SEM and X-ray CT Experiments
Experimental compressive studies have been carried out on unidirectional carbon fiber composite samples at the sub-ply scale in order to observe failure mechanisms within the microstructure. The experiments were conducted in-situ within an SEM and X-ray ÃŽÂ¼CT using a specially designed fixture allowing the same test to be conducted in both instruments without removing the test specimen. The combination of testing in SEM and CT allow for the gathering of high resolution images suitable for DIC during loading from SEM imaging and complete deformation understanding achieved through CT scans under load. Observed out of plane deformation through X-ray CT, proved essential to understanding the deformation. The testing scope of this project is from samples containing a handful of fibers to scores of fibers in order to link compressive failure to fiber micro buckling, global buckling, or crushing and fiber failure. Image analysis from CT reconstructions has been applied to track fiber rotation while in a deformed loading state and allow an exact model of the sample to be generated as the fiber locations throughout the sample are known.
DAVID MOLLENHAUER, BOB WHEELER, ALI KADHIM and NATHAN SESAR
A Single Fiber Peel Test to Measure Fibrillar Interactions in Ultra High Molecular Weight Polyethylene Fibers
This paper examines the effects of meso/nanostructure in ultra high molecular weight polyethylene (UHMWPE) fibers on their macroscopic mechanical properties. Tensile strength and stiffness in UHMWPE fibers is achieved through a drawing process that results in the extension and alignment of the polymer chains. However, the drawing process also yields a complex fibrillar hierarchy at the meso/nanoscale. Nanoscale fibrils, the fundamental structural feature of the fibers, are consolidated through the drawing process with physical branches connecting adjacent fibrils, creating a 3-dimensional network. The interaction of fibrils through consolidation and network junctions has implications for the way load is transferred through the fiber in applications involving tensile and transverse compressive loads. A single fiber peel test has been designed to quantify fibril interaction for fibers of different levels of draw and subsequently different mechanical properties. A miniature peel test apparatus, along with high speed cameras, is used to measure and observe the peeling of the fiber and separation of the fibrillar network. Results indicate an increase in adhesive fracture energy for fiber having undergone a higher draw ratio. Images acquired during the peel test indicate that higher fracture energy is directly related to consolidation of fibrils into larger bundles influencing the deformation of the fibril network. These results are explored and the implications for the relationship between processing, structure, and properties are discussed.
The lighter material is wished for a structure part of portable electronic devices and general home electric appliances. The compound material of the light weight and high strength is studied in recent years, but it is a problem to become high cost as a result because the raw material cost and the processing cost is high. ItÃ¢â‚¬â„¢s also difficult for recycle the reinforced compound material by a glass fiber and a carbon fiber. On the other hand, nonwoven fabric of core and sheath is cheap and high productivity with comparative. In addition, if one unity composite fibers are used for both core and sheath, the molded composites can be recycled easily. And with that, it was aimed that developing a composite material has both characteristics of low cost, light weight and high recyclability by using nonwoven fabric of core and sheath type one unity composite fiber. PP / PP and PET / PET are used for the raw nonwoven fabrics in this study. When they are molded, the core part becomes fiber part and the sheath part becomes the matrix part because the only sheath part is melted. The molding conditions is chosen by using DSC. Firstly, nonwoven fabric of core and sheath type one unity composite fiber was molded by using hot press machine. Subsequently, it is carried out that evaluation of tensile strength and tensile elastic modulus. Finally, cross section observation by SEM was carried out. As the result, the strength of PP/PP composites showed three times of only PP resin and the strength of PET/PET composites showed three times of only PET resin but these elastic modulus were not reinforced effectively.
TSUTOMU SETO, RYO MARUI, TAKAYUKI SAKAI and HIROYUKI HAMADA
Development of an Experimental Setup to Analyze Carbon/Epoxy Composite Subjected to Current Impulses
The use of composite materials is significantly growing in the aircraft industry because of their lightweight, high specific mechanical properties, such as strength and stiffness. These benefits are a potential improvement in aircraft design to save weight and reduce fuel consumption. A carbon fiber reinforced polymer (CFRP) composite is one of the types used for aircraft structural applications. Carbon fibers are mechanically stronger and lighter than aluminum. In a CFRP composite, carbon fiber acts as reinforcement and provides mechanical strength, and epoxy resin is the supporting matrix. Therefore, carbon fibers greatly improve compositeÃ¢â‚¬â„¢s overall material properties. They can be fabricated in a desired stacking sequence, with various fiber orientations. From an electrical point of view, a composite can be designed so that it will conduct current asymmetrically in one direction as compared to another. A fiber orientation in a CFRP composite results in strong anisotropic material properties. This indicates that the composite exhibits different conductivity in each direction, along the fibers or transverse to the fibers. In this paper, variation in electrical properties of CFRP caused by relatively low magnitude current impulses discharged through CFRP coupons are reported, and internal changes of the composite, due to current impulses, are studied. Based on electrical resistance measurements caused by these currents, property changes in CFRP composite coupons of two different carbon ply orientations are compared. Furthermore, it will be investigated if the changes caused by current impulses are mainly focused in the region of the current injection at the edges of composite coupons, or are they distributed uniformly through the bulk of the composite.
JUHYEONG LEE, MICHAEL MAZZOLA and THOMAS LACY
Through-thickness Electrical Conductivity of Toughened CFRP Laminate
CFRP laminated composites have orthotropic electric conductivity. Despite the existence of insulator resin-rich layers between each ply, the toughened CFRP laminates have electric conductivity in the thickness direction, which plays important role in delamination monitoring with changes in electrical resistance or numerical simulation on lightning current damage. However, the measured electric conductance has not been confirmed to be a material property for toughened CFRP. The objective of the present study is thus to clarify the mechanism of realizing electric conductivity in the through-thickness direction. During the curing process, alternating current is applied, and impedance is measured for toughened CFRP. It is found that conductivity in the through-thickness direction of toughened CFRP laminates is due to chance carbon fibre contact between plies during the curing process, and is not a material property.
Self-sensing of Viscoelastic Phenomena in Multiscale Composites by Using the Electrical Resistance Approach
The potential use of multiscale carbon nanostructures to sense viscoelastic phenomena in polymer composites using the electrical resistance method is explored, including glass fibers modified with multiwall carbon nanotubes (MWCNTs) and carbon nanotube yarns embedded in a vinyl ester matrix. Conventional carbon fibers are used as reference material. The intended application demands detailed knowledge of the electrical behavior of the investigated fibers. To this aim, DC current-voltage measurements of individual glass fibers covered with MWCNTs, carbon nanotube yarns, and conventional carbon fibers were conducted. The MWCNTs are deposited on glass fibers following an electrophoretic method assisted by ultrasonic MWCNT dispersion in distilled water, and a parametric analysis of the main factors affecting such electrophoretic deposition is carried out. The electrical resistance of 20 mm long- CNT-modified glass fiber tows is between 42-200 kÃŽÂ©/cm, and the best conditions were achieved with a deposition time of 30 min and an electric field intensity of 4.5 kV/m. Overall, current-voltage measurements of MWCNT-modified glass fibers, carbon nanotube yarns and carbon fibers shows a rather Ohmic behavior in a range of 0.1-2.5 ÃŽÂ¼A, but the measurements present local variations from linearity. These local variations from linearity are larger for carbon nanotube yarns (up to 90 %) and MWCNT-modified glass fibers (52 %). Preliminary tests of relaxation-induced piezoresistivity of monofilament composites showed that the electrical resistance of carbon fiber/vinyl ester specimens increases with elapsed time, while it decreases with time for carbon nanotube yarn/vinyl ester composites.
JOSE KU-HERRERA, OMAR RODRIGUEZ-UICAB, ALEJANDRO MAY-PAT, FIDEL GAMBOA, JANDRO ABOT and FR
Challenge Problems for the Benchmarking of Micromechanics Analysis: Multiscale Designer Level I Results
Following from Ã¢â‚¬Å“Challenge problems for the Benchmarking of Micromechanics Analysis: Level I Initial ResultsÃ¢â‚¬Â by Ritchey, Goodsell, Sertse, Yu, and Pipes [1]; this paper presents the results of all six level I micromechanics simulation challenge problems using Altair Engineering Multiscale Designer. The micromechanics simulation challenge problems consist of three levels. The challenge problems specified in level I and the focus of this paper consider only linear-elastic phase constituent behavior. The objective is to compute the homogeneous effective properties (homogenization) and extract the local fields (dehomogenization) along predefined paths though the unit cells of each of the six level I challenge problems. These results will be benchmarked against 3D FEA of the specified boundary value problems. Level II challenge problems will extend Level I by including non-linear phase constituent behavior including elastic-plastic constitutive relations. Level III challenge problems will extend Level II by including damage and failure prediction. It is the intent of the micromechanics challenge problems to compare the relative strengths and trade-offs of the participating tools and identify areas currently unaddressed by the micromechanics field at large. In 2015 Altair Engineering completed the acquisition of Multiscale Design Systems, LLC, the developers of MDS [2], a family of products focused on micromechanics, microstructural optimization, and life prediction of complex materials. MDS was renamed by Altair Engineering to Multiscale Designer [3] and is being released under the Altair HyperWorks suite of tools. Multiscale Designer is a mathematically rigorous practical engineering tool for the seamless integration of modeling, simulation, testing, and optimization of engineered products involving materials of multiple scales. It provides a complete environment for the design and analysis of engineered components made from heterogeneous material including; continuous fiber composites, long and short chopped fiber composites, honeycomb, metals, reinforced concrete, soil, bones, and various other heterogeneous materials. Multiscale Designer has macro solver plugins to commercial FEA codes; OptiStruct, RADIOSS, LS-Dyna, and Abaqus.
Improved Methods for Quantifying and Designing for Impact Damage Tolerance
Aerospace industry primes have indicated that improvements to current methods for characterizing damage tolerance of full-scale composite structures are needed. A methodology to address this need is being developed that utilizes experimental results correlated with analysis models to enable a reduction in time and funding resources required to demonstrate that damage tolerance requirements of a composite structure have been met. The subject of the work outlined in this document was a scale-up effort, extending the scope of traditional coupon level experimental and analysis approaches to include larger representative structural elements. This effort is intended to lay the foundation for designing a modified damage tolerance test procedure that more closely replicates loads and failure modes observed in composite airframe and nacelle structures. Once validated, this combined experimental/analysis approach will significantly reduce the number of tests required for airframe designers to demonstrate damage tolerance of composite structures. This paper outlines part of the validation effort undertaken to correlate a finite element (FE) nonlinear buckling numerical simulation to a set of coupon and scaled-up blade stiffened panel test results. The explicit FE analysis approach incorporates a user defined nonlinear material subroutine (VUMAT) to capture material softening behavior due to fiber and matrix failure modes. Two numerical models were evaluated, a 4Ã¢â‚¬Âx6Ã¢â‚¬Â test coupon, and a scaled up 10Ã¢â‚¬Âx12Ã¢â‚¬Â double blade stiffened panel, with both subjected to in-plane compression loads.
Rapid Integration of New Analysis Methods in Production
A framework for standardizing the evaluation of computational analysis methods for composite materials based on the verification and validation framework provided in ASME V&V 10-2006 is defined. Standardized cases and benchmarks for verifying method capability prior to test data correlation are developed, and the approach first focuses on benchmark analysis cases which must be completed to verify that the underlying constitutive equations, parameters, and coding produce results as expected. Second, a series of validation cases are identified for evaluating how well an analysis technique represents composite material behavior for increasingly complex configurations. Third, application class exercises are provided as a means for developing further confidence in method capability. Evaluating analysis techniques within this framework may provide an enhanced pathway towards effective implementation by ensuring the numeric implementation, material modeling, and design-level requirements are adequately captured prior to performing large scale analysis cases in which engineering judgment is required.
JOSEPH SCHAEFER, STEVEN WANTHAL, JORDAN HANDLER, GARY RENIERI and BRIAN JUSTUSSON
Failure Initiation and Crack Growth in Thick Adhesive Bonded Composite Joints: Computational Mechanics Modeling and Analysis
A computational mechanics study is conducted on crack initiation, growth and instability in thick adhesive bonded composite joints. An associated experimental program was previously reported on unique features of failure modes and strength characteristics in thick adhesive bonded joints of glass fabric/vinyl ester composite. Complementary computational mechanics modeling and analysis of the thick adhesive bonded composite joint are conducted in this study to advance further our understanding of the subject. Composite joints of similar geometry but with thin adhesive bonds are also examined and results are compared to elucidate the nature of failure mechanics in the thick adhesive-bonded joints. The computational study is formulated with composite laminate mechanics, nonlinear fracture mechanics and advanced finite element methods. The following issues on failure of thick adhesive bonded composite joints are addressed: (1) crack formation and mechanisms, (2) crack growth, its driving force and failure modes, and (3) the effect of adhesive bond thickness. Local material strength criteria are used to identify crack initiation in the joint. Driving force for crack growth in the nonlinear adhesive of the bonded composite joint is evaluated by energy release rates associated with the crack. The adhesive bond thickness is found to govern crack formation and subsequent growth in the joint failure. Comparison of numerical results with experimental observations and test data are made to assess the validity of the computational mechanics approach and to better understand the complex nature of thick-adhesive bonded composite joint failure.
Development of a Novel Health Monitoring System for Adhesively Bonded Composite Joints Using Magneto-Electric Nanoparticles
Composites are quickly becoming the predominant material in aerospace, automotive, and other industries because of its improved strength to weight ratio. Mechanical fasteners are currently being used as the primary method for joining composites but they do introduce a number of negatives effects. One solution to this is to use adhesive bonding; however, there is no non-invasive structural health monitoring system that can fully assure the health of the bond. The creation of a multifunctional adhesive using magneto-electric nanoparticles may provide a structural health monitoring system as well as a strengthening of the adhesive bond. This project will assess the change in mechanical properties of doping the adhesive by using single lap shear specimens. The doped lap shear samples show an increase of about 24% in shear strength compared to the undoped lap shear samples. The magnetic signatures, importantly the magnetic moments, of the adhesive are taken using a vibrating sample magnetometer in order to determine the effects of environmental exposure on the magnetic moment of the nanoparticles. Changes in the magnetic moment signatures are used to determine if the bond has aged from environmental exposure. The samples are placed in an environmental chamber for a four-week time period at 70 Ã‚Â°C and 95% relative humidity. The magnetic moment for the samples before exposure is approximately 3.7x10-5 emu and after is approximately 1.5x10-4 emu. An environmentally exposed sample is easily determined based on the magnitude of difference of the magnetic moments. Overall, the MENs doped adhesive shows promise to be a multifunctional adhesive that can act as a non-invasive structural health monitoring system as well as strengthen the bond.
KAO YANG, JORGE CORIA, PING WANG, BENJAMIN BOESL, SAKHRAT KHIZROEV and DWAYNE MCDANIEL
Mechanical Properties of Needle Punched Chopped Strand Mat Composites
Fiber Reinforced Plastics (FRP) have been expected to be used for structural materials in various field because of its high specific stiffness and strength. In the case of using FRP for automotive structures, joint structure must be needed. Most comfortable and reliable joint system is mechanical joint with bolt and nut or rivet. However, mechanical joint system for FRP have a problem; mechanical properties of composite laminates decrease because of delamination around hole by stress concentration. Therefore, improvement of interlaminar strength is important. In this study, needle punching techniques were focused on to improve interlaminar strength of FRP around open holes. This technique are usually used for fabrication of non-woven fabrics. In needle punching process for non-woven fabrics, fiber webs are punched and entangled by lots of special needles (called felt needles) with many barbs set on the metal plate. By using felt needles, a part of fibers of reinforcement in in-plane direction can be aligned in out-of-plane direction. In this study, needle punch process was applied to chopped strand mat. Tensile test, bending test and end notched flexure test were carried out, and effect of needle punch density on tensile, bending and interlaminar fracture toughness properties on fracture mechanism of FRPs with needle punched chopped strand mat were investigated.
An Experimental Study on the Failure Behavior of Composite Suture Joint Structures
The mechanics of nature suture joints have be concerned by researchers because of their designability of mechanical properties. Our previous numerical work [1] reveals the great potential of composite suture joint structure as an alternative engineering joint structure. The failure mechanism of suture joint structures are complicated due to the complex interfacial geometry, and the interactions between the damage and failure mechanism and interface geometry feature need further investigation. In this paper, a series of suture joint samples have been fabricated by using 3D printing technology. The tensile failure behavior and the strain distribution of suture samples were measured using Digital Image Correlation (DIC) method to study the phenomenon of interfacial crack initiation and propagation. The experimental results indicate that the tip angle plays as a main control parameter, which affects significantly the interface damage behavior and the overall structure load capacity. This work further improves our understanding on the relationship between mechanical strength, damage mechanism and suture geometry. It also provides validation data for the improvement of numercial models, which will be used for the design and optimization of novel high performance composite joint structures.
Phenomenological Investigation of NomexÂ® Core Damage Mechanics in Honeycomb Sandwich Panels under Transverse Impact and Quasi-Static Loading
The onset of damage on NomexÃ‚Â® paper based honeycomb core was investigated through a series of quasi-static and impact tests on different size sandwich coupons. Flatwise out-of-plane compression tests on core coupons were conducted to characterize the physical phenomena leading to local fracture. The extent of core damage was also quantified by performing local indentation impact tests with blunt metal tips. In all types of tests, the input energy was selected such that exposed composite skins exhibited minor residual dent. The flatwise compression tests revealed that the phenolic resin rich fillet columns surrounding the intersecting edges of adjacent NomexÃ‚Â® cells contribute to the core out-of-plane stiffness. As the out-ofplane displacement increases and the peak load capacity is reached, phenolic resin columns experience local fracture due to buckling resulting in a sudden decrease in stiffness and strength. During flatwise tests, the position of the localized damage can vary along the sandwich core thickness depending on the inherent flaws that the imperfect cells have. On the contrary, core damage is concentrated closer to the impacted skin for local indentation and impact tests. Finally, the effect of skin bending stiffness was taken into account by impacting specimens with different layup skin configurations. Local indentation impact tests on stiff face sheets (13 woven plies) left no residual dents or externally visible damage for kinetic energies even up to 28 J, despite the presence of crushed cell walls.
Improving Damage Tolerance of Composite Sandwich Structure Subjected to Low Velocity Impact Loading: Experimental Analysis
Sandwich structures with composite face-sheets have seen its applications in variety of sectors including aerospace and automobile, owing to its high specific mechanical properties. However, there is a need to develop better damage tolerant sandwich structure as composite face sheets have low impact resistance in transverse direction. Composite sandwich structures with different impact resistant core materials were tested experimentally and numerically to develop light weight impact resistant and damage tolerant sandwich structure. Cores were made of the materials that claimed to absorb impact energy using variety of different energy dissipating mechanisms. Coupons of 4in. x 6in. dimensions were fabricated with 2X2 Twill weave carbon fiber reinforced polymer (CFRP) prepreg face-sheets and core material of SorbothaneÃ‚Â®(energy absorption by dampening vibration),NomexÃ‚Â® honeycomb (conventional energy absorption through plastic deformation) and D3OÃ‚Â® mesh (strain sensitivity to lock molecules and absorb energy) respectively. Moreover, coupons of CFRP-only and Aluminum sheet were tested for the relative comparison of energy absorption. Impact tests at various low velocities was performed using the Instron 9250 HV Impact Test Instrument with 6.64 kg of hemispherical indenter following ASTM D7136/D7136M standards. Also, Compression after Impact (CAI) test was performed on the impacted specimen following ASTM D7137/D7137M standards. Force, Energy and Displacement curves with respect to time was generated through drop weight impact testing. Specific energy absorption value was then obtained for each candidate isotropic, neat composite and composite sandwich structure by dividing respective energy absorbed values by respective sample mass. Similarly, specific stress vs. strain curve was obtained from CAI testing for all the impacted coupons. Relatively ductile compression failure in CAI tests of CFRP sandwich with D3OÃ‚Â® mesh and SorbothaneÃ‚Â® cores was observed compared to CFRP sandwich with NomexÃ‚Â® core. Moreover, CFRP sandwiches with D3OÃ‚Â® mesh core absorbed more specific energy then its NomexÃ‚Â® and SorbothaneÃ‚Â® counterparts at relatively higher impact speeds of 3.0 m/s.
Energy Absorption and Impact Response of Meta-lattice Truss Core Sandwich Panels
A novel and original meta-lattice truss core sandwich structure, which combines the concepts of lattice sandwich structures and elastic metamaterials, is proposed and designed for impulsive attenuation and mitigation. This new type meta-lattice truss core sandwich structure has similar configurations as a normal lattice sandwich panel, but its truss bars are consisted of meta-lattice truss unit cells. The design philosophy of locally resonant elastic/mechanical metamaterials is integrated into the meta-lattice truss unit cell, where a relatively heavy metal core (the resonator) is coated by a soft material layer, then connected by a heavy outer shell. Based on this combination, several bandgaps are created by the local resonant behavior of the designed microstructures at particular frequencies of an impulsive wave, where the stress wave cannot propagate and energy is largely absorbed. The comparisons of responses between normal and meta-lattice truss core sandwich panels subjected to impulsive loadings are analyzed. The origins and mechanisms of energy absorption are also systematically investigated. Results evidently reveal that more superior ability of impact mitigation is achieved by the proposed new type structures; and higher energy absorption capability are caused by the local resonance of resonators and soft coats at different band-gap frequencies.
An Analytical Model for the Response of Carbon/Epoxy-Aluminum Honeycomb Core Sandwich Structures Under Quasi-Static Indentation Loading
An analytical model is developed to predict the loading and unloading response, as well as the residual dent diameter and dent depth, of carbon/epoxy-aluminum honeycomb core composite sandwich structures undergoing quasi-static indentation loading. The model considers damage created using spherical indentors and is valid up to the barely visible external damage threshold. The initial low load regime (until the onset of core crushing) is modeled using a combination of local Hertzian indentation of an elastic half-space and small deflection plate theory of a circular plate on an elastic foundation. For loads above those required to cause core crushing, the model uses the Rayleigh-Ritz method of energy minimization with the total system energy determined using a combination of face sheet bending energy, face sheet membrane energy and work done to the core during both elastic deformation and crushing. Degraded face sheet properties are used in the model beyond the onset of face sheet delamination, which are predicted using GriffithÃ¢â‚¬â„¢s energy criterion. The model is validated using experimental results for sandwich structures consisting of quasiisotropic 8 (thin) and 16 (thick) ply carbon/epoxy face sheets and aluminum honeycomb cores. The results show that the overall mechanics of the model is fundamentally correct and reflective of physical behavior. Thus, in its present form the model shows promise as a preliminary design tool.
BARRY DAVIDSON, ALAN ZEHNDER and BENJAMIN HASSELDINE
Validation of Surrogate Model-Based Life Prediction for a Composite Rotorcraft Hub Component
A composite rotorcraft hub component is subjected to fatigue due to thrust, centrifugal and bending loads during flights, leading to delamination between individual plies at the thick-to-taper transition resin pocket. In order to evaluate the delamination fatigue life of the rotorcraft hub, a finite element analysis (FEA) model is constructed for a quarter symmetry of the hub geometry. The virtual crack closure technique (VCCT) is used to assess the strain energy release rate associated with the fatigue delamination of the component under given geometry, material property, and loads. The fatigue life distribution is then obtained by propagating the uncertainty of material and loads through the simulation model. Validation of the life prediction model is required before the model is applied to the rotorcraft hub design. In this paper, a model validation methodology is developed for the fatigue life prediction model of the composite rotorcraft hub component. Since directly using the FEA model for fatigue life prediction is computationally expensive, a Kriging surrogate modelbased method is employed to reduce the computational burden. To validate the surrogate model-based fatigue life prediction model, in this work, Bayesian hypothesis testing is extended to include both statistical uncertainty and surrogate model uncertainty during model validation. The results show that the involvement of statistical uncertainty and surrogate uncertainty in model validation may reduce our confidence in rejecting or accepting the fatigue life prediction model of the composite component.
Multi-Scale Uncertainty Quantification of Fiber Reinforced Composites Using Polynomial Chaos Decomposition
A new non-intrusive polynomial chaos (PC) method is applied to quantify the uncertainties that exists at different scales of a fiber reinforced (FRP) composite to obtain the stochastic effective material properties as well as the responses: stresses and displacements of a composite structure. The inherent uncertainties in matrix and fiber properties at micro-level and the uncertainties in geometric properties-ply orientation and ply thickness- at component/macro level are used to obtain random responses. To account for the propagation of the uncertainties, a new approach known as Ã¢â‚¬Å“Polynomial Chaos Decomposition with Differentiation (PCDD)Ã¢â‚¬Â, which is based upon the differentiation of basis orthogonal polynomials as well as sensitivity calculation of the response is implemented that requires 50 % samples of the wellestablished PC methods. The sensitivities in PCDD are also calculated using a new technique known as Ã¢â‚¬ËœModFFDÃ¢â‚¬â„¢, which is a higher order finite difference. The multiscale modeling presented here consists of two stages. In the first stage, the PCDD is implemented to obtain the stochastic model of effective material properties of a graphite-fiber/epoxy-matrix FRP composite using Halpin Tsai micromechanics model. In the second stage, the stochastic model of effective material properties is used along with the uncertain geometric properties to determine the response behavior of a laminate. The responses of a composite plate are generated by commercial finite element analysis software for PCDD analysis. The results are then compared with the 50,000 LHS samples, which illustrates the computational advantage of implementing this new method for UQ in multi-scale modeling of a FRP laminate.
On Approaches to Combine Experimental Strength and Simulation with Application to Open-Hole-Tension Configuration
This paper compares three surrogate-based procedures to predict the mean strength of composite structures. They are (i) fitting experiments in design space; (ii) combining simulations with experiments based on an improved simple framework of multi-fidelity surrogates; and (iii) using experiments to calibrate parameters in a failure criterion. These are applied to a plate with an open hole under tensile loading. The ratio of the width of the plate to the diameter of the hole, and the fraction of Ã‚Â±45o plies in the laminate, are used as two design variables. The objective of the surrogate model is predicting the strength of untested plates from tests of six combinations of the design variables. An extension to predicting B-basis allowables at untested points is also discussed.
JOHN MEEKER, JACO SCHUTTE, NAM KIM and RAPHAEL HAFTKA
Lessons Learned in Certifying Space Structures
Oversight organizations share the goals of designers including the production of a robust structure with the desired performance that meets cost, schedule, and mission requirements. This paper provides lessons learned that have been gained during certification assessments of space flight structures. These lessons learned have been categorized into the following categories: communication, the building block approach, evaluation of the baseline structural design, and the evaluation of as-built hardware.
High-Toughness CFRP Laminates With Engineered Fracture Surfaces: A Shark-Teeth Design
Carefully-placed patterns of micro-cuts have been designed and then used to increase the translaminar work of fracture in thin-ply CFRP composite laminates. These patterns of micro-cuts are able to deviate a translaminar crack and force the formation of large bundle pull-outs. The technique led to a 68% increase in the laminate notched strength and a 460% increase in the laminate translaminar work of fracture when compared with the un-modied baseline material. Part of the improved performance was found to be due to the generated interaction of failure mechanisms between contiguous plies with dierent orientation; this process for generating interaction opens new possibilities for micro-structure design which will be explored in future works.
A Peridynamic Model for Analyzing Crack Propagation in Unidirectional Composite Lamina
In this paper, the bond-based peridynamic model is modified to study dynamic crack propagation in orthotropic composite materials. The peridynamic material constant is extended to a continuous micro-modulus constant CÃ´â‚¬Â°Â for orthotropic materials. CÃ´â‚¬Â°Â changes continuously from along the fiber direction to transverse to the fiber direction with a very good orthotropy compared to the change of YoungÃ¢â‚¬â„¢s modulus in a lamina from along the fiber direction to the transverse to the fiber direction. Moreover, unlike all previous peridynamic models predicting bondsÃ¢â‚¬â„¢ failure by using one critical strain energy release rate, this model investigated the impact dynamic crack propagation process by inputting an in-situ crack velocity related dynamic energy release rate. The crack propagation process and crack velocity is validated with comparison to the impact experimental results.
Enhancement of Delamination Resistance by Novel Z-pinning for Composite Laminated Structures
Z-pin reinforced laminates are usually manufactured by inserting the pins vertically into the laminate. However, in the present work, we consider a method in which pins are inserted at an angle to the vertical to enhance pullout strength via mechanical interlocking. This method of reinforcement not only increases the contact area of the z-pin with the laminate, but also reduces the stresses in the interface layer between the pin and the laminate, thus delaying delamination. Experiments were conducted to verify computational results under tensile loading conditions on reinforced laminates manufactured from IM8-epoxy prepregs. Test results showed increase in pullout strengths and thus the potential for increase in delamination resistance with increasing pin angles. In addition, a novel variety of pin, namely the threaded pin, was used to reinforce laminate specimens and tested under tensile loading. Results showed that the threaded pin provided superior pullout strengths when compared with laminates with smooth steel and carbon fiber inserts.
SOONWOOK KWON, MARK ROBESON and SUNG LEE
Mode III Cohesive Fracture of a Cylindrical Bar in Torsion
This paper examines the mechanics of mode III defect initiation and quasi-static growth by analyzing a torqued cylindrical bar separated at its midsection by a nonuniform, nonlinear cohesive interface. The analysis, which is exact, is based on the elasticity solution to the problem of a cylinder subjected to a nonuniform shear traction at one end cap and an equilibrating torque at the other. The formulation leads to a pair of interfacial integral equations governing the rigid body rotation and the interfacial slip field, i.e., the jump discontinuity in circumferential displacement across the interface at midsection. The cohesive interface is assumed to be modelled by a Needleman-type force-slip relation characterized by a shear interface strength and a characteristic force length. Radially symmetric interface defects are modeled by a shear interface strength which varies with radial interface coordinate. Infinitesimal strain equilibrium solutions, which allow for rigid body rotation, are sought by eigenfunction approximation of the solution of the governing interfacial integral equations. Solutions indicate that quasi-static defect initiation and propagation occur under increasing remote torque. For small values of characteristic force length, brittle behavior occurs that is readily identifiable with the growth of a sharp crack. At larger values of force length ductile response occurs which is more characteristic of a linear Ã¢â‚¬Å“springÃ¢â‚¬Â interface. Both behaviors ultimately give rise to abrupt failure of the interface. Results for the stiff, strong interface under a small applied torque show consistency with the static fracture mechanics solution of Benthem and Koiter [1] for the torsionally loaded cylindrical rod containing an annular crack The final section of the paper discusses preliminary results for the maximum principal stresses and associated principal planes which are used to help clarify the issue of the initiation of an array of oblique tensile cracks at the crack tip.
Maximum Clamping Force in Single and Double Lapped Joints
In this paper, progressive damage modeling was used in order to identify the maximum allowable clamping force in single (3DS) and double (3DD) bolted lapped joints. A constitutive model based on continuum damage mechanics was implemented and used in joint simulation. The model results were compared with experimental results and good agreement was evaluated. A 3D finite element model consists of a composite laminate plate, steel plates, steel washers, and steel bolts was adopted for the modeling of the single and double bolted joint. The effect of bolt clearance, friction coefficient, and joint type on the maximum clamping force was studied and analyzed. It was found that using the composite plate between two steel plates (double lap) increased joint clamping force capacity significantly. This leads to enhancement in the effect of clamping force and decreases the compression damage in the thickness direction of the composite plate. According to this study it is recommended to use washer bolt hole clearance less than or equal to the composite plate bolt hole clearance as bigger washer clearance can cause early damage due to bolt tightening. It was found that, the beak point of the stress destruction under the washer or the steel plate is shifted due to the softening behavior under the washer head. The clamping stiffness in the case of single lapped joint is about double the case of double lap.
ALAA EL-SISI, HESHAM EL-EMAM and HOSSAM SALLAM
Damage Initiation and Propagation Modeling in Laminated Composites under Fatigue Loading
Discrete Damage Modeling methodology and results of stress and fracture analysis in laminated composite materials are reported. This work is a direct continuation of efforts which begun in the early 90Ã¢â‚¬â„¢s at the Materials Directorate Wright Patterson Air Force Base, WPAFB OH and since the mid 90Ã¢â‚¬â„¢s were championed by Captain Jeffrey Schaff until the conclusion of his tenure with the Air Force. The essence of DDM is the insertion of true displacement discontinuities independent of mesh orientation to simulate matrix cracking and interaction with delaminations between plies. The fiber failure mode is modeled by property degradation methodology for progressive damage analysis in compression and statistical fiber macro failure criterion in tension. Static as well as fatigue analysis is enabled. Accurate tensile strength prediction was demonstrated in static analysis of pristine as well as fatigue cycled laminates. Compression strength was overpredicted in both cases and requires additional effort.
Correlation of Fuselage and Subcomponent Panel Responses Using ABAQUS Explicit Progressive Damage Analysis Tools
Analysis performed in this study substantiates the need for high fidelity vehicle level progressive damage analyses (PDA) structural models for use in the verification and validation of proposed sub-scale structural models and to support required full-scale vehicle level testing. PDA results are presented that capture and correlate the responses of sub-scale 3-stringer and 7-stringer panel models and an idealized 8-ft diameter fuselage model, which provides a vehicle level environment for the 7-stringer sub-scale panel model. Two unique skin-stringer attachment assumptions are considered and correlated in the models analyzed: the TIE constraint interface versus the cohesive element (COH3D8) interface. Evaluating different interfaces allows for assessing a range of predicted damage modes, including delamination and crack propagation responses. Damage models considered in this study are the ABAQUS built-in Hashin procedure and the COmplete STress Reduction (COSTR) damage procedure implemented through a VUMAT user subroutine using the ABAQUS/Explicit code.
ARUNKUMAR SATYANARAYANA and PHILIP BOGERT
Evaluation of the Mechanical Performance of a Multi-Cell Composite Overwrapped Pressure Vessel for Cryogenic Storage
This study focuses on establishing an operating window of a multi-cell structure for storing cryogenic propellants, with the use of an FE-based progressive failure analysis. The multi-cell is a Type IV structure (composite over-wrapped with a polymeric liner) that was modelled with temperature dependent properties, obtained by coupon testing and fitting functions. A optimized composite patch shape and number of reinforcing rings are identified based on minimization inplane shear during draping and spheres-intersections strain compatibility respectively.
DIMITRIOS ZAROUCHAS, OTTO BERGSMA, SOTIRIS KOUSSIOS and RINZE BENEDICTUS
Generalized Free-Edge Stress Analysis Using Mechanics of Structure Genome
This work reveals the potential of mechanics of structure genome (MSG) for the free-edge stress analysis of composite laminates. First the cross-sectional analysis specialized from MSG is formulated for solving a generalized free-edge problem of composite laminates. Then, MSG and the companion code SwiftCompTM are applied to the free-edge stress analysis of several composite laminates with arbitrary layups and general loads including extension, torsion, in-plane and out-of-plane bending and their combinations. The results of MSG are compared with various existing solutions for symmetric angle-ply laminates. New results are presented for the free-edge stress fields in general la minates for combined mechanical loads and compared with three dimensional (3D) finite element analysis (FEA) results, which agree very well.
JOHNATHAN GOODSELL, R. BYRON PIPES and WENBIN YU
Multiscale Modeling of Random Microstructures in SiC/SiC Ceramic Matrix Composites within MAC/GMC Framework
Advanced ceramic matrix composites (CMCs) are being aggressively pursued as potential candidates for many aerospace applications as they offer a unique combination of high temperature strength, creep resistance, low density, high thermal conductivity and low thermal expansion. Low fiber volume fraction composite systems like CMCs inherently have more microstructure variations (location of fiber centers Ã¢â‚¬â€œ disordered/random packing, coating thickness etc.) than higher volume fraction polymer matrix composite (PMC) systems. They rarely resemble the ordered packing (square or rectangular packing) that is generally assumed with periodic boundary conditions employed for computational expediency thus raising the question of how should one model such systems effectively. Prior work on PMCs has shown that random microstructures can be well approximated by an ordered repeating unit cell (RUC) with hexagonal fiber packing or an RUC containing multiple randomly located fibers; both with periodic boundary conditions. In order to keep a proper balance between accuracy and efficiency when performing multiscale composite analyses, it is preferable to limit the number of fibers in the RUC to just a few. NASAÃ¢â‚¬â„¢s in-house micromechanicsbased code MAC/GMC provides a framework to analyze such RUCs for the overall composite behavior. This framework embodies two basic micromechanics models, 1) Generalized Method of Cells (GMC) and 2) High Fidelity Generalized Method of Cells (HFGMC), that provide closed-form constitutive equations for composite effective properties, composite response, as well as local (microscale) stress and strain fields in the constituents of the composite. In this paper, the influence of micromechanics idealization (GMC and HFGMC), for both ordered and disordered microstructures, on the unidirectional and laminated composite effective properties, proportional limit stress (PLS) and fatigue life, will be examined for a typical ceramic matrix composite (CMC) with a weak interface.
SUBODH MITAL, PAPPU MURTHY and BRETT BEDNARCYK
Multi-Scale Computational Modeling of Short Fiber Reinforced Thermoplastics
Short-fiber reinforced polymer (SFRP) composites usually consist of particles that are slender, relatively short compared to the overall dimensions of the part, and imperfectly distributed in a continuous-phase matrix. Although not as stiff or as strong as their continuous counterpart, they have several attractive characteristics. In fact, their capability of being manufactured in complex geometries to conform to the desired shape without being damaged or distorted, their isotropic behavior, and their low fabrication costs are enough to make short-fiber reinforced composites the material of choice. Multi-scale modeling with de-homogenized approach is employed to consider the effect of fiber length increase and its associated manufacturing defects. Commonly used structural design software have difficulties predicting manufacturing parameters and constituent properties of short or long fiber reinforced polymer composites to meet mandated design requirements. A computational method is introduced for the virtual simulation of performance of chopped fibers in polymer composites. This new approach did lead to the development of a specialized multiscale material characterization of composite system comprising of: a) chopped fiber material modeling based on nano-mechanics failure theory considering fibers as inclusion, b) micro-macro mechanics, and damage failure theory, and c) structural durability and damage tolerance (D&DT) analysis under crush service loading. The material model established in MCQ-Chopped is integrated/coupled with FEA using GENOA platform software to perform multi-scale progressive failure analysis (MSPFA). The tensile properties and compressive properties of chopped tape thermoplastic composite were investigated by test and show saturation of mechanical properties. The fiber length shows different effect on each mechanical property which is consistent with computational material modeling results. Predictive software also indicated that as fiber length increased the contribution of failure mechanisms changed. Sensitivity analysis of mechanical response considering defects and fiber waviness are performed to design material with better performance.
MASSIMILIANO REPUPILLI, FRANK ABDI, YIN WAN, JUN TAKAHASHI and HSUMIN HUANG
Enhancement of Multiscale Modeling Methodology for Short Fiber Filled Injection Molded Parts Subjected to Uniaxial and Biaxial Loadings
To facilitate progressive failure structural simulation of short fiber filled injection molded parts, Autodesk has developed multiscale modeling methodology and software to seamlessly link the results of injection molding simulation with subsequent nonlinear multiscale structural response simulation. The key features of the methodology include: 1) Automated mapping of the predicted fiber orientation distribution onto the finite element mesh that will be used for the nonlinear structural response simulation, 2) Enhancement of the structural response simulation with a multiscale, progressive failure, constitutive model for short fiber filled plastic materials that accounts for plasticity and rupture of the matrix constituent material, resulting in a composite material that exhibits an anisotropic, nonlinear response, and 3) A robust, automated material characterization process that uses a minimal amount of simple tensile test data from the short fiber filled plastic material to fit the parameters of the multiscale, progressive failure, constitutive model. Recently, this multiscale modeling methodology has been improved to accurately capture the peak load for different load angles relative to the flow direction of the injection molded plaque in uniaxial experiments. To accurately resolve the uniaxial response at different load angles of short fiber filled injection molded parts, two different enhancements are required. First, the material characterization process must be enhanced to consider the actual predicted distribution of fiber orientation over the test specimens used for characterization. This enhancement is necessary because, the material stresses predicted by the model are sensitive to fiber orientation tensor which varies significantly through the thickness of the injection molded plaque. In addition, the basic multiscale material model must be enhanced by improving the failure criterion used to predict rupture of the short fiber plastic. Finally the improvements are compared against biaxial coupon results for different load ratios in tension.
Structurally Optimized Beams from Digital Composite Materials
Digital materials are an emerging materials concept, in which a material is fabricated from a (1) discrete set of components, with a (2) discrete set of relative positions and orientations, under (3) explicit control of these components and orientations [1]. Cheung and Gershenfeld created a reversibly assembled cuboctahedral composite structure that exhibited a 12.3 MPa elastic modulus from a 7.2 mg/cm3 density structure [3]. The structure takes advantage of the axial transmission of tensile and compressive forces, thereby simplifying the design to orient the fibers along the strut axis. Recent efforts at structural optimization via cellular materials has created extremely low density structures, typically composed of non-composite isotropic materials. Here, we investigate the design and fabrication of digital cellular composite materials in which the failure strength and buckling resistance of 1-D struts are optimized with respect to mass for NASA-oriented space applications. Struts containing microporous interiors are numerically investigated for specific mechanical performance. A genetic algorithm optimization of buckling performance, coupled with finite element studies, has been applied to investigate the axial buckling resistance of resulting beam designs. The resulting buckling strengths demonstrate that multi-cavity microporous beams have good buckling resistances, which are only marginally less per unit mass than for the unidirectional carbon fiber tubes. In order to manufacture these components, we have developed a circumferential rolling technique to produce carbon fiber composite beams composed of carbon fiber pre-preg sheets and sacrificial elements (e.g., poly(lactic acid) (PLA) filaments). Subsequent to the matrix cure, the sacrificial elements are depolymerized, leaving continuous channels that dramatically reduce the element mass and serve as the foundation for node joining processes. Via this approach, it is possible to produce composite beams with desired cross section (e.g., multi-cavity cellular cross sections). The fabricated struts are tested in compression and the performance compared to previous digital composite materials and predictions from numerical simulations.
RONEN YUDZINSKY, ANDREW BURKE and CHRISTOPHER HANSEN
Dynamic Deployment of Composite Tape Springs
Deployable aerospace structures are becoming more attractive to researchers due to their light weight and flexible design characteristics. Their ability to easily be deployed after a certain time period of stowage makes them very desirable joints in aerospace structures. Controlling the deployment process by controlling the rate of deployment is critical to the primary application of these structures. One of the most studied components of these deployable structures is a tape spring hinge. In this paper, we present an experimental study and finite element modelling of the deployment process of a tape spring specimen. A three-layered carbon fiber composite tape spring consisting of two Ã‚Â±45o plain weave fabric laminae and an intermediate 0Ã‚Â° unidirectional lamina was used for both the experimental and numerical simulations. The deployment displacement of the tape spring was experimentally tracked using a digital high-resolution camera. A finite element model in ABAQUS was developed to predict the dynamic behavior of the tape spring and was validated with the experimental measurements. The experimental and modelling efforts enable the inference of some fundamental aspects of the dynamic deployment of composite tape springs.
Identifying Critical Design Variables and Domains for Design Optimization of Deployable Tape Springs for Controlled Deployment
The deployment speed of tape springs is a critical factor effecting the structural performance of aerospace components. At either extreme, an undesirable speed of deployment could result in (a) only partial deployment if too slow or (b) damage to the tape spring and/or removal of attached articles if too fast. While traditionally tape springs have been fabricated to maintain a uniform distribution of material and geometric properties, this study proposes that an optimal distribution of material and geometric properties can be used to achieve a desirable speed of deployment. This paper develops the foundation for the design optimization of a FlexLam carbon fiber tape spring for desired deployment speed. The deployment response of the tape spring was simulated using finite element (FE) analysis. A parametric study was performed to identify the critical design variables in the tape spring optimization problem with the objective of achieving a desirable speed of deployment. Both uniform and gradient changes in composite tape spring thickness, width, stiffness, density, and ply angle were investigated. Results show that uniform changes in thickness and ply angle, as well as gradient changes in tape spring width, have a considerable effect on the deployment speed. The critical range for the variable thickness was bounded between 0.05 mm and 0.5 mm. The critical range for the fiber orientation of the middle unidirectional ply was between 0Ã‚Â° and 90Ã‚Â°. The critical range for the gradient width ranged between 12.7 mm and 25.7 mm, specifically with the width increasing from the fixed end of the tape spring to the free end. The results from the parametric study pave the road to conducting an effective multi-objective topological design optimization of deployable tape springs.
Analysis of Advanced Integrated Composite Thermal Structures for Space Applications
Optical systems experience severe performance degradation when their support structure or scientific instruments undergo shape distortion; thus, candidate materials for these systems must be sufficiently stiff and thermally stable. Carbon-fiberreinforced polymers (CFRPs), with their high stiffness-to-weight ratios and near-zero in-plane coefficients of thermal expansion (CTEs), are therefore appropriate for this application. However, despite the low CTE of the composite, thermal gradients can still stimulate warping of the structure. As such, the objective of the current research is to improve temperature uniformity of the CFRP and thereby enhance its dimensional stability. The program objective is to be realized using active control of flow through an interconnected network of channels embedded in the CFRP. Such vascular composites are an emerging class of multifunctional materials that show promise in applications for self-healing, structural health monitoring, and thermal regulation. However, due to the fledgling state of the field, the technology lacks proper tools for design, optimization, and analysis. This paper will introduce the application and describe development of a computational tool for analysis of thermovascular composites.
Carbon Nanotube Sheet Reinforced Laminated Composites
Laminated composite materials provide high performance, but weak failure modes such as delamination limit their applications. Poor thermal conductivity also makes conventional composites unsuitable to replace metals in hot parts. Fundamentally, the reinforcement architecture of laminated composite materials should be modified to allow composite materials to become more multifunctional (isotropic) to replace metals. Improving the low-strength failure modes and thermal and electrical conductivity of composites would have industry-wide value and significance including increasing the safety and performance of vehicles, wind turbines, aircraft, ships, and motors. In this research, carbon nanotube (CNT) sheet manufactured in the Nanoworld Laboratory at the University of Cincinnati has been integrated into laminated composites for superior interlaminar shear and electrical properties. The CNT sheet was chemically post-processed for better bonding with the carbon fiber laminate. The introduction of CNT sheet does not degrade the inplane properties. In addition to serving as a structural material, the CNT sheet can substantially increase the electrical and thermal conductivity of composites. The CNT sheet integrated with the carbon fiber plies has good electrical conductivity and therefore might be used to replace electrical wires. Therefore, CNT sheet reinforced multifunctional composites can save substantial weight by replacing conventional conductive wires with structural composite material. The multifunctional composites will be a new commercial product and will have uses ranging from aerospace, to wind power, to medicine.
DEVIKA CHAUHAN, GUANGFENG HOU, XINSHUO WEN, MICHAEL KATTOURA, CHRISTINE RYAN, VESSELIN SHA
Piezoresistive Response of Carbon Nanotube Yarns: Experimental Characterization and Phenomenology
Carbon nanotubes (CNTs) exhibit physical, mechanical and electrical properties that makes them ideal for use in multifunctional composites. They are highly electrically and thermally conductive, mechanically strong, have low density, and come from a readily available carbon source. The miniature dimensions of CNTs makes them very interesting for micro-and nano-scale applications. They also exhibit piezoimpedance properties that have drawn attention to the significant possibilities of utilizing them in sensing applications. Upon the application of mechanical strain, their electrical resistance changes. This behavior is known as piezoresistivity and can be tapped for strain measurement and damage monitoring when the CNT yarns are integrated into polymeric and composite materials. When the nanotubes are assembled into axially aligned yarns, its use can be extended into macroscopic applications. It is thus imperative to fully explore the electromechanical properties of the CNT yarns if they are to be utilized in large scale applications or deployed commercially. This paper provides the complete piezoresistive response of the free or unconstrained CNT yarns subjected to quasi-static and cyclic loading under uniaxial tension. By taking resistance measurements simultaneously with load and displacement histories, any relationship between the mechanical and electrical characteristics of the CNT yarn are determined including the effect of hysteresis, strain rate, and strain level. The coupled mechanical and electrical response of the CNT yarns for a relatively high strain rate were studied. The gauge factor is the most important parameter that allows the calculation of the strain from the resistance measurements. The values of the gauge factors are determined for the different strain levels and its variation with cycle. The phenomena occurring at the nanoscale level in the carbon nanotubes and at the microscale level in the carbon nanotube bundles, are hypothesized and used to explain the macroscopic response of the CNT yarns.
KHAWLA ALHAMDAN, KALAYU BELAY and JANDRO ABOT
Manufacturing of Self-healing Carbon-fiber/Thermoplastic-toughened Epoxy Prepreg
A self-healing prepreg containing solvent-filled microcapsules was manufactured using a specially designed and built laboratory scale prepregger. Carbon-fiber tow was used as reinforcing fabric and a mixture of poly (bisphenol a-co-epichlorohydrin) (PBAE) thermoplastic, EPON 828 epoxy, and 4,4Ã¢â‚¬Â²-diaminodiphenyl sulfone (DDS) hardener was used as matrix resin. Polymeric microcapsules with a double shell wall of polyurethane / ureaformaldehyde (PU/UF) containing ethyl phenylacetate (EPA) solvent were coated with polydopamine (PDA) to increase their solvent and thermal resistance and incorporated into the prepreg to impart self-healing functionality. The laboratory scale prepregger facilitated the fabrication of self-healing prepreg by continuously processing fiber sizing, resin impregnation, and fiber winding. A sizing agent was prepared by mixing microcapsules with butanone (MEK) solvent, and it was applied to a tow through a self-inking sizing drum which was covered by a consistent amount of the sizing agent. The tow then passed through matrix resin dissolved in MEK solvent to aid in an impregnation at room temperature, and the solvent was removed afterwards from the completed prepreg. Processing parameters for the prepreg fabrication were analyzed. Survival of microcapsules was examined using thermogravimetric analysis (TGA) and confocal fluorescent microscopy. A selfhealing laminated composite was manufactured using the prepreg after hot-pressing its laminate at 180 Ã‚Â°C. The co-continuous matrix structure of the composite was examined using a scanning electron microscope.
Piezoresistive and Thermoresistive Response of Constrained Carbon Nanotube Yarns towards Their Use as Integrated Sensors
Carbon nanotube (CNT) yarns are micron-size fibers that contain thousands of intertwined carbon nanotubes in their cross sections and exhibit piezoresistance and thermoresistance characteristics that can be tapped for sensing purposes including strain measurement, damage detection and temperature sensing in polymeric and composite materials. The sensing ability of these CNT yarns is thus determined by their coupled mechanical-electrical (piezoresistive) and thermal-electrical (thermoresistive) responses. The former had been studied in the case of the unconstrained carbon nanotube yarns revealing a piezoresistive response that is strongly dependent on the strain rate. In this study, the CNT yarns were embedded in polymeric beams and subjected to 4-point bending. Depending on the position of the CNT yarns within the beam, the piezoresistive response of the constrained CNT yarns was determined under either tension or compression. It is revealed that the effect of the lateral constraint of the CNT yarn on its piezoresistive response is very significant. The thermoresistive response of the CNT yarns embedded in polymeric beams was also determined by subjecting them to heating-cooling cycles. It was observed that their electrical resistance decreased with increasing temperature.
G.L. CARVALHO, M.K. BONARDI, C.R. COELHO, G.E. BRODEUR, M. CEN-PUC, J.J. KU-HERRERA, F. AV
Thermal Response to Simulated Lightning Currents on Stitched Composite Aircraft Structures
Integrally stitched composite structures have attracted significant interest for enhancing the structural integrity of aircraft. One recently developed stitched composite technology is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. While PRSEUS structures offer a high structural performanceto- weight ratio over traditional stiffened panels and sandwich panels, their thermal response is not well understood. The through-thickness stitching of PRSEUS panels provides a barrier to delamination initiation and growth. Therefore, the thermal and mechanical damage in the PRSEUS panel may be profoundly different than for traditional (unstitched) laminated structures. In this study, nonlinear transient coupled thermal-electrical finite element (FE) analyses and subsequent heat transfer analyses were performed to characterize the thermal response of a PRSEUS panel subjected to a simulated transient high electrical current waveform consistent with lightning strikes. Temperature dependent-material properties of each structural sub-component (i.e., foam core, rod, stringer, frames, and exterior skin) were employed to predict the electrical current and temperature distributions in the PRSEUS panel. Several special-purpose user subroutines were developed i) to simulate the standard lightning current waveform, ii) to predict the temperature distribution associated with high electrical currents, and iii) to update material properties based on temperature-time history. The objective of this study is to characterize the potential thermal damage in a PRSEUS panel exposed to simulated high current waveforms as well as to probe the effect of stitching on the through-the-thickness electrical and heat conduction.
THOMAS LACY, CHARLES PITTMAN, JR. and MICHAEL MAZZOLA
Free Edge Effect in Multi-directional Laminate under Uniaxial Loading
In this paper, a generalized plane strain model capable of predicting interface stresses is developed. The analysis of free edge stresses of composite laminates subjected to uniform axial strain by using the generalized plane strain model developed with help of commercially available FEA software Abaqus. The results are compared with previously published results and 3D analysis to validate the model. This model can predict the interlaminar stresses accurately and can be used as a computational tool to predict the interlaminar stresses of different laminate under mechanical loading.
Using Optimization to Improve the Quasi-Isotropic Status Quo
Anisotropy of composite materials is both a strength and a weakness. On the one hand, the extreme strength and stiffness provided by carbon fibers is often utilized in uniaxial loading environments to provide significantly improved structural performance as compared to metals. On the other hand, because of the weakness of unidirectional fiber-reinforced composites in the transverse direction, and because loading environments are rarely uniaxial, quasi-isotropic layups are often used to provide sufficient strength and stiffness in all directions. A methodology has been developed to tailor the layup and stacking sequence to local states of stress across a composite structure, which can provide improved strength or reduced weight relative to a quasi-isotropic baseline. This methodology uses geometry and external loads as fixed parameters, and optimizes layup parameters at the level of individual finite elements using advanced genetic algorithms.
Structural Optimization of Composite Helicopter Rotor Blades
Structural optimization of a helicopter rotor blade with uniform aerodynamic surface and twist at the functional region is performed for weight minimization subject to various constraints relevant to helicopter rotor blades. The genetic algorithm based optimization is performed only for the functional region of the blade. Design variables are taken as the number of unidirectional S-glass layers in the spar cap, position of the spar web with respect to the leading edge, nose mass diameter and position of the single span wise ply-drop-off. Constraints of the structural optimization are defined as maximum strain in the critical sections of the blade in the functional region, relative distances between the feathering axis, the mass center, shear center and the neutral axis and natural frequency limits. Optimization is performed in a stepwise fashion for the hover condition while sectional analysis of the blade is performed by the VABS, loads and natural frequencies of the blade are calculated by the multibody simulation tool Dymore. The initial sectional blade loads calculated by Dymore are kept constant and they are not updated in any design iteration during the optimization process. For the optimized blade properties, blade tuning is done by lumped mass attachment to the blade and the sectional blade loads are calculated by Dymore and optimization is performed again by keeping the sectional loads as constant in any design iteration of optimization process. Load calculation, blade tuning and optimization cycle is repeated until the sectional loads calculated by Dymore do not change within a prescribed tolerance to complete full blade optimization. With this approach, the time consuming sectional load calculation process by Dymore is eliminated.
PC-Based Numerical Modeling of Ballistic Impact into Multi-Layered Nonwoven Fibrous Targets
A numerical code has been developed to model ballistic impact into a multilayered, nonwoven composite thin plate. The target consists of N unidirectional layers that are arranged in an alternating, cross-plied configuration. The projectile is either a hard sphere or a right circular cylinder (RCC). Motivation for such a model is the commercial development of soft and plate-like body armor whose building blocks are thin, nonwoven ultrahigh molecular weight (UHMWPE) fibrous sheets consisting of four bonded unidirectional layers with alternating (0o/90o/0o/90o) orientations. The matrix is typically a very flexible polyurethane just sufficient to fill the voids between the fibers, so about 17 percent by mass. Our interest is in modeling various features seen in ballistic impact into such structures. The code, which was written in MATLAB and runs on a PC, involves a forward, finite-difference (FD) scheme of numerical solution. Layers are sub-divided into multiple elastic Ã¢â‚¬ËœyarnÃ¢â‚¬â„¢ elements with distributed masses. Elastic and viscous compression (normal) and shear (transverse) elements are used to model inter-layer yarn couplings. Sliding on the projectile follows a viscous friction law. Following impact, the evolution of tension and transverse waves in the layers are modeled, as well as projectile deceleration. Graphical results are presented for velocities, displacements, forces and strains versus time, which we interpret.
Dynamic reverse ballistics penetration of KevlarÂ® fabric with Different Indenter Geometries
In this study, the mechanical load on a bullet-shaped indenter when impacted by a single-ply Kevlar fabric was experimentally investigated using a reverse ballistics method at dynamic rates. Different indenter geometries, namely the 9mm Luger, .223 Remington, and .308 Winchester bullet geometries, were used. The penetration load of the stationary indenter was measured using a force transducer located behind the indenter, and the penetration load was then plotted against the impact velocity of the fabric sample. Different mechanisms of penetration were observed at different impact velocities. Penetration mechanisms were also found to be highly dependent on projectile nose geometry. A modified method to obtain an approximate ballistic limit based on the impact loads was used to compare the efficacy of different geometry types.
Computational Model for Woven Fabrics Subjected to Ballistic Impact by a Spherical Projectile
We present a numerical model that simulates ballistic impact of a rigid spherical projectile into a plain weave fabric target. The code, which runs in MATLAB on a highend PC or laptop, allows one to vary projectile radius, mass and velocity, as well as various fabric mechanical and geometric parameters. These include yarn spacing, stiffness and density, non-linear slip viscosity between fabric and projectile, non-linear viscoelastic contact stiffness (compressibility) at yarn crossovers together with viscoelastic shear stiffness resisting yarn sliding, and the angle of yarn crimp out-ofplane. The model accommodates a progressively spreading contact zone on the fabric, as occurs with an impacting spherical projectile. Fabric dimensions are chosen large enough to avoid any tension wave reflections at the boundary. After providing some details on the finite difference formulation of the model, we present a sampling of results obtained at various times after impact. The results include contour plots of out-of-plane displacements, out-of-plane and in-plane velocities and yarn strain distributions at a given time, as well as plots, over time, of maximum local strain level, and out-of-plane velocities and displacements in the contact region of the projectile. Some parameters will be varied to provide insight into the rich behavior possible.
The Ballistic Impact Response of Flexible Composite Body Armor
This paper presents results from an in-house, finite-difference (FD) code to model ballistic impact into flexible composite body armor. The code, written in MATLAB, runs well on a high-end PC. The version we consider in this paper models the postimpact, pre-failure behavior of non-woven, cross-plied sheets made from UHMWPE yarns (e.g., DSM DyneemaÃ‚Â®) in a polyurethane matrix at low volume fraction. This version can also model crimp imprinted in an up-down diamond pattern. Possible projectiles are right-circular cylinders (RCC) and spheres, though we focus on the former in this paper. We first review some assumptions regarding the element structure of the model where a single sheet is treated as a flat 2-D network of discrete, pin-jointed yarns and each node represents a non-sliding, yarn crossing point. We then adapt this model to apply also to a much thicker panel and then present various results, in graphical form, to illustrate the richness in behavior that the model captures.
Numerical Modeling of the Tensile Response of 3D Woven Textile Composites including Microstructure Imperfections
This paper presents results of numerical simulations on predicting the progressive damage and failure response of Z-fiber orthogonal interlock Ã¢â‚¬Å“hybridÃ¢â‚¬Â 3D woven textile composites (H3DWTCs) for uniaxial tension test using a novel two-scale computational mechanics framework. Orthogonal woven composites display high resistance to layer delamination, as observed in conventional laminated composites. Here the term Ã¢â‚¬Å“hybridÃ¢â‚¬Â refers to different constituent fibers, including carbon, glass and kevlar that are infused with SC-15 matrix and integrally woven into a single preform. The H3DWTCs are made through a 3D textile weaving process. This hybridized architecture is examined at Unit Cell level to determine the progression of damage and failure under tensile loading. The pre-peak nonlinearity, as caused by matrix microdamage is modeled using N-layers concentric cylinder model (NCYL) [1,2] and the post-peak softening failure response is modeled using a mesh-objective smeared crack approach (SCA) [3,4] in a multiscale framework. A micro-CT analysis of the architecture is conducted and FE model is generated directly from Micro-CT real data using the software tool Ã¢â‚¬ËœSimplewareÃ¢â‚¬â„¢. The numerical simulation results show the effect of microstructure imperfections on the prediction of progressive damage and failure response and are also verified with experimental results [6].
Simplified Analytical Stitch Model for Non-Crimp Fabrics
Non-crimp fabrics (NCFs) have become increasingly popular for cost effective applications of composite parts. Two dimensional NCFs are draped into three dimensional geometries in the preforming process and consolidated with resin via injection. The drape quality after forming is evaluated by the occurrence of unwanted drape effects like in-plane undulations, gaps and fisheyes and out of plane loops and folds. NCFs consist of unidirectional reinforcement tows that are fixed with a throughthickness stitching yarn. The drape effects undulation and loop occur in NCFs when the stitching yarn is strained during shearing. The strained stitching yarn prevents further rotation of the tows and the tows start to buckle. A simplified analytical stitch model based on the stitching yarn strain during shearing was developed to describe the relationship between the manufacturing parameters, the shearing process parameter and the drape behavior of NCFs. The occurrence of the drape effects gap and fisheye depends only on the manufacturing parameters. The gap-fisheye-index was developed to describe the formation of gaps and fisheyes in NCFs. NCFs with selected manufacturing parameters were produced and the developed models were correlated to experimental results.
Experimental Characterization and Numerical Modeling of the Behavior of 3D Interlock Textile Composite Used for Impact Loading
The study focuses on the development of a predictive numerical tool for the behavior of a special kind of 3-D woven composite called 3-D interlock. The woven fabric shows high performances in delamination resistance and damage tolerance because of yarns woven in the thickness-direction. Consequently, this material could be an interesting alternative of conventional laminated composites, which only have in-plane oriented fibers, for aircraft components under impact loading. This paper presents an experimental characterization campaign to understand the behavior of this material in the in-plane directions as well as in the though-thickness direction. Then suitable constitutive laws were developed to reproduce this behavior in each direction. Damage modeling including the inelastic strains were the main challenges. Developments were implemented into the finite-element software ABAQUS/Explicit using a VUMAT subroutine. Static simulations were firstly performed to recover the suitable behavior in all directions. Finally, a dynamic simulation of impact behavior was performed on the woven composite using a rigid impactor. First results demonstrate the ability of the model to correctly capture the overall behavior of the composite plate under impact loading.
MARIE-LAURE DANO, FREDERIC DAU and AUGUSTIN GAKWAY
Influence of 3D Warp Interlock Fabric Parameters on Final Geometry
The exact geometry of 3D warp interlock fabric during production is highly dependent on the weaving process. Several fabric parameters as the number of layers, the weave diagram of linking warp yarns and the end and pick densities can influence the final geometry. Several fabrics have been produced on the same dobby weaving machine using para-aramid yarns with different warp and weft densities. Thanks to these observations, differences between final geometries of 3D warp interlock fabric as the different position of yarn insertion and the cross-section shapes have been revealed.
CAROLINE CHEVALIER, CHRISTOPHE KERISIT and DANIEL COUTELLIER
Composite De-Tooling Simulation Using an Improved Plate and Shell Theory base on Mechanics of Structure Genome
Classical shell elements have difficulty to accurately capture the nonlinear throughthickness stress gradients and the stresses due to material anisotropy. The improved plate and shell theory by means of Mechanics of Structure Genome (MSG) appears as a feasible approach to reproduce high fidelity residual stress simulations in thermoset polymers while greatly reducing the computational time. The Structure Genome (SG) is defined as the smallest mathematical building block of the structure containing many such building blocks. For composite laminates, the reference surface can be described as a 2D continuum and the microstructure of every material point of this continuum is modeled by means of the specific SG, which is the material line of the transverse normal. Thereby, the original 3D anisotropic elasticity problem is cast in an intrinsic form and hence, arbitrarily large displacements and global rotations can be handled subjected only to strains being small. In this work the simulation of a continuous fiber-reinforced composite de-tooling process using two different approaches is presented, which are then compared against the detailed 3D FEA developed in the ABAQUS/COMPRO CCA platform. On the one hand, smeared though-thickness properties are considered in the ABAQUS/COMPRO CCA platform in order to minimize the amount three-dimensional solid elements required. On the other hand, the same simulation is carried out using the MSG-based plate and shell theory. The comparison of the results reveal that later methodology considerably reduces the computational time and cost without compromising the accuracy of displacement results and providing detailed stress distribution.
YUFEI LONG, JONATHAN GOODSELL, WENBIN YU and R. BYRON PIPES
Machining of FRP Composite Laminates with CD and UAD Techniques: A Comparative and Experimental Investigation
The enhanced properties and increased application of a natural hemp fibrereinforced polymer (HFRP) composites have emphasized the need for an innovative and a deeper understanding of their machinability in order to further improve their functionality and applicability. Hence, this paper presents a comparative and experimental investigation of the effects of a dry conventional drilling (CD) and hybrid ultrasonically-assisted drilling (UAD) techniques on both hemp fibrereinforced/ polycaprolactone and vinyl ester (HF/PCL and HF/VE) composite samples. The results obtained depict that HF/PCL composite sample of 19 aspect ratio (AR) has a minimum values of peak thrust force of 90N and 75N during UAD and CD respectively, with the lowest machining time of 30seconds for both techniques, among other ARs. These values produced an optimum drilling and best quality drilled holes, in terms of lowest thrust force and resultant drilling-induced damage. It is observed that there was a very little or no thrust force reduction between UAD and CD when drilling the HF/PCL composites. Consequently, PCL was replaced with VE, to produce another sample of HF/VE composites. Therefore, a substantial reduction in drilling forces (thrust force and torque) was obtained during HF/VE drilling, using UAD technique, when compared with the dry CD technique. Comparatively, the percentage of the forces reduction is approximately 70. The quality of the drilled sample depended on these primary forces. In addition, a significant lower ply delamination defect, burr formation, fibre-uncut/pull-out and surface roughness were observed, as well as an improved chip expulsion, hole roundness, material removal rate and surface finish (hole quality) were obtained with UAD, when compared with CD technique. Evidently, a non-traditional hybrid machining technique, UAD, has enhanced machinability (drilling) of HF/VE composite compared with the CD and HF/PCL composite samples.
HOM NATH DHAKAL, ANISH ROY, DONG WANG and IVAN POPOV
Smart Ultrasonic Welding Of Thermoplastic Composites
This paper presents an overview of the latest research on ultrasonic welding of composites performed at the Delft University of Technology. Firstly, we showed that for thermoplastic composites, a simple flat energy director shape, made of a loose film of neat resin, can be used to produce welds of high quality. Furthermore, for single lap shear coupons, it was shown that the use of a microprocessor-controlled welder allows for in-situ monitoring through power and sonotrode displacement data. As a result, a smart ultrasonic welding procedure was designed in which the feedback from the ultrasonic welder was used to define the processing parameters that yield optimum weld quality, significantly decreasing development times. Based on the knowledge developed at a lab-scale level, a welding strategy was developed to demonstrate the assembly of small and medium-sized components. Experimental comparison between ultrasonically spot-welded and mechanically fastened joints into double-lap shear and pull-through configurations further outlined the potential application of this technology, as well as its limitations. While ultrasonic welding is an efficient technique to join thermoplastic composites components, another potential application was shown to be the welding of thermoplastic and thermoset composites, enabled through the very short heating times in the ultrasonic welding process. This opens up new possibilities for ultrasonic welding where optimum design and manufacturing of aircraft parts call for the assembly of dissimilar materials, and could lead to significant costs and weight reduction as compared to mechanical fastening.
Drilling Conditions on Hole Quality for CFRP Laminates
Introducing circular holes in carbon fibre-reinforced polymer (CFRP) laminates by drilling is a typical process involved in making composite airframes. It is a complex process owing to materials heterogeneity, configuration, sharpness and feeding rates of drills, and to the fact that reinforcements and debris are very abrasive. CFRP laminates are prone to drilling-induced damage that comprises delamination and edge chipping. Drills of different configurations have been developed for practical usage, though current studies on drilling performance and integrity were mostly focused on twist drills. Examination of processing parameters on drilling quality and integrity is critical for assessment of integrity of holes and the tool life. In this study, drilling performance and integrity of circular holes in CFRP laminates for making the flat panel of a commercial aircraft were investigated using drills of three different configurations, Dagger drill, drill Reamer and twist carbide drills by employing drilling speeds of 500, 1000 and 2000 rpm and different back support. Several output responses such as thrust force and torque are measured. Quantification of quality and integrity of holes is conducted by evaluating surface roughness, heat distribution, roundness, chip size, and damage factor defining delamination at hole edge, which provide some guidelines for optimisation of machining parameters for introducing circular holes in practical operation.
A Physics-Based Fatigue Life Prediction for Composite Delamination Subject to Mode I Loading
Use of composite materials is wide reaching in numerous industries. Recent efforts in predicting composite fatigue life have shifted focus to utilizing physic-based methodologies. The developments in this field have reached as far as commercial implementation as a design tool. These efforts have been focused on fatigue within lamina and have not included the failure mode between the plies, known as delamination. In this paper, present the preliminary efforts in combining a proven delamination model with a prominent physics-based fatigue methodology. The framework for this approach is to use the bilinear traction-separation cohesive zone model and apply the kinetic theory of fracture (KTF) to predict fatigue life of the matrix layer separating composite plies. The present work is limited to mode-I delamination which is modeled with the double cantilever beam (DCB) experiment.
Fatigue Delamination Growth Behavior in Composite Materials under Block Loading
Delamination is one of the most important damage in composite materials. It can propagate under fatigue loading and cause failures of composite structures. With the application of damage tolerance design philosophy in engineering and requirement of light weight structures in advanced aircrafts, how to characterize fatigue delamination growth behavior in composite materials and develop reliable prediction models become critical issues for the application of these materials. A large amount of studies have been conducted on the characterization of fatigue delamination growth under constant amplitude loading in composite materials. However, little attention has ever been put into the block loading sequence effect on delamination growth. Referring to fatigue crack growth in metals, shielding mechanism of plasticity deformation around the crack front plays a dominant role in the loading sequence effect. Fibre bridging, acting as the shielding mechanism, therefore can contribute to the loading sequence effect on fatigue delamination growth in composite materials. Double Cantilever Beam (DCB) specimens were designed and manufactured for the mode I fatigue delamination tests with HI-HI and LO-HI block loading sequences. Paris relation was subsequently used to interpret the experimental results. It demonstrates there is an obvious loading sequence effect on fatigue delamination growth in composite materials. The fatigue crack growth in the HI-HI block loading is slower than it in the LO-HI block loading.
YI SUN, R.C. ALDERLIESTEN and R. BENEDICTUS
Towards the Fundamentals of Mode II Fatigue Delamination Growth
The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue in-plane shear delaminations. ENF tests were performed and the results showed that damage propagates ahead of the main crack tip in a process zone, which dissipates energy that should be accounted for when characterizing mode II delamination growth. It is not possible to measure appropriately the extent of the process zone in an ENF specimen from observations made from its side, and the definition of a crack tip is not advisable in mode II delamination studies. Instead, an effective crack length, which includes the damaged zone ahead of the main crack tip, should be defined. More studies are necessary in order to understand and define appropriately the extent of the process zone in mode II delaminations.
Strategies and Numerical Implementation of Fatigue Life Models for Continuous Fiber Reinforced Polymers
Structural components made of continuous fiber reinforced polymers are exposed to repeatedly-applied loads leading to fatigue failure during the service life. To predict the lifetime and achieve the proper industrial part design against fatigue failure, the development of numerical simulation tool is crucial. Even though fatigue phenomena in composites have been intensively investigated, having a unique and reliable numerical simulation strategy remains a great challenge. The primary issue to address is to establish a robust constitutive modeling including non-linear behaviors, namely progressive damage mechanism and inelastic behavior of constituents at micro and meso scales. Nonetheless, the complexity increases when time-dependent or cyclic behavior is involved. These issues on material modeling and lifetime prediction for composite materials were addressed in this work. Implementation is performed in environmental solution for structural design (Virtual Performance Solution. Solver Notes Manual, ESI Group, Paris, France. 2013).
ALAIN TRAMECON, STEPHANE PANIER and SEBASTIAN MUELLER
The Effect of Reduced Gravity on Ultrasonic Nanoparticle Dispersion (ERGUND)
Nanoparticle dispersion is an increasingly critical field, as composite materials become a more viable option for improved performance. While the properties of a single material can be well suited for the objective, combining several materials that possess advantageous properties can drastically improve the performance of a component. Composite materials can fulfill this need, but overall properties depend on the quality of its constituents. In this experiment, the quality of nanoclay Cloisite 30B exfoliation and dispersion in an Epon 828 epoxy matrix is sought to be improved by acoustic cavitation. This is attempted using a sonicator in a reduced-gravity environment to eliminate the buoyancy force. Buoyancy is a driving factor when cavitation occurs and eliminating it will result in higher energy cavitation. This higher energy cavitation will result in smaller sized nanoparticles that are better dispersed throughout the liquid. An even dispersion will cause the material to have better mechanical properties as shown in a tensile test than samples created in a 1G environment. However, the results are inconclusive. This discrepancy is attributed to the fact that the samples were not cured immediately after sonication. While the experiment yielded negative results, this work is intended to provide inspiration for future novel in-space manufacturing techniques.
MARK AGATE, BENTON PATTERSON, FELIPE GHEIMAN, STEPHEN MARKUS, MAURO FITTIPALDI, LUIS RODRI
Determining the Controlling Mechanism of Electrostatically Induced Carbon Nanotube Rotation Using In Situ, Real-Time Polarized Raman Spectroscopy
Carbon nanotube rotation induced by an electric field is analyzed using in situ, real-time polarized Raman spectroscopy. This technique for monitoring a dynamic system enables a measurement of the change in carbon nanotube alignment for various field strengths and temperatures as a function of time after the field is applied. The calculated activation energy of carbon nanotube rotation is similar to that of the viscous flow of the thermoset polymer matrix. The experimental results are compared to theoretical work from the literature concerning the diffusion of rigid rods in polymer melts, which indicate a bulk viscosity controlling mechanism for carbon nanotube rotation.
Scale-up and Continuous Highly Aligned Multi-Walled Carbon Nanotube Sheets for High-Performance CNT/Bismaleimide Nanocomposites
The mechanical properties of carbon nanotube (CNT) sheets, or buckypaper (BP), can be substantially enhanced by applying simple mechanical stretching on the random CNT network to achieve super alignment degree. However, the stretching media, serving as the lubricant, is crucial to the degree that random CNT networks can be elongated. Several mechanical stretching approaches with different stretching media were investigated. These included dry BP stretching, liquid solution assisted BP stretching, and resin assisted BP prepreg stretching. Different stretch ratios of BP were achieved ranging from 20% by dry stretching, to 40% by liquid solution assisted stretching, and up to 80% by resin assisted stretching. The different mechanical stretching methods were studied by optimizing the stretching speed, stretching pressure, and temperature. A manufacturing guidance for different stretching methods with consistent stretching quality was made through this study. The CNT alignment degree was characterized by Raman microscopy. The mechanical properties and failure modes of stretched BP tapes by different stretching media and their corresponding BP composites were also evaluated.
REBEKAH DOWNES, KHANH BUI, DEVIN JUSTICE, SOFIA GARCIA, JIN GYU PARK and RICHARD LIANG
Effects of the PopTube Approach CNT Synthesis Process on the Tensile Properties of Carbon Fibers and their Composites
In this study, the effects of CNT growth on the tensile properties of carbon fibers and their composites are examined. A novel CNT synthesis method Ã¢â‚¬â€œ the PopTube Approach Ã¢â‚¬â€œ is used to grow CNTs directly on carbon fibers. This technique utilizes microwave irradiation to synthesize CNTs on carbon fibers using the organometallic compound ferrocene as both the catalyst and primary carbon source. To study the effects of the CNT growth process at the fiber level, CNTreinforced single fibers are harvested and tested for tensile strength. To study the effects of the CNT growth process at the composite level, CNT-reinforced woven carbon fiber fabric is used to produce laminate composites Ã¢â‚¬â€œ which are also tested for tensile strength. In both components of this study, control groups are tested for comparison. Single fiber tensile tests are carried out according to ASTM D3379, while composite tensile tests are carried out according to ASTM D3039. Results show that single fiber tensile strength is reduced on the order of 10% as a result of the CNT growth process. This suggests that the CNT synthesis process causes some degree of damage to the fibers themselves, which is perhaps to be expected. Composite tensile strength, however, is reduced on the order of 30%. Factors contributing to this decline at the composite level are identified and examined. Along with damage to the fibers and a reduction in fiber volume fraction due to the presence of CNTs, a shift in failure mode is observed in the CNT-reinforced composites as a result of the increased interfacial shear strength provided by the CNTs. These observations suggest that a tradeoff may exist in CNT-reinforced composites between the desired improvements in delamination resistance and the undesired reduction of in-plane tensile properties.
Carbon Nanotube Sheet Scrolled Fiber Composite for Enhanced Interfacial Mechanical Properties
The high tensile strength of polymer matrix composites (PMC) is derived primarily from the high strength of the carbon fibers embedded in the polymer matrix. Fibers typically have high strength in tension. However, their compressive strength is generally much lower due to the fact that under compression, the fibers tend to fail through micro-buckling (or kinking) well before compressive fracture occurs. One approach to improve properties of the fiber/matrix interface in a PMC is the so-called Ã¢â‚¬Å“fuzzy fiberÃ¢â‚¬Â concept, where carbon nanotubes (CNT) are grown directly on the carbon fiber through CVD. However, that often leads to degradation of in-plane fiber properties. In our work, we considered multi walled CNT sheets (MWNTS) being wrapped around carbon fiber to improve interfacial properties. In order to understand the effect of CNT sheet orientation on composite strength, an atomistic model was developed to study the interface region between the epoxy (Epon-862), carbon fiber and the scrolled MWNT sheets. Molecular dynamics (MD) simulations were performed on this model using LAMMPS software to study its behavior under shear deformation. After proper equilibration, a uniform shear strain was imposed on the MD model with the appropriate boundary conditions at room temperature. Using a modified formula first proposed by Argon for composites, the compressive strength of the composite was calculated. Shear deformation was applied to the baseline MD model with and without the scrolled CNT sheets, and a similar procedure was followed to obtain the compressive strength for proper comparison as described in the paper.
Stochastic Failure Analysis of an Uncorrelated Volume Element Using Extended Finite Element Method
Variability present in composite microstructures was characterized using image analysis techniques. The quantified variability was incorporated in computersimulated microstructures. Microstructures were generated at a length scale associated with an uncorrelated volume element, which is a mesoscale for multiscale analysis. Nearest neighbor and radial distributions were used to evaluate the statistics of the generated microstructures. Fiber locations in the generated microstructures were rearranged to enforce statistical equivalence between all generated and actual microstructures. To predict mechanical properties, image-based finite element models were developed by importing fiber center locations and their corresponding radii. Extended finite element method (XFEM) was implemented in ABAQUS to model matrix cracking under transverse tension. Cohesive surfaces were defined at fibermatrix interface to model debonding between the two constituents. The failure initiation and the evolution path were predicted without assigning a preexisting crack. An ensemble of statistically equivalent microstructures was simulated to determine the variability in transverse strength. To evaluate the variability in strength under biaxial loading, stochastic failure envelopes were developed.
Low Rate Dynamic Fracture Simulation of Toughening in Polymers via Highly Ordered Nanoplatelets
This study employs finite element based computational fracture mechanics methodologies to evaluate microstructural toughening effects in epoxy reinforced with highly ordered nanoplatelets (achieved through magnetic field application during processing) to invoke crack redirection in addition to toughening. Despite a large amount of scientific study, much work remains to be done to understand and quantify nano-scale toughening mechanisms. Herein, simulations of active fracture mechanisms on the micron scale are used to identify and quantify various toughening or weakening effects due to nanoplatelet inclusions. This study develops strategies for determining critical strain energy release rates (GC) from simulation and examines some of the challenges involved therein. Simulation on the micron scale of fracture is critical to understanding different micro-scale toughening mechanisms and their relative contributions, but quantitative correlation to continuum level properties is demonstrably challenging. The current work examines successful strategies for bridging macro-scale and micro-scale simulation, such as replacing artificial damping with dynamic simulation of inertial effects to maintain computational fidelity and accurate physical representation. Methodologies are validated with experimental data at the macro-scale. Validated methodologies are applied to fracture simulations of highly ordered nanoplatelet inclusions in an epoxy matrix, and a parametric study quantifies the potential for toughening and crack redirection.
Progressive Damage Modeling of Notched Composites
There is an increased interest in using non-crimp fabric reinforced composites for primary and secondary structural components in high performance automobile applications in order to accomplish desired mass savings. However, one of the main challenges in implementing these composites is the lack of understanding of damage progression under a wide variety of loading conditions for general configurations. Towards that end, researchers at GM and NASA are developing new damage models to predict accurately the progressive failure of these composites. In this investigation, the developed progressive failure analysis model has been applied to study damage progression in center-notched and open-hole tension specimens for various laminate schemes. A segment of this effort was focused on a detailed study with respect to the effect of element size on the analysis outcome, which is presented here.
SATVIR AASHAT, HAMID KIA, ARUNKUMAR SATYANARAYANA and PHILIP BOGERT
Novel Carbon Nanotube-Based Non-Woven Composite Sensors: Processing, Characterization and Potential Applications
Carbon nanotube-based composites have exhibited tremendous potential for a variety of multifunctional applications. Recent advances in processing of carbon nanotube-based composites have allowed for the formation of conductive networks on the surfaces of a wide range of fibers. In this research, carbon nanotubes (CNTs) are selectively deposited onto randomly oriented, non-woven aramid fabric using a dipcoating process as well as an innovative electrophoretic deposition process. The formation of these electrically conductive networks enable carbon nanotube based nanocomposites to be used as sensors. Carbon nanotube-based nanocomposites exhibit piezoresistivity, that is, the resistance changes a lot more than the standard resistance change due to deformation. This unusually large resistance change is because of the nanotube-nanotube tunneling effect. These sensors are then characterized under different loading conditions and optimized for improved sensitivity. This novel approach of utilizing fibers as the backbone of the piezoresistive network enables broad application flexibility for integrating nanotubes into composites for strain and damage detection capabilities. Opportunities for future structural health monitoring will be discussed.
Acoustic Filter Design Using Temperature Tuning
The material properties selection for designing acoustic filters with optimal performance over a range of frequencies requires considerable effort to fabricate and test laboratory samples. To simplify this procedure, one may test a single sample at various temperatures to design an acoustic filter for a desired band-width. The essential idea is to fabricate a single layered periodic elastic composite with constituent materials that have temperature-dependent properties. As temperature is changed, such a composite exhibits a band structure that changes with the change in temperature. Once a desired band structure is attained and the corresponding constituent properties are identified, then new constituents that have those properties at the required temperature can be selected and new sample fabricated. We fabricated a 2-phase composite with periodic layers of polyurea and steel. The temperature is changed from -20Ã‚Â°C to 60Ã‚Â°C and ultrasonic measurements are performed on the sample over 0.15 to 2.2MHz at each temperature. The first three pass bands are captured experimentally and significant change in the band structure is observed over the test temperature range. Experimental transmission spectrum at each temperature is compared with the theoretical band structure and it is shown that good agreement exists for the observed variation in the band structure.
ANKIT SRIVASTAVA, ALIREZA AMIRKHIZI and SIA NEMAT-NASSER
A New Thermally Re-Mendable and Recyclable Epoxy Thermoset Based on Siloxane Equilibration
A new thermally re-mendable and recyclable epoxy thermoset was developed from diglycidyl ether of ethoxylated bisphenol-A (DGEBAEO) and bis (potassium) oligoaminopropylmethylsiloxanediolate (K-PAMS). The DGEBAEO/K-PAMS system was a highly crosslinked thermoset with Tg of 75 oC and elastic modulus about 2.2Gpa (Figure 1). K-PAMS contained active potassium aminopropylmethylsilanolate end groups that can catalyze the siloxane equilibration to restructure the crosslink network, enabling the fractured thermoset to be self-healed and the pulverized thermoset grains to be reprocessed like thermoplastics under elevated temperature (about 110 oC) (Figure 2). The healing efficiency of tensile strength achieved 80% after 24 h healing and the recyclable specimen showed fully recovery in storage modulus.
XIAO WU, XIAOJUAN ZHAO, YING ZHANG, RAN YU and WEI HUANG
Electrical Self-Sensing of Impact Damage in Multi-Scale Hierarchical Composites by Controlling the Location of the Carbon Nanotubes
The electrical sensitivity of a multiscale glass fiber reinforced polymer composite containing a hierarchically structured carbon nanotube (CNT) network, to self-sense damage under low velocity impact loading is investigated. The multiscale composites were manufactured into two hierarchical architectures depending on the location of the CNTs: a) with CNTs randomly dispersed within the matrix, and b) with CNTs located in the matrix and on the fibers. A squared grid of electrodes were instrumented on the panels in order to map the changes of their electrical resistance before and after impact. A finite element model was implemented in order to correlate the spatial distribution of stress components with the changes in electrical resistance of the panels upon impact loading. The effect of the CNT location into the composite on the energy absorption was also investigated. Using an electrical resistance mapping approach, both composite architectures were capable to detect impact damage. It was shown that the composite with CNTs dispersed only into the matrix is more sensitive to matrixdominated damage, while the composite with CNTs in the matrix and on the fibers more adequately capture damage associated to delamination. Composites with CNTs in the matrix and fibers absorbed less impact energy than the composites without CNTs and with CNTs dispersed only in the matrix.
ALEJANDRO ALONZO-GARCIA, JOSE KU-HERRERA, ALEJANDRO MAY-PAT, JAVIER CAUICH-CUPUL and FRANC
Identification of 4D Damage Precursors in 3D Woven Composites
The lack of strengthening or toughening mechanisms in the through-thickness or zdirection in two-dimensional (2D) laminated composite structures has been considered a cause for poor mechanical performance in demanding aerospace and automotive applications. To that effect, 3D woven composite systems consisting of z-binders have been introduced to potentially provide additional means for enhanced damage tolerance and reduced mechanical property degradation. To investigate the benefits of these novel composite architectures, this work employs a multimodal nondestructive evaluation and characterization approach consisting of in situ Digital Image Correlation (DIC) and Acoustic Emission (AE) monitoring coupled with ex situ X-ray micro computed tomography (micro-CT) to identify precursors to damage and link them with progressive failure patterns observed throughout the surface and volume. In general, the presence of the z-binder in 3D woven composite specimens, as opposed to their 2D counterpart, is found to be related with increased transverse reinforcement. Furthermore, local 3D surface strain patterns are observed over time (hence a 4D analysis is performed) and are coupled with AE monitoring and micro-CT data. This hybrid nondestructive evaluation setup indicated that a particular 4D mechanism related to the z-binders and consisting of sudden out-of-plane pop-outs is activated during both monotonic and cyclic testing. The activation of this damage precursor was further found to signal the transition of the material to a more advanced damage state. Post-mortem fracture pattern analysis was used to determine specific attributes of the failure patterns, while the micro-CT data confirmed the presence of lateral and transversal cracks, which were intimately related to the formation of this distinct damage precursor.
BRIAN WISNER, JEFFERSON CUADRA and ANTONIOS KONTSOS
Damage Precursor Detection and Identification in Composite Structures
In this work, the stiffness of a glass/epoxy composite material system subject to three-point bend fatigue is tracked throughout life. Linear curve fits of the data are used to develop phenomenological models of the response. Predictions of when failure will occur are made using the models and data for a particular specimen up to a prescribed number of cycles. Final fracture was predicted within 12%. The model based on the slope of the modulus vs. cycles curve was found to be less conservative than the model based on the intercept of the modulus vs. cycles curve. At 50% of life, the number of cycles to final fracture was predicted with an average error of 32%.
TODD HENRY, DANIEL COLE and VOLKER WEISS
Experimental Investigation on the Correlation between Damage and Thermal Conductivity of CFRP
By measuring the change in stiffness and thermal conductivity along with damage growth, the interaction of damage and thermal conductivity of the laminate is investigated. A quasi-isotropic carbon reinforced composites is subjected to tension Ã¢â‚¬â€œ tension fatigue loading and an in-situ DIC technique at high magnification is used to capture the initiation and propagation of micro cracks at different cycles. The thermal conductivity is measured using a modified transient plane source technique. The thermal conductivity, modulus of elasticity and the full-field strain across the thickness, are evaluated. The study has elucidated the gradual growth of matrix cracking and its transformation to inter-ply debonding up to fiber breaking. The growth of local cracks and its corresponding influence on the change in thermal conductivity at different fatigue cycles are acquired. Thereby, the effect of local damages on the heat flow and thermal conductivity of the laminate is understood. Similarly the change in modulus of elasticity with the crack growth is elaborated. Finally an explicit correlation between the degradation of thermal conductivity and modulus of elasticity is described.
NICHOLAS MYMERS, RONAK PATEL, SURAJ RAVINDRAN and ADDIS KIDANE
Characterizing and Predicting the Effects of Weave Geometry on Mode I Fracture Toughness of Composites
Characterization of Mode I interlaminar fracture toughness, GI, of unidirectional composite materials is well understood and standardized in ASTM standard D5528. However, many composite structures are made of woven fabrics. Knowledge of the effects that woven fabrics have on GI is lacking. An approach based on the double cantilever beam test was developed in this work for characterizing the fracture performance of non-woven and woven composites. Weaves were all manufactured inhouse for better control over their geometry. The effect of several architectural factors on GI were investigated, such as fibre orientation, yarn width, yarn thickness and stacking configurations of weaves. Results showed that for weaves, GI was mostly affected by the bifurcation of the delamination front around yarns and by subsequent yarn bridging. The framework for empirical models was proposed for describing fibre bridging and the effects of weaves on GI.
Designing and 3D Printing Continuous Fibre-Reinforced Composites with a High Fracture Toughness
The translaminar fracture toughness determines in many cases the damage tolerance of composite components, and this property is strongly influenced by the microstructure. A finite element model was developed that can aid in the optimisation of the microstructure for maximising the translaminar fracture toughness of composites hybridised at the tow level. The model proved that the toughness is increased when the crack has to grow through an interface between carbon and glass in a hybrid composite. This implies that the microstructure can be optimized to maximize the translaminar fracture toughness. Hybridisation at the tow level is now possible by using micro-tow placement machines. Therefore, samples were 3D-printed with continuous fibres. Therefore, samples were 3D printed with continuous fibres. Compact tension tests indicated that the toughness can indeed be increased by fibre-hybridization. This provides new avenues for improving the damage tolerance of composite materials.
Interlayer Fracture Toughness of Additively Manufactured Unreinforced and Carbon-Fiber-Reinforced Acrylonitrile Butadiene Styrene
Fused deposition modeling (FDM) has recently emerged as a promising method for rapid manufacturing of complex components in disciplines ranging from aerospace to biomedical. Despite the apparent advantage over other more traditional manufacturing methods, components manufactured using FDM are often highly anisotropic, having particularly low stiffness and strength in the direction of layer deposition. Additionally, poor adhesion between individual layers often leads to premature failure of FDM components, precluding their broader application in loadbearing structural applications. This study presents test method development for characterization of interlayer fracture toughness of FDM components with a modified double cantilever beam (DCB) test. DCB samples were manufactured with a commercially available 3D printer using unreinforced Acrylonitrile Butadiene Styrene (ABS) and chopped carbon-fiber-reinforced ABS (CF-ABS). Novel contributions of this work include 3D printing of DCB samples using unidirectional FDM layers, addition of reinforcing doublers to prevent DCB arm failure during loading, and insertion of 8 ÃŽÂ¼m Kapton starter cracks mid-print. Fracture toughness values measured for ABS and CF-ABS showed that lowering temperature of the filament during extrusion, and inclusion of chopped carbon fibers, can have a deleterious effect on the interlayer mode I fracture toughness of FDM components. The toughness data obtained using the proposed test methodology demonstrates its suitability for design and optimization of FDM processes for improved interlayer fracture performance.
The Effects of Hygrothermal Aging on the Impact Penetration Resistance of Triaxially Braided Composites
An experimental study was conducted to measure the effects of long term hygrothermal aging on the impact penetration resistance of triaxially braided polymer composites. Flat panels of three different materials were subjected to repeated cycles of high and low temperature and high and low humidity for two years. Samples of the panels were periodically tested under impact loading during the two year time period. The purpose of the study was to identify and quantify any degradation in impact penetration resistance of these composites under cyclic temperature and humidity conditions experienced by materials in the fan section of commercial gas turbine engines for a representative aircraft flight cycle. The materials tested consisted of Toray Ã‚Â® T700S carbon fibers in a 2D triaxial braid with three different resins, CycomÃ‚Â® PR520, a toughened resin, HerculesÃ‚Â® 3502, an untoughened resin and EPON 862, intermediate between the two. The fiber preforms consisted of a quasi-isotropic 0/+60/-60 braid with 24K tows in the axial direction and 12K tows in the bias directions. The composite panels were manufactured using a resin transfer molding process producing panels with a thickness of 0.125 inches. The materials were tested in their as-processed condition and again after one year and two years of aging (1.6 years in the case of E862). The aging process involved subjecting the test panels to two cycles per day of high and low temperature and high and low humidity. A temperature range of -60Ã‚Â°F to 250Ã‚Â°F and a humidity range of 0 to 85% rh was used to simulate extreme conditions for composite components in the fan section of a commercial gas turbine engine. Additional testing was conducted on the as-processed PR520 composite under cryogenic conditions. After aging there was some change in the failure pattern, but there was no reduction in impact penetration threshold for any of the three systems, and in the case of the 3502 system, a significant increase in penetration threshold. There was also an increase in the penetration resistance of the PR520 system impacted under cryogenic conditions.
DUANE REVILOCK, CHARLES RUGGERI, GARY ROBERTS, LEE KOHLMAN and SANDI MILLER
Experimental Studies on the Impact Response of 3D Fiberglass Fabric Subject to Different Size Impactors
The main objective of this study is to experimentally investigate the influence of impactorÃ¢â‚¬â„¢s size on the impact response of a truly 3D fiberglass fabric (3DFGF). The 3DFGF is a new generation of 3D fiberglass fabric, recently introduced to the market. The 3DFGF consists of two bidirectional woven fabrics, which are knitted together by a vertical series of fiberglass fibers (pillars). The vertical fibers create hollow cavities in between the top and bottom fabrics. In order to maximize the mechanical response of the resin reinforced version of 3DFGF, the cavities can be filled with foams (polyurethane foam in this study). The resulting 3DFGFs exhibit an excellent response under both flexural and impact loadings. Moreover, the damage mechanism exhibited by this composite is markedly different from that produced by the conventional laminated fiberglass fabrics. In other words, the damage created in 3DFGFs remains as a local damage and does not propagate throughout the fabrics. Moreover, because of the unique structure of this 3DFGF, no delamination is developed within the fabric when subjected to an impact loading. One of the main intended applications of the introduced fabric is in the automotive applications, specifically for formation of auto-body components. It should be noted that vehicles experience different levels of impact loading in their life spans, caused by different shape obstacles; consequently, it is imperative to investigate the influence of impactorÃ¢â‚¬â„¢s size on the response of this 3DFGF. For that, impactors with two different sizes (i.e., small-diameter and large-diameter hemispherical impactors) are considered in this investigation. Moreover, two different thickness of 3DFGF has been considered to study the influence of core thickness (or the overall structural stiffness) on their overall impact response. Force-time history, impact strength, maximum displacement and energy absorption capacity are presented and discussed for all tested specimens.
Mechanical Behavior and Damage Kinetics of Woven E-glass/vinylester Laminate Composites under High Strain Rate: Experimental and Numerical Investigation
Split Hopkinson pressure bar (SHPB) is one of the most important and recognized apparatus used for characterizing the dynamic behavior of materials. In the first part of this study, the results from a series of SHPB tests on the woven composites are presented in this paper. The compressive material properties are determined by testing the [0/90]26 laminate systems from low to high strain rates. Samples of cubic geometry are tested in in-plane direction. Preliminary compressive stress-strain vs. strain rates data obtained show that the dynamic material strength increases with increasing strain rates. For in-plane tests, there is a transitional strain rate, reflecting the dependencies on strain rate observed in experiments. The results indicate that the stressÃ¢â‚¬â€œstrain curves, maximum compressive stresses and strains evolve as strain rate changes. During the experiments, a high speed camera was used to determine the kinetics of damage. The specimens are mainly damaged in a crushing and shear failure mode under out-of-plane loading, as for in-plane test, the failure was dominated by fiber buckling and delamination. In the second part of this study, numerical models without damage are developed to investigate the validity of assumptions of compression Split- Hopkinson Pressure Bar technique. Abaqus software was used for the numerical simulation. The results obtained by numerical investigation of SHPB are compared with the in-plane compression test of a woven composite. A good correlation was noted between the experimental and numerical results, which allows validate the numerical approach.
Investigation of Collector Geometry and Speed on Orientation, Diameter Distribution and Mechanical Properties of Electrospun Nanofibers
Different collector geometry and speed are investigated to study their effects on alignment, diameter distribution and tensile properties of electrospun nanofibers. Nanofibers are fabricated from 10% polyacrylonitrile (PAN) in N,Ndimethylformamide (DMF) solution using a homemade electrospinning system. Copper collector of different geometry such as foil, mesh, and wire are attached to the surface of a rotating disk and rotates at different RPM to achieve surface speed in the range of 9.9- 29.8 m s-1. The as-spun nanofibers are subsequently stabilized at 260Ã‚Â°C in air for 180 min and carbonized at 1000Ã‚Â°C in nitrogen for 60 min. The nanofibers are characterized by scanning electron microscope (SEM) while tensile properties of nanofibrous yarns are measured using dynamic mechanical analyzer (DMA) under static condition. It is found that both collector geometry and speed affect the fiber diameter and alignment significantly which in terns affect the tensile strength of the fibrous yarns significantly.
The Effects of a Low Areal Weight Inter-layer Tackifier on Saturated Permeability of Carbon Fabrics
Accurate preform permeability characterization for liquid composite molding is paramount for producing process models, an understanding of resin saturation, and injection and outlet port placement. While, recent advances in computational tools have improved predictive methods for defining permeability, these techniques still lack robustness due to the complexity of advanced fiber reinforcements and the variables induced throughout the manufacturing process. Experimental techniques offer accurate methods for characterization and can detect trends in preforms that employ specific processing additives like tougheners or various types of preimpregnation. These additives have been shown to significantly alter resin flow, surface chemistry, and ultimately bulk reinforcement permeability. In this study, the saturated permeability of low areal weight inter-layer tackified preforms constructed with a four-harness satin IM7 carbon fabric were investigated. Both in-plane and through-thickness permeability measurements were made with different layups of the carbon fabric with and without the inter-layer tackifier. Additionally, the effects of a heated debulking cycle was explored through both compaction and permeability measurements. In general, the addition of tackifier was seen to reduce permeability by blocking the inter-tow flow channels. Interestingly, tackified preforms that underwent the debulking cycle showed a strong resistance to compaction and nesting, and this subsequently produced higher bulk permeability values compared to tackified preforms processed at room temperature. The debulking alleviated nesting enough to maintain inter-tow flow channels, and measurably higher permeability. Other results showed that the addition of tackifier produced a more consistent and isotropic measured permeability compared to their non-tackified counterparts. Ultimately, the study shows that preform properties may be tailored for specific liquid molding considerations by altering both tackifier content, and pre-processing.
Effect of Adding GFRTP Prepreg Sheet on the Properties of CFRTP Panel in Which Fine Cut Group was Induced to Prepreg
Strength of continuous fiber type CFRTP is large, however formability is restricted. On the contrary, discontinuous fiber type CFRTP is vice versa. Authors tried to obtain better balance between strength and formability by following process. (1) To induce fine cut group to CFRTP woven fabric prepreg sheet. (2) Sheets of prepreg were layered and consolidated to panel. Fine cut group was induced by fiber laser. Cutting condition was selected so as not to as much as possible to cause heat affected zone. Then GFRTP prepreg sheet was added on both surface to improve strength. Tested materials were TPS and PA66 matrix CFRTP. Formability test and strength test was conducted. Hemispherical shape forming test was conducted as formability test. Results were as follows. 1) Formability was improved by inducing fine cut group, especially panel in which both 0/90 Ã‚Â°layer and Ã‚Â±45Ã‚Â°layer were mixed could be formed. Strength of panel with fine cut group was about half of the panel without fine cut group. This value was sufficiently strong compared with the CFRTP panel of discontinuous fiber. 2) Adding GFRTP prepreg sheet to CFRTP panel with fine cut group on both surface did not restrict formability, but showed better effect to forming products. 3) Fracture load of panel by adding GFRTP prepreg sheet showed large increase. The amount of increase was much larger than the added strength of GFRTP.
Monitoring Crack Growth along the Interface in a Microdroplet Specimen using Non-Invasive Carbon Nanotube Sensors
Crack onset and propagation along the fiber-matrix interface in an S-Glass/Epoxy microdroplet specimen is studied under quasi static loading. Carbon nanotubes (CNT) are deposited on to the surface of glass fibers using electrophoretic deposition method to create nonintrusive nanoscale interface sensor for crack detection. Based on the experimental loaddeformation response and electrical resistance of the CNT sensor, the initiation and propagation of the crack through the interfacial region is determined. Crack initiates in the interfacial region near the loading end and grows around the fiber circumference followed by unstable crack growth along the interface. The sensor provides an accurate determination of load at initiation for interface failure.
SUBRAMANI SOCKALINGAM, ERIK THOSTENSON, BAZLE HAQUE and JOHN GILLESPIE, JR.
On the Role of Shear Transfer Mechanisms in the Longitudinal Tensile Failure of CFRP Composites
A three-Dimensional (3D) Fibre Bundle Model (FBM) has been developed to simulate the longitudinal tensile failure and predict the statistical strength distribution of CFRP bundles of dierent sizes. The model incorporates an original semi-analytical approach to determine the stress eld inside the bundle and to predict the evolution of the failure process. The present model can account for the eects of bre-matrix debonding, as well as the eect of the size of the cluster of broken bres on the stress recovery length and, in turn, on the bundle strength. The predictions from the model are shown to compare favourably with experimental results from the literature, thus demonstrating the suitability of the modelling approach.
A Finite Element Study of Dynamic Stress Concentrations Due to a Single Fiber Break in a Unidirectional Composite
In high-performance unidirectional composites, even under static tensile loading, breaking of a fiber is a locally dynamic process which leads to propagation of stress waves in the interface, matrix and neighboring fibers. Current Finite Element modeling approaches consider the fiber break to be a quasi-static process and do not account for these wave propagation mechanisms. To gain better understanding of this event, a fiber-level finite element model of a 2-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Inclusion of these dynamic effects shows that there is not only a significant increase in the peak stress concentrations, but also a 5x increase in the the zone-of-influence of a single fiber break. The FE modeling approach adopted here enables us to visualize the dynamic wave propagation mechanisms associated with brittle fracture within the composite. The results are qualitatively compared with experimental observations of damage evolution within unidirectional composites and they are consistent with the experiments.
SUBRAMANI SOCKALINGAM, BAZLE HAQUE and JOHN GILLESPIE, JR.
Relation between Morphology and Thermo-Elastic Properties of Carbon Nanotube Polymer/Carbon Fiber Hybrid Composites
The morphology of polyimide nanocomposite with single-walled carbon nanotubes (CNT) was considered. The effect of CNT agglomeration and twisting into a bundle was incorporated by the proposed hierarchical helical bundle model. The bundle model was used in predicting the effective thermo-elastic properties of the nanocomposite. The micromechanical analytical models were found useful in predicting the effective thermo-elastic properties of the nanocomposite. Finite element micromechanical analysis was used to calculate the residual cooling stresses inside the nanocomposite matrix with carbon fiber reinforced composites. The reduced residual stresses and transverse coefficient of thermal expansion of unidirectional lamina in the case of nanocomposite matrix were attributed to the reduced mismatch between the thermo-elastic properties of matrix and fiber. Reduction in the residual processing stresses of the hybrid composite opens the way for the use of polymeric nanocomposites as matrices in carbon fiber reinforced materials.
ROCIO MISIEGO, XIN QIAN, SERGII KRAVCHENKO, R. BYRON PIPES and ICA MANAS-ZLOCZOWER
Thermal Properties of Hybrid Carbon Nanotube/Carbon Fiber Polymer Composites
Carbon fiber reinforced polymer (CFRP) composites possess many advantages for aircraft structures over conventional aluminum alloys: light weight, higher strengthand stiffness-to-weight ratio, and low life-cycle maintenance costs. However, the relatively low thermal and electrical conductivities of CFRP composites are deficient in providing structural safety under certain operational conditions such as lightning strikes. One possible solution to these issues is to interleave carbon nanotube (CNT) sheets between conventional carbon fiber (CF) composite layers. However, the thermal and electrical properties of the orthotropic hybrid CNT/CF composites have not been fully understood. In this study, hybrid CNT/CF polymer composites were fabricated by interleaving layers of CNT sheets with HexcelÃ‚Â® IM7/8852 prepreg. The CNT sheets were infused with a 5% solution of a compatible epoxy resin prior to composite fabrication. Orthotropic thermal and electrical conductivities of the hybrid polymer composites were evaluated. The interleaved CNT sheets improved the in-plane thermal conductivity of the hybrid composite laminates by about 400% and the electrical conductivity by about 3 orders of magnitude.
ROBERTO CANO, HOA LUONG, JAMES RATCLIFFE, BRIAN GRIMSLEY and EMILIE SIOCHI
Characterization of Nanosilica Filled Bis F Epoxide with Diamino Diphenyl Sulfone Curing Agents
The incorporation of nano-sized particles into epoxy has received increasing attention in recent years as a means of improving the mechanical performance of continuous fiber reinforced epoxy composites. Developments in surface chemistry have allowed high concentrations of spherical nanosilica particles to be uniformly dispersed in epoxide without agglomeration, filtration, or excessive increase in mix viscosity. Until the present time, however, only limited mechanical performance data have been published on such highly dispersable nanosilica in high temperature epoxies, especially in resin systems with high glass transition temperatures. In the current research, surface modified nanosilica in concentrations of up to 34.4% by weight in a diglycidyl ether of bisphenol F epoxide with 3,3Ã¢â‚¬â„¢ or 4-4Ã¢â‚¬â„¢ diamino diphenyl sulfone curing agents was investigated to determine the resulting mechanical properties. It was observed that Mode I fracture toughness and modulus improved significantly on addition of nanosilica without significantly affecting the glass transition temperature. Although the density of the resin system increased with increasing nanosilica concentration, specific modulus and specific fracture toughness also improved. Based on the experimental results, micromechanical models were used to back-calculate plausible elastic moduli of assumed interphase regions between the NS particles and the epoxy.
Precursor Damage Inception Quantification in Composites Using Coda Wave Interferometry based on Taylor Series Expansion Technique
Precursor damages in composite material develops in the form of matrix cracking, fiber breakage, deboning, wrinkling, micro buckling etc. However, it is extremely challenging to detect those damages in their early stage using conventional Structural Health Monitoring (SHM) methods. Most of the times, these damages remain undetected for the long time. Ultrasonic coda wave interferometry (CWI) technique based on Taylor series expansion theory is extremely sensitive to small changes in the medium. Here, we implement this technique for the first time to detect damage precursor in woven carbon fiber composite laminates (CFRP). One American Society of Testing and Materials (ASTM) standard specimen were tested under high cycle (10Hz) tensile-tensile fatigue loading. Two high frequency piezoelectric (PZT) sensors were mounted on the specimen as sensor and actuator. Pitch-catch experiments were performed at a regular interval of 5,000 cycles until 300,000 cycles. A standard 5 count tone burst signal was used for the actuation. Stretch parameter, which is measure of relative velocity change in the medium is calculated at 5,000 cycleÃ¢â‚¬â„¢s interval. Scanning Acoustic Microscopy with 25MHz transducer was used to Quasi-longitudinal (QL) wave velocities over the gage length of the specimen. QL velocities were calculated at the interval of 10,000 cycle. Cumulative damage growth in the composite specimen from both the methods are correlated. To verify both the techniques, optical microcopy imaging was also performed at the interval of 20,000 cycles to investigate the development of the precursor damages in the specimen.
On Quantitative Coda Wave NDE for Carbon-Fiber Reinforced Polymers
Ultrasonic coda waves have been applied to concrete and metallic specimens to assess damage with some success. The capability of coda wave NDE for damage diagnosis in fibrous composite materials has not been widely addressed. One method of interpreting the changes in long, complex signals caused by the introduction or growth of damage is to isolate and quantify the difference between baseline and damage signals. Presented here is an overview of efforts to extend coda wave NDE to Carbon-Fiber Reinforced Polymers (CFRP). Experimental results show that coda wave differential features are effective in detecting drilled holes and discriminating between increasing hole diameter and increasing number of holes. The differential features are shown to have an underlying basis function that is dependent on the hole volume and can be scaled by a material dependent coefficient to estimate the feature amplitude and size holes. Measurements were found to be repeatable and three of the features were found to be largely insensitive to noise. The potential for micro-crack detection and monitoring is also demonstrated.
Nondestructive Evaluation of Adhesive Bonds via Ultrasonic Phase Measurements
The use of advanced composites utilizing adhesively bonded structures offers advantages in weight and cost for both the aerospace and automotive industries. Conventional nondestructive evaluation (NDE) has proved unable to reliably detect weak bonds or bond deterioration during service life conditions. A new nondestructive technique for quantitatively measuring adhesive bond strength is demonstrated. In this paper, an ultrasonic technique employing constant frequency pulsed phased-locked loop (CFPPLL) circuitry to monitor the phase response of a bonded structure from change in thermal stress is discussed. Theoretical research suggests that the thermal response of a bonded interface relates well with the quality of the adhesive bond. In particular, the effective stiffness of the adhesive-adherent interface may be extracted from the thermal phase response of the structure. The sensitivity of the CFPPLL instrument allows detection of bond pathologies that have been previously difficult-to-detect. Theoretical results with this ultrasonic technique on single epoxy lap joint (SLJ) specimens are presented and discussed. This technique has the potential to advance the use of adhesive bonds Ã¢â‚¬â€œ and by association, advanced composite structures Ã¢â‚¬â€œ by providing a reliable method to measure adhesive bond strength, thus permitting more complex, lightweight, and safe designs. 1
DANIEL PEREY, WILLIAM YOST, ELLIOTT CRAMER and MOOL GUPTA
Experimental Characterization of Progressive Damage in Countersunk Composite Laminates Loaded in Bearing
Composite materials are more widely used in the aerospace industry due to their superior stiffness and weight-savings when compared to metals. Although composites significant reduce the need for fasteners, they are not completely eliminated. The failure of these composite laminates at fastener sites has been a long recognized as a problem and investigated by many researchers. The focus of this work is on countersunk bolted joints in composite laminates. Countersunk fasteners are preferred in aerospace to satisfy aerodynamic smoothness of airframe or wing outer surface. The ASTM D5961 Procedure A is the standard test to obtain bearing strengths of composite laminates. The ASTM D5961 Procedure C is recommended to use in testing countersunk composite joints. For thick laminates and large fastener diameter to laminate thickness (d/t) ratios, Procedure C is not very representative. In this paper we perform bearing tests of laminated composites, using a modified ASTM D5961 fixture designed for testing countersunk bolted joints. The progressive failure behaviour and the damage resulting from the modified test fixture are quantified and compared to laminates tested using the standard ASTM D5961 Procedure A (double shear with straight pin) and Procedure C (countersunk single lap shear).
Digital Image Correlation as an Improved Technique for Adhesive Shear Strain Measurement in the ASTM D5656 Test
The purpose of this study was to develop and validate an improved technique for measuring adhesive shear deformation during the Thick Adherend Shear Test. The ASTM standard defining this test (D5656) requires the use of a unique extensometer system (KGR-1) that is expensive, tedious, and time consuming to calibrate and operate, and is susceptible to several experimental errors. This work focused on development of two different measurement procedures, both based on two-dimensional Digital Image Correlation (2D DIC) as potential alternatives to the KGR-1. In the first, adhesive shear data was generated from the extraction of point data from the DIC results from the region of the adherend in a manner similar to the KGR-1 (i.e. 3 points on the adherend surface). In the second, adhesive shear strains were directly extracted from the DIC data set from a defined area in the bond line. All work was performed with specimens prepared and loaded in accordance with ASTM D5656. Half the specimens were tested using the KGR-1 system, while the others were tested using DIC to obtain surface displacements and strains. The two different processing methods for the DIC data were applied during post-test analysis . The results showed both DIC processing methods to be in very good agreement with the KGR-1. Detailed analysis of the results found both DIC techniques provided significantly reduced variation in most properties compared to the KGR-1, with the DIC adherend displacement technique providing the more reliable results as well as the best comparison to the KGR-1 data. These tests also found both DIC techniques offer significant potential for time savings during adhesive shear properties testing as compared to the standard KGR-1 method. This study demonstrates the potential for the use of 2D-DIC as a more precise, and time-saving alternative to the KGR-1 extensometer method.
RICHARD COLE, JEREMY LALIBERTE and DAVID BACKMAN
On the Relationship between Fracture Toughness and Specimen Thickness for Quasi-isotropic Carbon/Epoxy Laminates
Trans-laminar fracture toughness was measured in a series of centre-notched quasi-isotropic IM7/8552 carbon/epoxy laminates with scaled specimens of thickness between 2 and 8 mm. The aim was to find out if there is a transition laminate thickness at which there is a change in fracture toughness. No dependency on specimen thickness was found for specimens with 12.7 mm and 25.4 mm centre notches. There is a discrepancy compared with the findings of the previous literature. The potential causes are discussed.
Carbon Fiber Sensors for Strain and Temperature Measurement of Composite Overwrapped Pressure Vessels
Previous researchers have demonstrated carbon fiber (CF) based sensors for strain/temperature measurement and health monitoring of composite structures1,2. Aurora Flight Sciences is developing this technique for the unique challenges related to qualifying and monitoring Composite Overwrapped Pressure Vessels (COPVs). CF strain sensors can be placed directly within a composite structure and aligned with the fibers or directions of interest. The sensors can be configured for small area measurement, such as the polar regions where there is a complex state of stress, or for large global strains, such as boss to boss displacement or circumferential growth measurement. CF temperature sensors rely on fiber junctions which exploit the Seebeck effect. By placing junctions in a network fashion, a temperature sensing network can be constructed.
Sustained Release of Bioactive Compounds from Polymer Microcapsules for Smart Dental Composites
Natural plant extracts have been shown as effective and attractive dental biomaterials with promising properties for restorative/reparative procedures. These bioactive extracts, however, may lose their stability when exposed to air, moisture, or light, if not well-protected. Here, we present a novel technique for sequestering bioactive extracts within polymer microcapsules for long-term stability and controlled release of active compounds as needed. We use a combination of water-oil-water (W/O/W) double emulsion and solvent evaporation techniques to encapsulate watersoluble bioactives within polymer microcapsules. Polylactide acid was used as the encapsulating polymer to provide sustained release of bioactive compounds from microcapsules. Manufactured microcapsules possess a polynuclear inner structure as a result of two-step double emulsion technique. We demonstrate the ability to control the size and core loading of microcapsules with varying the key microencapsulation parameters.
In-Plane Shear Characterization of Sandwich Laminates Using a Picture-Frame Test Configuration
An investigation was conducted to characterize the accuracy of Ã¢â‚¬Å“picture-frameÃ¢â‚¬Â test configurations for measuring the in-plane shear strength of sandwich constructions as may be limited by local instability of the face sheets including wrinkling and dimpling. Two configurations were studied: a conventional configuration featuring corner pins which extend between the two sides of the specimen and fixture, and a modified configuration in which the corner pivot axes coincide with the corners of the square gage area bounded by edge doublers. Shear strains were predicted using finite element analysis and measured experimentally with strain gages and digital image correlation. The conventional configuration produced shear strain at the specimen center approximately 15% below the nominal value predicted for uniform shear stress, and the strain was significantly non-uniform over the gage area. The modified configuration provided a much more uniform shear strain distribution, at a strain level corresponding closely to the nominal value. Ultimate strength measurements are provided for four sandwich designs featuring both plain foam core and web reinforced core, tested in both configurations. Practical details of the hardware implementation for the modified configuration are presented.
The Separated End Notched Flexure Test for Mode II Fracture Toughness Characterization of Sandwich Composites
The Separated End Notched Flexure (S-ENF) test is proposed for Mode II fracture toughness characterization of facesheet/core disbonding in sandwich composites. While similar to the three-point End Notched Flexure test for monolithic composites and the Cracked Sandwich Beam test for sandwich composites, the proposed test utilizes a tensioned wire placed underneath the top facesheet to maintain separation and prevent frictional forces during loading. The use of a tensioned wire eliminates the need to remove core material from the test specimen to achieve the desired separation. Of particular interest in the development of the S-ENF test was the prevention of unstable disbond growth for a variety of sandwich composite configurations. A combined numerical and experimental investigation was performed to identify factors contributing to stable Mode II disbond growth. Three-dimensional finite element analysis was used to investigate effects of the initial disbond length and core thickness of the sandwich configuration as well as the span length of the flexure test fixture. Numerical results suggest that ratio of the initial disbond length relative to the span length must be selected properly for stable disbond growth. Mechanical testing confirmed that stable disbond growth can be produced using composite sandwich specimens with Nomex honeycomb and polyurethane foam cores. However, further investigation is needed to better understand the considerations leading to suitable S-ENF specimen design.
Single Cantilever Beam Test for Honeycomb Sandwich Materials with Very Thin Facesheetsâ€”Effects of Test Conditions and Material Properties
This paper deals with the facesheet/core disbonding characterization of CFRP/Honeycomb sandwich structures using the Single Cantilever Beam (SCB) test. Facesheet/core disbonding is characterized by determination of the corresponding critical strain energy release rate Gc. Motivated by the high relevance of damage tolerance analysis of complex shaped and loaded lightweight sandwich structures, e.g. for aerospace application, the SCB test has been widely investigated over the last years and guidelines are available to properly apply the SCB test to various kinds of sandwich materials. Standardization of the test method is currently being prepared. Nevertheless, particularly in the case of lightweight honeycomb sandwich materials with very thin facesheets, various effects are observed, which are not yet entirely understood. The proposed test conditions are still under discussion. In aerospace applications, often low density aramid paper honeycomb core material is combined with thin CFRP facings. In the case of thin facesheets with a thickness of 1 mm or less and a high disbond toughness, the deflections and rotations of the loaded upper skin cantilever beam of the SCB specimen is considerably high Ã¢â‚¬â€œ test evaluation based on small deformation theory, like the Compliance Calibration Method (CC) or the Modified Beam Theory (MBT), are no longer applicable. In this case, based on a strain energy concept, the Area Method (AM) is under consideration for fracture toughness determination. Alternatively, to avoid improper nonlinearities during loading and crack propagation, the use of doublers, applied to the upper facesheet, is often recommended. Further investigation is presented here to understand specifics of thin facesheet sandwich SCB testing and to assess the applicability of the simple and robust SCB test set-up for facesheet/core disbonding fracture mechanical characterization. Different test modifications and situations are numerically analysed by means of Finite Elements (FE) and compared to each other. In addition, mesoscopic in-situ observation of the fracture region during the test was carried out using X-ray CT measurements.
Mode I Quasi-Static Delamination Growth in Multidirectional Composite Laminates with Different Thicknesses
Multidirectional DCB specimens with different thicknesses were manufactured and tested to have in-depth understanding of delamination behavior in composite laminates. The initiation crack growth is demonstrated to be ply orientation and thickness independent. However, interlaminar resistance and damage mechanisms in delamination growth are significantly related to the interface configuration as well as thickness. Fractography analysis demonstrated that resistance difference between unidirectional and multidirectional DCB specimens is related to damage mechanisms. Optical microscope observation revealed that crack path in the multidirectional specimens is zigzag. This phenomenon becomes vague with thickness increase. The appearance of zigzag crack indicates both interlaminar and intralaminar damage can occur in the delamination growth. However, interlaminar damage becomes the dominant failure mode in the thick specimens. SEM (Scanning Electron Microscope) observation demonstrated fibre prints and cusps are two typical morphologies located on the fracture surface. These features are even more significant in the multidirectional interface with thickness decrease. This paper will conclude that interface configuration and specimen thickness have significant effects on the damage mechanisms and interlaminar resistance during delamination growth in composite laminates. It is, therefore, insufficient to apply the unidirectional DCB specimen with a given thickness to determine delamination growth and damage mechanisms of a composite material.
LIAOJUN YAO, R.C. ALDERLIESTEN and R. BENEDICTUS
A Modified Edge Crack Torsion Test for Measurement of Mode-III Fracture Toughness of Laminated Tape Composites
Modifications to the edge crack torsion (ECT) test are studied to improve the reliability of this test for measuring the mode-III fracture toughness, GIIIc, of laminated tape fiber-reinforced polymeric (FRP) composites. First, the data reduction methods currently used in the ECT test are evaluated and deficiencies in their accuracy are discussed. An alternative data reduction technique, which uses a polynomial form to represent ECT specimen compliance solution, is evaluated and compared to FEA results. Second, seven batches of ECT specimens are tested, each batch containing specimens with a preimplanted midplane edge delamination and midplane plies with orientations of +ÃŽÂ¸/Ã¢Ë†â€™ÃŽÂ¸, with ÃŽÂ¸ ranging from 0Ã‚Â° to 90Ã‚Â° in 15Ã‚Â° increments. Tests on these specimens show that intralaminar cracking occurs in specimens from all batches except for which ÃŽÂ¸ = 15Ã‚Â° and 30Ã‚Â°. Tests on specimens of these two batches are shown to result in mode-III delamination growth at the intended ply interface. The findings from this study are encouraging steps towards the use of the ECT test as a standardized method for measuring GIIIc, although further modification to the data reduction method is required to make it suitable for use as part of a standardized test method.
Prediction of Energy Release Rates for Echelon Crack Formation in Mode III Delamination Toughness Tests
The energetics leading to the formation and growth of an array of echelon cracks along a planar delamination front in a split laminate subjected to anti-plane shear loading are studied. The finite element method is used to model both single and multiple echelon cracks that intersect the planar delamination front. Energy release rates are determined along the echelon crack peripheries and along the planar delamination front. Various echelon crack shapes are evaluated in order to represent progressive stages of growth. It is shown that the echelon cracks advance due to primarily mode I conditions, whereas a mode III criterion is appropriate for predicting advance of the planar delamination. It is further shown that mode I advance of the echelon crack and mode III advance of the delamination are competing yet coupled processes, and that the sequence of events predicted by this approach agrees with what has been observed experimentally.
Isogeometric Analysis of Damage and Residual-Strength in Aerospace Composite Structures Subjected to Low-Velocity Impact
A multi-layered shell modelling approach is developed in the framework of the Isogeometric Analysis (IGA), based on a higher-order accurate and higher-order continuous Non-Uniform Rational B-Splines (NURBS), which allows us to adopt a computationally efficient rotation-free shell formulation. The proposed model is capable of predicting the interlaminar damage modes such as delamination, and the intralaminar damage modes such as matrix cracking due to transverse shear stress and fiber breaking. The IGA shell layers, which represent a single lamina or a groups of lamina, are connected through cohesive interfaces governed by local traction-separation laws. The intralaminar elasto-plastic damage response and the in-plane damage modes related to tension, compression and shear, are evaluated layer by layer at the level of each ply. Two numerical examples are shown. The End-Notched Flexure (ENF) test is first presented as a benchmark case to validate the cohesive-interface formulation. Then, a simple low-velocity impact scenario is considered. The smoothness of the NURBS shell discretization is shown to be beneficial for the penalty formulations adopted for both the cohesive interface and contact with the impactor. Also, due to the higheraccuracy geometry and solution representation enabled via NURBS basis functions, it is found that with the proposed IGA approach, issues that typically affect impact problems, such as through-thickness element distortion/inversion which may cause accuracy loss, shortening of time steps, and numerical instability/crashing of the code, are often circumvented.
YURI BAZILEVS, DAVID BENSON, HYONNY KIM and andREW ELLISON
Micro-CT Inspection of Impact Damage in Carbon/Epoxy Rods
Various configurations of unidirectional carbon/epoxy composite rods were impacted radially, inspected using micro-CT scanning equipment, and tested in axial compression to measure the residual strength after impact. The micro-CT data was used to calculate the peak crack area at a cross-section, and the total integrated crack volume along the entire length of the rods. This data was used to correlate the relationship between impact energy, internal damage, and residual strength. Solid cylindrical structural rods represent local members in three-dimensional composite lattice structures (e.g., based on Isogrid or IsoTrussÃ‚Â® geometries). Unidirectional core specimens, 8 mm (5/16Ã¢â‚¬Â) in diameter, were consolidated with various sleeve configurations and materials: i.e., sleeves differed in types (bidirectional braided or unidirectional spiral wraps), nominal sleeve coverage (full or half), and sleeve material (Nomex thread or Dunstone Hi-Shrink Tape). The unsupported length of the specimens used in this research was 50.8 mm (2Ã¢â‚¬Â), ensuring a strength-controlled compression failure. The specimens were radially impacted with 2.5, 5.0, 7.5, 10.0, 15.0, and 20.0 J (1.9, 3.7, 5.6, 7.4, 11.1, and 14.8 ft-lbs), and compared to undamaged control specimens. After impact, the specimens were scanned using a Micro-CT Scanner at resolutions of 50 and 35 microns and subsequently tested in axial compression. The micro-CT scan images were analyzed and the peak crack area and total crack volume along the length of the specimen were measured. Similar to related research, as the impact energy increases, the residual compression-strength-afterimpact decreases. Specimens with shrink tape sleeves had the largest increase in peak crack area and overall crack volume while specimens with full spiral sleeves had the lowest increase in peak crack area and overall crack volume. A minor correlation exists between increases in peak crack area and overall crack volume and decreases in residual compression strength after impact.
Simulation of Barely Visible Impact Damage (BVID) and Compression After Impact (CAI) Sstrength of Carbon Fiber Reinforced Composite Laminates
This work is focused on development of computational tools to predict the barely visible impact damage (BVID) and its effect on the residual strength measured through the compression after impact (CAI). A Finite Element (FE) based approach is used to model each of the 24 layers of the laminate individually and cohesive laws are used to model the delamination between the layers. The model is capable of capturing the damage due to low velocity impact (LVI) and the BVID predictions are measured and compared against the experimental results. Residual strength after impact is modeled using a second step, capturing the progressive failure under compression. A strong correlation is found between the experimental observations and model predictions. Details about the delamination between all the individual layers have been monitored and compared against the experimental data. Correlations between the BVID and residual strength are drawn to predict failure trends.
Manufacturing Energy Intensity and Opportunity Analysis for Fiber-Reinforced Polymer Composites and Other Lightweight Materials
With their high strength-to-weight ratios, fiber-reinforced polymer (FRP) composites are important materials for lightweighting in structural applications; however, manufacturing challenges such as low process throughput and poor quality control can lead to high costs and variable performance, limiting their use in commercial applications. One of the most significant challenges for advanced composite materials is their high manufacturing energy intensity. This study explored the energy intensities of two lightweight FRP composite materials (glass- and carbon-fiber-reinforced polymers), with three lightweight metals (aluminum, magnesium, and titanium) and structural steel (as a reference material) included for comparison. Energy consumption for current typical and state-of-the-art manufacturing processes were estimated for each material, deconstructing manufacturing process energy use by sub-process and manufacturing pathway in order to better understand the most energy intensive steps. Energy saving opportunities were identified and quantified for each production step based on a review of applied R&D technologies currently under development in order to estimate the practical minimum energy intensity. Results demonstrate that while carbon fiber reinforced polymer (CFRP) composites have the highest current manufacturing energy intensity of all materials considered, the large differences between current typical and state-of-the-art energy intensity levels (the Ã¢â‚¬Å“current opportunityÃ¢â‚¬Â) and between state-of-the-art and practical minimum energy intensity levels (the Ã¢â‚¬Å“R&D opportunityÃ¢â‚¬Â) suggest that large-scale energy savings are within reach.
SABINE BRUESKE, ALBERTA CARPENTER and JOSEPH CRESKO
An Overview of the NIST FIBERS Roadmap to Advance the State of Composites Manufacturing in the U.S.
The FIBERS (Facilitating Industry By Engineering Roadmapping and Science) roadmap encompasses all sectors of composites manufacturing and will be a valuable tool to assist in developing pathways that can lead to new and expanded markets for composites and to guide future calls for research programs on composites manufacturing. This paper summarizes the current state-of-the-art, drivers and barriers for five topical areas associated with the U.S. Composites Manufacturing industry and actions that can be taken to address the associated barriers. The major topics presented include (1) Materials, (2) Processing methods, (3) Recycling, (4) Predictive models and (5) Workforce development.
CHRISTOPHER HANSEN, PATRICK DRANE, JENNIFER GORCZYCA, KARI WHITE, EMMANUELLE REYNAUD, DANI
Semi Empirical Modeling of Magnetic Field Assisted ED Machining of Metal Matrix Composites
This study investigates the hybrid EDM technique performed in a magnetic field environment for refining process execution. Earlier magnetic-field-assisted EDM (MFAEDM) techniques were confined to use with a category of magnetic materials. In this investigation, the magnetic field was paired with traditional EDM plasma zone to examine this hybrid process on Al-based metal matrix composites. The parameters, for instance, peak current, pulse on duration and electrode variant were selected to determine their after effects on the response such as material removal rate (MRR) and surface integrity. A semi-empirical model for MRR on the MMC has been built by incorporating dimensional analysis depending on relevant process parameters selected out by the design of experiments (DOE). The experimental results witness improved recast layer formation along with an improvement of 11.7% MRR at higher spark energy. As the experimental results inferred that the MFAEDM facilitated the process stability, predictions of the semi-empirical model have conferred a good correspondence with experimental verifications.
Role of Inelastic Transverse Compressive Behavior on Kevlar KM2 Single Fiber Transverse Impact
KevlarÃ‚Â® KM2 fibers used in high velocity impact applications exhibits a nonlinear inelastic behavior in transverse compression with an elastic limit less than 1.5% strain. The effect of this transverse behavior on a single KM2 fiber subjected to a cylindrical and a fragment-simulating projectile (FSP) transverse impact is studied with a 3D finite element model. The inelastic behavior results in a significant reduction of fiber bounce velocity and projectile-fiber contact forces by 40% compared to an elastic behavior. The inelastic dissipation and reduced bounce leads to an inelastic collision rather an elastic collision. The fiber experiences multi-axial stress states during impact including transverse compression, axial tension, axial compression and interlaminar shear at the location of failure. In addition, for a FSP impact, significant multi-axial strain concentrations are predicted. Thus, the experimentally measured yarn breaking speeds are lower than classical Smith theory and may be attributed to the multi-axial loading stress concentration and property degradation mechanisms.
High Strain Flexural Characterization of Thin CFRP Unidirectional Composite Lamina
Thin composite flexures possess unique characteristics desirable to the aerospace deployables community. These high strain composite (HSC) laminates have been found to exhibit elevated strength and deformation capacities prior to failure in comparison to those observed in thicker traditional composite structures. For this reason, HSCs are regarded as a prime candidate for strain energy driven deployable space structures for their ability to endure high levels of stress and strain enforced during the stowage process. Means of quantifying this phenomenon have been met with difficulty due to current limitations in testing methodologies; typically resulting in lower material capacities non-representative of those observed in thin HSCs in bending. In attempts to describe this occurrence, a nonlinear fiber based constitutive material model is used along with high strain lamina flexural mechanics. A workflow is laid out, detailing the various test approaches employed to determine the essential material constants (composite constituent moduli, tensile/compressive nonlinear parameters, etc.) required to identify the failure modes at play. Such testing methods include ASTM tensile testing, optical micrographing, Large Deformation Four Point Bending, and Platen Flexure. To demonstrate the validity of this approach, the entire process is performed for an IM10 carbon fiber/PMT-F7 aerospace toughened epoxy based unidirectional composite. Matrix modulus, fiber initial modulus, tensile and compressive nonlinear parameters were found to be 3.53 GPa, 284.2 GPa, 23.6, and 34.0 respectively. These values were utilized to fit Platen Flexure test data to determine ultimate stress-strain values at representative failure conditions. The IM10/PMT-F7 unidirectional displayed capability in achieving tensile strain levels up to 25% greater than manufacturers stated limits (2.0% observed vs 1.6% reported). Compressive strains calculated at failure also showed notable values, approaching close to 3% at its best.
Load-Transfer-Based Micromechanical Simulation for Evaluating Elastic-Plastic Response of Discontinuous Carbon Fiber Reinforced Thermoplastics
The present study investigated nonlinear elastic-plastic stress-strain relations of discontinuous carbon fiber reinforced thermoplastics (CFRTPs) using experiments and numerical simulations. In the experiments, we conducted uniaxial tensile tests and three-point bending tests for two types of CF/PA6 specimens; injection-molded specimens with short fiber length and aligned fiber orientation and compression-molded specimens with long fiber length and random fiber orientation. Comparing the experimental results, we found that the injection-molded specimens showed a nonlinear stress-strain response, while the response of compression-molded specimens was almost linear. The results implied that long discontinuous fibers were effective for increasing the yielding point of the composites, even if the composites had a random fiber orientation, which was effective to eliminate the orientation dependence on mechanical properties of the composites. Furthermore, we attempted to simulate elastic-plastic stress-strain relations of discontinuous CFRTPs, in order to understand the effect of microstructure including fiber length. For this purpose, we employed fiber-based simulations to deal with the microstructure of fibers and matrix and the constitution law of matrix. The simulated results found that the fiber length is an influential factor on the nonlinearity of the stress-strain relations of discontinuous CFRTP composites.
AKIRA FUKUZO, NAOKI MATSUDA and MASAKI HOJO
Predicting Thermal Conductivity of Graphene Nanoplatelet/Epoxy Nanocomposite using Non-Equilibrium Molecular Dynamics
The thermal properties of Cycloaliphatic Epoxies (CE) and Anhydride Curing Agent (ACA) are predicted using Molecular Dynamics (MD) simulations with the Optimized Potential for Liquid Simulation (OPLS) force field. A modeling procedure has been developed to simulate the unique continuous crosslinking process of this material system, which has not been previously achieved. The predicted glass-transition temperature is in good agreement with experiment. The thermal conductivity is determined by using Non-Equilibrium MD (NEMD) techniques and is also close to the experimental value.
M.S. RADUE, D.R. KLIMEK-MCDONALD, S. GOWTHAM, J.A. KING and G.M. ODEGARD
Accelerated Hydrothermal Aging of Cycloaliphatic Epoxy/Graphene Nanoparticle Composites
Cycloaliphatic epoxy (CE) resin systems are of primary interest in applications that require improved resistance to harsh conditions relative to other epoxy systems. Because other epoxy systems have demonstrated improved resistance to hydrothermal aging with the addition of carbon-based nanoparticle reinforcement, it is expected that the hydrothermal resistance of CE resins will likewise be improved with incorporation of nanoparticles. Therefore, the objective of this study is to determine the influence of graphene nanoparticles (GNP) on the hydrothermal aging resistance of CE resins. CE specimens are fabricated with varying levels of GNP and exposed to elevated temperatures and moisture levels for varying amounts of time up to 400 hours. The results from flexure testing indicate that the addition of GNP provides modest improvements in the stiffness for all aging levels, while the strength is improved for aging times below 400 hours.
I.D. HELMAN, W.A. PISANI, D.R. KLIMEK-MCDONALD, S. CHINKANJANAROT, I. MISKIOGLU, J.A. KING
Molecular Dynamics Model of Graphene Nanoplatelet in EPON 862/DETDA Polymer
A computational molecular dynamic model involving multilayer graphene nanoplatelets in epoxy composite was developed in order to characterize its elastic behavior when subjected to thermo-mechanical loadings. In this investigation, the approach for constructing the molecular structure of graphene/epoxy composite using Optimized Potential Liquid Simulation (OPLS) force field is documented. Also, the cross-linking and the method for equilibrating the systems are defined. The techniques used for evaluating the models under different loading conditions are given. The predicted results are made available for the user of composite materials.
Acoustic Emission of Large PRSEUS Structures
In the role of structural health monitoring (SHM), Acoustic Emission (AE) analysis is being investigated as an effective method for tracking damage development in large composite structures under load. Structures made using Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) for damage tolerant, light, and economical airframe construction are being pursued by The Boeing Company and NASA under the Environmentally Responsible Aircraft initiative (ERA). The failure tests of two PRSEUS substructures based on the Boeing Hybrid Wing Body fuselage concept were conducted during third quarter 2011 and second quarter 2015. One fundamental concern of these tests was determining the effectiveness of the stitched integral stiffeners to inhibit damage progression. By design, severe degradation of load carrying capability should not occur prior to Design Ultimate Load (DUL). While minor damage prior to DUL was anticipated, the integral stitching should not fail since this would allow a stiffener-skin delamination to progress rapidly and alter the transfer of load into the stiffeners. In addition, the stiffeners should not fracture because they are fundamental to structural integrity. Getting the best information from each AE sensor is a primary consideration because a sparse network of sensors is implemented. Sensitivity to stiffener-contiguous degradation is supported by sensors near the stiffeners, which increases the coverage per sensor via AE waveguide actions. Some sensors are located near potentially critical areas or Ã¢â‚¬Å“critical zonesÃ¢â‚¬Â as identified by numerical analyses. The approach is compared with the damage progression monitored by other techniques (e.g. ultrasonic C-scan).
Delamination Detection Using Guided Wave Phased Arrays
This paper presents a method for detecting multiple delaminations in composite laminates using non-contact phased arrays. The phased arrays are implemented with a non-contact scanning laser Doppler vibrometer (SLDV). The array imaging algorithm is performed in the frequency domain where both the guided wave dispersion effect and direction dependent wave properties are considered. By using the non-contact SLDV array with a frequency domain imaging algorithm, an intensity image of the composite plate can be generated for delamination detection. For the proof of concept, a laboratory test is performed using a non-contact phased array to detect two delaminations (created through quasi-static impact test) at different locations in a composite plate. Using the non-contact phased array and frequency domain imaging, the two impact-induced delaminations are successfully detected. This study shows that the non-contact phased array method is a potentially effective method for rapid delamination inspection in large composite structures.
A Low Cost Microwave Imaging System Using a 6-Port Reflectometer for NDE of Composites
The proliferation of composites in aerospace, automotive, and other industries has continued to increase largely due to the high strength to weight ratio. Improving the effective use of these materials is a continuing effort. If the material is compromised due to delamination or disbond, during manufacturing or use, the results can be catastrophic [1]. There is an urgent need for accurately detecting such defects in a rapid and reliable manner using cost effective technologies. Microwave NDE is well suited for analysis of low loss, or lossless fiber reinforced polymer (GFRP) composite structures or composites adhesively bonded to metal structures. In low loss dielectric materials, microwaves can penetrate and scatter off discontinuities. The scattered waves can provide information about the structural integrity of the material. In contrast to other NDE techniques, microwave NDE offers advantages such as non-contact detection, rapid far field inspection, and requires no coupling medium. However, microwave instruments such as a vector network analyzer (VNA) are expensive and difficult to transport to conduct experiments in the field. Such a system can be potentially replaced by a low-cost, single frequency device utilizing a 6-port measurement technique. In this paper, the feasibility of using a single frequency microstrip line 6-port reflectometer for NDE of composite samples has been studied. The first part of the paper deals with experiments that have been conducted using a VNA and X-band horn antenna to demonstrate the application of microwaves for defect detection in adhesively bonded metal-composite joints. The second part of the paper focuses on the design of the 6-port reflectometer to be used as a low cost imaging device for NDE of composite samples. Preliminary simulation results have been presented in this paper, followed by a fabricated prototype. Future work involves a thorough evaluation of device performance by conducting a variety of parametric studies and real-time experiments for detecting defects in composites.
SAPTARSHI MUKHERJEE, LALITA UDPA and PREM CHAHAL
Effects of Density and Cell Rise Ratio on Compressive Stiffness and Strength of PVC Structural Foam
A study has been conducted to investigate the important effects of foam density and cell rise ratio on compressive mechanical properties of closed-cell PVC structural foam. Taking advantage of density variation in an as-received large PVC foam panel, foam compression specimens with different densities from the panel were prepared. Straight-side specimens with different gage-section length-to-width ratios were used for foam out-of-plane compressive stiffness study. For strength and failure mode evaluation, foam specimens with reduced gage sections were used. Test results show that foam compressive stiffness did not correlate linearly with either foam density or cell rise ratio. Similar behavior was observed for the foam compressive failure strength. Compressive failure modes were found to include foam cell buckling and formation of a localized shear band across the entire specimen cross-section. Recently developed foam compressive stiffness and strength models based on foam microstructure and micromechanics were used to correlate analytical predictions with experimentally determined foam compressive stiffness and strength. The results indicate that foam density and its cell rise ratio alone cannot properly address the effect of individual material and microstructure parameters on foam compressive properties. Their interactive roles and combined effect must be considered simultaneously in the modeling, analysis and experimental evaluation of foam mechanical properties. For the light-weight closed-cell PVC foam tested in the study, an approximately linear relationship with the product of foam density and cell rise ratio was found for both foam compressive stiffness and compressive strength.
Thermal and Thickness Effects in Para-Aramid Core
This paper presents results from a recent test program on para-aramid (KevlarÃ‚Â®) honeycomb core. Test results from several suppliers are combined to obtain knockdown factor curves for thermal and thickness effects. The results were compared to curves published for non-metallic core in the HexcelÃ‚Â® core design guide. Some historical test data from the suppliers was also used in the analyses.
Tensile Behavior of Compression Molded Glass Microballoon/HDPE Syntactic Foams
Tensile behavior of glass microballoon (GMB) reinforced high density polyethylene (HDPE) matrix syntactic foams is investigated in the present study. GMB's having true particle density 350 kg/m3 are varied in 0, 20, 40 and 60 by vol. % in HDPE matrix using brabender and subsequently compression molded to form the syntactic foam sheets. Experimental results show that the modulus increases while strength decreases with increase in microballoon content. Syntactic foams present lower fracture strain as compared to neat HDPE. For designing syntactic foam microstructures with desired properties theoretical model can be effectively utilized.
B. R. KUMAR, MRITYUNJAY DODDAMANI, STEVEN ZELTMANN and NIKHIL GUPTA
Crashworthiness of Microvascular Fiber-Reinforced Composites
Microchannels can provide fiber-reinforced composites with many functions such as self-healing and thermal management. However, microchannels have the potential to disrupt composite structural integrity. Here we demonstrate for the first time that microchannels can be incorporated in a carbon fiber composite without decreasing crashworthiness. Corrugated panels were manufactured containing 400 ÃŽÂ¼m microchannels at a channel volume fraction of 5%. Channels were sequestered within non-load-bearing plies to reduce their impact on mechanical properties. Compression tests using a chamfer damage trigger revealed no loss in specific energy absorbed (SEA) for vascular panels vs. controls. Non-chamfered bars were also tested to demonstrate that channels can actually enhance crashworthiness by triggering stable failure. Control bars buckled when compressed while vascular samples had stable failure initiated at the channels.
Molecular Dynamics Study for Experimental Design Guideline of Dimeric Anthracene-based Mechanophore in the Thermoset Polymer Matrix
This paper presents a molecular level trade-off analysis on mechanophore embedded thermoset polymer matrix between its self-sensing capability and critical mechanical properties affecting performance. Dimeric 9-anthracene carboxylic acid (Di-AC), a mechanophore, was synthesized in epoxy polymer, and with the application of mechanical load, fluorescence was observed under ultraviolet (UV) excitation prior to the yield point. Experimental results, conducted with 5% Di-AC weight fraction, showed successful early damage detection capability; however, it was also observed that the inclusion of Di-AC reduced the material yield strength. To mimic this experimentally observed phenomenon and understand the complex mechanochemistry of this novel material, the molecular dynamic (MD) methodology simulates mechanophore synthesis, epoxy curing, and the mechanical loading test. Mechanophore sensitivity is investigated with different Di-AC weight fractions and the corresponding effects on the yield strength are analyzed. An initial degradation and subsequent recovery in yield strength over a certain range of weight fraction is observed. The possibility of early damage detection capability using higher Di-AC weight fractions is also investigated. Results indicate that the sensitivity to external stress is an inherent mechanophore property and is independent of the weight fraction. However, visible fluorescence (intensity), which is essential to damage detection, is highly dependent on the amount of Di-AC in the polymer.
Icing Protection System for Composite Structures Using Carbon Fiber Heating Elements
Long ago, carbon fibers were considered an exotic material. Only high performance applications could justify the use of carbon fiber composite parts. Over the years, production processes have accelerated and the cost of the formerly esoteric material has dropped. Their strength-to-weight ratio is the root of their high demand, nevertheless, they exhibit other advantageous physical properties, such as electrical conductivity. Processing of metal wires or films in a fiber-reinforced composite can be cumbersome and require advanced tooling. If carbon fibers could replace metallic wires and films as an effective embedded heating element in composite structures, then the processing of embedded composite heaters could be done at a lower cost; assuming that the textile weaving process could be automated, and that any postprocessing of the textile to maintain its quality would be less costly than that of the aforementioned metals. In this study, composite plates laminated with a carbon fiber heater have been fabricated and tested in a small scale icing wind tunnel. The performance of the carbon fiber heater matched that of a similar laminate made with a metallic heating filament for icing protection applications.
Design and Testing of Damage Tolerant Composite Airframe
Requirements for current and future aircraft platforms have moved the aerospace industry to the widespread adoption of lightweight composite structures as the basis of modern airframe design. Beyond the benefit of weight reduction, composites offer inherent fatigue and corrosion resistance, resulting in reduced routine maintenance and higher aircraft readiness rates. However to fully realize this potential, further improvements in durability and damage tolerance are being sought. This is especially true in the case of military platforms where service conditions are harsh and aircraft availability is at a premium. The Combat Tempered Platform Demonstration, being conducted by the U.S. Army Aviation Development Directorate Ã¢â‚¬â€œ Aviation Applied Technology Directorate and Sikorsky Aircraft, is focused demonstrating an integrated platform solution that exemplifies both operational durability and total survivability. The project includes the demonstration of a lightweight, highly durable and damage tolerant airframe structure based on advanced composite design and construction methods. Integrated health management sensor system capable of detecting and monitoring structural damage were also incorporated to reduce the risk associated with adopting a damage tolerance approach. Additional structural life enhancement was achieved through the implementation of load reduction using load alleviating flight control laws. This research was partially funded by the U.S. Government under Agreement No. W911W6-12-2-0005. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Aviation Development Directorate Ã¢â‚¬â€œ Aviation Applied Technology Directorate or the U.S. Government.
Multi-Scale Simulation of Delamination Migration
A Discrete Damage Modeling (DDM) approach using a Regularized eXtended- Finite Element Method (Rx-FEM) formulation has been applied to a test specimen for characterizing delamination migration in laminated composites. The migration specimen is designed such that as a delamination propagates, the stress state alters such that delamination migration through a neighboring ply becomes favorable. The present research aims to use the DDM framework to simulate the local, micro-scale behavior at a delamination migration point. Additional functionality to the Rx-FEM formulation, required for this task, allows for Rx-FEM cracks of arbitrary shape. A four-point shear fracture specimen was used as empirical guidance in validating this capability. Excellent agreement between model and experiment was shown. A laminated effective-modulus representation of the test was then modeled using the DDM framework, establishing the point of delamination migration. Location of delamination migration was predicted accurately. A local, micro-scale model was constructed and applied at the location of migration. The micro-scale model consisted of arbitrary interacting cracks in resin-rich zones using the modified Rx-FEM formulation and standard cohesive zone disbonding between fibers and resin. Model predictions show great similarity to the initial shape of the migrating transverse cracks observed in experiment.
ERIC ZHOU, KEVIN HOOS, ENDEL IARVE, MICHAEL BRAGINSKY, TIMOTHY BREITZMAN and DANIEL RAPKIN
Probabilistic Characterization of Interlaminar Toughness for Reliability Analysis of Aircraft Composite Structures
Interlaminar areas are usually the weakest links in composite primary structural elements, and objectives of this work are the development of a methodology for efficient probabilistic characterization of Mode II interlaminar toughness; its demonstration on examples of typical laminated carbon-fiber composite materials; and generation of representative data quantifying its statistical scatter and distribution trends. The developed methodology is based on the assumption that, in contrast with average properties, parameters of variability can be sensitive to considered statistical populations. Therefore, it is based on analysis of variability at different scales, namely, within a relatively small area of materials (e.g., one beam-type coupon), larger domains (e.g., square panels), and populations of multiple panels. In addition, effects of material thickness are captured through consideration of composites with different numbers of layers. For efficient testing, a modified variant of Mode II characterization was suggested and successfully implemented. A systematic test program was planned and performed to generate a significant population of results sufficient to make conclusions on statistical variability with high confidence. Typical uni-directional IM7/8552 carbon-fiber laminated composites with 20, 30, and 40 individual layers were used as representative examples for the demonstration. Finally, applicability of the generated results to probabilistic Finite Element Analysis for damage tolerance based analysis was suggested.
Fatigue Damage Accumulation under Biaxial Cyclic Loading of Off-Axis Composites
The mechanism of fatigue damage accumulation in laminates is one of the most important subjects in the engineering predictions of composite structures fatigue life. This paper presents a non-linear fatigue damage accumulation model to predict fatigue life of off-axis composites under constant-amplitude biaxial cyclic loading. This type of loading is typical for many elements of various composite structures, therefore the generation of fatigue damage accumulation model for these elements is evidently relevant.
Predicting the Strength and Failure Envelops of High-Performance Discontinuous Composites
CarbonÃ¢â‚¬â€œfibre towÃ¢â‚¬â€œbased discontinuous composites are novel materials combining the highÃ¢â‚¬â€œperformance of carbonÃ¢â‚¬â€œfibres with the manufacturability of metals. These composites are reinforced by a network of discontinuous and randomlyÃ¢â‚¬â€œ oriented fibreÃ¢â‚¬â€œtows (each composed of thousands of aligned fibres), creating a multiscale and unstructured architecture. Consequently, they can be moulded into complex 3D shapes using fully automated and highÃ¢â‚¬â€œrate processes, which makes them suitable for massÃ¢â‚¬â€œproduction applications. Moreover, towÃ¢â‚¬â€œbased discontinuous composites can achieve fibre contents up to 60% in volume, and a stiffness similar to that of quasi-isotropic continuous composites. While the complex and multiscale microstructure of towÃ¢â‚¬â€œbased discontinuous composites is key for their manufacturability and good mechanical performance, it also creates a challenge for predicting their properties. This work therefore proposes a multi-scale model to predict the strength and failure envelopes of towÃ¢â‚¬â€œ based discontinuous composites, using the mechanical properties of the tows and their geometry as inputs. The model predicts full failure envelopes for any tensionÃ¢â‚¬â€œtension or tensionÃ¢â‚¬â€œ shear in-plane stress state in less than 5 min, making the model suitable for design of structural components under complex stress states. The model is also able to predict the effect of changing the properties and geometry of the tows on the overall response of the composite, and can therefore be used to design optimal material configurations with improved performance.
Characterization and Modeling of Progressive Damage in Angle-ply Composite Laminates under Varying Strain Rate Loading
The strain-rate-dependent damage progression of toughened-matrix carbon/epoxy composite (IM7/8552) angle-ply laminates was characterized over the range of quasi-static (10-4 s-1) to dynamic (103 s-1) strain rates. A progressive damage model was proposed to describe the matrix-dominated transition from linear elastic to non-linear material behavior. The Northwestern Failure Theory was adapted to provide a set of physical-based yield criteria for predicting the matrix-dominated damage initiation using the lamina transverse tension, transverse compression, and shear yield strengths. The characteristic damage state of the angle-ply laminates was determined for the tested strain rates to provide a length scale-relevant benchmark for characterizing progressive damage in the angle-ply laminates. The framework evaluated the angle-ply laminate yield, characteristic damage state, and ultimate failure based on matrix-dominated response and the Northwestern Criteria were found to provide superior prediction of the matrix-dominated behavior for all evaluated cases compared to the classical approaches. The results indicate the potential to use the Northwestern Criteria to provide the predictive baseline for damage propagation in composite laminates based on experimentally identified damage response on a length-scale-relevant basis.
Development of a Constitutive Material Model with Progressive Failure and Damage for Woven Thermoplastic Composites
This paper describes the behavior of the DuPontÃ¢â€žÂ¢ VizilonÃ¢â€žÂ¢ thermoplastic composite sheet materials and matches this to composite models as implemented in Abaqus [1] and LS-DYNA [2]. The performance of the models with the corresponding material parameters are evaluated by simulating material specimen tests and structural beam component. The results show limitations of current composite material models in Abaqus [1] when applied to thermoplastics. To obtain a more accurate description of the actual material behavior, DuPont has developed a User material model by combining both plastic deformation of the thermoplastic matrix material with a more classical composite damage model for the description of the nonlinear permanent deformation of the woven continuous fiber reinforcement. The results obtained with this model are presented, demonstrating accurate prediction results and the potential of this model for thermoplastic composites.
A Mesh Insensitive Composite Damage Model for Crash Simulations
The usage of advanced lightweight materials such as polymer based composites is one direction to meet todayÃ¢â‚¬â„¢s requirements and regulations regarding energy efficiency. This is especially true for the automotive industry, where the usage of lightweight components can substantially lower the fuel consumption. However, sometimes these components carry critical structural loads and represent key safety parts. Therefore being able to predict by simulation the crash and strength performance of such materials is crucial starting in the early stages of the design process. State of the art mechanical simulation software not only provide elastic stiffness properties, they aim at predicting the progressive damage and failure limits of the composite part. However, the available damage and failure models may only be able to predict the point of damage initiation or suffer from numerical drawbacks such as a strong mesh dependence. Therefore, numerous research teams focus on the development of both robust and accurate progressive damage models for composites. A promising approach has been published recently by Pineda and Waas [1]. It is based on the definition of characteristic failure planes and the internal transition from a continuum to a cohesive based formulation. The damage model has been successfully implemented in the FE software package ESI Virtual Performance Solution (VPS). This allows for an explicit simulation of complex composite structures under dynamic loading such as they occur in a crash simulation. The implementation is subsequently evaluated for a variety of test cases and compared with standard continuum based formulations. The investigated models range from simple coupon tests to complex crash models.
Fatigue Behavior of Unidirectional Carbon/Epoxy AFP Laminates Containing Gaps
Using automated fiber placement (AFP) provides highly repeatable and flexible manufacturing of complex and large composite components. Existence of manufacturing defects such as gaps can affect the performance of these components. In this paper, an experimental investigation was performed to study the effect of gaps on fatigue performance of AFP laminates. Tension-tension fatigue tests were conducted on unidirectional carbon/epoxy laminates containing gaps and the behavior was compared to that of base line laminates free from defects. Monitoring of damage evolution during fatigue tests was performed using infrared thermography technique. This was observed to be a powerful nondestructive technique capable of detecting the spreading of non-uniformity in structures such as induced gaps as a result of AFP manufacturing. Experimental results revealed that the effect of gaps depends on the maximum applied stress during fatigue loading. The higher is the applied stress, the higher is the reduction in fatigue life.
Effects of Interfacial Defects on Properties of Laminated Composite Materials and Their Bonded Joints
Composite materials are widely used in a range of applications and their reliability depends on the quality of manufactured structure. Defects can be produced in composite materials during the manufacturing and can also evolve over the entire service life. A number of different defects can initiate during the manufacturing, adhesive bonding and repair process. For example, voids and interfacial debond can be very detrimental to the performance of the composite structure. Debond at bonded joints is often difficult to characterize (for example, Ã¢â‚¬Å“kissing bondÃ¢â‚¬Â). Although progress has been made in detection of defects using popular nondestructive methods, there are still significant challenges remain in quantifying the distribution of defects and the corresponding property loss. The objective of this study is to explore effects of defects (micro-void and debond) at interlaminar locations within the laminate and also at adhesive joints. To study the effect of interlaminar defect, glass microspheres and Teflon inserts are used to create debond region between two carbon fiber plies. On the other hand, coatings of mold release agent and release films were used in the adhesive joint to mimic kissing bond between glass fiber substrates. We will use a new concept that heterogeneous charge polarization inside a material is related to the microstructural degradation and can be quantified using Broadband Dielectric Spectroscopy (BbDS) technique. Using both bulk and scanning mode, we can quantify the distribution of dielectric properties. Corresponding mechanical property loss due to defects in the material has been determined and related to changes in dielectric characteristics such as changes is complex permittivity. Results show that real permittivity captures the interfacial defect in both interlaminar and kissing bond experiments. A 3D X-ray microscope will be used to validate experimental observations. Details of experimental results and observations are documented in the paper.
PRASUN MAJUMDAR, PRASHANT KATIYAR and ROBERT WILKES
Experimental Study of Laminated Composites Containing Manufacturing Defects Under Combined Stress States
Manufacturing defects are known to cause a knock-down in the mechanical properties of composite materials. These knock-downs are however usually only tested in simple stress states and not the more complex loading as found in real structural applications. Here a modified Arcan test fixture has been designed and manufactured to apply interlaminar shear in combination with through thickness compression to carbon fibre/epoxy composite laminates. The fixture is designed to accommodate a 30 Ãƒâ€” 30 mm size specimen and allows the ratio of interlaminar shear to through-thickness compression applied to the specimen to be varied by altering the angle of the fixture. The material used in this study is the HexplyÃ‚Â® IM7/8552 and the ply stacking sequence of the laminates is [+452/902/-452/02]3S, giving a nominal specimen thickness of 6 mm. The study included pristine specimens as well as specimens with an artificially embedded out-of-plane wrinkle defect which was generated by inserting a precise pattern of narrow pre-preg strips with 90Ã‚Â° orientation across the width of the laminate during lay-up, which during cure merged with adjacent 90Ã‚Â° plies in the stack. This experimental study has investigated failure stresses and failure modes for both pristine and wrinkle defect specimens, tested over a range of interlaminar shear to through thickness compression stress ratios.
MIKE JONES, LUIZ KAWASHITA and STEPHEN HALLETT
Effect of Manufacturing Induced Fiber Break on Local Tensile Failure in Composites
Fiber breakages are commonly found during composites manufacturing process. In the current study, the effect of manufacturing induced fiber break on local tensile failure in unidirectional (UD) composites is investigated numerically. In the finite element (FE) model, a broken fiber is placed centrally in a hexagonally packed UD composite and is assumed to be perfectly bonded to the matrix. Since the stress perturbation caused by a single fiber breakage is local, only the six most affected nearest-neighbor fibers are modeled and are placed as a ring of concentric axisymmetric cylinder embedded in the matrix. The discrete fiber region is surrounded by a concentric outer cylinder ring of homogenized composite. The entire FE model is subjected to axial tensile loading. Upon loading, it is found that matrix crack would most likely initiate perpendicular to fiber axis by cavitation due to tri-axial stress state near fiber break, and the thermal residual stress is found to promote the cavitation process. Once the matrix crack initiates from fiber break, fracture mechanics methodology is adopted by using extended finite element method (XFEM) to simulate the matrix crack propagation. The stress concentration factors (SCF) along the neighboring fibers are calculated during matrix crack propagation and obtained results show that the maximum SCF is the highest when matrix crack reaches a neighboring fiber. Finally, the statistical consideration regarding neighboring fiber failure is incorporated and it is found that the initial fiber breakage, together with the matrix cracking that follows, greatly enhance the probability of neighboring fiber failing at the local region close to the original fiber-break plane, which indicates that a planar fracture plane is expected if final tensile failure of UD composite starts from a manufacturing induced fiber break.
Characterization of Water-Epoxy Interactions with Spectroscopic Methods in Epoxy
Composite materials are widely used in the aerospace and wind energy industries. In most applications these materials undergo mechanical loading coupled with the effects of environmental degradation from moisture and temperature variation. It has been widely reported that absorbed moisture in high humidity environments can significantly reduce the strength of epoxy, while increasing its strain to failure. However this behavior varies significantly depending on the type of the epoxy. Zhou and Lucas [1, 2] show that at a molecular level, water molecules interact with polymer chains in two ways: i) H2O forming one hydrogen bond (H-bond) with a single polymer chains and ii) H2O forming H-bonds with two or more polymer chains. Both interactions change the polymer chain configuration. Based on the type and number of interactions of H2O with polymer chains, either plasticity or stiffness is locally enhanced, which leads to changes in material properties at the macroscopic level. In the current study, Fourier Transform Infrared (FT-IR) spectroscopy along with 2D correlation analysis is utilized to identify various water-epoxy interactions for two different types of epoxies. Furthermore, microtensile tests and dynamic mechanical analysis is performed on hydrated epoxy samples. These results are compared with their dry counterpart. We identify the possible deformation mechanisms based on water-epoxy interactions and changes in mechanical properties.
Flexural, Thermomechanical and Low-Velocity Impact Studies of CFRP Composites with Nanoclay and Multiwalled Carbon Nanotubes
Fiber reinforced polymer (FRP) composites show significantly superior performance over many traditional metallic materials because of their superior strength to weight ratio and higher stiffness. Significant development in the use of nanoparticles for modification of epoxy matrix has led to improved mechanical properties of the FRP composites. Carbon fiber reinforced epoxy composites were modified with 2 wt. % Montmorillonite nanoclay and 0.3 wt. % multi-walled carbon nanotubes (MWCNTs). A hybrid system wherein 2 wt. % of nanoclay and 0.1 wt. % of MWCNTs was also developed. Dynamic Mechanical Analysis (DMA), Thermo- Mechanical Analysis (TMA), and 3-point bending tests were conducted for characterizing samples. Low Velocity Impact (LVI) tests were conducted at different energy levels. Impact damage was characterized through thermography. Results obtained from these experiments were compared with unmodified carbon/epoxy composites. Reinforcement with nanoparticles was found to significantly improve the mechanical and thermo-mechanical properties of composites. Addition of nanoparticles in composites caused significant improvement in mechanical and viscoelastic properties compared to those of control ones. Best results were obtained for addition of 0.1% MWCNT/2% MMT hybrid nanoparticles in epoxy. Composites modified with hybrid nanoparticles exhibited about 15% increase in storage modulus as well as 10Ã‚Â° C increase in glass transition temperature. Flexural modulus for hybrid nanoparticle modified composites depicted about 30% improvement compared to control ones. Viscoelastic properties like storage and loss modulus increased by over 40% and coefficient of thermal expansion decreased both in the inplane and through the thickness directions. Carbon fiber reinforced epoxy composites modified with MMT/MWCNTs hybrid nanoparticles displayed better impact resistance than control samples. Improved impact resistance of modified composites will help to address some of the major issues with using composite materials in naval vessels and offshore structures without sacrificing the advantages of composite structures.
TANJHEEL MAHDI, EKRAMUL ISLAM and SHAIK JEELANI
Compressive Behavior of Cenosphere/HDPE Syntactic Foams under Different Strain Rates
An industrial scale injection molding machine is used to prepare fly ash cenosphere reinforced high density polyethylene (HDPE) syntactic foams. Thermosetting matrix foams with glass microspheres are being used in marine and aerospace applications owing to higher specific properties. Thermoplastic matrix syntactic foams have not been studied extensively despite interest in them for lightweight underwater vehicle structures and consumer products. Syntactic foams are fabricated with 20 and 40% cenospheres by weight. The studies on the manufacturing process suggest that a small percentage of cenospheres fracture in syntactic foams containing up to 40 wt.% cenospheres. Incorporation of particles, which are inexpensive, helps in fabricating low cost syntactic foams. Quasi-static compression tests are conducted at 10-4, 10-3 and 10-2 s-1 strain rates. The compressive strength of syntactic foams is higher than that of HDPE resin at the same strain rate due to the incorporation of ceramic particles. Yield strength shows an increasing trend with strain rate.
ASHISH SINGH, MRITYUNJAY DODDAMANI, DUNG LUONG and NIKHIL GUPTA
Flexural Investigation of Woven Composites with Sea Water Exposure
Naval structures tend to be exposed to hazardous environments and extreme temperatures. Composite materials have encompassed a large percentage in the manufacturing process of the same, appealing to a new focus in naval material research. In this paper, the adverse effects of sea water and arctic temperatures on carbon fiber composite laminates were studied in the form of moisture uptake, flexural moduli impact, strength impact, and structural damage. Moreover, an effort was made to calculate diffusion coefficients accounting for anisotropy within the composite laminate. Diffusivities were 6.25E-6 Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â¦ for in-plane Dx=Dy, 1.44E-6 Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â¦ for transverse Dz, and an effective coefficient D of 1.81E-6 Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â Ã´â‚¬Â¯Â¦ . Flexural testing showed a strong decline in material strength with a 19.45% reduction, while the flexural moduli displayed a fluctuating behavior in the form of linear equation y = -0.0011x3 + 0.0579x2 - 0.7933x + 42.289 throughout the experiments. Scanning electron microscopy was implemented for comparison between the microstructure of dry and wet composite samples; images showed a smoother, saturated surface for the wet samples compared to the rougher, dented surface of the dry samples.
Behavior of Single and Double Bolted Staggered Joint in Thick Composite Plates
Although staggered arrangements in bolted connections can offer good area usage especially for narrow members, it has the most complex stress fields. In this paper, a 3D finite element model was developed to analyze single and double lapped bolted composite plates. The modeling technique was validated with published experimental and numerical works. Different bolt arrangements were selected combined with other parameters like number of bolts, clamping force, and friction coefficient. The staggered bolt arrangements in composite plates were studied and compared with other different bolting patterns. It was found that in the absence of clamping force, part of load will be transferred by friction due to the effect of secondary stresses, which can be increased by increasing the friction coefficient. Furthermore, the staggered patterns produced horizontal forces that could cause cracks in the composite material if it was not taken into consideration during the composite laminates design.
HESHAM EL-EMAM, HANI SALIM, HOSSAM SALLAM and OSAMA EL-HUSSEINY
Effect of Pre-Strained CFRP Composite Patch on Cracked Steel Plates
Externally bonding carbon fiber reinforced polymer (CFRP) patches to the damaged elements to arrest the crack propagation can be used to extend the fatigue life of steel bridges. In this paper, a numerical study was performed to highlight the effect of bonded pre-stressed composite patches on the fracture parameters, such as stress intensity factors, crack tip opening displacement and the plastic zone. A threedimensional finite element model of the double sided CFRP patch-strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of loading modes I and II. The influence of different prestrain levels, 0.058, 0.1, and 0.2% on the fracture parameters are also investigated. It is found that the introduction of a compressive stress by pre-tensioning of the CFRP patch prior to bonding produced a significant reduction of up to 70% in SIFs for different crack lengths, which led to increasing the remaining fatigue life of the steel member. For a given level of pre-stressing, the SIF obtained is zero until a specific level of the external applied stress is reached. The pretension of the composite patch developed crack closure mechanism, which reduced the crack tip opening displacement and plastic zone size by 42% and 80% respectively for a/w=0.3 at 0.2% prestrain level. Based on these results, the pre-strain level needed to accomplish a complete crack arrest can be predicted using the crack length/plate width ratio (a/W) and the fatigue load.
ALAA ELSISI, HANI SALM, MOHAMED SELEEM and HOSSAM SALLAM
Analysis of Laminated Composite Stiffener with Unsymmetrical C-section
A closed-form analytical solution is developed for analyzing laminated composite beam with unsymmetrical C cross-section. The developed expression takes into consideration of the structural deformation characteristics of narrow beam. The sectional properties such as centroid, shear center, equivalent axial stiffness and equivalent bending stiffness as well as the ply stresses of flanges and web laminates are computed for composite C-stiffener under axial and bending loads. The ply stresses of uneven flanges of the C-stiffener subjected to axial load and bending moment are also calculated analytically and the results give excellent agreement with the results from ANSYSÃ¢â€žÂ¢. A parametric study of shear center and centroid with different layup sequences was investigated using the developed solution. It is found that the centroid and shear center locations of a composite C-stiffener structure is dependent on the laminate material properties and stacking sequence besides its structural configuration if the flange and web laminates are of different family of laminates. The location of the centroid/shear center of a composite structure is dependent of its structural configuration only if all of the flanges and web of the beam have same family laminates with symmetrical and balanced, unbalanced or unsymmetrical layup regardless the ply orientation and the stacking sequence of laminate. The ply stresses of uneven flanges of the C beam subjected to axial load and bending moment is also calculated analytically and is verified by finite element analysis. The result from the proposed theory gives excellent agreement with the ANSYSÃ¢â€žÂ¢.
The paper suggests a protocol for qualifying bridge deck systems based on experience obtained during a Federal Highway funded Highways for LIFE project that was completed in 2013. A full range of qualification tests was conducted, starting with characterization of materials at the coupon level, then subcomponent testing of custom pultruded tubes, mechanical testing of full scale structural panels in a laboratory, fire exposure, followed by service-critical details that included panel-to-panel field joints, connections to the bridge superstructure, railing attachment, and durability of the wearing surface. After installation on a bridge, load testing was done to evaluate the system as a whole, then, after three years of service, observations were made based on performance under actual operational and environmental conditions. Good correlation was obtained between the global finite element model analysis and the behavior of the as-built structure. Observations of unanticipated opening of the centerline field joint after an exceptionally cold winter indicate that a more detailed thermal analysis may be necessary to provide a model that accurately predicts joint behavior during seasonal thermal cycling.
AMJAD AREF, STEPHEN AYERS and MARIA LOPEZ
Meso-Scale Constitutive Response of Woven Composites Subjected to Large Deformation
This work aims to study the local large deformation response of woven composites at mesoscopic scales. Woven carbon reinforced composite specimens are subjected to in-plane axial quasi-static tension in parallel (on-axis) and inclined (off-axis) directions relative to the primary fiber directions, while the deformation response on the fiber-rich and matrix-rich regions on the surface of the sample is captured and analyzed in-situ using stereo-vision digital image correlation at sub-millimeter length scale. The evolution of local strain components are extracted and analyzed, and later used to explain the local constitutive response of the composite samples at various locations on the surface of the composite laminate. Appreciable strain heterogeneity is observed in both on- and off-axis specimens; whereas the degree of the heterogeneity depends on the angle between the principal fiber bundles and the axial loading direction. Attempts are also made to extract and characterize the local constitutive curves over fiber bundles and matrix-rich areas within the material, allowing for the acquisition of effective stress-strain curves at meso-scales. The approach proposed and discussed in this work has the potential to facilitate the establishment of a correlation between the globally applied load and locally developed strains used to model the progressive damage at mesoscopic scales in woven composite laminates under large deformation conditions.
Identification of Material Parameters for Damage Model of Ductile Failure in Thermoplastic Polymers
In recent years, polymer nano-composites (PNCs) have increasingly gained more attention due to their improved mechanical, barrier, and thermal as well as optical, electrical, and biodegradable properties in comparison with the conventional microcomposites or pristine polymer . Although PNCs offer enormous opportunities to design novel material systems, development of an effective numerical modeling approach to predict their properties based on the complex multi-phase and multiscale structure of these composites is still at an early stage. Authors recently developed a multiscale computational framework to predict the mechanical properties of PNC [2]. Upon the large deformation, fiber debonding occurs due to the significant difference between the properties of inclusion and the host polymer in polymer nanocomposite.The finite element model of fiber debonding in nano-reinforced composite has been developed based on the cohesive-zone model of the interface [3]. Matrix cracking is another failure mechanism in composite materials. Thermoplastic polymer undergoes large ductile deformation before losing all load-carrying capacity. The loss of load-carrying capacity results in the progressive degradation of material stiffness. In this paper the modified Gurson-Tvegaard- Needleman (GTN) model was applied to thermoplastic polymer (here high-density polyethylene) to model its plasticity and damage behavior. GTN material parameters and elastic and hardening parameters has been obtained using experimental results (tensile and shear tests, Digital Image Correlation). This technique can be applied to other materials and it is not limited to thermoplastic polymers.
FARHANG POURBOGHRAT, TAEJOON PARK and FADI ABU-FARHA
Estimating the Process Zone Length of Fracture Tests
This paper provides closed-form expressions for theprocess zone lengths and load-displacement responses of cohesive zone modeling,for the popular mode Idou- ble cantileverbeam (DCB) test and the mode II end notched flexure (ENF) testofcom- posite materials. By incorporating traction-separation lawsthat consist of onlyone quasi-brittle softening segment,the problems are formulated and analytically solved for homogenous materialor mid-plane symmetric laminates within the framework of classical laminationtheory(CLT).Th eexpressions indicate that the process zone length isa system parameter that is influenced by themodemixity,the material and interfacial fracture properties as well as the specimen geometry,including the thick- ness and the crack length.Based on the parametric studies and the comparison against finite element(FE)simulations,suggestions for estimating the process zone length are provided.
Observations and Lessons Learned from Composite Progressive Damage Analysis Benchmarking Exercise
AFRL led an effort with Lockheed Martin to evaluate composite progressive damage analysis methods in static and fatigue. This paper provides an overall summary of the effort, but concentrates on the prediction of fatigue damage growth of a [60/0/-60]3S open hole coupon. The fatigue experiments yielded non-intuitive results, resulting in poor correlation between the simulations and experiments during the blind prediction phase of the program. The accuracy of the models were improved by correcting the calibration approach to better capture the actual physical failure mechanisms.
Ultrasonic NDE Simulation for Composite Manufacturing Defects
The increased use of composites in aerospace components is expected to continue into the future. The large scale use of composites in aerospace necessitates the development of composite-appropriate nondestructive evaluation (NDE) methods to quantitatively characterize defects in as-manufactured parts and damage incurred during or post manufacturing. Ultrasonic techniques are one of the most common approaches for defect/damage detection in composite materials. One key technical challenge area included in NASAÃ¢â‚¬â„¢s Advanced CompositeÃ¢â‚¬â„¢s Project is to develop optimized rapid inspection methods for composite materials. Common manufacturing defects in carbon fiber reinforced polymer (CFRP) composites include fiber waviness (in-plane and out-of-plane), porosity, and disbonds; among others. This paper is an overview of ongoing work to develop ultrasonic wavefield based methods for characterizing manufacturing waviness defects. The paper describes the development and implementation of a custom ultrasound simulation tool that is used to model ultrasonic wave interaction with in-plane fiber waviness (also known as marcelling). Wavefield data processing methods are applied to the simulation data to explore possible routes for quantitative defect characterization.
Benchmarking of Computational Models for NDE and SHM of Composites
Ultrasonic wave phenomena constitute the leading physical mechanism for nondestructive evaluation (NDE) and structural health monitoring (SHM) of solid composite materials such as carbon-fiber-reinforced polymer (CFRP) laminates. Computational models of ultrasonic guided-wave excitation, propagation, scattering, and detection in quasi-isotropic laminates can be extremely valuable in designing practically realizable NDE and SHM hardware and software with desired accuracy, reliability, efficiency, and coverage. This paper presents comparisons of guided-wave simulations for CFRP composites implemented using three different simulation codes: two commercial finite-element analysis packages, COMSOL and ABAQUS, and a custom code implementing the Elastodynamic Finite Integration Technique (EFIT). Comparisons are also made to experimental laser Doppler vibrometry data and theoretical dispersion curves.
CARA LECKEY, VASYL HAFIYCHUK, PETER JUAREZ, DOGAN TIMUCIN, STEFAN SCHUET and HALYNA HAFIYC
DPSM Modeling of Wave Propagation in Anisotropic Half Space
Distributed point source method (DPSM) is a recently developed mesh free semi-analytical technique. It has been successfully used in modeling wave propagation in many different fluid and isotropic solid media. However, for the first time in this paper, it has been implemented for modeling the wave propagation behavior in anisotropic half space. In the past decades, methods such as finite element method (FEM), Rayleigh-Sommerfeld Integral (RSF), Spectral Element Method (SEM), Multi-Gaussian Beam Model (MGBM), Boundary Element Method (BEM), Charge Simulation Technique (CST), Elastodynamic Finite Integration Technique (EFIT), and Multiple Multi-pole Technique (MMP) has been developed and instigated in the field of computational nondestructive evaluation (CNDE) for modelling wave. Among all the techniques mentioned, FEM is considered the most efficient. In FEM, the entire problem is discretized which is time consuming. However, DPSM doesnÃ¢â‚¬â„¢t require the discretization of the problem geometry. Instead, point sources are placed near boundaries and interfaces building a model that proficiently accounts for the radiation condition at interfaces. Through previous works, it has been proven that the DPSM is more accurate than the FEM at the higher frequencies. The ChristoffelÃ¢â‚¬â„¢s equation is solved to calculate the phase velocity of different wave front present at any point in the problem geometry. The calculated phase velocity is then used to develop the GreenÃ¢â‚¬â„¢s function, which in turn is used to simulate and understand the wave propagation behavior in the anisotropic half space. MATLAB is used for coding and simulation.
Model-Based Inversion of Flash Thermography Nondestructive Evaluation Measurements of Composites
Flash thermography is a nondestructive evaluation (NDE) technique that finds flaws such as delaminations in composites through the blockage of heat flow and resulting change in surface temperature. In this presentation we will discuss a model-based inversion approach to identifying and localizing damage from a thermal image sequence in a composite material. The approach is built on the observation that the influence of defects in blocking heat flows on surface temperature can be approximated by replacing the defect with a heat source, and using the well known thermal Green's function to predict the resulting surface temperature. Linear inversion is then used to represent the measured thermal image sequence as a sum of these Green's functions. The method has been tested in both simulation and experiment. In general, this method seems to give reasonable approximations of the first subsurface insulating layer. Inversion results behind that layer are not meaningful and often not physical (for example, negative source intensity). As would be expected for any inversion process, the inversion process also results in noise gain. Nevertheless, this process offers for the first time a direct inversion of flash thermography measurement data to localize the source of observed contrast in regimes where 3D heat flow is significant.
ELIZABETH GREGORY and BRYAN SCHIEFELBEIN
Tribological Behavior of PTFE/PEEK Composite
A study was conducted on tribological behavior of a PEEK-matrix composite with 20% (by volume) PTFE particulates. Neat PEEK and PTFE were also investigated to provide reference information for identifying the effect of PTFE inclusion on sliding friction and wear behavior of the composite. A comprehensive experimental program on friction and wear tests of the materials was developed and carried out with a precision pin-on-disk tribometer. Theoretical evaluation of the composite tribology was also made based on friction and wear mechanisms, and mechanical and thermal properties of individual constituents of the composite. The results show that solidstate lubricant PTFE particulates in the PEEK matrix decrease the PTFE/PEEK composite friction coefficient and wear rate appreciably by continuous formation of PTFE transfer films over the contact surface between the composite and the steel counterface. The specific wear rate of the composite was six times lower than that of the neat PEEK and two hundred times lower than that of the neat PTFE. Main friction and wear mechanisms of the composite included abrasive friction and wear of the PEEK matrix, PTFE adhesion and pull-out, formation of low-friction-coefficient solid-state transfer films, and shear flow of solid transfer films under combined compression and surface traction. High flash temperatures at the sliding contact surface were observed and determined in friction and wear tests of neat PEEK, PTFE and the composite. The parameter, i.e., the Peclet number, governing the temperature rise during sliding was obtained for each individual material. The temperature rise affected significantly the friction and wear behavior of the PTFE/PEEK composite, as both yield strength and hardness of the composite constituents decreased with the temperature rise, especially near their glass-transition temperatures.
Four Probe Electrical Resistance Characterization of Carbon Fiber and Carbon Nanotube Buckypaper Composites
In this work, the electrical resistance of carbon fiber reinforced polymer (CFRP) and carbon nanotube buckypaper (BP) composites has been studied. The specimens were prepared according to three different layup configurations. Three 16-ply unidirectional carbon fiber (CF) composites IM7/977-3 [0]16T, or CF16, were prepared and used as the baseline for the study. Additional specimens were prepared with the addition of 4 and 7 layers of BP. Experimental measurements of the electrical resistance have been performed using two methods. In the first method, six probes were attached to the top and bottom of the specimens and measurements were performed using the traditional 4-probe electrical resistance method (i.e. the electrodes span the entire width of the specimen) for composite laminates. The second method was based on a modification of the four-point probe method of the Japanese Industrial Standard (JIS) K7194. Results were analyzed in order to observe the effects of the highly conductive buckypaper on the overall resistance of the composite laminates as well as to illuminate any trends between the number of BP layers added and the resulting changes to electrical resistance. The specimens with layers of BP were found to have lower resistance compared to the specimens with no BP. Moreover, as the number of BP layers increased from 4 to 7, the resistance continued to decrease. These results demonstrated two significant findings. First, the current not only flowed through the top CF layer but penetrated through the thickness, as evidenced by the lower resistance measured on the specimens with BP. Second, the decrease in resistance between the specimens with 4 and 7 layers of BP measured by the traditional 4-probe electrical resistance method was greater than predicted by finite element analysis (FEA) due to nonlinear edge effects. The method based on the JIS K7194 was found to reduce the influence of edge and geometry effects on resistance.
Similitude Analysis of the Strain Field for Loaded Composite I-Beams Emulating Wind Turbine Blades
The characterization of new materials for the large wind turbine blades starts with the testing of material coupon and culminates with the full-scale testing of a blade using these materials. Coupon testing is a cost-effective means to characterize a material, but it is not necessarily representative of the structural performance of the blade. Thus, expensive and time consuming full-scale testing is the current means for demonstrating the capabilities of a material system for wind turbine blades. Subcomponent testing has the potential to bridge the gap between the coupon and full-scale tests. To develop a subcomponent test, similitude analysis is utilized to design scaled models that are accurate representations of their parent full-scale component. In this study, a similitude analysis is developed to design the scaled-down models for the I-beam geometry of a utility-scale wind turbine blade. Similarity of the strain field between scaled models and the prototype is investigated to design the scaled-down I-beams for characterization of the structural performance of the new materials. The scaling laws for strain distribution on the flanges of the I-beam are derived and used as a metric to design models with different scales and lamination schemes that withstand the same strain field as the prototype under adjusted loads. The designed models are manufactured and loaded using a three-point bending test. Digital image correlation is used to measure the full-field strain distribution in the flanges of the models. Measured strain distribution for incremental loads is used to verify the analytical and finite element models. The strain distribution of the prototype is predicted with the analytical model and compared to the experimental values to demonstrate the similarity of the strain field between model and prototype.
CHRISTOPHER NIEZRECKI, JAMES SHERWOOD and PETER AVITABILE
Development of a Carbon Composite Rocket Motor Case
In the scope of insensitive and lightweight motor case development study, design, prototype production, development and verification phases of a carbon composite rocket motor case are presented. The development phase is completed by performing hydrostatic burst tests for two composite motor cases and static firing tests for four prototypes. Static firing tests following the combined environmental tests of four prototypes and insensitive munitions tests of two prototypes are performed to fulfill the requirements of the verification phase. At the end of the test series, it was shown that all the prototypes satisfy ballistic and mechanical/structural performance requirements together with Type V insensitive munitions requirements for fast cookoff and bullet impact threats according to STANAG 4240 and 4241, respectively. Hence, the motor with composite case development and verification studies were proved to be successful.
Precise Monitoring of Damage Evolution in Laminated Composite Materials Using Integrated Carbon Nanotube Fiber Sensors: Experimental Results and Validation
Laminated composite materials are susceptible to catastrophic failure due to their relatively low interlaminar mechanical properties and poor resistance to low-velocity impact loading. This is heightened by the fact that it is not easy to detect the damage caused by such loading at the onset as it is most commonly likely to occur at interlaminate locations that are not easy to observe on the surface. The purpose of this research is to study this damage as it evolves and propagates through the laminated composite material and monitor its progression. Carbon nanotube (CNT) yarns are integrated into laminated composite materials and used as a sensing mechanism. CNT yarns with their unique piezoresistive property, can measure the change in the resistance of the materials under strain without altering the innate properties or integrity of the laminated composite material. Strain or damage in close proximity to the CNT yarn alters its electrical properties and the change in its resistance measurement could be used to detect and monitor the damage. This paper includes the experimental results of a study about the precise detection of delamination and its evolution in laminated polymeric composite materials using CNT yarn sensors. The experimental results are presented for glass/epoxy laminated composites and include the mechanical loading data of the samples and the electrical resistance data of the CNT yarn sensors. A combination of sensor configurations including CNT yarns stitched through the thickness direction of the composite laminate and CNT yarns placed straight cross wide to the laminate are used to detect the presence of damage as well as its extent and propagation pattern. The results corresponding to a sample implemented with these integrated sensors demonstrate their ability to detect the damage and provide its location as a function of the applied load and in real time. In order to validate the observations from the integrated CNT yarn sensors, x-ray tomography images of the tested samples are used to provide the exact locations of the damage areas and thus corroborate the readings from the CNT yarn sensors.
JUDE ANIKE, SAM MORTIN, JOSEPH BILLS, VANESSA GONZAGA, GLEYSON OLIVEIRA, PEDRO SILVA, RONA
Electro-Mechanical Simulation of Multifunctional Composite Structural Batteries
Multifunctional structural battery composites are able to store and deliver electrical energy while replacing or being integrated into the structure of the system to support operational loadings. These composites can be employed in a wide variety of systems including automobiles and aircraft to provide auxiliary electric charge while simultaneously reducing weight for extended range and efficiency. These efforts focused on investigating laminate mechanical properties, as well as preliminary coupled electrical-structural-thermal micromechanical analyses. Several carbon reinforcing materials and potential laminate orientations were analyzed through both FEM and analytical methods to determine laminate flexural properties. A simulated three point bend test was employed to determine the flexural stiffness and strength. The stiffnesses and strengths for the composite were determined using carbon fiber paper, AS4 unidirectional carbon fiber, P100 carbon fiber, and a carbon paper reinforcement with interstitial honeycomb core material. This honeycomb core was investigated as a means to assuage the tradeoff between mechanical and electrical properties of the structural battery constituent materials. Three coupled electrical-structural micromechanics models were analyzed: the single carbon fiber unit cell, a 3Ãƒâ€”3 array of fibers, and a 5Ãƒâ€”5 array. Voltage boundary conditions were applied to examine the current distribution throughout the material system, including the response under structural deformation. Improvements and expansions to these efforts are being investigated to examine the effects of large deformation on electrical performance, current distribution, and potential short-outs.
Toughness Enhancement Mechanisms in Polymer Nanocomposites Due to Length Scale Effects at the Nanoscale
This paper presents fracture studies conducted on nanographene platelet (NGP) reinforced EPON 862 polymer composite tested in pure Mode I and mixed mode. Compact Tension (CT) tests were performed to determine Mode I fracture toughness for baseline (0 wt%), 0.1 wt% and 0.5 wt% NGP reinforced EPON 862 neat resin. Remarkable improvement in fracture toughness, KIC (~197%), and critical strain energy release rate, GIC (~572%), was observed for only 0.5 wt% NGP addition to the polymer system. SEM and AFM studies revealed significantly higher roughness in the fracture surface morphology for NGP reinforced specimens. Asymmetric four-point bend tests were also performed for three different mode mix (tan-1(KII/KI)) ratios (38o, 57o, and 89.5o). Significant enhancement in toughness for each mode mix case was observed. In order to ascertain if the toughness enhancements translated from the neat resin to the carbon composite, IM7/EPON 862 unidirectional laminates were manufactured using hand-layup with 0.1 and 0.5 wt% of NGP reinforcement. Double Cantilever Beam (DCB) experiments were performed to characterize delamination toughness in Mode I (initiation toughness as well as fracture resistance). Significant increases of 48% and 100% in initiation toughness and resistance to crack propagation was observed respectively for 0.5 wt% NGP reinforced EPON 862. The reinforcement potential of NGPs is explained using a novel nanoscale toughness enhancement mechanism, which can be employed to predict the length and orientation of the NGPs for maximum fracture toughness improvement.
Analysis-Driven Design of Vascular Antennas Embedded in Multifunctional Composites
Research on structurally embedded microvascular systems in laminated structures continues to provide new insights into the areas of thermal transport and self-healing. At the same time, the number of studies considering uses of non-toxic liquid metal (LM) alloys for reconfigurable electronics is also increasing. This work benefits from these efforts and proposes a concept for a reconfigurable Structurally Embedded Vascular Antenna (SEVA). A set of fully coupled multi-physical engineering models of the driven antenna consider the electromagnetic, fluid, thermal, and mechanical responses. A full-wave electromagnetic model (frequency domain) is used to assess antenna Radio Frequency (RF) performance and compute dissipative heat generation (dielectric and resistive). The full field heat source data is then passed to a conjugate heat transfer model, which accounts for the thermal coupling between the solid composite and flowing fluid based on the LM pumping pressure. Finally, a representative volume element representing a local region of the highest temperature is used to compute mechanical knockdown ratios relative to a similar panel with no microvascular network. The entire modeling framework is implemented using a combination of COMSOL, Matlab, Abaqus FEA, and Python tools and libraries. It is shown that the increased resistive heating that results from the use of LM can be offset by the action of circulating the liquid metal such that heat is continuously removed from the system. A simulation process management framework is used to automate design of experiment (DOE) and multi-objective optimization studies. Because of the high computational cost associated with performing the multi-physical computational analysis, a surrogate model-based optimization approach is employed to explore the design space associated with this concept and to arrive at SEVA configurations that simultaneously maximize antenna effectiveness and structural strength.
GEOFFREY FRANK, GREGORY HUF and JEFFERY BAUR
Flexural Stiffness of Thick Walled Composite Tubes
Composite tubes have been used in many applications such as pipes, robot arms, drive shafts, electrical conduits, printing rollers, tube structures for sports equipment, rocket structures, satellite truss structures, landing gears for helicopters, and structural building members etc. For thin wall tubes made of isotropic materials, the flexural stiffness is usually determined by using strength of material approach with the expression EI, where E is the material modulus and I the cross section inertia. For thin walled composite tubes where many layers with different orientations are involved, one may tempt to use the same equation with E replaced by Ex to take care of the fiber orientations. However this may not be correct. Adding to this the increasing thickness of the cylinder, then the inaccuracy of the equation may be even more. By using elasticity approach, an equation for the determination of the stiffness of thick composite cylinders was derived by previous researchers. It was shown by experimental work that the equation based on elasticity provides more accurate results that that based on strength of materials approach. This paper presents some behavior of the bending stiffness of thick walled composite tubes.
Modelling of Variable Stiffness Plates Based on Mechanics of Structure Genome
A plate model for laminates with variable stiffness materials is constructed based on the mechanics of structure genome. The modelling aims at minimizing the difference between the potential energy of original 3D model and the equivalent plate model. In this study, a line through the thickness of the plate is chosen as the structure genome, so the the analysis of the original 3D model is separated into a 1D through the thickness analysis and a 2D global plate analysis. Energy of the 3D model is expressed in terms of 3D strains, obtained from continuum mechanics, and energy of the equivalent plate model is expressed in terms of 2D membrane strains and curvatures of the reference surface as well as the warping functions. Warping functions are solved in terms of 2D strains and are used for deriving the constitutive relation as well as recovering the local fields. Several numerical examples are studied to verify the presented theory.
Analytical Modeling for Stress Distribution around Composite Interference Fit Joints with Elastic Pins
Interference fit joints can bring benefits for composite structures such as fatigue life improvement. In these structures, as fasteners are oversized, plates have already deformed before loading, and residual stress exists around holes. As a result of composite anisotropic property and contact relationships between fasteners and plates, the residual stress is uneven around the hole and depends on several factors. All of these make the problem quiet complex, and bring difficulties in estimating the loading situation of joints. An analytical method considering pin elastic deformation was proposed to analyze the residual stress around composite interference fit joints in this paper. The method was based on the Lekhnitskii complex variable approach. Stress functions which satisfied the load condition of interference fit pinned plates were built. Unknown coefficients in stress functions were determined according to displacement boundary expressions. By this method, stress components could be obtained if elastic constants and interference value were known. 3D finite element models were built to verify the validity of the analytical method, and results were in good agreement. The effects of material properties and interference value were discussed: (1) Material properties had obvious effects on stress distribution. Radial and circumferential stress distributed like circle on quasi isotropic plates, while like figure eight on zero dominated plates. Rigid pins obtained apparently higher stress level than elastic pins; (2) Larger interference value led to higher stress level. But the shape of stress curve would not change when interference value varied.
KAIFU ZHANG, HUI CHENG, PING LIU, YI LIANG and YUAN LI
Plasticity Tool for Predicting Shear Nonlinearity of Unidirectional Laminates under Multiaxial Loading
This study implements a plasticity tool to predict the nonlinear shear behavior of unidirectional composite laminates under multiaxial loadings, with an intent to further develop the tool for use in composite progressive damage analysis. The steps for developing the plasticity tool include establishing a general quadratic yield function, deriving the incremental elasto-plastic stress-strain relations using the yield function with associated flow rule, and integrating the elasto-plastic stress-strain relations with a modified Euler method and a substepping scheme. Micromechanics analyses are performed to obtain normal and shear stress-strain curves that are used in determining the plasticity parameters of the yield function. By analyzing a micromechanics model, a virtual testing approach is used to replace costly experimental tests for obtaining stress-strain responses of composites under various loadings. The predicted elastic moduli and PoissonÃ¢â‚¬â„¢s ratios are in good agreement with experimental data. The substepping scheme for integrating the elasto-plastic stress-strain relations is suitable for working with displacement-based finite element codes. An illustration problem is solved to show that the plasticity tool can predict the nonlinear shear behavior for a unidirectional laminate subjected to multiaxial loadings.
Experimental Methods to Characterize the Woven Composite Prepreg Behavior during the Preforming Process
This paper reports several characterization methods of the properties of the uncured woven prepreg during the preforming process. The uniaxial tension, biasextension, and bending tests are conducted to measure the in-plane properties of the material. The friction tests utilized to reveal the prepreg-prepreg and prepreg-forming tool interactions. All these tests are performed within the temperature range of the real manufacturing process. The results serve as the inputs to the numerical simulation for the product prediction and preforming process parameter optimization.
HUAQING REN, JIE LU, ZIXUAN ZHANG, LINGXUAN SU, JANE WANG, DANIELLE ZENG, XUMING SU and JI
An Evaluation of the *FABRIC Material Model in ABAQUS/EXPLICIT for Composite Preforming Analysis Suitability
Carbon fiber composites are becoming important in addressing global design challenges. To support the use of carbon fiber composites, modeling of the fabric preforming is an important tool. In this work, the existing *FABRIC material model in ABAQUSÃ‚Â®/EXPLICIT was evaluated for composite preforming analysis suitability. Specifically, picture frame and bias-extension fabric in-plane shear characterization tests and deep-drawing preforming tests using a hemisphere and an in-house square clamping tool were simulated using this material model. This *FABRIC material model employs shell or membrane elements to capture the wrinkling of fabric in a smeared fashion and track the shear angle change during analysis. Reasonably good prediction-experiment correlations of shear angle changes and loading curves were achieved for both of the material in-plane shear characterization tests; i.e., picture frame and bias-extension tests. However, the accuracy in the predicted loaddisplacement curves for the hemisphere deep-drawing preforming tests was found to vary with the fiber orientation relative to the clamping tool. Hypotheses attributing to the deviation are discussed. All the material inputs can be determined in advance from well-established physical tests. Despite its modeling efficiency advantage, this material model was only valid for materials that exhibit two Ã¢â‚¬Å“structuralÃ¢â‚¬Â directions.
3D Representative Volume Element Reconstruction of Fiber Composites via Orientation Tensor and Substructure Features
To provide a seamless integration of manufacturing processing simulation and fiber microstructure modeling, two new stochastic 3D microstructure reconstruction methods are proposed for two types of random fiber composites: random short fiber composites, and Sheet Molding Compounds (SMC) chopped fiber composites. A Random Sequential Adsorption (RSA) algorithm is first developed to embed statistical orientation information into 3D RVE reconstruction of random short fiber composites. For the SMC composites, an optimized Voronoi diagram based approach is developed for capturing the substructure features of SMC chopped fiber composites. The proposed methods are distinguished from other reconstruction works by providing a way of integrating statistical information (fiber orientation tensor) obtained from material processing simulation, as well as capturing the multiscale substructures of the SMC composites.
Lightweight Sheet Molding Compound (SMC) Composites Containing Cellulose Nanocrystals
Lightweight composites with high strength and stiffness have been identified as a key cross-cutting technology by automotive industry, considering that a 10% reduction in the vehicle weight can result in 6-8 % increase in fuel efficiency. In this study, a scalable technique was introduced to make lightweight hybrid sheet molding compound (SMC) composites for automotive applications by replacing the heavier components, i.e. glass fibers (GF), with a small amount of cellulose nanocrystals (CNC) with no penalty on mechanical performance. We report that incorporation of 1 and 1.5 wt% CNC in the epoxy matrix of short GF/epoxy SMC composites allows removing 10 wt% glass fibers (GF), producing 7.5% lighter composites with no compromise in tensile and flexural properties. The addition of 1 wt% CNC in 25GF/CNC-epoxy SMC composites resulted in increases of 15% in elastic modulus, 11% in flexural modulus and 14% in flexural strength, reaching the corresponding properties of 35GF/epoxy SMC composites. The results of this study will enlighten the path toward high volume production of lightweight SMC composites with a better performance in automotive composite production.
MARK MILLER, ARJUN SINGH, ROBERT MOON, KYRIAKI KALAITZIDOU
Studies on the Synthesis and Characterization of Epoxidized Soybean Oil
Traditionally, polymers are synthesized from petrochemical by-products. The greenhouse gas emissions from petroleum based polymers make them unattractive options as there has been an increase in climate changes and adverse weather conditions. Additionally, they are non-degradable, creating issues when used as landfills. For these reasons and more, there is an urgent need to find an alternative source of polymeric materials. Hence, this research focuses on synthesizing a completely bio-based polymer using soybean oil to achieve a high bio-content. Determining an optimum stoichiometric ratio for high oxirane oxygen content is investigated taking into consideration the effect of temperature, reaction time, and concentration of hydrogen peroxide. These parameters are monitored to achieve the highest oxirane oxygen content. Determination of the epoxide groups was done through manual titration process using ASTM D1652-11. The highest percent oxirane oxygen content of 2.86 and 2.58% was achieved when using 1 mole of hydrogen peroxide for a reaction time of 4 hours at temperatures of 50 and 60 Ã‚ÂºC. Fourier transform infrared [FTIR) spectroscopy verified presence of low epoxide groups as determined by ASTM D1652-11 which revealed possible side reactions indicated by the disappearance of the epoxide groups. Rheological characterization plotting the viscosity versus shear rate indicated a pseudoplastic behavior at initial shear rates, but Newtonian, shear thickening, and shear thinning properties beyond 75 s-1. It is concluded that time, temperature, and variation in the concentration of hydrogen peroxide affect the epoxidation process while optimum conditions are suggested to require longer reaction times using 1 mole of hydrogen peroxide at temperatures between 50 and 60 Ã‚ÂºC.
CHRISTINA YOUNG, DANIEL ABUGRI, MAHESH HOSUR and SHAIK JEELANI
Composite materials grown from fungi, known as mycology materials, provide a novel option for a natural biocomposite material Ã¢â‚¬â€œ the entirely natural composition allows for their disposal through composting, and provides the benefit of reduced impact from their manufacture due to being grown from agricultural or forestry wastes. As more uses are identified for mycology materials and the number of applications increases, there is a greater need for information on the materialÃ¢â‚¬â„¢s behavior during fabrication and manufacture. The purpose of this research is to determine the response of mycology materials to commonly used manufacturing methods, including cutting processes. Determining the suitability and resulting finish achieved from traditional manufacturing processes for mycology materials is a new area of research, useful to both commercial and artisan users; the results of which can be used to help inform the development of applications that utilize the biocomposite material. Mycology materials are grown from carbon containing feedstocks, such as sawdust, by digesting the feedstockÃ¢â‚¬â„¢s ligneous material to form the structure of the mycology material. The mycology material is made from the vegetative part of the selected fungi species that grow a network of individual strands of mycelium, called hyphae, to form a cellular material. Mycology materials form a biocomposite comprised of the mycelium and the original feedstock, in this case wooden agricultural waste, with the mycelium and feedstock analogous to the matrix and reinforcing fiber of a typical synthetic composite. Due to being a natural material, fabrication and manufacturing includes the growth and processing of the fungi species that form the material, and the following manufacturing methods used to shape the material. The growth and processing of mycology materials includes the use of molds for near net-shape forming, and the subsequent denaturing of the growing material into an inert material.
Mode I Interlaminar Fracture Toughness of Natural Fiber Stitched Flax/Epoxy Composite Laminates â€“ Experimental and Numerical Analysis
The mode I interlaminar fracture toughness of stitched and unstitched composite laminates of continuous UD flax fiber and epoxy matrix was experimentally and numerically investigated. A twistless flax fiber yarn was used for stitching purpose. The mode I interlaminar energy release rate of the unstitched and stitched laminates was experimentally determined through Double Cantilever Beam (DCB) test. Tensile test was conducted to characterize the tensile properties of the dry and resin impregnated forms of the stitch yarn. A 3D Finite Element (FE) model of DCB specimen to analyze delamination propagation of stitched flax fiber laminate was developed using cohesive element with nonlinear softening law and embedded 2-node beam element. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves were achieved.
W.S. TEO, L.Q.N. TRAN, M.S. YONG and T.E. TAY
Carbon-Polymer Composites From Natural Precursors: Micro-Structural, Thermal and Mechanical Properties
Agricultural waste materials have been explored in the preparation of activated carbon and carbon black employing chemical activation using H3PO4 and physical activation method respectively. Various biomasses based carbon blacks and activated carbons derived from bamboo stem, coconut shell and oil palm empty fiber bunch were characterized, compared and used as filler in epoxy composites. The microscopic structure, thermal properties and mechanical properties of the composites were studied. Incorporation of biomass based carbon blacks and activated carbons resulted in significant increases in thermal and flexural properties of the epoxy composites. The microstructure of the composites indicated that the addition of 5% filler greatly improved the properties of the composites and increased the fillers adhesion unto the epoxy matrix. This work provides new opportunities for advanced applications bioresource based materials in high performance composites applications.
Effect of Adding Boron Nitride Nanotubes on Mechanical Properties of Epoxy 862 Nanocomposite
Mechanical properties of Epoxy/Boron Nitride Nanotubes (BNNTs) nanocomposite was predicted using Molecular Dynamic (MD) simulations. Four different structures were constructed where the bonding between BNNTs and epoxy molecules was altered. The structures were subjected to energy minimization and geometry optimization. MD simulations were then conducted on epoxy/BNNTs using ensemble NPT (fixed number of atoms, pressure and temperature) and the YoungÃ¢â‚¬â„¢s modulus of the structures were obtained. MD simulations results show that by introducing bonding between BNNTs and epoxy molecules, the YoungÃ¢â‚¬â„¢s modulus increases. Also, the changes in density reveal that increasing the number of bonds between epoxy molecules and BNNTs from 2 to 4 results in a more compact structure.
JOSEPH ESTEVEZ, AJIT KELKAR and JAMES RYAN
Multidisciplinary Optimization of Cross-Linked Polymers Based on Molecular Dynamics Simulation
We propose a systematic method for the appropriate materials design for thermosetting resin based on computer simulations. The proposed method is a combination of the molecular dynamics (MD) simulation and self-organizing maps (SOMs). The MD simulation is alternative to experiments that consists the curing reaction and mechanical test of the epoxy resin. In particular, in order to obtain the natural cross-linked structure of the epoxy resin, we conducted curing simulation wherein the activation energy and heat of formation are considered for the chemical reaction. On the other hand, SOMs enables us to classify the materials species in accordance with the similarity in their characteristics. From the classification of the 25 species in the epoxy resins, we found that epoxy systems that have base resins with multi-functional groups express superior mechanical properties. Moreover, the molecular scale mechanism of the higher mechanical properties was hydrogen-bonding extended throughout the system.
KOICHI TANABE, GOTA KIKUGAWA and TOMONAGA OKABE
Molecular Dynamics Study of the Mechanical Properties of Silica Glass using ReaxFF
Using molecular dynamics (MD) simulations, we study the mechanical properties of amorphous silica using reactive force field ReaxFF. Amorphous silica is created from crystalline structure through annealing process. Silica model is then subjected to tensile loading to determine mechanical properties. Wide range of strain rates (2.3 Ãƒâ€” 10Ã´â‚¬Â¬Â¼ Ã´â‚¬ÂÂÃ´â‚¬Â¬Â¿Ã´â‚¬Â¬Âµ ~ 1.0 Ãƒâ€” 10Ã´â‚¬Â¬ÂµÃ´â‚¬Â¬Â¹ Ã´â‚¬ÂÂÃ´â‚¬Â¬Â¿Ã´â‚¬Â¬Âµ) is considered to allow comparison with results reported in the literature for other force fields. Quasi-static modulus obtained from power-law fitting of the low stain rate modulus predicted by ReaxFF is in good agreement with experimental results. A transition strain rate of approximately 2.5 Ãƒâ€” 10Ã´â‚¬Â¬ÂµÃ´â‚¬Â¬Âµ Ã´â‚¬ÂÂÃ´â‚¬Â¬Â¿Ã´â‚¬Â¬Âµ is identified where modulus increases rapidly to a plateau level. Tensile strength also increases significantly in this range of strain rate and plateaus at the theoretical upper bound for silica. Damage mechanism at different strain rates is presented.
ROBERT ELDER, TIMOTHY SIRK, BAZLE HAQUE, JAN andZELM and JOHN GILLESPIE, JR.
Molecular Modeling of Crosslinked High-Temperature Bismaleimide Resins: Matrimid-5292
The crosslinked structure of Matrimid-5292 was built for Molecular Dynamics simulation using a novel, adaptable crosslinking framework to execute a total of 5 distinct cure reactions. The influence of cure reactions modeled and extent of curing were analyzed on the density, glass transition temperature, coefficient of thermal expansion, elastic moduli, and thermal conductivity. The developed models correctly predict experimentally observed trends for various properties.
VIKAS VARSHNEY, JEFFERY BAUR, AJIT ROY and GREGORY ODEGARD
Achieving Worldwide Code Acceptance for the Use of Advanced Composite Materials to Strengthen Civil Structures
In the 1990s, the use of advanced composite materials for infrastructure initiated with the California Department of Transportation (Caltrans), as an alternative to steel jacketing bridge columns for seismic retrofit. Caltrans turned to an independent material testing facility, Aerospace Corporation, to assist in the development of the first prequalification test program for advanced composite materials in civil infrastructure, which focused on durability requirements of composite materials under various environmental conditions. After the Northridge earthquake in 1994, the International Code Council Evaluation Service (ICC-ES), formerly International Conference of Building Officials (ICBO) developed an acceptance criteria (AC125) for the use of advanced composite materials to strengthen non-ductile reinforced concrete and masonry structures. The AC125 criteria was substantially influenced by structural testing at the University of California, San Diego and has been adopted by numerous countries, including New Zealand and Canada, as a preferred criterion to evaluate various composite systems. The American Concrete Institute (ACI) has also been influential in the development of guidelines for civil structures, with the first guideline published in 2002 and since adopted and referenced internationally. Other industries (e.g. water and waste water transmission) have developed their own unique requirements for durability testing and inspection. In the UK, the Concrete Society developed guidelines (TR55 & TR57) for the design, acceptance, inspection, and monitoring of composite materials, while Germany developed a general building approval program (Zulassung by DIBt), similar to ICC but requires more rigorous durability testing. These requirements have evolved over the years and will continue to change along with the associated codes and guidelines. This paper will review the various durability and materials testing requirements that have been developed, with a brief overview of the guidelines and codes being developed to allow these advanced composite materials to become more accepted for use in civil structures.
Finite Element Analysis into the Flexural Response of CFRP Strengthened Prestressed Concrete Girders
Prestressed concrete bridges require strengthening due to aging, frequent over load, deterioration, etc. Carbon fiber reinforced polymer (CFRP) composites have been successfully used for enhancing flexural, shear, and axial strength of RC and PC members. Increase in traffic service load on highway bridges dictates replacement of the bridge super structure or adoption of a structural upgrade system. Strengthening of existing concrete bridges with conventional construction materials results in an increase in the dead load, and possible decrease in bridge clearance. The use of CFRP composites is considered a practical alternative for flexural strengthening of prestressed concrete bridge girders. The effectiveness of the use of CFRP depends on several factors, which are not yet quantified. This paper presents a finite element investigation on the effectiveness of CFRP composites in flexural strengthening on AASHTO type II prestressed girder.
TIAN BAI, RIYAD ABOUTAHA and HOSSEIN ATAEI
Shear Resistance of GFRP Composite Bars for Concrete Pavement Joints
Dowel bars are used in concrete pavement to transfer loads between adjacent panels across construction, control, and expansion joints. Such load transfer is secured through shear action in the dowel bar. Steel dowel bars tend to corrode prematurely, and cause deterioration of the concrete pavement joints. In order to retrofit concrete pavement with corroded steel dowel bars, a glass fiber reinforced polymer (GFRP) composite bars are considered durable alternative for replacement of corroded steel dowel bars. GFRP bars have high corrosion resistance, however, the transverse shear properties of GFRP bar vary widely, depending on their chemical and physical properties. This paper presents comprehensive review of literature along with experimental investigation of GFRP bars for use as dowel bars in concrete pavement, at the construction and control joints. Two series of GFRP bars having diameters of 16mm and 32mm were investigated and reported.
Ductility of CFRP Strengthened Reinforced Concrete Flexural Members
Carbon fiber reinforced polymer (CFRP) composites are used to enhance flexural, shear, and axial strength of reinforced concrete (RC) members. The degree of strength increase depends on the detailing of the CFRP system and the properties of the RC beam section. For CFRP strengthened RC beams with straight CFRP sheets/strips for increasing flexural strength, the ultimate mode of failure is dominated by sudden debonding of the CFRP system. Such bond failure is controlled by shear-tension failure of the concrete cover. Several researchers investigated the use of various types of end transverse and diagonal CFRP anchor to enhance the ductility of CFRP strengthened RC beams. However, the ductility of such members also depends on the properties of the RC beam section and the reinforcement ratio. This paper presents a comprehensive investigation into the properties of CFRP strengthened RC beams, along with experimental and analytical studies. The results of this investigation suggest that the ductility of a CFRP strengthened RC beam depends on various parameters, which include the reinforcement ratio, CFRP ratio, shear span to depth ratio, and detailing of the CFRP system.
Characterization of Energy Dissipation in Fiber/Matrix Composites under Transverse Tension
Cohesive zone elements were used to model diffuse cracking and the eventual formation of a dominant crack in fiber/matrix composites under transverse tension. The effect of the fiber/matrix interfacial and matrix strengths on the overall RVE behavior was studied using an ensemble of random realizations. The RVE strength was slightly more sensitive to a change in the matrix strength compared to the fiber/matrix interfacial strength and was significantly more sensitive to a decrease in either constituent strength compared to an increase because the transverse tensile strength of the composite is more dependent on the weaker constituent. The energy dissipated by diffuse damage and along the dominant crack was separated, and the effect of the microscale strengths on the energy dissipation was studied. Although varying the microscale strengths changed the ratio of the energy dissipated diffusely versus along the dominant crack, the total energy dissipated was insensitive to the changes in the microscale strengths, except when increasing the matrix strength, which resulted in an overall increase in energy dissipated. The energy dissipated within each cohesive zone was visualized for the load increments just before the dominant crack formed and well after the dominant crack formed. The location of the dominant crack was related to the intensity and arrangement of diffuse damage within the RVE, but further research is needed to understand the relationship well. Tracking the diffuse damage and dominant crack separately, using an ensemble of realizations to predict statistically significant results, and visualizing the energy dissipated within the RVE were shown to be useful for understanding progressive failure when significant diffuse damage exists.
Progressive Failure Analysis of a Stack of Aligned Prepreg Platelets
A computational, three-dimensional model for simulation of progressive failure in a stack of aligned prepeg platelets was developed. The model was based on a representative volume element (RVE). Two possible failure mechanisms were considered for the platelet stack: (i) the failure of platelets and (ii) the dis-bonding between platelets. Continuum and discrete damage mechanics models were used to represent the constitutive behavior of RVE elements during the progressive failure of a stack. Direct coupling of the two failure models was accomplished by incorporating both damage models within a single finite element model. A numerical study was conducted to investigate the effect of platelet geometry and stack topology on the macroscopic failure response of the platelet-stacked, composite system. Increased platelet length-to-thickness ratio and platelet overlap length were found to improve the homogenized tensile strength of a stack. Enhanced stress sharing between platelets was shown to be responsible for stack strength improvements. In such a case, the platelet stack failure was governed by the platelet fracture rather than platelet disbonding. Critical platelet aspect ratio and overlap length ratio were determined from the analysis.
An Efficient Virtual Testing Framework to Simulate the Interacting Effect of Intra-laminar and Inter-laminar Damage Progression in Composite Laminates
A reliable prediction of the complex inelastic behaviour of laminated composite materials requires that the computational model be able to capture the interactive effect of several failure/damage mechanisms including matrix cracking, fibre breakage, splitting and delamination. Depending on the layup and loading geometry, the accurate prediction of the onset and propagation of certain discrete damage modes (e.g. splitting and delamination), is critical in predicting the progressive damage response of the laminate up to its ultimate failure. In capturing such details, it is important to maintain the efficiency of the computational models so that they can be readily used by engineers to conduct failure analysis of composite sub-components and components. In this study, an efficient combination of the continuum and discrete approaches will be used to capture the interacting effect of discrete and smeared cracks. In contrast to some of the previous studies that employ a fully discrete approach, in the current framework, delamination is captured by a discrete approach while all intralaminar forms of damage, including splitting are modelled using the nonlocal continuum damage approach. Through judicious placement of the discrete delamination interface and synchronizing the onsets of delamination and splitting, the computational effort is markedly reduced.
Progressive Damage and Failure Analysis of Composite Laminates Using XFEM/CZM Coupled Approach
The aim of this study is to establish a high-fidelity mesoscale numerical simulation tool which can predict the progressive damages and resultant failure of carbon fiber reinforced plastics (CFRPs) laminates. In the proposed tool, the plastic behavior (i.e. pre-peak nonlinearity in the local stress-strain response) is characterized through the pressure-dependent elasto-plastic constitutive law. Moreover, the evolution of matrix crack and delamination, which result in post-peak softening in the local stress-strain response, is modelled through cohesive zone model (CZM). While the CZM for delamination is introduced through the interface element, the CZM for matrix crack is introduced through the extended finite element method (XFEM). Additionally, fiber failure which typically depends on the specimen size is modelled by Weibull criterion. Finally, the validity of proposed methodology was tested against the off-axis compression (OAC) test of unidirectional laminates and the open-hole tensile (OHT) test of quasi-isotropic laminates.
TOMONAGA OKABE, KENICHI YOSHIOKA and TOSHIO MAGASHIMA
Public Speaking and Media Interactions: Avenues for Outreach and Dissemination of Research Outcomes
The total federal research spending in the United States was close to $130 billion in the year 2015. National Science Foundation (NSF) is a leading non-defense agency that defines the trends in scientific and engineering research. Among the mandatory sections of grant proposals written to NSF include Scientific Merit, Broader Impact, and Dissemination Plan. The rationale behind including broader impacts section is twofold (a) ensuring a wide area of impact for the proposed research and (b) justifying the investment of public funds in the proposed research. Journal publications and conference presentations are commonly used as elements in the dissemination plans. With ubiquitous presence of smartphones and tablet devices, the outreach and dissemination plan can be transformed to reach a wider set of audience. However, opportunities for interacting with news media and making use of internet based resources are utilized by only a small set of researchers. This presentation will cover aspects related to interacting with news media through general purpose articles, videos, and interviews developed for people that may not have scientific background. Scientists carry the responsibility of making a strong case to lawmakers and general public for increasing the investment of tax dollars in research and development. Such efforts can only be successful when a larger set of people can understand the direct or indirect benefits of the research outcomes.
Uniting Composite Manufacturing Theory and Application: Practical Manufacturing Methods in a Team-Based Curriculum
Four solid carbon fiber composite propeller blades were manufactured through unique processes as the final project in the AAE 590/MSE 597 Manufacturing of Advanced Composites course taught by Prof. R. Byron Pipes during the Fall 2015 semester at Purdue University. The first portion of the course focused on laboratory experiments, exploring topics such as cure kinetics of thermoset polymers, heat transfer during cure, fiber bed permeability, and consolidation. The second portion of the course concentrated on a manufacturing project culminating in the production of small propeller blades. Four groups of three students each were formed, with each group selecting a composites manufacturing method: an autoclave process, an oven cure process, a Vacuum Assisted Resin Transfer Molding (VARTM) process, and a discontinuous chopped pre-impregnated composite fiber compression molding process. To utilize concepts previously learned in class and to demonstrate the complexities within each manufacturing method, the overall objective of the final project was to create a part with a complex three-dimensional geometry through four mutually exclusive methods. Students decided upon a common propeller blade geometry, after which each team was required to conduct their own analyses using the tools previously learned in class to produce a functional part according to their assigned process. All teams successfully created their respective parts with final dimensions within approximately 1.5 mm of the design value. This exercise demonstrated the difficulty in taking conceptual composite designs and translating these to finished products by going through the entire engineering and manufacturing processes.
EDUARDO BAROCIO, JUSTIN MILLER, MATTHEW PRALL, ORZURI GARAIZAR, DREW SOMMER, ORION WINGFIE
The Composites Design and Manufacturing HUB: Advancing Composites Education in the Classroom
The Composites Design and Manufacturing HUB (cdmHUB) offers novel and useful capabilities for composites education. The cdmHUB is a cloud-based platform focused on advancing the best practice use of composites simulation. The distinguishing feature of the cdmHUB is the ability to run composites simulation tools over the browser. As of January 2016, the cdmHUB hosts 20 simulation tools with models for cure kinetics, laminate theory, lamina analysis, micromechanics, strength prediction, shear lag and multi-scale structural analysis. In addition, the cdmHUB offers collaborative and management features for community building and classroom management including groups, announcements, projects, calendaring, and discussion threads. At Purdue University, we have used the cdmHUB in teaching the graduate-level Mechanics of Composite Materials and Experimental Characterization of Advanced Composite Materials courses, including in distance-education classroom formats. In the mechanics course, the simulation tools allow the students to interact more readily and thoroughly with the models and theories presented throughout the course, such as micromechanics, lamina analysis, laminated plate theory and elementary strength prediction. In the experimental course, the tools facilitate comparison of experimental data to theory, for example comparison of off-axis strength data to off-axis failure theories; and fitting of model parameters, such as reverse engineering fiber strengths from lamina data. Browser-based access to the tools on the cdmHUB increases the freedom of the instructor to present real-time examples and demonstrations, including on-thefly responses to student questions and interests, and allows students to focus on exercising the model and exploring what the model can teach them rather than on the computer implementation of the model. Providing students with cdmHUBaccess to the simulation tools gives them a portable toolkit companion for their career. In the present, we will focus on the use of computational models and simulation tools and briefly touch on the use of the collaborative and management features offered by the cdmHUB. 1
Design, Build, Test of Composites for Supersonic Ping Pong Balls
Education of composite materials at the undergraduate (junior year) level requires a different set and knowledge of understanding than is taught at the graduate level. Additionally, undergraduates learn by doing; seeing things and physically understanding how they perform helps to reinforce the different concepts. This paper will cover a semester based project that has students design build and test a composite material that must withstand the impact a supersonic Ping-Pong ball. This extreme use helps motivate the students in two ways. First it provides a very different type of experiment keeping them motivated. Second it provides an open ended solution that allows students to think differently and enables creativity in solutions. During the semester students fabricate the materials and test them to get a better understanding of their response and material constants. The final competition is a weight based solution compared to the other groups in the class. During the impact event high speed videos of the impact are taken so that students can get a better idea of how their structures withstand the impact.
Buckling Tests of Sandwich Cylindrical Shells With and Without Cut-outs
The results of buckling tests performed during the project DESICOS funded by the European Commission in the FP7 Programme are here presented. The tested structures are sandwich cylindrical shells that consist of reduced models of a component of the Ariane 5 launcher: the Dual Launch System. In particular, the scaled component is studied with and without the presence of cut-outs. Before performing the tests, the geometric imperfections as well as the thickness variations were measured. The tests were performed using the buckling testing equipment of Politecnico di Milano. The results of the tests contributed to understand the complex phenomenon of buckling of sandwich cylindrical shells, and to study the effect of initial geometric imperfections. They were also used to validate finite element models useful for the design of future launcher structures, and to set-up a probabilistic approach for the buckling analysis of cylindrical shells.
Dynamic Instability of Antisymmetric Cross-Ply Laminated Composite Rectangular Thin Plates Based On Large Deflections Theory
In this paper, the dynamic instability of thin laminated composite plates subjected to harmonic in-plane loading is studied based on nonlinear analysis. The non-linear von Karman strains associated with large deflections and curvatures are considered. Introducing AiryÃ¢â‚¬â„¢s stress function, GalerkinÃ¢â‚¬â„¢s technique is then applied to the problem and it leads to a system of non-linear Mathieu-Hill equations. Applying the BolotinÃ¢â‚¬â„¢s method, the dynamically-unstable regions, and both stable- and unstable-solutions amplitudes of the steady-state vibrations are obtained. The effect of various lamination schemes and also magnitude of both tensile and compressive longitudinal periodic loadings on the dynamically-unstable regions and the amplitudes of the steady-state vibrations are examined and compared.
Buckling Design and Imperfection Sensitivity of Sandwich Composite Launch-Vehicle Shell Structures
Composite materials are increasingly being considered and used for launch-vehicle structures. For shell structures, such as interstages, skirts, and shrouds, honeycomb-core sandwich composites are often selected for their structural efficiency. Therefore, it is becoming increasingly important to understand the structural response, including buckling, of sandwich composite shell structures. Additionally, small geometric imperfections can significantly influence the buckling response, including considerably reducing the buckling load, of shell structures. Thus, both the response of the theoretically perfect structure and the buckling imperfection sensitivity must be considered during the design of such structures. To address the latter, empirically derived design factors, called buckling knockdown factors (KDFs), were developed by NASA in the 1960s to account for this buckling imperfection sensitivity during design. However, most of the test-article designs used in the development of these recommendations are not relevant to modern launch-vehicle constructions and material systems, and in particular, no composite test articles were considered. Herein, a twopart study on composite sandwich shells to (1) examine the relationship between the buckling knockdown factor and the areal mass of optimized designs, and (2) to interrogate the imperfection sensitivity of those optimized designs is presented. Four structures from recent NASA launch-vehicle development activities are considered. First, designs optimized for both strength and stability were generated for each of these structures using design optimization software and a range of buckling knockdown factors; it was found that the designed areal masses varied by between 6.1% and 19.6%
DAVID SLEIGHT, DAVID MYERS, ALLEN WATERS, JR., PRASAD CHUNCHU, ANDREW LOVEJOY and MARK HIL
Delamination Buckling Response of 3D Fiber-Metal Laminates Subjected to different loading Rates
In this paper, the buckling response of magnesium-based 3D fiber metal laminated composites (3D-FML) subject to four different strain-rates (static, 0.01, 0.1 and 1 s-1) is experimentally investigated. The effect of varying initial delamination lengths on the response is also investigated. The buckling capacity, maximum loadcarrying capacity and damage mechanisms are presented and the overall response is discussed for all the cases. As expected, specimens without initial delamination carried a higher load under all the strain-rate values. Moreover, the length of the initial delamination had a negligible effect on the maximum load-carrying capacity. The recorded images of the specimens during tests show that the buckling of the first skin always occurs prior to failure of the core, while for the second skin, buckling occurs almost simultaneously along with the onset of coreÃ¢â‚¬â„¢s failure.
Influence of Multiwalled Carbon Nanotube on Interfacial Fatigue Performance of Glass Epoxy Polyvinyl Chloride Core Sandwich Composites
Fatigue performances of glass epoxy PVC core sandwich composites with and without multi walled carbon nanotube (MWCNT) are investigated through modified DCB specimen subjected to tension-tension cyclic load under different frequencies (1Hz, 2 Hz and 3 Hz) and several loading ratio (0.1 and 0.25). Results demonstrate increase in static fracture load values as well as significant retardation in crack initiation and propagation in samples with MWCNT over those of the conventional samples under predominantly mode I cyclic load tested at or below 75% of static fracture loads of conventional samples. Sandwich samples were prepared with MWCNT dispersed resin system through sonication and vacuum resin infusion (VRI) technology. The results are supported by high resolution transmission electron microscope (HRTEM) analysis and fracture micrograph of field emission scanning electron microscope (FESEM) investigation.
The Influence of Surface Finishing of Core on the Impact Behaviour of Polymer Foam-cored Sandwich Structures
The interest on sandwich composite structures is increasing due to their good mechanical properties, the ability to tailor the sandwich structure to each component, and good formability. These materials are used in aerospace, wind energy and automotive industries, where minimising the weight of vehicle or components is very important. The increasing use of polymer foams in composite structures has encouraged the polymer industry to improve the mechanical properties of the core materials, by redesigning the materials and applying different types of surface finishing. In this research, the effect of surface finishing options on the impact behaviour of sandwich composite structures was investigated by performing low velocity drop weight impact tests, because impact is often critical in this kind of components. The sandwich composites used had a PVC polymer foam core with various surface finishing options (plain, flexi cut, knife cut, saw cut and mini cut), and were manufactured with Glass Fibre Reinforced Polymer (GFRP) skins through Resin Infusion under Flexible Tooling (RIFT). After the impact test, the delaminated areas at the back face of the samples with different finishes were measured and compared against each other. The surface finishing options affected the impact behaviour of the sandwich composite structure, and increased the ductility and energy-absorption capacity of the structure.
SORAIA PIMENTA, ANDY MORRIS and JOHN DEAR
Strengthening of Honeycomb Cardboard by FRP as Promising Green Panels
The main objective of the present investigation is to adopt green material like sandwich honeycomb cardboard to be part of the production of structural elements. With the development of technology and the materials, the people figured out the preserving the earth materials is the most important issue that should be maintained to last the human gender. This can be secured by adopting green thoughts on manufacturing and building things needed for our life. Unfortunately, sandwich honeycomb cardboard has low mechanical properties, therefore, it is not proper for many industries except for packaging and limited applications, but definitely it is a green material worldwide available, cheap as well as recyclable. With advances of the FRP, it becomes possible to produce hybrid sections, where it can sustain higher levels of loads and at the same time being flexible, lightweight and cheap. This needs blending diverse knowledge, such as design experience, materials, and applications as well as manufacturing to come up with a creative idea to resolve challenges and problems as well as to implement green thoughts. This research proposal is a green attempt to enhance recyclable material to be used for aerospace applications through using little amounts of FRP to strengthening purposes. The honeycomb cardboard panels were strengthened using woven carbon fiber mat and epoxy. The main challenge of the research was the absorption of the epoxy by the honeycomb; therefore, different techniques were adopted to overcome this problem which led eventually to different results. The samples were tested under 3- point bending test, where the original sample results are taken as a reference to compare the other strengthened sample to figure out the percentage increase of the stiffness.
ALI HILAL-ALNAQBI, ANAS HALLALO and AHMAD ALTELBANI
Technical Challenges and R&D Needs for Compressed Hydrogen Storage On-Board Fuel Cell Electric Vehicles
A primary goal of the Hydrogen Storage Program (the Program), part of the U.S. Department of EnergyÃ¢â‚¬â„¢s Fuel Cell Technologies Office, is to develop and demonstrate advanced hydrogen storage technologies to enable successful commercialization of fuel cell products. Compact, reliable, safe, and cost-effective storage of hydrogen is a key technology requirement for the widespread commercialization of Fuel Cell Electric Vehicles (FCEVs) and other hydrogen fuel cell applications. While some light-duty FCEVs with a driving range of about 300 miles are emerging in limited markets, affordable onboard storage still remains a roadblock to commercialization beyond limited vehicle platforms and niche markets. A key challenge is how to cost effectively store sufficient quantities of hydrogen onboard without sacrificing passenger and cargo space. While the energy per mass of hydrogen is substantially greater than most other fuels, its energy by volume is much less than liquid fuels, such as gasoline. The current state of the art is to store hydrogen in Composite Overwrapped Pressure Vessels (COPVs) with polymer liners at 700-bar pressure. The following is an overview of technical challenges and R&D needs to enable successful deployment of high-pressure hydrogen storage systems into commercial FCEVs. These technical challenges primarily concern the contributions of composite materials to compressed hydrogen storage system cost, mass, and volume. To achieve the performance requirements for FCEVs, the Program, with input of US automobile manufacturers, has developed 2020 performance targets for onboard hydrogen storage that include costs of $10/kWh, gravimetric density at 1.8 kWh/kg system, and volumetric density at 1.3 kg H2/L system to provide for a driving range of at least 300 miles. The Program is developing strategies and supports R&D efforts to address composite materials for high-pressure COPV use with FCEVs. Examples of projects and activities the Program has supported are provided for illustrative purposes.
JR., GRACE ORDAZ, JESSE ADAMS and NED STETSON
Crush Analysis of Compression Modeled Chopped Fiber Tubes
Chopped fiber reinforced thermoplastic composites can be very useful in developing lightweight high strength recyclable components for automobiles. Such parts can be produced using injection or compression molding process. In general compression molding is preferred as it can help retain the fiber length compared to injection molding process. The longer fiber results in improved structural properties. A computational method is introduced for the virtual simulation of performance of chopped fibers in polymer composites. This new approach did lead to the development of a specialized Multi-Scale Material Characterization of composite system comprising of: a) chopped fiber material modeling based on nano-mechanics failure theory considering fibers as inclusion, b) micro-macro mechanics, and damage failure theory, c) tensor stiffness averaging technique; and d) structural durability and damage tolerance (D&DT) analysis under crush service loading. The material model established in MCQ-Chopped is integrated/coupled with FEA with GENOA platform software to perform Multi Scale Progressive Failure Analysis (MS-PFA). The crush modeling of composite crushed tube was test validated using GENOA software in 3 distinct integrated steps using, such as follows: a) Characterizing Material Properties - of composite materials composed of chopped fibers using MCQ-Chopped and validating against Toyota coupon test data; b) Mapping and Transformation - of statistical average tensor orientation from unstructured Moldex3D detailed model to LS-DYNA FE solver. In this regard GENOA platform software algorithm was used to perform 3D models data management and visualize the mapping error between two dissimilar meshes. And c) De-Homogenized Multi-Scale Progressive Failure Dynamic Analysis (MSPFDA) was employed to capture the damage and failure at multi scale levels, namely micro (constituents fiber/matrix) and macro through-the-thickness. The simulation results are compared with test provided load vs. displacement and acceleration vs. time curve. In addition the software prediction provided the damage and fracture evolution and the contributing failure mechanism. Keywords: Compression Molding, Chopped Fiber, crushed tube, De-homogenized modeling, Tensor orientation stiffness, Effect of defects, Strain Rate Effect.
DADE HUANG, MASSIMILIANO REPUPILLI, FRANK ABDI, YUYANG SONG and UMESH GANDHI
Modeling of Crush Behavior of Carbon Fiber Composites
The use of continuous carbon fiber composites in load bearing automotive applications can potentially offer significant benefits in weight reduction provided that the key issues regarding the prediction of energy absorption and crash energy management are thoroughly addressed. In this paper, the development of a framework to address the challenges in modeling the crush of non-crimp carbon fiber composites (NCF) is presented. Material models for intra and inter-laminar damage mechanisms were first developed and validated with tensile, bending, and impact experiments. Furthermore, these models were extended to dynamic axial crush of flat specimens and the predicted results were correlated with the experimental results for samples with and without delamination suppression. Good correlations were observed for the crush load and crush morphology, allowing one to unambiguously assess the accuracy of the developed models.
HAMID KIA, SATVIR AASHAT and VAMSI PULUGURTHA
Free Vibration of Doubly Tapered Laminated Composite Beams using Hierarchical Finite Element Method
In the present study, free vibration response of symmetric thickness-and-widthtapered (doubly-tapered) laminated composite beams is considered. Considering a variety of tapered configurations according to different types of ply drop-off configurations, both conventional and hierarchical finite element formulations are developed based on cylindrical bending theory of laminated beam. Natural frequencies of different types of internally-tapered composite beams are determined. Comparison of the hierarchical finite element solution with the Rayleigh-Ritz solution and a higherorder finite element solution is performed. A parametric study is conducted to investigate the effects of boundary conditions, width-taper-ratio, thickness-taper configurations, thickness-tapering angle, laminate configuration and compressive axial force on the free vibration response of doubly-tapered laminated composite beams.
Hygrothermal Analysis of Composite Beams under Moving Loads
With the expanded use of composite materials in civil structures such as bridge decks, a moving-load analysis would simulate a truck moving over a composite bridge. Available results concerning moving-load analysis are in the form of deflections and dynamic load factors; a natural extension is the behavior of such a beam under a hygrothermal environment. In this research a finite element method employing h-p version finite elements is made and analyzed to determine the deflection distribution and the dynamic magnification factor of a composite beam under the action of a moving load. A computer code is written in MATLAB to accomplish this. Classical laminate plate theory (CLPT) and first order shear deformation theory (FSDT) are considered for the analysis of composite beams. A consistent formulation is used to reduce the composite plate theory to beams. The governing equations are obtained using HamiltonÃ¢â‚¬â„¢s principle. NewmarkÃ¢â‚¬â„¢s time integration scheme is used to determine the dynamic response in the form of the displacement field of the beam. The maximum deflection obtained for the moving load case is compared with that of the static load to determine the dynamic magnification factor when subjected to a hygrothermal environment. Results are presented for a simply-supported beam subjected to a moving load that traverses the entire span of the composite beam. The displacement field and the magnification factors based on the CLPT is compared with that of FSDT. A parametric study is conducted by considering different fiber volume fractions and different laminates. The major conclusions are listed here. The ply orientation influences the dynamic behavior of a beam subjected to moving load. The dynamic magnification factors in dry and hygrothermal conditions for the laminates considered generally follow similar pattern. As the fiber volume fraction increases, the dynamic magnification factor generally decreases.
Failure Analysis of Composite Beams under Moving Loads
With the expanded use of composite materials in civil structures such as bridge decks, a moving-load analysis would simulate a truck moving over a composite bridge. Almost all the available research concerning moving-load analysis concerns with results in the form of deflections and dynamic load factors; very little attention has been paid to the stress analysis and using the stress field to ascertain failure. In this research a finite element method employing h-p version finite elements is made and analyzed to determine the failure of a composite beam under the action of a moving load. A computer code is written in MATLAB to accomplish this. Classical laminate plate theory (CLPT) and first order shear deformation theory (FSDT) are considered for the analysis of composite beams. A consistent formulation is used to reduce the composite plate theory to beams. The governing equations are obtained using HamiltonÃ¢â‚¬â„¢s principle. NewmarkÃ¢â‚¬â„¢s time integration scheme is used to determine the dynamic response in the form of the displacement field of the beam. The strain field is obtained using the kinematic relations and then the stress field using the appropriate constitutive equations. Finally, the maximum stress failure theory is applied to determine the factor of safety. Results are presented for a simply-supported beam subjected to a moving load that traverses the entire span of the composite beam. The stress field and factors of safety based on the CLPT is compared with that of FSDT. A parametric study is conducted by considering different laminates and different fiber volume fractions.
Simulation of the Automation of Composite Wind Turbine Blade Manufacture
The majority of large wind turbine blades are manufactured from textile-reinforced resin-infused composites using an open mold. The placement of the textile reinforcements in the mold is traditionally accomplished by a manual process where dozens of workers hand place each dry fabric in the mold. Depending on the level of skill and experience of each worker and the relative complexity of the mold geometry, local areas may exhibit out-of-plane wrinkling and in-plane waviness. Fabric imperfections such as these can adversely impact the strength and stiffness of the blade, thereby compromising its durability in service. In an effort to reduce the variabilities associated with a manual-labor process, an automated piecewise shifting method has been proposed for fabric placement. This automated layup method shows potential to save time on the preform process and to reduce blade-to-blade variability. In the current research, the automated shifting layup method is investigated using the finite element modeling approach. A user-defined material model is linked with Abaqus to capture the evolution of the fabric shear stiffness, and the modeling approach allows for the automatic tracking of the changes in the fiber orientations during the fabric-placement process. The simulation approach is demonstrated for the geometry of the trailing edge of a typical wind turbine blade. The simulation considers the mechanical behavior of the fabric and reliably predicts fabric deformation.
Modeling of Resin Transfer Molding of Carbon Fiber Composites
Resin transfer molding (RTM) has become increasingly popular for the manufacturing of composite parts. To enable high volume manufacturing and obtain good quality parts at an acceptable cost to automotive industry, accurate process simulation tools are necessary to optimize the process conditions. Towards that goal, General Motors and the ESI-group are involved in developing a state of the art process simulation tool for composite manufacturing in a project supported by the Department of Energy. This paper describes the modeling of various stages in resin transfer molding such as resin injection, resin curing, and part distortion. An instrumented RTM system located at the General Motors Research and Development center was used to perform flat plaque molding experiments. The experimental measurements of fill time, in-mold pressure versus time, cure variation with time, and part deformation were compared with the model predictions and very good correlations were observed.
HANG YU, SELINA ZHAO, JOHN OWENS, PRAVEEN PASUPULETI and MARK DOROUDIAN
Understanding and Prediction of Fibre Waviness Defect Generation
Fibre waviness is one of the most significant defects that occurs in composites, due to the severe knockdown in mechanical properties that it causes. This paper summarises a substantial research programme that has been undertaken at the University of Bristol to investigate the generation of fibre waviness defects during prepreg composites processing and develop predictive numerical models. Generation of fibre waviness can come from a number of sources, here the focus has been on thick sections, where consolidation of the ply stack leads to out of plane ply movement. This out of plane movement can directly lead to waviness generation or it can indirectly create the excess length in a ply that in turn leads to the formation of waviness. Initial experimental results on compaction of simplified specimens led to the development of a model that was embedded in the finite element software Abaqus as a user material. A number of case studies were then investigated using this model to show how the formation of wrinkle defects in typical component features can be predicted. Experimental testing has been undertaken to validate the model results.
JONATHAN BELNOUE, OLIVER NIXON-PEARSON, TASSOS MESOGITIS, JAMES KRATZ, DMITRY IVANOV, IVAN
Evolution of Damage Mechanisms and Remaining Properties in Carbon Fiber Composite Materials
Fiber reinforced composite materials are materials are increasingly being used in many applications. Composite materials go through a complex process of material state changes (e.g. distributed damage development, accumulation, and interaction) which affect global property (strength and stiffness) degradation and eventual failure. Despite significant progress, different modes of local changes are often difficult to characterize and estimation of remaining properties can be quite challenging. The objective of this paper is to enhance understanding of local mechanisms and property evolution to facilitate design for damage tolerance and durability. For this purpose, a new material state description is provided based on the understanding that microstructural degradation causes heterogeneous charge polarization inside composite materials. This frequency dependent dielectric response measured using broadband dielectric spectroscopy (BbDS) provides important features which directly relate to degradation behavior and associated loss of properties (strength and stiffness). In this study, material state change has been discussed in two example cases: i) Damage distribution and remaining property (strength and stiffness) after low velocity impact until barely visible impact damage (BVID) limit, and ii) Evolution of damage mechanisms in different laminates during tension loading. A 3D X-ray microscope has been used to visualize and hence validate evolution of damage. Validated results from these test cases show that degradation process, and evolution of properties can be represented by material state variables with a high sensitivity. Relationship of property degradation (mechanical and dielectric) with modes of damage evolution has been discussed at the fundamental level. This provides an opportunity for the development of predictive modeling which of interest in aerospace and other applications. Details of experimental methods and results will be included in the paper.
JALLISA CLIFFORD, HUNTER GOMAN and KEVIN EPLEY
Hybridization Strategy for Improving Damage Tolerance of Three Phase Composites
This paper presents the development of novel three phase composite materials consisting of reinforcing fibers and continuous thermoplastic fibers in a thermoset matrix. In this work, two hybridization strategies have been employed for blending thermoplastic fibers with reinforcing fibers- comingling and micro-wrapping. S-glass and Polypropylene have been blended using a) air-jet comingling process and b) intratow micro-wrapping. Damage resistance and tolerance of the two material systems have been evaluated with drop-weight impact and compression after impact (CAI) tests. Results indicate that thermoplastic fibers improve the total absorbed energy of the composite by toughing mechanism through extensive plastic deformation as well as providing a protection to glass fibers to enhance damage tolerance of the hybrid laminates.
Modeling, Synthesising and Testing Nacre-inspired CFRP Structures for Improved Damage Tolerance
In this work, a bio-inspired carbon/epoxy composite with nacre-like tiled microstructure is designed and synthesised. Analytical models were developed to predict the strength and energy dissipation of such composite, and the predictions for the stress-strain response during tile pull-out were validated against FE. Suitable configurations for tile geometry with interlocks were then identified and used for the subsequent manufacturing, based on their anticipated energy dissipation capability. The dimensions of the tiles considered were of the order of 0.7 mm. In-situ three-point bending tests were then carried out in an SEM environment, showing the capability of the interlocking microstructure in deflecting the crack.
A Continuum Damage Mechanics Model to Predict Kink-Band Propagation Using Deformation Gradient Tensor Decomposition
A new model is proposed that represents the kinematics of kink-band formation and propagation within the framework of a mesoscale continuum damage mechanics (CDM) model. The model uses the recently proposed deformation gradient decomposition approach to represent a kink band as a displacement jump via a cohesive interface that is embedded in an elastic bulk material. The model is capable of representing the combination of matrix failure in the frame of a misaligned fiber and instability due to shear nonlinearity. In contrast to conventional linear or bilinear strain softening laws used in most mesoscale CDM models for longitudinal compression, the constitutive response of the proposed model includes features predicted by detailed micromechanical models. These features include: 1) the rotational kinematics of the kink band, 2) an instability when the peak load is reached, and 3) a nonzero plateau stress under large strains.
Analysis and Characterization of Damage and Failure Utilizing a Generalized Composite Material Model Suitable for Use in Impact Problems
A material model which incorporates several key capabilities which have been identified by the aerospace community as lacking in state-of-the art composite impact models is under development. In particular, a next generation composite impact material model, jointly developed by the FAA and NASA, is being implemented into the commercial transient dynamic finite element code LS-DYNA. The material model, which incorporates plasticity, damage, and failure, utilizes experimentally based tabulated input to define the evolution of plasticity and damage and the initiation of failure as opposed to specifying discrete input parameters (such as modulus and strength). The plasticity portion of the orthotropic, three-dimensional, macroscopic composite constitutive model is based on an extension of the Tsai-Wu composite failure model into a generalized yield function with a non-associative flow rule. For the damage model, a strain equivalent formulation is utilized to allow for the uncoupling of the deformation and damage analyses. In the damage model, a semicoupled approach is employed where the overall damage in a particular coordinate direction is assumed to be a multiplicative combination of the damage in that direction resulting from the applied loads in the various coordinate directions. Due to the fact that the plasticity and damage models are uncoupled, test procedures and methods to both characterize the damage model and to covert the material stress-strain curves from the true (damaged) stress space to the effective (undamaged) stress space have been developed. A methodology has been developed to input the experimentally determined composite failure surface in a tabulated manner. An analytical approach is then utilized to track how close the current stress state is to the failure surface.
KELLY CARNEY, PAUL DUBOIS, BILAL KHALED, CANIO HOFFARTH, SUBRAMANIAM RAJAN and GUNTHER BLA
Modelling Failure Mechanisms in Composites Subjected to Impact and Post-impact compression
Composites subjected to low-velocity impact are prone to abrupt failure during post-impact compression. Impact reduces the compressive residual strength of composites significantly. In many cases, compression after impact (CAI) strength is the critical factor in design. This paper presents an integrated finite element model that aims to predict CAI strength without simplifying or idealizing the damage due to the impact event. Composite laminates impacted at energy levels of 1.6 J, 6.5 J and 17 J are subjected to compression to failure. The CAI strength values as well as delamination and damage patterns correlate well with experiments taken from literature. It was found that failure during post-impact compression is governed by local buckling, which triggers damage growth. An increase in mode Ãâ€ Ãâ€ fracture toughness can essentially reduce delamination size induced during impact, consequently improving residual strength by delaying the onset of buckling (and therefore damage growth) during compression.
Fiber Orientation Assessment on Laminated Carbon Fiber Composites using Eddy Current Probe
Fiber orientation is a crucial feature of carbon fiber reinforced composite materials, with fiber layup chosen to achieve optimum part performance in the load environment for which the part is designed. This is especially important for compression molded composite parts, where fiber orientation can change during the molding process. Therefore, good quality control requires verification that the manufactured part meets design specifications, and a rapid technique to verify orientation of carbon fiber plies is needed. In this paper, a technique using a flexible eddy current sensor is proposed to assess the fiber orientation. The directional eddy current sensor is configured to measure part conductivity as a function of applied current direction. Experimental results show a correlation between the peaks in the measured conductivity as a function of angle and the fiber orientations in the part. This nondestructive evaluation (NDE) technique has a fast inspection rate and is relatively cost effective. A four-layer quasi-isotropic carbon fiber composite made by compression molding was used to calibrate the system in this study. Several specimens were fabricated with nonsymmetric ply layups using unidirectional preimpregnated carbon fiber with varying orientations. These specimens were tested using the calibrated system. The final fiber orientations in the specimens were confirmed by cross sectional microscopy and microfocus X-ray computed tomography analyses.
Advances in In-Situ Inspection of Automated Fiber Placement Systems
The advent of Automated Fiber Placement (AFP) systems have aided the rapid manufacturing of composite aerospace structures. One of the challenges that AFP systems present is the uniformity of the deposited prepreg tape layers, which are prone to laps, gaps, overlaps and twists. The current detection modus operandi involves halting fabrication and performing a time consuming visual inspection of each tape layer. Typical AFP systems use a quartz lamp to heat the base layer to make the surface tacky as it deposits another tape layer. The idea was proposed to use the preheated base layer as a through transmission heat source and inspecting the newly added tape layer using a thermographic camera. As a preliminary study of this concept a laboratory proof of concept device was designed and constructed to simulate the through transmission heat source. Using the proof of concept device, we inspected an AFP-built uncured composite specimen with artificial manufacturing defects. This paper will discuss the results of this preliminary study and the implications involved with deploying a fullscale AFP inspection system.
Development of a Fully Automated Guided Wave System for In-process Cure Monitoring of CFRP Composite Laminates
A guided wave-based in-process cure monitoring technique for carbon fiber reinforced polymer (CFRP) composites was investigated at NASA Langley Research Center. A key cure transition point (vitrification) was identified and the degree of cure was monitored using metrics such as amplitude and time of arrival (TOA) of guided waves. Using an automated system preliminarily developed in this work, high-temperature piezoelectric transducers were utilized to interrogate a twenty-four ply unidirectional composite panel fabricated from HexcelÃ‚Â® IM7/8552 prepreg during cure. It was shown that the amplitude of the guided wave increased sharply around vitrification and the TOA curve possessed an inverse relationship with degree of cure. The work is a first step in demonstrating the feasibility of transitioning the technique to perform in-process cure monitoring in an autoclave, defect detection during cure, and ultimately a closed-loop process control to maximize composite part quality and consistency.
Mechanical Performance of Repaired Sandwich Panels: Experimental characterization and Finite-Element Modelling
This paper describes the static performance of adhesively bonded repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behavior of pristine and repaired panels under tensile loading were studied. All tests were conducted at room temperature conditions. Then finite-element analyses were performed to predict the repair strength. Two material models were developed for the adhesive joint: one was linear elastic and the second was elastic-plastic with a shear failure criterion. For the composite skins, an orthotropic linear elastic model was used.
MARIE-LAURE DANO, AUGUSTIN GAKWAYA and CHARLES-OLIVIER AMYOT
Composite Box-Beam Failure Modes and Strength: 3D Modeling and Analysis and Comparison with Experimental Results
This paper presents a study to develop detailed understanding of the structural strength and failure modes observed in the testing of adhesively bonded composite box-beam specimens. The box-beam specimens were scaled structures which were designed and fabricated to emulate the characteristics of the main load-bearing component of a large composite wind turbine rotor blade (i.e., the box beam formed with the turbine blade spar caps and shear webs). Several composite box-beam specimens were assembled, using thick adhesive layers between spar caps and shear webs. The spar caps and shear webs were fabricated with glass fabric composites using a vacuum assisted resin infusion molding (VARIM) process. The structural integrity of the box-beam specimens was evaluated in both 3- and 4-point bending experiments. To obtain proper understanding of the observed bending test results, detailed deformation and strength behavior of the tested box beams were modeled and analyzed using the finite element methods. A series of linear eigenvalue analyses were conducted to determine the buckling modes and loads of the tested box beams. Incremental loading analyses were conducted to investigate the progressive development of local nonlinear deformation, instability and stress distributions in the box beam spar caps, shear webs and adhesive layers during the bending tests. The numerical results were correlated and compared with the observed bending test results to develop detailed understanding of damage and failure progression in the tested box beams. The factors that influenced the ability of the box-beam specimens to carry additional loads beyond the first buckling load were assessed. The good agreement between bending test and numerical analysis results suggests that the analysis procedures taken in the study could be used to analyze and predict ultimate strength and failure modes of the composite box-beam specimens under different bending loads.
AKIRA MIYASE, KING HIM LO and SU SU WANG
The Mechanical Behavior of Foam-filled Corrugated Core Sandwich Panels in Lateral Compression
A series of experimental investigations and finite element analyses is presented in the compression response and subsequent failure modes in foam-filled corrugated core sandwich panels. The corrugated cores based on carbon fiber reinforced plastic (CFRP) and glass fiber reinforced plastic (GFRP) were fabricated using a hot press molding process and then bonded to face sheets based on the same material, to produce a range of lightweight sandwich panels. A liquid of low-density Rigid Polyurethane (PUR) foam was injected to fill the empty space of the panels, where to investigate the influence of filling the corrugation with the foam. The experiments also provide an insight into the post-failure response of the sandwich panels. The results are compared with the numerical predictions offered by a finite element analysis (FEA). Buckling of the cell walls has been found to be initial failure mode in these corrugated core panels. Continued loading resulted in fracture of the cell walls, localized delamination as well as debonding between the skins and the core. The predictions of the FEA show reasonably good agreement with the experimental measurements, for semi-filled and fully-filled foams. Finally, filling the inside of the corrugated core sandwich panels with the foam significantly improved the specific energy absorption (SEA) characteristics of the panels. The SEA for the GFRP and the CFRP fully-filled specimens were improved by almost 160% and 105%, respectively. This improvement was due to the constraint effects between the foam and the corrugated core members.
D. BACHTIAR, J.P. SIREGAR, P. PARUKA, S.H.S.M. FADZULLAH, B. ZHANG and W.J. CANTWELL
Heterogeneous Fracture Mechanics Representations of the Effects of Defects from Manufacturing to End of Life
Composite materials are essential for many modern applications, including airplanes and cars, energy conversion and storage devices, medical prosthetics, and civil structures. The strength and life of such materials are determined by a complex sequence of progressive nucleation, accumulation, and coalescence of micro-damage that is always related to the micro-morphology of the constituents and their properties. Although detecting and modeling all of the discrete events in that sequence is quite difficult, and in some cases not feasible, it would be very useful to identify observable parameters that indicate the onset of different stages of damage development so that remaining strength and life could be estimated. Recently, the authors have developed a method of following the effect of multi-defect nucleation, growth, coalescence, and fracture plane development based on the measurement of dielectric response of composite materials. The method also enables the prediction of as-manufactured individual sample strength, suggesting that the birth-to-death effect of defects can be followed and interpreted. The present paper postulates the construction of a fundamental fracture mechanics methodology based on this general set of observables that could enable the consistent relationship of all defect and material states during the processing and application life of composite materials as a foundation for a birth-to-death assessment of the effect of defects. The Ã¢â‚¬Å“heterogeneous fracture mechanicsÃ¢â‚¬Â concept is defined and discussed with example data and observations that indicate the success and limitation of the method for as manufactured materials, static and fatigue loadings.
KENNETH REIFSNIDER, RASSEL RAIHAN and FAZLE RABBI
Multiscale Analysis of Stitched CFRP Composites Including the Effect of Geometrical Imperfection
In this paper, multiscale analysis is conducted for through-the-thickness stitched CFRP composites in order to investigate the effect of unintended imperfections in the microstructure. The analytical method is based on the homogenization method, and the effect of the imperfection is introduced by using perturbation method. Throughthe- thickness stitched CFRP laminates have prominent mechanical properties such as higher delamination toughness, better impact resistance under low-velocity impact, and higher CAI strength than that of non-stitched laminates. However, through-thethickness stitching process inevitably induces microscopic geometrical imperfection such as fiber waviness and resin pocket. Therefore, in order to analyze material behavior precisely, the evaluation of the effect of the imperfection is essential. In this paper, the effects of the imperfection on the macroscopic material properties and on the microscopic stress distribution are both analyzed by using multiscale analysis. The multiscale simulation method is based on the homogenization method. Geometrical imperfection is considered by using perturbation method, and it is introduced as an input to the calculation. The calculation to analyze the effect of the imperfection is always done using the idealized perfect geometry. In this study, several realistic geometrical imperfection patterns in the stitched CFRP composite are measured by using sub-micro focus X-ray CT devise, and they are used as inputs to the calculation method. The calculated results are compared to the experimental results. Simulation results reveal that the microscopic geometrical imperfection significantly affects the macroscopic material properties and microscopic stress distribution of the through-thethickness stitched CFRP composites.
ANTHONY WAAS, HAYATO FUKUI, MASAHIRO NAKAYAMA and RYOSUKE MATSUZAKI
Coupling Process and Structural Simulations in Crash Application
The energy absorbed during crushing of composite structures is strongly dependent on the layup, fiber architecture and type of resin used. Modeling of the crash behavior of composites is therefore highly influenced by the composite material system chosen, and current constitutive models must be improved to include/account for the inherent properties from the manufacturing step. The ultimate goal of this contribution is to optimize the material system and manufacturing method for the required crushing performance in terms of energy absorption and cost. A first outcome of the study will be to provide information regarding the properties of the final manufactured composite material such as residual stresses and effects of defects. These properties are then used in the development of crash models. A robust link between manufacturing, experiments and crushing simulations is vital where there should be a generic routine towards the data transfer and constitutive models. The study of effects of defects will affect the input data into the material and constitutive models in form of change in strength and stiffness properties of the material. In this contribution, an experimental study on the material response under quasistatic crushing is performed where the manufacturing effects on the material properties are considered based on estimated data provided from vacuum infusion simulation. The crushing simulations are performed with ABAQUS where the material model developed in-house, which is a physically based damage model based on the LaRC05 failure criterion and progressive damage, is chosen to model the constitutive behavior. The parameters that are transferred to the system from manufacturing simulation are fiber content and voids. Consideration of these parameters into the constitutive behavior of the structure will directly influence the structural response. A parametric study is completed and results are discussed.
SERGIO COSTA, MACIEJ WYSOCKI and RENAUD GUTKIN
Evaluation of Tow-Steering Effectsâ€”Mechanical Coupon Testing
Tow-steering of composites using Automated Fiber Placement (AFP) provides significant flexibility for the composite designer to spatially tailor fiber orientation within a given ply. However, tow-steering is not without drawbacks which may negatively impact the strength and stiffness of the material. In general, there are two sources of strength reduction of concern: inherent geometric complexities due to tow convergence zones (e.g., gaps and overlaps) and manufacturing defects related to the in-plane curvature of the tows. The purpose of the present work is to identify the mechanical properties of towsteered laminates relative to those of an unsteered laminate for the application of a 39- foot passive-aeroelastically tailored wingbox. The tow-steered laminate currently studied employs a fixed ply fraction wherein all tows are steered together and maintain fixed relative angles to each other, thus minimizing the number of unique effective layup sequences. An alternate approach wherein half the plies (the equivalent quasiisotropic component) are unsteered is also considered. The design and fabrication of tow-steered and unsteered panels for mechanical coupon testing is described. Normalized test results for modulus and strength are presented for tow-steered laminates, relative to equivalent unsteered laminates.
Dynamic Mechanical Analysis of Cenosphere/HDPE Syntactic Foams
High density polyethylene (HDPE) syntactic foams containing fly ash cenospheres as the hollow filler are fabricated using an industrial scale injection molding machine and studied for their dynamic mechanical behavior. Syntactic foams using thermoset matrix materials and engineered glass hollow particles have long been used as buoyancy devices and thermal insulation in the marine sector and as a lightweight sandwich core in the aerospace industry. This class of materials is attractive because of high mechanical properties in compression, tailorable density, and improved thermal properties. The constituents are used in as-received condition, without surface treatments. These lightweight composites can be highly beneficial in developing consumer goods by reducing consumption of HDPE. Syntactic foams are produced containing 20, 40, and 60% cenospheres by weight. A temperature sweep from 35-130Ã‚Â°C and a frequency sweep from 1-100 Hz are conducted on the fabricated syntactic foams. At all temperatures, syntactic foams show higher storage and loss moduli and lower damping than neat HDPE. Syntactic foams with 60 wt.% cenospheres show only a small increase in properties compared to those with 40 vol.% due to particle breakage during processing. However, high particle loading has the benefit of reducing consumption of HDPE. The time-temperature superposition principle is used to extend the frequency response to cover the range 10-2-106 Hz.
NIKHIL GUPTA, BHARATH KUMAR and MRITYUNJAY DODDAMANI
Microvoid Formation in Fiber Tows with Non-Uniformly Spaced Fibers
Liquid composite molding is a common manufacturing method to fabricate complex and net shape composite parts in which a liquid resin is introduced into a dry fibrous preform. The fiber preforms usually contain fiber tows and resin impregnation is driven by an applied pressure and capillary forces. The fiber tows are usually less permeable than the regions in between them hence the resin will typically flow around the fiber tows at much faster rate than into them. This paper predicts microbubble formation in fiber tows with non-uniformly spaced fibers and the dissolution of the microbubbles before the resin cures. Any microbubbles trapped within the tow after curing will be microvoids in the final composite part. A parametric study is conducted to determine the influence of fiber packing and manufacturing process parameters on microvoid formation within tows.
Effect of Fiber-Orientation on the Long-Term Thermo-Oxidative Degradation in Composite Laminates
Polymer composites have a limited service-life in high temperature environments due to oxidation-induced material degradation. The degradation processes are controlled by oxygen diffusion and reaction kinetics during the initial hours of exposure and over a longer-term by the oxidation-induced chemical and dimensional changes as well as damage evolution. Longer-term aging after damage initiation causes formation of discrete crack surfaces that adsorb oxygen and lead to acceleration in oxidative degradation and the permeation of oxidation deeper into the composite. This work focuses on model-based calculation of oxidation growth rates over thousands of hours of aging in both axial and transverse directions of a polymer matrix composite lamina. In a G30 Carbon/PMR-15 Polyimide lamina that is exposed along its edges and on the transverse surfaces, the axial (along the fiber-axis) oxidation growth rate before the crack initiation starts at ~4.12 Ã´â‚¬Å¸Â¤Ã´â‚¬Ââ€°/Ã¢â€žÅ½ and reduces to ~ 2.18 Ã´â‚¬Å¸Â¤Ã´â‚¬Ââ€°/Ã¢â€žÅ½, if a surface oxidation layer grows without crack initiation. Similarly, the transverse oxidation growth rate before crack initiation starts from a relatively high value (~ 1.12 Ã´â‚¬Å¸Â¤Ã´â‚¬Ââ€°/Ã¢â€žÅ½) and decreases to its lowest value (~ 0.09 Ã´â‚¬Å¸Â¤Ã´â‚¬Ââ€°/Ã¢â€žÅ½) as the active reaction zone moves deeper into the material. These rates estimation of the use-life or conversely a durability-substantiated design of surface ply thickness.
Pseudo-Ductile Hybrid Composites with Overload Sensing Capability
Fragmenting hybrid laminates create a pseudo-ductile response and a striped pattern of fractures and delaminations which can be used as a simple visual strain overload sensor. The mechanism is explained and it is shown how it may be used to detect if a specified strain threshold or a range of different strains have been exceeded. The novel hybrid composite sensor may be used as an indicator patch cocured with composite components or retrofitted/bonded onto the surface of composite or metal components, or be incorporated as an integral load-carrying surface layer with sensing functionality.
GERGELY CZEL, MEISAM JALAVAND and KEVIN POTTER
Mechanical Properties of Hierarchical Discontinuous Composites
This work proposes bioÃ¢â‚¬â€œinspired hierarchical brickÃ¢â‚¬â€œandÃ¢â‚¬â€œmortar microstructures Ã¢â‚¬â€ where each Ã¢â‚¬Å“brickÃ¢â‚¬Â is itself composed of smaller platelets regularly staggered in a matrix Ã¢â‚¬â€to add damage tolerance to conventionally brittle composites. Finite element simulations on such geometries are performed, using cohesive elements to model damage in the matrix; models with multiple repetitions of unitÃ¢â‚¬â€œcells are also used to evaluate damage dispersion at a macroscopic scale. Finally, hierarchical brickÃ¢â‚¬â€œandÃ¢â‚¬â€œmortar specimens are manufactured and tested, and showed good agreement with the model predictions. It is shown that optimised hierarchical brickÃ¢â‚¬â€œandÃ¢â‚¬â€œmortar architectures create a markedly non-linear material response, with extensive energy dissipation before failure, and also delay damage localisation in larger structures.
Characterization Inspired Manufacturing of Carbon Fiber Composites Considering Multiscale Response
A characterization technique for carbon fiber at its single filament level using a highly precise nano-tensile testing data is presented. Using a continuous stiffness method (CSM), instantaneous moduli of single carbon fibers (~7 micron diameter) are acquired as a function of global deformation by applying small sinusoidal harmonic oscillations to a single carbon fiber being deformed in tension. This provides insight on response of small diameter carbon fibers with unprecedented resolution to precisely evaluate the non-linear stiffening response with increasing tensile strain. Precise estimates of modulus and its dependency on strain for single carbon fibers is essential for optimizing multi-scale response of carbon fiber reinforced composites. The rate of increase of tensile modulus with axial strain amplitude will help form a basis to relate the heterogeneity and microstructure of single carbon fibers with its mechanical properties. Microstructure of carbon fibers is characterized by using Wide Angle X-ray Diffraction and Small Angle X-ray Scattering in addition to electron microscopy (SEM and TEM). The current industry norm for specifying carbon fiber properties is based on tensile testing of a composite of unidirectional tow of fibers optimally aligned and tensioned to obtain the tensile modulus and failure strength. This paper will address the role of tow specimen preparation procedure, resulting microstructure for a manual manufacturing processes employed, and implications to the estimation of translation for predicting the upper bound properties of carbon fiber composites. This part of the study will elucidate the role of characterization to further optimize mechanical properties in a composite form considering the linear density and fiber density of carbon fibers. As an application example, we will consider results associated with the multi-scale response of a pultruded carbon fiber plate considering single fiber mechanical testing, interface/interphase with resin system, alignment and tensioning during manufacturing for an epoxy resin system.
JOSHUA CRABTREE, MATTHEW KANT, STEPHEN YOUNG and NATHAN MEEK
Cure Monitoring of Carbon Fiber Reinforced Composite via Laser Vibrometry
Effective in-line cure monitoring is crucial for quality and cost optimization of carbon fiber reinforced (CFR) composites manufacturing. The capability to ensure high quality and low cost production of CFR composites through improved monitoring methods will make the material more accessible to various industries. In the present study, 3D scanning laser Doppler vibrometry (LDV) is used as a means of remote, non-invasive monitoring of the degree of cure in unidirectional CFR epoxy prepreg tape. Analysis of the surface velocity of the test samples shows the changes in vibration characteristics that relate to the viscoelastic properties of the material. These properties are directly affected by phase transitions that occur as the sample cures. Differential scanning calorimetry (DSC) is used to develop a baseline model against which the laser vibrometry data are analyzed. The presented findings will be leveraged in further development of real-time cure monitoring technologies.
A Special Case of the Brinkman Equation for 2D Flow in Composite Laminates
A composite fluid flow equation is developed as a special case of the Brinkman equation where only squeeze flow is considered transverse to the fibers and only Darcy flow is considered along the fiber direction. The model is tested at several different layups to observe the flow behavior and the percentages of squeeze and Darcy flow are compared as a function of lamina thickness, aspect ratio, and fiber volume fraction. Darcy flow contribution is shown to be greatest at low aspect ratio, low thickness, and low fiber volume fraction.
Automated Data Analysis Algorithms for Ultrasonic Nondestructive Evaluation of Complex Composite Panels
To address the data review burden and improve the reliability of the ultrasonic (UT) inspection of large composite structures, automated data analysis (ADA) algorithms have been developed to make calls on indications that satisfy the call criteria. Certain complex composite structures with varying shape, thickness transitions, front-wall and back-wall curvature, and the presence of bonds can greatly complicate this interpretation process and produce false calls. In this effort, enhancements to the automated data analysis algorithms are introduced to address these challenges for complex composite panels. First, the thickness of the part and presence of bonds is estimated by tracking potential backwall signals, detecting the presence of multiple signals and step changes which are indicators of bonded sections, and through the application of smart spatial filters for estimating the panel thickness and additional bonded sections with varying signal levels. Once part boundaries, thickness transitions and bonded regions are identified, feature extraction algorithms are applied to multiple sets of through-thickness and backwall C-scan images, for evaluation of both first layer through thickness and layers under bonds. To verify the algorithm performance, a set of test data was selected to challenge the ADA algorithms that includes a wide range of complex parts and artificial defects located both above and below bond lines. Results are presented for a variety of test specimens that include inserted materials and discontinuities produced under poor manufacturing conditions, demonstrating the desired detection capability while minimizing false indication calls to a manageable level.
A Roadmap to Account for Potential Uncertainties in Non-Destructive Testing during Structural Health Monitoring of Composites
In the context of structural health monitoring (SHM) applications, this article first presents an example of damage detection uncertainty in a typical fiberglass woven thermoplastic plate structure with pre-embedded disbonds of various sizes and locations. A Taguchi experimental design has been implemented to statistically assess the significance of uncertainty in the performed ultrasonic NDT tests. Next, a Gaussian Process (GP)-based SHM framework has been suggested to account for such possible uncertainties in training datasets of SHM applications, and hence arriving at more robust damage systems.
D. FREY, R. SEETHALER, J. RAMKUMAR, B. CRAWFORD, H. HEIMOURI, F. ISLAM and P. PAL
Numerical Simulation of Induction Thermography on a Laminated Composite Panel
A three-dimensional finite element model was developed for simulating induction heating of a composite panel using the COMSOL multiphysics software, version 5.1. Equivalent anisotropic material thermal and electrical conductivities of the composite panel were used in the simulation. The model was validated using experimental temperatures obtained from an infrared camera. Good agreement was obtained between the experimental and numerical results for a pristine panel and a specific flawed panel. Then, this methodology was used to simulate the induction heating of a panel within different flaw scenarios. The correlation between the flaw scenario and temperature distribution was investigated. Flaws led to high gradients in the temperature distributions. The numerical results suggest that temperature variation on the panel coil side (outer) surface could be used to detect some types of flaws. In addition, due to low thermal conductivity, the induction heating period should be carefully controlled to avoid potential material degradation caused by overheating when using this thermography inspection technique.