American Society of Composites - 30th Technical Conference
A Shell-Beam Modeling Method for Crash Simulation of Thin-Walled Composite Tubes
There are extensive studies tried to model composite material tubes under axial impact. However, very few of them is able to predict both failure morphologies and force-displacement responses of a crushing tube. In order to solve this problem, this study proposes a new modeling technique, the shell-beam method. Essentially, this method enhances shell elements with real out-of-plane properties with beam elements. Both one element tests and tube crash tests are performed to fully evaluate this method. An in-house material model, Enhanced Continuum Damage Mechanics (ECDM) model, is used in this study. It is proved that the shell-beam method is very effective in terms of conquering the defects, like instability, of some traditional elements.
Characteristics of Thermoplastic Composite Tube Under Axial Loading
Research into the design of lightweight automobile is driven by the need to reduce fuel consumption, to preserve dwindling hydrocarbon resources without compromising other attributes such as safety, performance, recyclability and costs. The automobile industry is seeing an increased need for the application of plastics and their derivatives in various forms, such as fiber reinforced plastics. These materials are used in the design and manufacture of various structural components, to reduce weight, cost and to improve fuel efficiency. A lot of effort is being directed at the development of structural plastics, to meet specific automotive requirements such as stiffness, safety, strength, durability and environmental standards like the recyclability. Crashworthiness of the automotive parts plays a key role in the safety of the vehicles. The use of thermoplastic composite materials in the automotive components can provide the advantages of weight, cost reduction and recyclability, in addition to eco-efficiency and renewability, compared to conventional materials. This paper presents an experimental study on quasi-static and dynamic axial crushing of composite circular tubes with various geometric characteristics; such structures were fabricated with a thermoplastic material both for the fibers and for the matrix. The data were recorded and analyzed in terms of load-displacement curves, specific energy absorption (SEA), crush force efficiency (CFE), stroke efficiency (SE) and crushing stress. The results obtained with the thermoplastic specimens were compared with those obtained with traditional thermoset. The failure mechanisms, obtained varying the geometric parameters and the boundary conditions, will be also presented and discussed.
Axial Crush and Bending Collapse Analysis of Non-Crimped Fabric Composite Structures
In this paper, the investigation of the energy absorption capability of non-crimped carbon fiber composite tubes in axial compression and bending will be documented. The tubular beams were comprised of an eight layer (0/-45/45/90/90/45/-45/0) unidirectional carbon fiber architecture manufactured using a compression molding process. The experiments for axial compression and bending were conducted using drop tower and Instron three-point bend fixtures, respectively. The COmplete STress Reduction (COSTR) damage model, developed for predicting intra-layer failures, was extended to predict the response under axial crush and bending collapse through a user written subroutine developed in the framework of the LS-DYNA computer program. Inter-layer cohesion and failure were modeled using a computationally efficient cohesive zone model governed by a bi-linear traction-separation law with a quadratic mixed mode delamination criterion and damage formulation. The comparison of numerical and experimental results for the crush and bending tests are also presented and show good correlation.
Comparative Composite Fatigue Damage Models for Life Prediction of Laminated Composite Structures
Two approaches have been compared in this paper for the failure prediction of composite laminate. The first approach is based on a progressive damage model, with the application of LaRC04 as the damage initiation criterion and softening curves to describe damage evolution. Failure strain is calculated elementwise based on a mesh characteristic length in order to ensure its energy dissipation independence of the mesh size. The fatigue damage is evaluated based on stress states and S-N curves, which will be added to the damage calculated from the static balance analysis. The second approach is a hybrid continuum damage and discrete crack representation of the composite matrix failure. While the fatigue damage is computed the same way as in the first approach,, a phantom pair element technique is applied to explicitly characterize the matrix cracking induced displacement discontinuities but the fiber failure is still simulated via a continuum damage approach. Also, the delamination in the second approach is modeled through the element stiffness degradation.
A Fatigue Damage Model for Failure Analysis of Single-lap Multi-bolt Joints
As various kinds of composite connections are widely used in aircraft and space structures, the connection areas are generally the critical places in structures. In this article, the aim is to develop a damage model to study the fatigue behavior of fibrereinforced (FR) composite single-lap multi-bolt joints. In order to accurately simulate the fatigue evolution of composite bolted joints (CBJs), a user-defined three dimensional (3D) damage model was implemented in the commercial implicit finite element solver, ABAQUS, taking into account of material degradation and fatigue damage accumulation. S-N curves are calibrated against fatigue tests respectively in longitudinal, transverse and shear directions of laminates basing on Shokrieh’s empirical model which takes stress ratios into consideration. A two-parameter combined degradation model of the residual stiffness and residual strength is achieved from calibration of maximum stress ( max σ ) and fatigue life ratio ( n / N ). To accurately predict the stress-strain loops during each single cycle, a new intra-laminar exponential plasticity model including the coupling effects of shear and transverse stress responses is incorporated into present fatigue damage model, this plasticity model is capable to predict the nonlinear responses of shear and transverse stresses. Additionally, with the implementation of irreversible cyclic strain accumulation and Puck’s multi-axial failure criteria for the inter-fibre failure, the fatigue damage model successfully simulated the growth of hole elongation due to material degradation, bearing damage and unrecoverable deformation. Validation across a range of stress levels for single-lap CBJs indicates that the fatigue damage model proposed in this article is capable to predict the failure, fatigue life and damage evolution of single-lap CBJs.
Closed-form Solutions for Cohesive Zone Modeling
Closed-form solutions for cohesive zone modeling within the framework of classical lamination theory (CLT), for the popular mode I double cantilever beam (DCB) test and the mode II end notched flexure (ENF) test are provided in this paper. Zerothickness virtual multi-linear traction-separation laws are introduced to simulate the cohesive interactions between potential crack surfaces. The problem is formulated by solution forms corresponding to each segment of the cohesive laws and using appropriate boundary and continuity conditions. Detailed algorithms for each case is provided to clearly show the influence of the cohesive zone law on the ensuing crack growth. Comprehensive parametric studies are performed on the crack growth response and the process zone length, revealing their relations to the delamination length, cohesive parameters and the shape of the traction-separation laws. With applicability to general multilayered structures, and the capability to incorporate arbitrary, piecewise linear cohesive constitutive laws, the closed-form solutions can serve as a tool to determine the accuracy of numerical solutions used in conjunction with the finite element simulations.
Hypervelocity Impacts of Shear Thickening Fluid Imbibed Metallic Foam Core Sandwich Panels
Hypervelocity impact (HVI) experiments were carried out on two configurations of open cell foam core sandwich composite panels both infused with a shear thickening fluid (STF). The first configuration consisted of an aluminum facesheet, 1.27 cm thick 6.35 pores per cm open cell aluminum foam core, and a rear aluminum facesheet. The second configuration consisted of an aluminum facesheet, 1.27 cm thick aluminum honeycomb core, an intermediate aluminum sheet, 0.64 cm thick 6.35 pores per cm open cell aluminum foam core, and a rear aluminum facesheet. The open cell foam core of each configuration was infused with a STF consisting of 0.225 mass fraction of Aerosil 200 fumed silica in 200 molecular weight polyethylene glycol (PEG). HVI experiments were conducted for two specimens of each configuration employing a 1 mm diameter aluminum projectile and impact velocities of approximately 3-5 km/s. For comparison, the HVI experiments were repeated using additional specimens where the open cell foam cores were infused with the STF’s liquid phase only, PEG. All specimens regardless of configuration or imbibed fluid prevented penetration of the rear facesheet. However, the STF infused core of the second configuration was not perforated. For the first configuration, in addition to the projectile entrance holes, the front facesheet sustained out of plane deformation local to the point of impact, and the amount of deformation was similar for the PEG and STF infused specimens. Additionally, the PEG infused specimens sustained a small amount of out of plane deformation of the rear facesheet, while the STF infused specimens did not. Non-destructive computed tomography (CT) scans of the impacted specimens were conducted to investigate the internal damage. Comparisons of the CT scans indicated that the PEG infused specimens sustained a slightly greater volume of core damage than the STF infused specimens.
Impact Testing and Simulation of a Sinusoid Foam Sandwich Energy Absorber
A sinusoidal-shaped foam sandwich energy absorber was developed and evaluated at NASA Langley Research Center through multi-level testing and simulation performed under the Transport Rotorcraft Airframe Crash Testbed (TRACT) research project. The energy absorber, designated the “sinusoid,â€ consisted of hybrid carbon- Kevlar® plain weave fabric face sheets, two layers for each face sheet oriented at ±45° with respect to the vertical or crush direction, and a closed-cell ELFOAM® P200 polyisocyanurate (2.0-lb/ft3) foam core. The design goal for the energy absorber was to achieve an average floor-level acceleration of between 25- and 40-g during the fullscale crash test of a retrofitted CH-46E helicopter airframe, designated TRACT 2. Variations in the design were assessed through quasi-static and dynamic crush testing of component specimens. Once the design was finalized, a 5-ft-long subfloor beam was fabricated and retrofitted into a barrel section of a CH-46E helicopter. A vertical drop test of the barrel section was conducted onto concrete to evaluate the performance of the energy absorber prior to retrofit into TRACT 2. Finite element models were developed of all test articles and simulations were performed using LSDYNA ®, a commercial nonlinear explicit transient dynamic finite element code. Testanalysis results are presented for the sinusoid foam sandwich energy absorber as comparisons of load-displacement and acceleration-time-history responses, as well as predicted and experimental structural deformations and progressive damage for each evaluation level (component testing through barrel section drop testing).
Internal Impact Damage to Lightweight Composite Sandwich Panels
Impact damage developing within lightweight Nomex® honeycomb core sandwich panels were investigated when subjected to impacts by low velocity metaltips, and high speed ice spheres representing hail ice. The metal tips had radius ranging from 12.7 to 76.2 mm, whereas all ice spheres were 50.8 mm. Of interest is the observation of damage modes and severity formed by impacts of varying radius for metal tips, and of varying glancing angle for ice spheres, with particular focus on the severity of internal damage modes to the core vs. external visibility. For the low velocity metal tip impacts, four modes of internal core damage were identified (initial slight wrinkling of core walls, visible wrinkling, buckling/kinking, and core wall fracture). Damage depth within the core was radius-dependent, but stabilized at constant depth independent of impact energy. Internal damage span increased with increasing impact energy, but not with increasing tip radius. Fractured core typically corresponded with readily-visible dents, when viewed from the external surface. For the high speed glancing ice impacts, the four modes of core damage were also observed. However, impact damage tended to either cause face sheet rupture (readily visible), or leave a very shallow surface dent (often non-visible) while also causing significant internal core crushing and even core fracture. Cases of core fracture with no external visibility present a safety concern, as subsequent separation of the facesheet can occur under specific loading conditions (e.g., facesheet in compression or in-plane shear, core cell pressurization as aircraft climbs to high altitude).
Autonomic Cooling of Composites Using Transpiration
In nature, plants use transpiration to transport water against gravity from the soil to their leaves, powering the flow of nutrients throughout the plant. During transpiration, water evaporates from the leaves (i.e. porous network) and is replenished by the xylem (i.e. vascular channels) from the water stored in the soil (i.e. reservoir). In our synthetic system, autonomic cooling of a structural composite is accomplished using a leaf inspired vascularized coating. In response to an external heat load, evaporation occurs from pores in the coating to remove heat. The fluid is then replaced by capillary action from an externally located reservoir, allowing for continuous operation. Experiments demonstrate the cooling performance of the system, its ability to self-start and stop, and the ability to remotely locate the reservoir.
Nonlinear Deformations of Smart Plates Under Electro-Mechanical Actuations
This work presents a three-dimensional nonlinear analysis of a flexible smart thin plate consisting of a thin elastic plate and arbitrary numbers of thin piezoelectric patches distributed and bonded on the top and bottom surfaces of the plate. The patches are perfectly bonded to the elastic plate and their thicknesses are negligible compared to the thickness of the plate. Electric fields are prescribed to the piezoelectric patches, which induce axial forces and bending moments to the plate. These internal forces are determined by imposing equilibrium equations and displacement continuity conditions at the interfaces between the piezoelectric patches and elastic plate. Since the overall planar dimension of the plate is much larger than its thickness (thin plates), the deformed structure can undergo large rotations but maintain relatively small strains. Finite elements method (FEM) based on co-rotational Lagrangian description, is used for numerically determining the deformations of the active plate. Since, in practice, large deformation/displacement of the smart systems under electric actuation often requires large electric field inputs, a nonlinear electromechanical coupling constitutive model is considered for the piezoelectric material. As shown through a few examples, by varying number, location and/or size of the piezoelectric actuators or changing magnitude of the electric field inputs, it is possible to obtain desired shape configurations for the active flexible plate.
Effect of Nanoclay Addition on Fatigue Behavior of Glass/Epoxy Tapered Laminates
Tapered laminates have been used in many engineering applications such as composite yokes to hold the blades in helicopters, wing structures etc. The tapering configuration requires cut fibers along the length of the laminate. This provides flexibility. However this also creates stress concentration which can accelerate fatigue crack propagation. One possibility to improve the performance of this type of structure is to modify the epoxy matrix with nanoclay. In our previous work, it was shown that addition of clay can improve the fatigue performance of regular laminates of glass/epoxy. In this work, epoxy modified with nanoclay was incorporated into glass fibers and tapered laminates were made using these materials. Three different types of laminate stacking sequence were investigated. Tension-tension fatigue tests were performed. The results show that significant improvement in fatigue lives was indeed obtained. The incorporation of nanoclay seems to speed up the delamination at the initial stage of the fatigue. However the rate of crack propagation is slower subsequently. The net result is longer fatigue live to reach a certain crack length. It is believed that the improvement is due to the enhancement in mode II fracture toughness. Stacking sequence also has an effect on rate of fatigue crack propagation.
Environmental Degradation of Nano Enhanced Polymers in Low Earth Orbit
Satellites and space shuttles face severe environmental conditions like ultraviolet radiation and orbital debris when placed in low earth orbit. These harsh conditions could have a direct effect on spacecraft’s materials. Since polymers are used extensively in this application, due to its unique properties, it is important to investigate the methods to protect these materials against harsh environment. One of the most degrading effects that polymers have to withstand is Atomic Oxygen (AO) which is the main degrading component in low earth orbit (LEO). The atmosphere in LEO is comprised of 96% atomic oxygen and orbits the earth at an approximate speed of 8 km/s which makes AO particles collide with a kinetic energy of approximately 5 eV, this energy is enough to facilitate the degradation of Kapton® polymer. In this paper, different polyimide-based nanocomposite materials are studied. Nano particles are used to enhance their performance against AO. Effect of glassification (plasma treatment is evaluated). Seven samples of the different chemical and composition and different plasma treatment levels (with or without) were subjected to AO erosion at Marshal Space Flight Center (MSFC). Effect of AO on the mass change and topography are investigated. Results show that the incorporation of polyhedral oligomeric silsesquioxane (POSS) substantially enhanced the resistance against AO attack.
Carbon Based Hybrid Nanostructures for High Performance Composites: The Surfactant Investigation
The addition of nanofillers has been shown an alternative to improve the fiber/matrix interaction and the epoxy matrix mechanical properties. Many studies have reported the addition of carbon based nanostructures, i.e. carbon nanotubes (CNT) and graphene, as nanofillers in different composite materials, with an increase on its mechanical properties. Nevertheless, the individual nanostructures have the tendency to agglomerate when the energy provided by the dispersion process, such as sonication, high shear mixing, etc., is removed. The use of surfactants seems to be a promising route to overcome this problem. This research goal is to investigate the surfactant effects on carbon/epoxy composites nano-modified by graphene and carbon nanotubes. Multi-layered graphene and multiwall carbon nanotubes were dispersed with two different surfactants, one ionic and another non-ionic, i.e. Sodium dodecylbenzenesulfonate (SDBS) and Polyoxyethylenenonylphenyl ether (IGEPAL® CO-890). The nanoscale analysis was based on morphological/nature observations via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while the macro analysis follows the mechanical characterization by tensile tests (ASTM D 3039) and three point bending test (ASTM D 790).
Thermal Evaluation of Epoxy Resin Using n-hexadecane Based Shape Stabilized PCM for Energy Saving in Building
Epoxy resin has excellent characteristics of moisture, low toughness, solvent and chemical resistance, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. In this experiment, we prepared epoxy resin with shape stabilized PCM, to enhance the thermal properties of epoxy resin. The SSPCM (shape stabilized PCM) was prepared through the vacuum impregnation method, and the SSPCM/epoxy resin composites were prepared through the shear stirring process and curing process. In the preparation process, the epoxy resin and hardener were mixed in a beaker at a one-to-one ratio. Then, 5wt%, 10wt%, 15wt% and 20wt% of the SSPCM was added to the mixture. The thermal properties of epoxy resin with SSPCM were analyzed from DSC, TGA and UTM analyzer. From the analysis, we determined that the prepared epoxy resin with SSPCM has heat storage capacity and high thermal conductivity, compared with the epoxy resin.
High Temperature Nanocharacterization of Al/SiC Laminated Composite
Metal-ceramic laminated composites exhibit attractive mechanical properties including high strength, high toughness, and damage tolerance. However, there is very little information available about their behavior at elevated temperatures due to the limitations of macro scale high temperature characterization techniques. In this study, the hardness and elastic modulus of aluminum/silicon carbide (Al/SiC) nanolaminates have been investigated within the temperature range of 23 to 450ºC using isothermal hot stage nanoindentation. Alternating layers of aluminum and silicon carbide -each with an average thickness of 20 and 22nm, respectively- were deposited on a silicon wafer, Si (111) substrate using a combination of DC and RF magnetron sputtering techniques. The hot stage nanoindentation tests were carried out using thermally stable cubic boron nitride Berkovich nanoindenter. Both the sample and the indenter were heated separately during the experiments to temperatures of 150, 300, and 450ºC while the nanoindentation chamber was purged with argon gas to retain oxygen level below 1.5% to deter oxidation of the sample. The cross section of the sample was also examined by scanning electron microscopy (SEM). The experimental results revealed that the hardness decreased with increase of temperature up to 300ºC. However, the elastic modulus increased up to at 150ºC after then it consistently declined with elevating the temperature. Interestingly, slight increase of the hardness was observed at 450ºC. Moreover, the plastic deformation of the sample at this temperature was smaller than that observed at 300ºC. This behavior is likely due to microstructural changes of the Al/SiC nano-films following a chemical reaction between thin layers at temperatures beyond 300ºC.
Manufacturing Process Induced Effects on the Strength of Carbon Fiber-Epoxy Composites
Fiber-reinforced polymer matrix composites (FRPCs) are light-weight materials widely used in many engineering fields; their high-strength and light weight make them perfect candidates for aerospace applications. During manufacture of FRPCs, fibers and non-cured resin placed in a mold are subjected to a prescribed temperature cycle. The chemical reaction during cure is highly exothermic and thermal stresses develop. During this process, as the matrix hardens, it shrinks due to the formation of polymer networks and crosslinks, causing additional selfequilibrating mechanical stresses. Depending on the severity of cure shrinkage and thermal stresses, the material can degrade during the cure process. Determining the effect of curing is important to determine the strength and to improve the design of composite structures. In the present work, a network curing model is proposed. The crack band model of Bazant-Oh (CBBO) is used to include the possibility of damage and failure during curing and to estimate the actual strength of the structure. Analyses are conducted by performing virtual tests at the micro-level on the virtually cured RVEs with periodic boundary conditions (PBC) enforced. The analysis is divided in three steps; a thermochemical analysis is performed to compute the degree of cure and the cure rate in the matrix for a given cure temperature profile (Step 1). Then, the self-equilibrating stress that builds up during the process is computed (Step 2). At the end of the curing cycle, the RVE strengths are determined by subjecting the virtually cured model to mechanical loading (Step 3). In this paper, results related to the effects of cure on the RVE shear strengths, S12 and S23, will be presented.
Failure Modes and Strength of Composite Box Beam Structures
This paper presents a study conducted to investigate potential failure modes and strength of the main structural element of a composite wind turbine blade, i.e., the box beam formed with blade spar caps and shear webs. The box beam, a critical element of the wind turbine blade, is usually constructed by adhesively bonding the blade shear webs onto the blade spar caps. Several composite box beam specimens were fabricated and their structural integrities were evaluated using 3-point and 4-point bending tests. The blade spar caps and shear webs were fabricated with glass fabric composites using a vacuum assisted resin infusion molding (VARIM) process. They were bonded together with an epoxy-based adhesive. Box beams with different geometries were designed, fabricated and tested. Bending tests of the composite box beams were conducted using an in-house designed and constructed servo-hydraulic test system with a loading capacity of 25 kips and a displacement range of ±4 inches. Based on the composite structural model and numerical simulations of the beam deformation, strain gages were placed on strategic locations to measure the strains during the bending tests. LVDTs were also used to monitor the composite beam deflections. Strain gage and LVDT signals as well as loads and actuator position were recorded, using an in-house built computer data acquisition system. The bending tests were run under a displacement-controlled mode in order to capture damage initiation, propagation and the final failure of the box beam. After the tests, damage and failure modes were examined macroscopically. Finite element analyses were conducted to determine the strength and buckling behavior of the composite box beams and to evaluate the effect of debonding between the spar caps and the shear webs on box beam structural performance. Correlation between bending test results and numerical simulations were obtained.
Fatigue Life Prediction in Composite Wind Turbine Blades: Effects of Variable Wind Load and Non-proportional Multi-axial Stress States
Conventional fatigue analysis of composite wind turbine blades has applied a fixed wind load distribution that does not fully capture the effect of variable wind load on fatigue life prediction. Another simplification in the conventional fatigue analysis is that the blades are often treated as a typical beam-like structure in which fatigue life calculation is limited in considering the normal stress component in the beam axis direction. Consequently, the effect of non-proportional multi-axial complex stress states on blade fatigue life prediction has been ignored. In order to study the fatigue effects of variable wind load and non-proportional multi-axial stress states, a comprehensive fatigue analysis that includes variable wind load, wind field simulation and aerodynamic analysis, stress analysis by finite element analysis, and fatigue damage evaluation based on tested fatigue data has been developed for composite wind turbine blades. The variable wind load is represented by a joint distribution of 10-minute mean wind speed and 10-minute turbulence intensity. The non-proportional multi-axial complex stress states are involved when calculating 10-minute fatigue damage of section points through laminate thickness. The annual fatigue damage is calculated based on the 10-minute fatigue damage and the joint distribution. Consequently, the blade fatigue effects due to both the variable wind load in a large spatiotemporal range and the non-proportional multi-axial complex stress states can be investigated. The case study reveals that the variable wind load has a significant influence on the fatigue life of composite wind turbine blades. It is also shown that neglecting transverse normal and shear stresses in the blade fatigue analysis could lead to substantially overestimated blade fatigue life.
Lightning-Strike-Induced Heat Transfer in Glass Fiber Polymer Matrix Composite Blades
The extent of thermal damage in the glass fiber reinforced composite blade is highly dependent on the severity of the lightning current, which also determines the mechanism of lightning-strike-induced heat transfer in the composite. Lightning attachment at the electrically non-conductive glass fiber reinforced polymer (GFRP) composite surface may come as a direct heat injection. However, if the electric field at the attachment point is sufficient, the breakdown of the GFRP composite occurs. In this case, in addition to the direct heat injection at the surface, Joule heating will be produced in the newly conductive glass fiber reinforced composite. Therefore, it is crucial to determine whether dielectric breakdown can occur prior to formulating the problem. In this work, lightning-strike induced heat transfer in a GFRP composite wind turbine blade is analyzed. First, several models are employed to estimate the electric field in the vicinity of a wind turbine blade to determine if dielectric breakdown occurs. The wind turbine blade under consideration is the Sandia 100-meter All-glass Baseline Wind Turbine Blade (SNL 100-00). Results show that the dielectric breakdown of the composite blade is not likely to occur for a lightning stepped leader under three lightning protection levels (LPL I, LPL II and LPL III). Therefore, only the direct heat injection problem at the composite blade surface is considered, and the nonlinear transient heat transfer problem for a layered temperature-dependent GFRP composite panel is solved using finite element analysis.
Thermoforming Process Optimization for Reduction of Inter-Ply Friction of Out-of-Autoclave Material
Thermoforming of composite materials includes deforming the flat sheets of prepreg laminates into complex shaped composite parts. This deformation of flat laminates into complex shapes can cause wrinkles in the final part. External processing conditions of forming process must be chosen in a way to minimize wrinkles. Taguchi method is the statistical way to optimize the processing conditions and improve the quality of final product. In this paper, a numerical approach is described to find the optimum processing parameters in thermoforming of composites. The three main processing parameters are temperature, normal pressure and forming rate. Three levels were selected for each processing parameter. A test rig was designed and manufactured to perform experiments. This test rig can simulate the conditions of different temperatures and normal pressures. It was installed on a tensile testing machine which can simulate different forming rates by pulling the prepreg plies at different speeds. Load cell mounted on the tensile testing machine was used to measure frictional force between plies. An appropriate orthogonal array was selected and experiments were performed according to the combinations of process parameters in the array. After performing the experiments, inter-ply friction was analyzed. Using smaller-the-better formula Signal to Noise ratio was calculated. Analysis of Variance was used to find the contribution of each parameter. It was observed that optimum processing conditions for lowest inter-ply friction are high temperature, low normal pressure and low forming rate. Temperature appeared as the most influencing processing parameter.
Residual Cure Stress Buildup in Thermosetting Composite: Modeling and Experimental Validation
The evolution of resin properties was measured using differential scanning calorimetry for cure rate characterization, digital image correlation for measuring coefficients of chemical shrinkage and thermal expansion and dynamic mechanical analysis for storage modulus. These experiments provided necessary information for deriving homogenized composite properties during the cure cycle, which were implemented into a thermo-mechanical constitutive model of the composite using finite elements. To observe residual stress development in the thermosetting composite, a bi-lamina strip configuration that consisted of [0/904] laminate stacking sequence was used. The proposed experimental procedure allowed separating the contribution of the chemical and thermal shrinkage to the constrained deformation inside of the composite. Modeling of bi-lamina strip during the cure cycle predicted the evolution of the deflection profiles of the sample during cure, which agreed closely with the experimentally observed deflection profiles.
A Novel Through-Thickness Reinforcement Method of Cured Composites
A through-thickness reinforcement method for a fully cured carbon/epoxy composite laminate is proposed and characterized. The method consists of embedding the interlaminar rods into the holes drilled in the cured laminate with further bonding the rods to the laminate with an epoxy resin. Such a technique has the same rod/laminate stress transfer mechanism responsible for the improvement in laminate fracture resistance as the conventional z-pinning. The composite resulting from the proposed interlaminar reinforcement technique possesses the benefits of a uniform surface quality and consolidation of the original unreinforced laminate. The technique was found to be highly effective in suppressing the damage propagation in DCB test specimens under mode I loading conditions. It was experimentally shown with DCB specimens in the paper that increased aspect ratio (length to diameter ratio, AR) of through-thickness rods significantly elevates mode I maximum delamination fracture resistance of a laminate. This effect is explained through the analysis of rod/laminate interfacial stress transfer.
Polymer Derived In-Situ Nano Metal Matrix Composites Obtained by Friction Stir Processing
Friction Stir Processing (FSP) is a solid state technique used for material processing. Tool wear and the agglomeration of ceramic particles have been serious issues in FSP of metal matrix composites. In the present study, FSP has been employed to disperse the nanoscale particles of a polymer-derived silicon carbonitride (SiCN) ceramic phase into copper by an in-situ process. SiCN cross linked polymer particles were incorporated using multi-pass FSP into pure copper to form bulk particulate metal matrix composites. The polymer was then converted into ceramic through an in-situ pyrolysis process and dispersed by FSP. Multi-pass processing was carried out to remove porosity from the samples and also for the uniform dispersion of polymer derived ceramic particles. Microstructural observations were carried out using Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) of the composite. The results indicate a uniform distribution of ~100 nm size particles of the ceramic phase in the copper matrix after FSP. The nanocomposite exhibits a five fold increase in microhardness (260HV100) which is attributed to the nano scale dispersion of ceramic particles. Tensile properties of the produced metal matrix composite were significantly higher than processed plain specimen and base metal.
Progressive Composite Damage Modeling in LS-DYNA Using MAT162: Part A—Properties and Parameters
Rate dependent progressive composite damage model MAT162 in LS-DYNA is the state-of-the-art in composite damage modeling in 3D. Two progressive damage modules for Uni-directional (UD) and woven fabric composites (FABRIC) are available in MAT162. These two damage models have been used to develop mesomechanical RVE of 3D woven composites and very thick composite structures in modeling damage at both meso- and macro-mechanical RVE levels. While the fundamental elastic and strength parameters can be determined via standard and nonstandard mechanical testing, in modeling the semi-empirical erosion of finite volume elements to allow penetration, model validating experiments need to be simulated to determine the necessary modeling parameters required for MAT162. Low velocity impact (LVI), ballistic ô€œ¸ô€¬¹ô€¬´ testing, and depth of penetration (DOP) experiments have been proven sufficient in determining the modeling parameters. This two part paper will present the methodology of determining the MAT162 properties and parameters by simulating LVI, ô€œ¸ô€¬¹ô€¬´, and DOP experiments. Part A will focus on describing the properties & parameters needed for MAT162, and Part B will focus on parametric determination of modeling parameters simulating model validating experiments.
Progressive Composite Damage Modeling in LS-DYNA Using MAT162: Part B—Model Validating Experiments
Simulating the Mixed-Mode Progressive Delamination in Composite Laminates
Delamination, or interlaminar debonding, is a commonly observed failure mechanism in composite laminates. The objective of this paper is to develop a 2D and a 3D numerical approach to simulating the double-cantilever beam (DCB), the endnotched flexure (ENF), and the mixed-mode bending (MMB) tests with a commercial software, Abaqus, and to seek for appropriate simplifications of the simulations. An explicit dynamic time integration scheme and a viscosity parameter is adopted to yield close predictions of the specimen responses. The load rate and the specimen mass density are properly selected to approximate a quasi-static loading process. A linear cohesive zone model is used to characterize the mechanical response of the interface. The validity of different strategies of simplifying the finite element model is discussed. The 2D and the 3D predictions are both found to fit the experimental results well, but the 2D predictions are found to be more accurate and stable than the 3D predictions. The 2D approach is found to better handle the mixed-mode progressive delamination in composite laminates satisfying plane stress condition. Although the present approaches are developed with Abaqus/Explicit, they are also amendable to other finite element codes.
Top Down Design Using Bayesian Network Classifiers for Composite Panels
Optimization of composite panels has historically been approached using bottom-up design techniques. Such strategies require evaluating a large number of designs and then exploring these solutions to better understand performance impacts. This is often time consuming and is not computationally suitable for more difficult problems. Top-down approaches have been recently introduced as a way of quickly zeroing in on desired performance characteristics and only evaluating designs that exist within that region. In this presentation Bayesian Network Classifiers (BNC) will be used to enable this top-down strategy. We will evaluate the effectiveness of this approach on a problem that has two loading scenarios - Tensile in the 1 direction and then compressive buckling in the 2 direction. 4, 8 and 16 ply laminates will be evaluated to examine the benefits and tradeoffs as problem complexity increases. This top-down approach will be compared to the bottom-up approach to evaluate the effectiveness of the BNC in the design process. Finally we will examine simple problems with tow steering and how this new method can be implemented into this framework.
Modeling to Predict Micro-scale Permeability for Fiber Reinforcement in Liquid Composite Molding
In liquid composite manufacturing, permeability is the driving process parameter for mold fills and is critical for understanding the infusion flow and pressure distribution that results. Permeability has been identified as a complex variable which can vary significantly in magnitude for similar test cases. Permeability has also been isolated at different levels because of the multi-scale nature of composite fiber reinforcement. In the micro-scale, fibers are formed into randomly aligned tows composed of thousands of fibers. A second scale, the meso-scale, considers the tow dimensions and weave parameters, but inputs a Darcy based permeability to make up for physical geometry variations. On the micro-scale, fibers are generally considered as ordered in some kind of idealized packing arrangement, for example hexagonal or square packing. This is not always realistic and defining permeability as a function of porosity alone may not be enough to achieve an accurate permeability prediction on the micro-scale. Here, we isolate the micro-scale structures of unidirectional fiber reinforcements and investigate flows across aligned fiber geometries and infusion characteristics during manufacturing. Reduced geometries are utilized to represent the fiber and matrix interactions during a liquid infusion. The overarching goal of this research is to use numerical tools to create better understanding of composite manufacturing processes. A systematic computational approach is utilized to understand how material packing changes the fluid flow regime during composite manufacturing, helping to select appropriate infusion parameters to produce a part. This approach incorporates a numerical modelling procedure to predict unidirectional permeability on the microscale. The results set up a baseline that compares well with analytical models as a function of fiber diameter and volume fraction. Results show that variations due to fiber packing can be identified independently of fiber volume fraction.
Manufacturing of Carbon/Epoxy IsoBeam Lattice Structures
This research demonstrated the fabrication of nominally 5 ft. (1.5 m) and 10 ft. (3 m) long carbon/epoxy IsoBeam™ structures, with a rectangular 5â€ (12.7 cm) wide by 10â€ (25.4 cm) deep cross-section. The IsoBeam structure is a composite lattice structure that is a geometric derivative of the IsoTruss® structure. IsoTruss composite structures have proven to be stiff, light-weight, and efficient in a variety of loadbearing applications. Modifying the geometry to yield a rectangular cross-section provides additional applications for these beams as structural elements in buildings, aircraft, vehicles, and other structures. Three IsoBeam structures were manufactured – two 5-foot (1.5 m) long beams and one 10-foot (3 m) long beam – with carbon/epoxy tows comprising Toray T700SC-12K-50C carbon fibers pre-impregnated with TCR Composites UF3369- 100 epoxy resin. An aluminum 3-dimensional modified pin-mandrel was designed and manufactured. The composite tows were positioned on the mandrel under tension using a combination of hand and machine filament winding to create the complex geometry of the IsoBeam structure (the complex IsoBeam geometry prohibits the filament-winding process from producing a full IsoBeam structure without hand intervention). Each member was circumferentially wrapped with overlapping 1 inch (2.5 cm) wide strips of Dunstone Hi-Shrink Tape (polyester) to consolidate the tows during the curing process. The results of this research indicate high quality manufacturing as evidenced by high volume fraction and low void ratio.
Development of Stitched Composite Structure for Advanced Aircraft
NASA has created the Environmentally Responsible Aviation Project to develop technologies which will reduce the impact of aviation on the environment. A critical aspect of this pursuit is the development of a lighter, more robust airframe that will enable the introduction of unconventional aircraft configurations. NASA and The Boeing Company are working together to develop a structural concept that is lightweight and an advancement beyond state-of-the-art composites. The Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) is an integrally stiffened panel design where elements are stitched together and designed to maintain residual load-carrying capabilities under a variety of damage scenarios. With the PRSEUS concept, through-the-thickness stitches are applied through dry fabric prior to resin infusion, and replace fasteners throughout each integral panel. Through-the-thickness reinforcement at discontinuities, such as along flange edges, has been shown to suppress delamination and turn cracks, which expands the design space and leads to lighter designs. The pultruded rod provides stiffening away from the more vulnerable skin surface and improves bending stiffness. A series of building blocks were evaluated to explore the fundamental assumptions related to the capability and advantages of PRSEUS panels. These building blocks addressed tension, compression, and pressure loading conditions. The emphasis of the development work has been to assess the loading capability, damage arrestment features, repairability, post-buckling behavior, and response of PRSEUS flat panels to out-ofplane pressure loading. The results of this building-block program from coupons through an 80%-scale pressure box have demonstrated the viability of a PRSEUS center body for the Hybrid Wing Body (HWB) transport aircraft. This development program shows that the PRSEUS benefits are also applicable to traditional tube-andwing aircraft, those of advanced configurations, and other structures where weight and through-the-thickness strength are design considerations. An overview of the development of PRSEUS technology for commercial transport aircraft is the subject of this paper.
Impact-induced Damage in Tri-axially Woven Composites
As the use of fiber composites extends to automotive applications, there is a need to characterize the composite properties at low strain rates, such as below 100/s. In performing such low strain rate tests, the commonly used split Hopkinson’s pressure bar for high strain rate investigations should become inadequate. A high-speed hydraulic testing machine may be suitable for the low strain rate testing. However, it is not necessary within the reach of general engineers due to its high cost. Based on a conventional drop weight impact tester, this study presents an affordable testing technique for characterizing a carbon composite at low strain rates up to 100/s. Experimental results from static testing and slip Hopkinson’s bar testing were also performed for comparisons.
Incorporation of Plasticity and Damage Into an Orthotropic Three-Dimensional Model with Tabulated Input Suitable for Use in Composite Impact Problems
The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. While there are several composite material models currently available within commercial transient dynamic finite element codes, several features have been identified as being lacking in the currently available material models that could increase the predictive capability of the impact simulations. A specific desired feature includes the incorporation of both plasticity and damage within the material model. Another desired feature includes using experimentally based tabulated stress-strain input to define the evolution of plasticity and damage as opposed to specifying discrete input parameters (such as modulus and strength) and employing analytical functions to track the response of the material. To begin to address these needs, a combined plasticity and damage model suitable for use with both solid and shell elements is being developed for implementation within the commercial code LS-DYNA. The plasticity model is based on extending the Tsai-Wu composite failure model into a strain-hardening based orthotropic plasticity model with a non-associative flow rule. The evolution of the yield surface is determined based on tabulated stress-strain curves in the various normal and shear directions and is tracked using the effective plastic strain. The effective plastic strain is computed by using the non-associative flow rule in combination with appropriate numerical methods. To compute the evolution of damage, a strain equivalent semi-coupled formulation is used, in which a load in one direction results in a stiffness reduction in multiple coordinate directions. A specific laminated composite is examined to demonstrate the process of characterizing and analyzing the response of a composite using the developed model.
Characterizing Composites at Low Strain Rates
As the use of fiber-reinforced polymer-matrix composites gains momentum in automotive designs, there is a need to characterize the composite properties at low strain rates, such as below 100/s. In performing such low strain rate tests, the commonly used split Hopkinson’s pressure bar for high strain rates should become overqualified. A high-speed hydraulic testing machine may be suitable for the low strain rate testing. However, due to its high cost, it is not necessarily available to general laboratories. Based on a conventional drop weight impact tester (DWIT), this study presents an affordable testing technique for characterizing a glass woven composite at low strain rates, such as up to 100/s.
Impact Crush Modeling of Chopped Fiber Reinforced Polymers
Chopped fibers reinforced polymer composites are considered for use in many automotive applications. 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 Multi-Scale Material Characterization of composite system comprising of: a) chopped fiber based on Eshelby and Mori Tanaka failure theory, b) micro-macro mechanics, and damage failure theory, c) tensor stiffness averaging technique; and d) optimizer as part of the durability analysis software GENOA. The material model established in MCQ-Chopped is used in FEA interface with GENOA 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, 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 (MS-PFDA) - of Toyota provided 3D model of tube structure. MS-PFDA-LS-DYNA Crush worthiness analysis was performed to predict the damage and fracture evolution process. The simulation results were compared with Toyota 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.
Hysteresis and Time-Dependent Effects on the Coupled Mechanical-Electrical Response of Unconstrained Carbon Nanotube Yarns Subjected to Uniaxial Tensile Loading
Carbon nanotube (CNTs) yarns or fibers have extraordinary mechanical, electrical and thermal properties that make them attractive as reinforcements in high-performance and multifunctional composites. They also exhibit a unique piezoresistance response when subjected to mechanical strain. This characteristic is of interest for sensing applications in strain measurement and damage monitoring when integrated into polymeric and composite materials. Thus there is a need to understand the coupled mechanical and electrical behavior of the CNT yarns to fully comprehend the entire scope of their sensing applications. Of particular interest is to determine those characteristics when the CNT yarns are used as piezoresistive strain sensors in structures that are subjected to dynamic loading (fatigue or impact) or quasi-static loading and unloading. This paper presents a study about the presence of hysteresis and other time-dependent effects observed in carbon nanotube yarns during repeated loading and unloading cycles under uniaxial tension. By simultaneously taking four probe electrical resistance history measurements and load and displacement history measurements, any direct correlations between the mechanical and electrical characteristics are investigated including the effect of hysteresis and other time-dependent nonlinearities.
Structural and Conductive Adhesives Enabled by Single-Walled Carbon Nanotubes
Adhesives are increasingly being employed as alternatives to mechanical fasteners in aerospace and other engineering applications. While structural adhesives are commonly employed on aircraft, electrical bonding continues to be achieved with rivets as the existing electrically conductive adhesives suffer from low strength due to the high loading (typically 25-30 vol.%) of conductive filler particles. Carbon nanotubes are one of the more attractive candidates for development of multifunctional adhesives for both structural and conductive bonding because of their high conductivity and high aspect ratio. The latter enables creation of conductive pathways at much lower loading (< 1 vol.%) and, therefore, conductivity can be achieved without degrading mechanical performance. Single-walled carbon nanotubes (SWCNTs) offer the highest intrinsic conductivity and aspect ratio as well as the lowest percolation threshold, which is associated with higher conductivity at a fixed loading. In this work, SWCNTs were incorporated at low loading (0.5 - 3 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability. Mechanical properties of composite-tocomposite joints were evaluated using ASTM-based lap shear and peel tests. Bulk electrical conductivities over 1 S/m were achieved without degrading the joint structural performance in the selected test methods. The mechanical and electrical performance of a SWCNT-modified epoxy adhesive was also studied for aluminumto- aluminum bonding. It was found that the integration of 1 wt% SWCNTs can considerably improve joint Mode I fracture toughness by ~ 35% due to mechanisms such as crack bridging. The electrical resistance of the bondline was also consistent with the electrical conductivity of SWCNT-modified adhesive films (~10-3 S/m), but slightly lower than the bulk electrical conductivity measured on thicker samples.
Development and Implementation of the Experimental Procedure to Examine the Response of CFRP Composites Subjected to a High-intensity Pulsed Electric Field
In this work, the response of carbon fiber reinforced polymer (CFRP) composites subjected to a high-intensity pulsed electric field has been studied. A new fully automated experimental setup was developed that allows for real time measurements of the pulsed electric current and voltage on the carbon fiber polymer matrix composite laminates. The experimental setup included a new custom-built current pulse generator that utilizes a bank of capacitor modules capable of producing a 20 millisecond current pulse with an amplitude of up to 2500 A. The entire setup is controlled through a single Agilent VEE Pro 8.5 program via a computer and USB connections. A series of electrical characterization tests were performed on 16-ply IM7/977-2 and 32-ply IM7/977-3 unidirectional and symmetric cross-ply CFRP composites to assess the ability of the composites to withstand application of a pulsed electric current and determine the effects of the lay-up and thickness on the electrical response. It was found that the electrical resistance of specimens decreased with an increase in the electric current magnitude and lay-up and thickness have a significant effect on the electrical resistance. The 16-ply cross-ply specimens exhibited the largest specimen-to specimen variation in the electrical resistance values among all specimens tested. In addition, both unidirectional and symmetric cross-ply 32-ply specimens exhibited less specimen-to-specimen variation in the electrical resistance as compared to the 16-ply unidirectional and symmetric cross-ply specimens. A decrease in the electrical resistance with an increase in an electric current was observed for all specimens, however, a stronger trend was observed for 16-ply specimens. It was also determined that 32-ply laminates are capable to withstand higher electric current without signs of thermal damage compared to the 16-ply laminates.
Carbon Nanotube Based Network Heaters for Composite Adhesive Bonding
Adhesive bonding and bonded repair of metallic and composite aircraft structures have been used as effective methods for manufacturing and for restoring structural integrity of aircraft structures. Adhesive bonding requires the application of heat in order to cure the adhesive and complete the bonding operation; however, conventional heating methods are subject to several drawbacks and might be undesirable, particularly in cases involving repair of new, exotic aerospace materials. The motivation of this work is to develop a novel, cost-effective bonding method and apparatus using a network of carbon nanotubes (CNTs) to provide heating with uniform bondline temperature, rapid temperature response, and minimal energy cost by producing heat directly at the bondline. Such a solution is achieved through integrating a paper-like CNT network (i.e., buckypaper sheet) within film adhesive. This self-heated adhesive layer is applied in the same was as conventional film adhesive and cured through application of a voltage (or current) across the CNT network to cause Joule heating. The approach gives excellent temperature uniformity, fast response, and low energy consumption among other advantages.
A Study of Mechanical Behavior of Polycarbonate Nanocomposites Using Molded Disk Specimens
This paper presents an experimental study of the mechanical behavior of polycarbonate modified with boron nitride nanotubes, an emerging type of nanotube with extraordinary mechanical performance that is optically transparent. Using molded disk specimens with the same geometry, equibiaxial flexural, supported punch shear, and low-speed impact tests were performed to determine the flexural stiffness, punch shear strength, and impact performance of nanocomposites. Polycarbonate nanocomposite samples with different nanotube content (up to 1 wt%) were investigated. Within the scope of the study, mechanical properties of the nanocomposite samples exhibited only very limited variations depending on the reinforcement content; however, noticeable improvement in flexural stiffness and dissipated energy in punch shear were observed, with the low-speed impact performance remaining essentially unchanged. In addition, the complex failure behavior in punch shear was studied with the aid of scanning electron microscopy and the preliminary results are reported.
Effects of Nanoparticles on the Shear Properties of Polymer Composites
The effect of nanoparticles on the in-plane and interlaminar shear properties of composites is investigated. Two different batches of materials are made by hot pressing of woven carbon fiber prepregs without and with nanoparticles respectively. For batches of the specimen with nanoparticles added, and Silica nanopowders are used as nanoparticles. The nanoparticles are dispersed between the prepregs laminas’ and the stack is set under hot press at 350oF and 3.5 Tons, resulting a fiver layers laminated composite where the matrix between the lamina are reinforced with nanoparticles. By using this technique, it was able to fabricate composites that are reinforced with uniformly distributed nanoparticles between the laminas. The other batch is fabricated following similar procedure except the nanoparticles are absent. Both batches are tested under uniaxial tension and shear tests, and using 3D digital image correlation and load cell data the in-plane and interlaminar shear properties are characterized. The effects of nanoparticles on the elastic properties are investigated. It is found that the presence of nanoparticles improved the in-plane shear strength of the composites by about 15 % and shear modulus by 16%.
A Novel Computational Framework for the Oxidation of C/C Composites Under Thermal Shock
In this paper, the influence of thermal shock conditions on both, the extent of carbon materials decomposition and the through-thickness compressive mechanical response of 2D woven C/C composites is investigated by computational efforts using ABAQUS. First, C/C composite specimens and carbon fibers are exposed to thermogravimetric (TG) experiments under isothermal conditions at 400°C, 600°C and 800°C. The weight loss with time is recorded and TG curves are obtained for the C/C composite specimens and carbon fibers, which are utilized to predict the degree of decomposition of the carbon fiber tow and carbon matrix by using rule of mixtures. Finally, these TG curves are related to the Arrhenius equation to determine the corresponding kinetic parameters, which are the inputs to the proposed computational scheme. A novel computational framework consisting of two main steps is proposed. In the first step, a finite element radiation heat transfer analysis is developed on a meso-scale representative model of a C/C composite specimen exposed to thermal shock conditions with peak temperatures in the range of 400°C to 800°C. The radiation heat transfer analysis is coupled to a HETVAL subroutine to account for the degree of decomposition at every integration point and time within the meso-scale model. In the second step, a UMAT subroutine is added to the computational framework to account for the carbon materials stiffness degradation with temperature and time. Following this, a static analysis is performed by applying a through-thickness compressive load on the meso-scale model to determine its compressive stiffness. Finally, the predicted compressive responses of the meso-scale model under various thermal shock conditions are compared with experimental results provided in the literature, resulting in good agreement. In conclusion, this computational model can also be extended to other composites by changing individual material properties, fiber architecture, weave pattern and/or fiber volume fraction.
Effect of Curing Induced Parameters During Manufacturing of Textile Composites
Fiber-reinforced composite materials are widely used in the aerospace and automobile industries. Textile composites have shown promising capabilities for light-weight and high strength applications. During the compaction of the VARTM process, due to the presence of woven yarns, the adjacent layers of fabric compress against each other, resulting in a formation known as nesting. Nesting causes obstructions to the resin flow during the manufacturing process, leading to voids and/or resin rich areas in the composite. Another important parameter during manufacturing of composites is the curing temperature. Higher curing temperature accelerates the curing process but at the same time residual stress develops within the composites. Which leads to poor mechanical properties of the composites. So, it is very important to know the optimum curing temperature for a particular composites. Different curing temperatures is investigated in this paper to observe any damage on the matrix due to rapid curing. The computational framework under development will encompass the effects is compaction on the dry fabric, curing of the liquid resin, and subsequent response to service loads (example tensile loading).
Effect of Elevated Temperature on Adhesively Bonded Lightweight Material Single Lap Joints
This study investigates the effect of elevated temperature on the static load transfer capacity of adhesively bonded composite-based single lap joints. Two different types of temperature resistant adhesives as well as various joint material combinations are studied. The static load transfer capacity and corresponding failure mode are evaluated at room temperature and at elevated temperature of 285°F.
Thermo-Mechanical Behavior of Spatially Tailored Functionally Graded Materials in a High Temperature Environment
Airframes operating in hypersonic regime are subjected to complex structural and thermal loads. Structural loads are a result of aggressive high-g maneuvers, rapid vehicle acceleration, and dynamic pressure. Thermal loads are a result of aerodynamic heating. Thus, hypersonic airframes are typically designed with external insulation, active cooling or a thermal protection system (TPS) is added to the structure. TPS may consist of adhesively bonded, pinned, and bolted thermal protection layers over exterior panels. These types of attachments create abrupt change in thermal expansion and stiffness that make the structure susceptible to cracking and debonding. Functionally graded materials (FGMs) with metal-rich interior surfaces and ceramicrich exterior surfaces can potentially overcome the issues related to high thermal gradients and stress concentrations as well as selectively provide enhancements in strength and toughness and, thus, are a particularly appealing solution for thermal protection of structures operating at extreme temperatures. In this paper thermo-mechanical response of metal-ceramic Ti-6Al-4V/TiB2 functionally graded composite structural panels is studied and compared against a Titanium structural panel with Exelis Acusil® syntactic foam TPS. Effective thermal and mechanical properties of Ti-6Al-4V/Ti-B2 composite are obtained using micromechanical models and finite element analysis (FEA) of representative volume elements. The results of the micromechanical modeling are incorporated into the structural analysis of the Ti-6Al-4V/TiB2 FGM panels subjected to extreme thermomechanical loading. Realistic structural and thermal loads and time history, obtained by hypersonic flight trajectory simulation, are used in the analysis. The obtained computational results indicate reduction in deflection and weight with an increase in stiffness when Ti-6Al-4V with TPS is replaced with Ti-6Al-4V/TiB2 metal-ceramic functionally graded composite.
New Method to Analyze Numerical Crash Simulation Data to Assess the Suitability of Hybrid Composite Structures in Automotive Applications
In order to enhance development processes for multi material vehicle architecture concepts involving composites or hybrid composite-metal material systems a method is presented that aims to rationalize engineering judgements by means of quantifiable criteria for the suitability of a certain hybrid material system for a particular structural application. A number of significant crash load cases for a structure is considered simultaneously by superimposing the output data of several numerical crash simulations and defining criteria to qualify the suitability of a particular hybrid material system. The work introduced here focusses on the analysis of the global anisotropy of the crash loading situation as an indicator for a rather composite- or metal-intensive hybrid material system. The definition of the anisotropy criterion and the implementation of the method into the development infrastructure of automotive engineering is presented. A first step validation using simple flat coupon specimen simulations shows reasonable results and proves the applicability of the method.
Screw Geometry Design and Performance Effects on Fiber Breakage Study
Long glass fiber pellets has emerged as a potential substitute for metals in the making of non-load bearing automotive structural components and panels for the past five years. Their weight can be reduced and the part design can be more flexible. Long fiber now finds its balance between the mechanical performance and the easiness of processing. Since maintaining the fiber length is crucial to the superior performance of the long fiber, it is crucial to consider the machinery design along the melt pathway, including the screw, nozzle, runner, and the gate so as not to pose high stress on the fiber causing breakage. We presented in this paper a simulation method to predict the fiber length degradation during the plastication of the injection molding process. The screw design and the processing setting proved to be important for the fiber breakage. This method serves as the missing element by considering the effect of screw and barrel, which has not been addressed before with injection molding simulation.
Design and Development of Automotive Bumper Subsystem for Light Weight and Vehicular Crashworthiness
Reducing fuel consumption, satisfying current regulation and legislation regarding GHG emission and customer safety requirements are the main concerns for automotive industries. Today, Scholars and research and development department of car industries are trying to address such issue trough substitution the traditional steel by lightweight advanced composite materials. However, the complex composite failure behavior particularly under certain sever conditions like crash and trade-off between light vehicles and vehicle crashworthiness, makes the substitution of metallic to composite materials very difficult and remains the main obstacle for their rapid development. The current development in die forming manufacturing and injection overmolding technologies makes the production of open composite beam shells with the desired end profile and structurally integrating crash box with beam as a single unit possible. Therefore, if the strength/stiffness of an open structure is maintained, the capability of component integration will offer part reduction, increase production rate and reduce mechanical joints . The fact is, an open-channel thin-wall beam component is structurally weak and can readily buckle under lateral and compressive loads. However, with very little lateral support, provided by a thin-wall rib-like injectionmolded plastic subcomponent, the buckling resistance (and the stiffness) of the component can be greatly increased. In the current study, three composite materials considered for automotive bumper beam for light weight and better crashworthiness. Rib pattern selection and beam end section and rib geometry optimization are conducted numerically . Low velocity impact test were conducted and parameters, such as impact energy, peak load, crash resistance and energy absorption have been taken as evaluation criteria to compare the proposed materials solutions with steel solutions. The result shows that with a proper end section and ribs geometry optimization and reinforcing ribs, selection an open section beam can yield a structural performance comparable with that of the close section beam.
Extended Lamination Theory for Bridging Laminate Analysis to Composite Structures in Teaching Fundamentals of Composite Materials
The senior/first year graduate level course on fundamentals of composites has been widely offered by most of universities in US. The course provides students with both the theory and applications of the fundamental principles of composite materials that used in structural design. Analysis of composite structures in laminate level has widely employed “Classical Laminated Plate Theoryâ€ in determining the ply stresses. However, analysis in structural level often employs either finite element method or non-FEM approach using equivalent property of laminate. Hence, teaching this ‘fundamentals of compositesâ€ course often contains up to analysis of the laminate level. This paper demonstrates the applicability of lamination theory usually for laminate level that can be extended to analysis of composite beam structural level with open- and closed- sections, such as I-beam and circular and n-side polygonal tubular structures. The expressions for sectional properties such as bending, torsional and warping stiffness as well as centroid and shear center are also included.
Student Simulation Challenge: A Competition-based Composites Education Opportunity
Professional societies perceive the need to attract younger members and students to their professions and are increasingly holding competitions and challenges in various fields. The objectives for organizing the student challenges are to promote shared, innovative and open solutions to challenging real-world problems. In 2014, the American Society for Mechanical Engineers (ASME) conducted seven student competitions in conjunction with its annual IMECE conference and the IEEE Robotics and Automation Society regularly held five student challenges at the ICRA conference. The American Society for Composites initiated the student simulation challenge on a pilot basis in 2012. The objectives for the development of the student challenge are a) to encourage students to adopt the emerging simulation-based materials design and deployment as exemplified by the Integrative Computational Materials Engineering approach and the Materials Genome Initiative; b) to educate students in the application of modeling, interpretation of simulation results, and the significance of verification in the modeling and simulation loop. This paper describes the objectives, design of the challenge, outcomes, assessment of learning outcomes and lessons learnt from the first two years.
Composite Catapult Arm Design: A Case Study of Composite Design, Manufacture, and Education
In modern times, catapults have been used extensively at all levels as an educational tool. Recently, many communities, universities, and organizations have begun to hold contests where participants are tasked to build a catapult to throw pumpkins at targets or for distance. In 2013, Team ETHOS (Experimental Torsion Hybrid Onager System), a non-profit group promoting STEM education, began design and manufacture of a carbon-fiber throwing arm for their catapult. This article describes the process Team ETHOS and partners went through to design and manufacture a new arm for their machine while using the journey as an educational tool for themselves and student team members. The team developed requirements and models to demonstrate the impact of various design strategies to create a reliable structural arm. A hybrid filament winding process was chosen as a manufacturing method for the new arm. This approach consisted of building a semi-structural mandrel suitable for filament winding and incorporating pultruded unidirectional materials into the filament wind process. A unique ply-drop technique was developed in the filament winding process to address the need for skin thickness reductions from the arm root to tip. Experimental measurements of mass, center of gravity, mass moment of inertia (MOI), and internal strains of the new arm were quite close to those specified in the design process. The as-manufactured arm displayed a mass 1.80% higher and a center of gravity 15mm away from specified in the design of the 4.6m arm. Most importantly, the MOI about the arm pivot was less than 0.5% higher than predicted. Field-measurements of strain compare favorably with models.
Laminate Strain Allowables as Design Parameters in Teaching an Introductory Course on Composite Materials
A common practice in the aerospace industry is to use a laminated composite failure criterion based only upon laminate strain allowables, for which failure in any lamina denotes first-ply failure. Therefore, first-ply failure is a single event that can be used in composite design. The main assumptions of a balanced and symmetric laminate subjected to membrane forces constant along the edges (i.e., no external bending or twisting), are utilized. In order to prepare our students for future work and generate interest in the field of composites, we have introduced an active component into our introductory composites course in which student teams fabricate, test and analyze laminates to determine laminate strain allowables. Four student teams comprised of 3-4 members were given different laminate configurations with varying percentages of 0°, ±45°, and 90° plies. Students were required to fabricate their laminates using unidirectional carbon fiber/epoxy prepreg. The primary outcomes of this activity were to (a) enable students to use their own design laminate code to determine the critical stress that can be applied so that the given stress resultants can be supported without exceeding the “allowableâ€ strains, (b) compare the results to the measured data and (c) introduce students to a common and important industry practice in composite analysis and strength prediction. The exercise demonstrates that the critical laminate strain is not a function of the laminate configuration, but of the percentage of plies in the four orthogonal directions, thereby validating the use of laminate strain allowables as design parameters.
Reducing the Notch Sensitivity of Quasi-isotropic layups Using Thin-ply Hybrid Laminates
The concept of stress concentration suppression using thin-ply pseudo-ductile hybrid composites is presented in this paper. In previous works, it has been shown that it is possible to achieve high amounts of pseudo-ductility in Uni-Directional (UD) and multi-directional laminates. In this work, the requirements for thin-ply hybrid sublaminates to suppress/reduce notch sensitivity in quasi-isotropic laminates with an open hole is studied. A User Material subroutine (UMAT) has been developed in ABAQUS which can replicate the typical response of pseudo-ductile thin-ply hybrids. This UMAT was then used to study several idealised responses and find the important parameters controlling the fibre failure in notched specimens. The stress concentration factor in the 0° layer has been selected as a measure of notch sensitivity. The ratio of pseudo-ductile strain to initiation strain has also been found to be a useful parameter in comparing different fibre direction tensile responses. The stress concentration factors of the 0° layer found from two different tensile response shapes of UD sub-laminates were found to be very similar when they were plotted versus the ratio of pseudo-ductile strain over initiation strain. These results show that the stress concentration factor varies between 3 for linear elastic material response to 1 for sub-laminates with pseudo-ductile strain to initiation strain ratio of 3. This suggests that optimal structural response can be achieved with thin ply hybrids if the failure strain of the low strain material to high strain material is about 25%.
Composite Patch Repair of Cracked Aluminum Structures
A bonded composite patch repair of damaged metallic structures has many advantages over traditional welding and bolted patch repair, such as no new stress concentration, design flexibility, high fatigue and corrosion resistance, and low cost. The effectiveness and reliability of a repair are highly reliant on the patch design, bondline properties, and processing and fabrication parameters. To explore an optimal repair within a design space, most of the current modeling and simulations are performed based on a single failure mechanism along the bondline without considering other aging and damage mechanisms in the composite patch and aluminum structures. A Hybrid Structure Evaluation and Fatigue Damage Assessment (HYSEFDA) toolkit based on an Abaqus/Implicit platform has been developed to access the residual strength and fatigue life of a composite patch repaired metallic structure. A mesh independent, phantom-paired element based extended finite element method (XFEM) and a cohesive spring element is employed to describe the discrete cracks in the metallic layer and the adhesive interface between the metal layer and the composite patch, respectively. A virtual crack closure technique (VCCT) approach is adopted to extract the energy release rate at the crack tip and delamination front, which is tracked by a developed geometrical mapping algorithm. In order to capture other damage and aging mechanisms, a continuum damage mechanics (CDM) approach is applied to characterize inter and intra ply damage. In addition, the plasticity induced crack closure effect is also investigated by implementing a plastic hardening model into the HYSEFDA toolkit. The HYSEFDA toolkit is used to evaluate the fatigue life of an aluminum plate repaired by E-Glass epoxy composite patch tested by the Naval Surface Warfare Center, Carderock Division. Parametric studies are performed to examine the effects of patch damage on the bridging of a fatigue crack in an aluminum plate.
Simulation of Progressive Damage in Composites Using the Enhanced Embedded Element Technique
Analysis of complex composite structures requires a fine contiguous mesh of threedimensional (3D) solid elements. The embedded element technique is a promising technique for predicting stiffness and stress. This paper presents a new method for enhancing the embedded element with continuum damage mechanics methods for predicting the evolution of damage in fiber reinforced composite structures. Comparison of the model prediction with experimental results reveals an excellent correlation between the tensile strength of quasi-isotropic laminate with an open hole. The embedded element technique allows the fiber reinforcement and matrix domains to be meshed independently and failure is evaluated separately in each domain. The enhanced embedded element approach allows the failure modes to be observed, specifically, the evolution of matrix cracking and fiber rupture. Compared to the traditional contiguous mesh finite element method, the present modelling technique demonstrates a clear advantage in predicting the experimentally observed failure modes and accurate characterisation of intralaminar fracture.
Characterization of Fracture Properties of Nanographene Reinforced EPON 862 Polymer and Their Carbon Fiber Composites
This paper describes the increase in Mode I fracture properties (stress intensity factor and energy release rate) of a thermosetting polymer EPON 862 and its nanographene reinforced counterpart. Low (0.1 and 0.5) weight percent nano-graphene platelets (NGP) were mechanically dispersed in EPON 862 and compact tension (CT) fracture experiments were conducted at quasi-static strain rates. Significant enhancement in fracture toughness (190%), and energy release rate (570%), was observed for nano-graphene reinforced matrix for only 0.5 wt% of graphene platelets. Fractography analysis of the fracture surface using SEM was performed to qualitatively visualize and understand the mechanism(s) responsible for the enhancement in these properties. Evidence of crack deflection (i.e., crack tilting and/or twisting) leading to increased surface roughness, graphene platelet pullout, and plastic deformation of the matrix caused by filler-matrix debonding corroborated by the presence of voids on the surface from SEM. AFM was employed to quantify the magnitude of surface roughness changes between the NGP reinforced and unreinforced fracture specimens, and correlate surface roughness changes to increased fracture toughness. Mode I Double Cantilever Beam (DCB) experiments were carried out to measure the delamination fracture toughness of carbon fiber reinforced laminates with NGP reinforced epoxy matrices. An enhancement of 42% in the fracture toughness was obtained for only 0.1 wt% NGP matrix laminates and a higher resistance behavior (Rcurve) to crack propagation was observed for the NGP matrix laminates in comparison to the baseline matrix counterparts.
Nanoscale Self Assembly of Triblock Copolymers in Epoxy Polymer Composites: A Comparison to Traditional Rubber Toughening
This research investigated the ability of a traditional rubber toughening agent, namely carboxyl-terminated butadiene-acryltonitrile (CTBN) and the triblock copolymer (poly)-styene-block-(poly)-butadiene-block-(poly)-methylmethacrylate (SBM) (Arkema E21) to toughen a diglycidyl ether of bisphenol-A (DGEBA) epoxy resin cured with m-phenylenediamine (mPDA). It was found that both CTBN and SBM additives led to decreases in epoxy polymer composite (EPC) flexural properties and increases in fracture toughness (KQ). Fracture surface investigation by scanning electron microscopy showed that CTBN phase separated into particles with diameters on the order of microns, while SBM self assembled into 100 nm spherical micelles which, at larger concentrations in the epoxy, tended to aggregate to form an interpenetrated network. In both systems, cavitation of spherical rubbery domains and matrix shear yielding were identified as toughening mechanisms. Additionally, the SBM EPC fracture surfaces showed pullout of SBM particles. The finer particle size of the SBM terpolymer in DGEBA led to a 215% increase in KQ at 10 phr SBM, while 10 phr CTBNx13 led to a 90% increase in KQ. Also, it was found that KQ increased linearly with concentration for the SBM EPCs, while CTBN EPC KQ did not increase linearly with concentration. Further, dynamic mechanical analysis showed that CTBN plasticizes the epoxy matrix, leading to decreases in glass transition temperature (Tg). A decrease in cured epoxy Tg by the addition of SBM at 5, 10 and 15 phr was not observed. Finally, the influence of curative concentration on cured DGEBA mechanical, thermomechanical and KQ was explored. Maxima in flexural strength and modulus were observed for epoxide rich networks and maximum KQ was observed for amine rich networks. Therefore, SBM was used to modify amine rich networks, which showed more ‘toughenibility’ and a 300% increase in KQ at 10 phr SBM additive.
Smart Hybrid ZnO Nanowire/carbon Fiber Reinforced Polymer Composites with In-situ Structural Health Monitoring Capability
Structural fiber reinforced plastics (FRPs) composites are increasingly replacing metals due to their high specific stiffness and strength. However, FRPs are prone to delamination failure due to insufficient adhesion between the fiber and matrix or due to unexpected overloading. As delaminations reduce the structural stability and durability of FRPs leading to failure, structural health monitoring (SHM) is needed to detect delamination at early stages. This study suggests growing nanoscale ZnO piezoelectric nanowires (NWs) on the surface of carbon fabrics to act both as (1) sensor/transducer for SHM and (2) as an interfacial strengthening mechanism to mitigate delamination in the first place. The study employs a synergistic approach involving processing and characterization of hybrid polymer matrix composites based on ZnO NWs/carbon fibers and characterizing them mechanically and piezoelectrically. The in-situ SHM system utilizes electromechanical impedance based methods for early delamination detection and diagnosis. Utilizing a low temperature (85 °C) hydrothermal synthesis technique, ZnO nanowires were grown on the surface of woven carbon fiber (CF) and woven glass fiber (GF) fabrics. Hybrid Carbon FRPs were fabricated and tested via high velocity (~90 m/s) impact tests, and the impact energy dissipation and delamination failure mitigation capabilities of the FRPs were investigated. A 20% improvement in impact energy dissipation, and 16% reduction in delamination length of the FRPs were observed through whiskeriaztion of the carbon fiber fabrics with surface grown ZnO NWs. Hybrid CF-GF-CF composite laminates comprising different predesignated delamination depths were also fabricated to investigate the effect of damage severity on the electro-mechanical impedance of the FRPs. A correlation between the delamination severity and change in the FRPs’ impedance was established. The developed ZnO NWs based SHM provides a truly coupled structure-actuator system and benefits from large arrays of active regions within the composite which has many advantages for early detection of delamination in FRPs.
Experimental and Computational Investigations of Hybrid Interfaces in Hybrid Composite Laminates
In this work, hybrid laminates, whose constituents are a shape memory alloy NiTi foil, carbon fabric and epoxy matrix, were manufactured using the vacuum assisted resin transfer molding process. Acid etching and sol-gel surface treatment techniques were applied on both surfaces of the NiTi foil prior to fabricating the hybrid laminates. The interface between NiTi and polymer matrix composite was investigated both experimentally and computationally. The double cantilever beam tests at both room (25oC) and elevated (60oC) temperatures were conducted to study the mode I dominated delamination behavior at the hybrid interface. Distributed strain profiles were measured on both the top and bottom of the specimen using Rayleigh backscattering fiber optics technique. Extensive fractographic analysis utilizing optical and scanning electron microscopy were performed to investigate the fracture modes and mechanisms. Finite element analyses were carried out to assist with the examination and interpretation of experimental results. Load-displacement and strain profiles obtained numerically agreed well with those measured from the experiment.
Improving Damage Tolerance with Energy Absorbing Mesh in Composite Laminates: An Experimental Study
This study aims to evaluate the impact resistance of carbon fiber reinforced polymer (CFRP) skin sandwich structures having strain sensitive, energy absorbing D3O® mesh cores. CFRP plates were fabricated having 5, 7, 10 and 14 layers respectively. The 5 and 7 layer plates were used as facing skins for sandwich structures containing D3O® mesh as the cores. Impact energy absorption characteristics were assessed using Charpy tests and 4 J low velocity drop weight tests. Sandwich structures with D3O® cores absorbed more energy compared to the CFRP only samples. Furthermore, the sandwich structures showed lower peak loads, ductile behavior and were able to survive greater impact damage than CFRP only. Post impact thermography was performed on the front and back of impacted samples and compared with the undamaged state. The results revealed that lesser subsurface delamination occurred in the mesh incorporated structures compared to the CFRP only samples after impacting. Thus, the approach increased the impact resistance of the CFRP incorporated mesh structures.
Damage Tolerance of Sandwich Foam Cores: Experimental Characterization and Numerical Modelling
In this paper, a novel analytical model for predicting the post-crushing compressive response of crushable foams is presented. The calibration of the model is performed using experimental measurements obtained exclusively from standard monotonic compressive tests. Therefore, the need of performing time-consuming compressive tests including multiple unloading-reloading cycles is avoided. Model predictions have been validated against experimental measurements available for three different foam materials. The model is shown to accurately predict the thickness of the crushed material layer during progressive crushing and the residual after-crushing strain (with a maximum error of about 12%). When compared to existing models, whose calibrations require the same experimental measurements of the present model, the latter is capable to predict the residual after-crushing strain with a significantly smaller error (error reduction over 56%). Furthermore, it is shown that the proposed model is able to accurately capture the characteristic features of the post-crushing compressive response of crushable foams. The results presented in this work demonstrate the relevance of the proposed model for a damage-tolerant design of foam-cored composite sandwich structures.
A Numerical Study on the Effect of Facesheet-Core Disbonds on the Buckling Load of Curved Honeycomb Sandwich Panels
A numerical study on the effect of facesheet-core disbonds on the post-buckling response of curved honeycomb sandwich panels is presented herein. This work was conducted as part of the development of a damage tolerance approach for the nextgeneration Space Launch System heavy lift vehicle payload fairing. As such, the study utilized full-scale fairing barrel segments as the structure of interest. The panels were composed of carbon fiber reinforced polymer facesheets and aluminum honeycomb core. The panels were analyzed numerically using the finite element method. Facesheet and core nodes in a predetermined circular region were detached to simulate a disbond induced via low-speed impact between the outer mold line facesheet and honeycomb core. Surface-to-surface contact in the disbonded region was invoked to prevent interpenetration of the facesheet and core elements. The diameter of this disbonded region was varied and the effect of the size of the disbond on the post-buckling response was observed. A significant change in the slope of the edge load-deflection response was used to determine the onset of global buckling and corresponding buckling load.
Degradation of Carbon Fiber Composite Materials Due to Electrical Current and Potential Impact on Synergistic Durability
Composites materials are often subjected to multi-physical conditions in different applications where in addition to mechanical load, they also need to sustain other types of loads such as electrical currents. Hence durability and damage tolerance of composite materials for both mechanical and electrical loads need to be studied. Although the durability of composite materials under mechanical loading has been studied over several decades, their response to electrical currents is still not fully understood. Damage tolerance for mechanical response may or may not be sufficient for a desired durability in electrical response. Electrical effects are often coupled with structural integrity and thermal behavior. This inherently multi-physical behavior needs to be understood and analyzed to facilitate new multi-functional material design. An essential first step towards that goal is to understand how multi-physics properties depend on local micro-structural details. In this study, anisotropic electrical behavior is measured experimentally and threshold of nonlinear behavior has been quantified. This directly relates electrical response with microstructural changes. The thermal response is also studied with thermography tests and the temperature distributions were compared to those of the electrical measurements to facilitate identification of potential damage locations. A 3D X-ray microscope has been used to visualize and quantify (down to 1 micron) such local material state changes due to electrical current. This ongoing work is expected to form a foundation for future multifunctional material development.
Characterization of Delamination in Translaminar Reinforced Composites Due to Low Velocity Impact
Z-pinning has been used to improve the delamination strength of laminated composites as has been shown in Mode-I and Mode-II fracture toughness. Additionally the degradation of in-plane properties has been studied in the past and documented in the literature. This paper presents a preliminary study on the use of zpinning to improve the delamination resistance against barely visible impact damage. Experimental results indicated that the use of pinning can considerably improve the compression strength after impact (CSAI). Non-destructive observations revealed a lower delamination area after impact when the samples were fully covered with zpins. Additionally, samples with a surrounding z-pin boundary were able to provide a containment mechanism for an area with sub-surface delamination. Computational models have been developed to help predict the delamination area and CSAI with impact damage.
Evaluation of Test Methods for Triaxially Braided Composites Using a Meso-scale Finite Element Model
The characterization of triaxially braided composite is complicate due to the nonuniformity of deformation within the unit cell as well as the possibility of the freeedge effect related to the large size of the unit cell. Extensive experimental investigation has been conducted to develop more accurate test approaches in characterizing the actual mechanical properties of the material we are studying. In this work, a meso-scale finite element model is utilized to simulate two complex specimens: notched tensile specimen and tube tensile specimen, which are designed to avoid the free-edge effect and free-edge effect induced premature edge damage. The full field strain data is predicted numerically and compared with experimental data obtained by Digit Image Correlation. The numerically predicted tensile strength values are compared with experimentally measured results. The discrepancy between numerically predicted and experimentally measured data, the capability of different test approaches are analyzed and discussed. The presented numerical model could serve as assistance to the evaluation of different test methods, and is especially useful in identifying potential local damage events.
Characterization of the Mechanical and Impact Response of a New-Generation 3D Fiberglass Fabric
The main objective of the present study is to characterize the mechanical behavior and impact response of a recently developed 3D woven/braided fiberglass fabric. The 3D-fiberglass fabric (3DFGF) consists of two bi-directional woven fabrics, knitted together by vertical braided glass fiber pillars. In addition to, or instead of glass fibers, carbon and even basalt fibers, as well as hybridizations of these fibers, could be used to form such 3D clothes. The unique configuration of fibers in this particular 3DFGF provides outstanding impact resistance to panels formed by this fabric. Moreover, the two bi-directional woven fabrics of 3DFGF create hollow core regions, surrounded by vertical glass pillars, which could be filled with a foam, thus further enhancing the resilience of the panel. The weakest link in any fiber-reinforced plastic (FRP) laminates is the interlaminar shear properties of the FRP, thus making them susceptible to impact loading. However, owing to the unique and resilient configuration of fibers in this new generation of 3DFGF, delamination type failure is seldom experienced by 3DFGFs. Moreover, to the best of authors’ knowledge, there has been no investigation characterizing the impact response of such 3DFGFs. Three-point bending and flatwise compressive tests with two different core thicknesses are conducted in two principal directions of 3DFGF to investigate its mechanical behavior. For each specimen and thickness, two different configurations are considered: (i) panels with their hollow core cavities filled with a foam, and (ii) panels without foam. Moreover, the low-velocity impact (LVI) response and failure modes of this new fabric are also investigated experimentally and also by computational simulations. The latter is done to address the clear lack of a robust computational framework by which one could predict the response of such 3DFGFs. Therefore, a major part of this study deals with predicting the behavior of such a complex 3D materials configuration by the finite-element method, using a commercial code (the ABAQUS). It will be demonstrated that the good agreementobtained between the experimental and simulation results would render the adopted framework as an effective and reliable means for predicting and optimizing the response of such 3DFGFs subject to complex loading conditions.
High-Velocity Impact Behavior of Stitched CFRP Laminates
In this paper, high velocity impact (HVI) behavior of the through-the-thickness stitched CFRP laminates is investigated. Both rigid body impact tests and soft body impact tests were conducted. The authors developed finite element model, in which the effect of the stitching was considered. Tests and simulation results revealed that through-the-thickness stitching significantly affects the damage process, and that it can effectively suppress the propagation of the delamination. Through thickness stitched CFRP laminates have prominent mechanical properties such as higher delamination toughness and higher CAI strength than that of non-stitched laminates. However, HVI behavior of the stitched CFRP laminate has not been investigated sufficiently. In this study, effect of stitching on HVI damage of stitched composites is investigated. HVI tests were conducted using single-stage air gun. 7.5mm-diameter steel spheres and 20 mm gelatin spheres are used as projectiles. The impact velocity ranges from 130 to 240m/s. HVI damage of each specimen was investigated by NDI techniques. Authors developed finite element model in which the damage was considered by using stress criterion, continuum damage mechanics (CDM), and cohesive zone model (CZM). The effect of the stitch threads were introduced by specially developed nonlinear spring elements. The simulation results revealed that delamination bridging of the stitch threads significantly affects the damage behavior of the laminate. Delamination area was reduced by crack closure traction generated by stitch threads. Experimental and simulation results demonstrated that through-the-thickness stitching is effective to suppress the delamination under high velocity impact load.
Enhancement of Electrical Conductivity and Assessment of Thermal and Mechanical Properties of Graphene/Epoxy Nanocomposites
A three roll mill processing technique was used to disperse graphene nanoplatelets in epon 828 epoxy system. As a first step of this research, processing graphene/epoxy nanocomposites was explored with different weight percent of graphene. After establishing a repeatable process to achieve good electrical properties, the materials were tested for thermal conductivity and mechanical properties. The as received graphene nanoplatelets had average diameter of 25 microns, thickness 6-10 nanometers and epon 828 epoxy system were selected. Conventional mechanical mixing, sonication and three roll mill dispersion techniques were investigated. Based on this study three roll dispersion was found to be repeatable, consistent and scalable. Through the thickness (T-T-T) or volume electrical conductivity increased by nine log cycle, thermal conductivity doubled and fracture toughness increased by one-third over the base with 1.0 wt.% dispersion of graphene to epon 828. However, tensile and flexure properties changed little.
High Shear Strain Characterization of Plain Weave Fiber Reinforced Lamina
Strain energy driven deployable structures have recently become of high interest to the deployable space structures community. Made out of thin composite sections, these structures possess increased stiffness and stabilization while reducing payload masses significantly. The added capability of self-deploying provides additional benefit in reducing structural complexity. One challenge to using such a structure is the high deployment kinetic energy resulting from conservation of strain energy during stowage. At the end of deployment, excessive kinetic energy can damage the onboard payload through high shock loads. The research presented herein suggests that viscoelastic laminates can effectively dissipate energy during deployment and greatly reduce shock loads. One method to achieve this is tailoring the matrix constituent of the composite materials. An optimal laminate for such members is described to include discrete elastic and viscoelastic plies. It has been identified that a pure shear stress state is induced in the viscoelastic plies of typical cylindrical members (tape springs, slit tubes, etc.) when subjected to high shear strains. The intention is to key on this respective viscoelastic lamina and tailor it in a way that provides a sufficient amount of energy dissipation while still preserving the deployment torque and energy required for successful deployment. A custom combined loading Picture Frame Shear (PFS) fixture was designed to test plain weave composite lamina in high shear strain load cases similar to operational conditions. Before viscoelastic characterization can occur, the PFS fixture was tested using plain weave materials of interest. Material selection consisted of an IM10/PMTF7 carbon/epoxy plain weave lamina. Shear strain rate dependency was investigated in the form of variable load head displacement rate testing. Varying content levels of rubberized toughener matrix additive were examined.
Asymptotical Modelling of Thermopiezoelastic Laminates
A general anisotropic laminated plate model with thermal deformation and twoway coupled piezoelectric effect and pyroelectric effect is constructed using the variational asymptotic method, without any ad hoc assumptions. Total potential energy contains strain energy, electric potential energy and energy caused by temperature change. Three-dimensional strain field is built based on the concept of warping function and decomposition of the rotation tensor. The feature of small thickness and large in-plane dimension of plate structures helped to asymptotically simplify the three-dimensional analysis to a two-dimensional analysis on the reference surface and a one-dimensional analysis through the thickness. Several numerical examples are studied. The present model is validated by the excellent agreements between the results from 3D finite element analyses and the presented model.
Methodology for the Design and Optimization of Hybrid Structures in the Early Phase of Vehicle Development
Hybrid structures hold a high potential of lightweight design which is important for the automotive industry. To find an appropriate design of those multi material parts is a key criterion for its application in a car body. In this paper a methodology is presented which allows the definition of an advantageous hybrid part design considering demands on structure stiffness, strength, local stability and weight. The analytical approach allows a rapid optimization of parameters, like the dimensions of a cross section, materials and boundary conditions. In order to improve understanding of the implementation and working method, an analogous model of a real car door sill is optimized. Two different concepts to reinforce the steel structure with CFRP are defined and investigated. A) CFRP reinforcement on each side of a rectangular crosssection leads to a weight saving of 31% compared to the reference structure with the same bending stiffness made of high-strength steel. At the same time the maximum transferable load causing first failure is increased by 54%. B) A second example, a beam reinforced on three sides results in a weight saving of 20% and an increase of load capacity of 16%.34
Cyclic Corrosion of Composite-Based Lightweight Material Single Lap Joints
This study investigates the effect of cyclic salt-fog spray on the static performance of bonded multi-material lightweight single lap joints. Adherends material combinations include composite-composite, composite-aluminum and compositemagnesium. Tensile shear tests are carried out to monitor the effect of the number of corrosion cycles on the decay of joint static load transfer capacity, as compared to the baseline values before corrosion exposure. Failure analysis of the fracture surfaces is conducted using an optical stereoscopy.
Non-destructive Evaluation and Bonds Between Fiberglass Composites and Metals
This study aims to develop a reliable and portable NDE system for service-level adhesive inspection in the automotive industry. To assess the integrity and reliability of an adhesive joint in an automotive composite component, several non-destructive evaluation (NDE) methodologies are correlated to lap shear bond strengths. A glassfabric- reinforced composite structure was bonded to a metallic structure with a two-part epoxy adhesive. Samples were subsequently cut and tested in shear, and flaws were observed in some areas after lap shear test. Fiberglass-to-metal bonding is the ideal configuration for NDE via thermography using excitation with induction heating, due to the conductive metal and non-conductive glass-fiber-reinforced composites. Excitation can be either by a research-grade induction heater of highly defined frequency and intensity, or by a service-level heater, such as those used for sealing windshields in a body shop. The thermographs thus produced can be captured via a high-resolution infrared camera, with principal component analysis and 2D spatial Laplacian processing. Alternatively, the thermographs can be captured by low resolution thermochromic microencapsulated liquid crystal film imaging, which needs no post-processing and can be very inexpensive. These samples were also examined with phased-array ultrasound. The NDE methods were compared to the lap shear values and to each other for approximate cost, accuracy, and time and level of expertise needed.
The Flexural Strength and Modulus of a Randomly Oriented Carbon Fiber PEEK Composite
The objective of this investigation was to compare the flexural properties of highmodulus carbon fiber, thermoplastic matrix composite. With increased pressure from government regulations and consumers to create lighter, more fuel-efficient cars, the automotive industry has long been looking toward composite materials. Thermoplastics offer the strength and toughness required for many applications as well as the promise of reduced cycle times over thermosetting resins. In this study, a papermaking process was used to randomly and uniformly disperse matrix and reinforcement fibers in a non-woven, fibrous mat, which could then be molded into composite panels. A thermoplastic PEEK matrix was paired with a high-modulus, high-strength carbon fiber to produce the composite plates. Test blanks were then cut at multiple locations from these panels and subjected to visual inspection and flexural strength testing. Insights were then drawn from the comparisons giving specific attention to overall flexural strength properties. As a secondary objective, this papermaking process was evaluated as a viable production method for consistent, randomly oriented composite material.
Micro-Scale Chemical, Morphological and Mechanical Characterization of Polymer-Matrix Textile Composites
Polymer matrix composites (PMC) with textile fiber architectures are of significant interest in advanced aerospace applications. Despite the great promise, there are significant challenges associated with the uncertainties in material response caused by the complexity and variability of the geometry and the characteristic materials properties. These variabilities include local dissimilarities in fiber alignment, fiber volume fraction, resin rich pockets and variations in matrix dimension (interphases, local area covered by matrix, voids between the fibers or in resin pockets). Understanding of these variations is critical, and it must be taken into account for stiffness and strength calculations. This study illustrates our initial efforts on understanding the variability and its corresponding effect on the properties of textile PMC using nanoscale characterization techniques combining spectroscopy, microscopy, and mechanical testing. We have chosen first to examine micro-scale variations in laminated tape prepreg composites as a simpler beginning. Using atomic forced microscopy coupled with infrared spectroscopy (AFM-IR), we were able to evaluate thickness, chemistry and relative modulus of the interphase. This has also allowed us to distinguish resin rich pockets and variation of fibers distribution. Similarly, combining it with electron microscopic techniques (SEM) and mechanical (Nanoindentation) characterization techniques, this study illustrates deeper understanding of the composite laminate in terms of the chemical mapping, morphology and the mechanical properties. This will provide guidance for future advancement of composites in terms of composite processing as well as toward developing advanced modelling methodologies.
High-Speed 3D Digital Image Correlation of Low-Velocity Impacts on Composite Plates
Composites are beginning to be used in a variety of different applications throughout industry. However, certification and damage tolerance is a growing concern in many aerospace and marine applications. Transverse impact of composites results in damage in the form of transverse matrix crack, delamination, fiber breakage and permanent deformation. Low-velocity impact experiments were conducted on 3.2 mm thick IM7/977-3 composites with 10 joules of energy (1.888 m/s). High-speed digital photography was used to capture the impact phenomenon of the composite back surface. The specimens were speckled to perform 3D digital image correlation to analyze the displacements and strains that occurred on the back surface. The results from this study provide basic knowledge of the impact event such as deformation, strains, residual strains, damage threshold load, crack initiation and propagation.
Repeated Tensile Loading of Thin-ply Pseudo-ductile Laminates
The loading-unloading-reloading response of thin-ply hybrid and angle-ply laminates is presented. Fibre fragmentation in 0Ëš plies leads to a loss of modulus. Considerable non-linearity arises in the angle plies, with some permanent deformation. However there is little loss of initial modulus on reloading. Experimental results are presented for different configurations, and the results compared to illustrate the mechanisms controlling the cyclic behaviour and changes in modulus.
Experimental and Theoretical Investigation of Discrete Damage and Residual Strength Prediction in Fatigue-Loaded CFRP Speciemns
Ply level detailed prediction of fatigue performance of a nontraditional carbon fiber reinforced composite laminate with open hole has been accomplished and compared with experimental data. Discrete Damage Modeling (DDM) method was used for this purpose. The essence of this technique is the insertion of true displacement discontinuities independent of mesh orientation to simulate matrix cracking. Multiple cracking in each ply is allowed. All plies are tied together by using cohesive interfaces, which are allowed to delaminate. Matrix cracks in two adjacent plies interact through the interface cohesive model and their presence is a major delamination initiator. A material history variable in each integration point is introduced and updated after each loading increment, corresponding to certain load amplitude and number of cycles. The computation begins without any matrix cracks present. A matrix crack is inserted when the material history variable value reaches unity. Failure criterion and cohesive zone model for initiation and propagation of cracking and delamination under fatigue loading were proposed. The delamination and matrix cracking extent under fatigue loading in open hole [30/60/90/-60/-30]2s laminate was predicted and showed good correlation with experimental micro X ray CT imaging. In addition, static residual strength in tension and compression for the same laminate after 200,000 fatigue cycles was predicted and showed slight increase in tension and slight decrease in compression residual strength as compared to pristine laminate, which is in qualitative agreement with experimental observations.
Low Velocity Impact Damage Analysis of Nylon-66 Nanofiber Interleaf Composite Laminate
Electrospun Nylon-66 nanofiber was used in AS4/3501-6 carbon/ epoxy prepreg to fabricate a quasi-isotropic base and interleaved laminates. The areal density of Nylon-66 nanofiber in the interleaved laminate was varied from 0.5, 1.5 and 2.5 g/m2, which translate to 0.23, 0.69 and 1.15% interleaved weight percentage. The base and interleaved laminate were impact tested. The Nylon-66 nanofiber interleaving increased the overall thickness by maximum 2.5%, the mass and the ply areal density of the three interleaved laminates increased by maximum 1.4% and 1.4%, respectively. The interleave effect on the threshold impact force and energy for 0.23% interleaved laminate are slightly 8% and 16% respectively. The threshold impact force and energy for 0.69 and 1.15% interleaved laminates improved significantly by 42 and 89 for 0.69% interleaved and for 1.15% interleaved is 45 and 97%, respectively. The damage growth rates for the three interleaved laminates decreased by 12, 32 and 48%.
Low Velocity Impact Response of Bio-inspired Fiberglass Woven Composites
For many years, nature has provided some of the most effective solutions to challenging problems that men have faced. Scientists and researchers have tried to mimic natures approach in order to improve and advance human`s life. In this study, the multistate structure of turtle shell is used to improve the impact resistance of out of autoclaved polymeric composite laminate. The optimally designed turtle shell possesses three layers: The exterior and interior which made out of cortical bone and the middle layer which consists of cancerous bone. This interesting structure provides high stiffness on the outside, especially under out if plane loadings while giving a possibility of small elastic deformations on the inner side. The shell comprises of a multiphase sandwich composite structure of functionally graded material having exterior bone layers and a foam-like bony network of closed-cells between the two exterior bone layers. This concept is used in designing an out of autoclaved woven composite in order to minimize the deformation and damage in the composite when subjected to low velocity impact loading. The progressive deformation and damage behavior of composite panels with polymer membrane impacted by drop-weights at four different velocities were investigated. The specimens tested were made of woven fiberglass fabric/ toughened epoxy (cured at 1200 C). During these low-velocity impact tests, the time-histories of impact-induced dynamic strains and impact forces were recorded. The damaged specimens were inspected visually. Test results revealed that the polymer membrane layer plays a significant role on the overall deformation and damage behavior of the composites under impact loading. These findings could provide fundamental understanding of structure-property phenomena and biological pathways to design efficient energy absorbing bio-inspired composite materials for various composite structures which are susceptible to the impact damage.
Impact Performance of a Hybrid ZnO Nanorod/Carbon Nanotube/Carbon Fiber Reinforced Polymer Composite
Carbon fiber reinforced polymer composites (CFRPs) are replacing traditional structural materials, due to their outstanding in-plane strength and stiffness. However, their out-of-plane and interlaminar performances are inadequate. Incorporating nanosized reinforcement into the CFRP structures was shown to potentially eradicate these vulnerabilities. Zinc oxide (ZnO) nanorods and carbon nanotubes (CNTs) are among possible nano reinforcement additives. In this study, a low temperature hydrothermal synthesis technique is utilized to grow ZnO nanorods on the surface of woven carbon fiber fabrics. A relatively low temperature CNT synthesis technique; graphitic structures by design (GSD), is also employed for grafting multi-walled carbon nanotubes (MWCNTs) on the surface of carbon fibers and on the ZnO nanorods to form hierarchical reinforcements. These two synthesis techniques provoke different surface treatment on the carbon fibers including the exposure to elevated temperatures, amorphous ZnO coating, water-soaking, ZnO nanorods growth and CNTs growth. Carbon fiber fabrics based on these different surface treatments were utilized to fabricate composite laminates. The quality of the synthesized nano reinforcements was studied via scanning electron microscopy (SEM). The energy absorption capabilities of the different hybrid composites were examined via out-of-plane high velocity (~90m/s) impact tests utilizing a spherical impactor. In order to study the progressive damage mechanism of the hybrid CFRPs, spherical punch tests were also carried out utilizing a punch identical in size to the high speed impactor projectile. Delamination damage of the hybrid CFRPs was also investigated using SEM micrographs of the samples’ damage region after impact tests. A 21% improvement in the impact absorption energy of the CFRPs comprising surface grown ZnO nanorods and MWCNTs was achieved, compared to the composite laminates based on the raw carbon fibers with no surface treatments.
Low Velocity Impact Strength of CFRP Composites Incorporating Nanoclay
Carbon fiber reinforced polymer (CFRP) composites are the most commonly used material for aerospace applications. Damage of CFRP composites due to low-velocity impact from hail or small cosmic particles on airplanes or aerospace structures have been reported as a critical issue in aerospace applications. Since CFRP off-axis and out-of-plane responses strongly depend on matrix properties, we hypothesize that altering the matrix properties of CFRP using nanomaterials might result in a significant improvement in the low-velocity impact strength of CFRP. In this work, we present the use of nanoclay as a natural material with good chemical modification responses and a relatively low cost. Two loadings of nanoclay by weight of epoxy resin were examined; 4.0 wt.% and 10.0 wt.%. CFRP composite plates were produced using the epoxy-clay nanocomposites. The CFRP plates were then subjected to impact energy of 9 and 18 J representing a typical range for lowvelocity impact. Time-history response of load, displacement and velocity were monitored and reported. Moreover, 3D computer models generated from x-ray computed tomography (CT) scans of the impacted CFRP were examined to quantify the level of damage in CFRP. Finally, microstructural characterization was also performed to understand the significance of nanoclay intercalation on the behavior of CFRP composites fabricated using the modified nanocomposite. It was concluded that low content of nanoclay (4.0 wt.%), representing exfoliated platelets, does not have a significant effect on the low-velocity impact strength of CFRP. However, a relatively high content of nanoclay (10.0 wt.%), representing intercalated nanoclay platelets, has a significant improvement of the low velocity impact strength of CFRP. However, it was observed that these enhancements at 10.0 wt.% are associated with embrittlement of CFRP composite under impact. It is demonstrated that quantifying low velocity impact damage in CFRP using CT-scans might be possible for relatively low damage levels.
The Role of Phase Shifts on Homogenized Modulus and Maximum Strength of Woven Laminated Composites
This research focuses on responses of 64 woven laminated composites whose phase shift in microstructure is different under uniaxial compression and biaxial compression. The motivation of this paper is to understand how the microstructure arrangement affects laminate responses, particularly on the maximum strength and the tangential stiffness. Each laminated composite consists of four layers of laminae. Each layer of lamina has the same constituent microstructures. The laminae in different layers are shifted in the x and y direction to create phase shift. The first (bottom) layer of lamina in laminates remains stationary while the second layer of lamina, third layer of lamina, and fourth (top) layer of lamina are shifted in the x direction by 0mm, 2mm, 4mm, and 5mm, respectively. The results show that, under uniaxial compression, phase shift has significant influence on the homogenized tangential stiffness and has significant influence on the maximum failure strength. Under biaxial compression, phase shift also affects maximum strength. No significant difference on the tangential modulus of different laminates is observed under biaxial compression. By optimally arranging phase shift, the maximum strength of laminates under uniaxial compression or biaxial compression can be reached. For laminates consisting of the same microstructures in each layer of lamina, merely using homogenized modulus obtained from one layer of lamina cannot predict the responses of multi-layer-laminae composites due to phase shift.
CZM-Based FVDAM Analysis of Damage Evolution in Cross-Ply Laminates
Progressive cracking of 90ï¯ plies and concomitant delamination of the 0ï¯ / 90ï¯ ply interface in graphite/epoxy cross-ply laminates are analyzed using the 0th-order parametric FVDAM theory with recently incorporated damage evolution capability based on the cohesive-zone model (CZM). Gradual separation of adjacent phases as well as crack evolution may be simulated with this capability within a unified framework based on discontinuity functions recently introduced into the FVDAM framework. The FVDAM simulations of evolving damage in graphite/epoxy cross-ply laminates are compared with available experimental results on the homogenized axial stress-strain and transverse Poisson's responses, as well as crack density as a function of applied load. The effects of resin-rich region along the 0ï¯ / 90ï¯ ply interface, residual stress and variable strength of the 90ï¯ plies are included in the simulations. The newly developed framework offers a unified methodology in simulating on the fly damage evolution in a class of composite laminates due to cracking or progressive interfacial degradation, and in identifying underpinning damage mechanisms that manifest themselves in the homogenized laminate response.
Challenge Problems for the Benchmarking of Micromechanics Analysis
Because of the inherent heterogeneity in composites, the field of micromechanics provides essential tools for the understanding and analyzing composite materials and structures. Micromechanics can serve two purposes: homogenization or prediction of effective properties and dehomogenization or recovery of the local fields in the original heterogeneous material. Many micromechanical tools have been developed and codified, including many commercially-available software packages that offer micromechanical analyses as either a stand-alone tool or as part of a chain of analyses. However, with the increasing number of tools available, the practitioner must determine which tool(s) provides the most value for the problem at hand given budget, time and resource constraints. To date, simple benchmarking examples have been reported in the literature. The present paper suggests a series of comprehensive benchmarking exercises in the field of micromechanics against which such tools can be compared. The microstructures include aligned, continuous fibers in a matrix, with and without an interphase; a 0/90 laminate; spherical inclusions; a plain-weave fabric; and a “randomâ€ short-fiber microstructure. In each case, the material constitutive relations are restricted to linear-elastic. Results from DIGMAT-MF, MAC/GMC, FVDAM, Altair MDS, SwiftComp, and 3D finite element analysis are reported.
Effective Characteristics of Composites with Non-linearly Elastic Components
Majority of fibers used in composite production are linearly elastic in the main range of possible acting stresses. Some nonlinearity can occur at the stresses close to strength. Some polymeric fibers have wider range of non-linearity. Metallic fibers usually have significant range of non-linearity and/or plasticity. In active loading non-linearity is not distinguished from plasticity. Many polymeric binders have some non-linearity of stress-strain diagrams at tension, at compression, and especially at shear. It contributes to the effective nonlinearity of stress-strain diagrams, especially for angle-plies. Metallic matrices have some non-linearity and plasticity. Modeling of fracture of composites based on linearly-elastic theory might be not completely adequate. Development of non-linear models was done mostly for the metal-matrix composites. It was based mainly on applications of methods of theory of plasticity. The model presented in the report was initially developed by the first author at the beginning of seventies. Report is devoted to more advanced version of the model. There are several difficulties to overcome. Using condition that the strains in the constituents of composite and effective strains of composite are much smaller than unity, it is possible to describe diagrams in form of power series of strains. However, the typical shape of diagrams makes it inconvenient because it is necessary to keep many terms to prevent diagrams from decreasing in practical range of strains. Keeping many terms means too many constants have to be found experimentally. Power series in stresses is more convenient, because for practical purposes it is enough to keep linear and cubic terms. The role of quadratic term is to describe differences of diagrams of tension and compression. Inverse dependence can contain many terms, which can be recalculated from few initial characteristics. Another difficulty is related to complex stress state and necessity to use tensor form describing nonlinear dependence of each component of strain on each component of stress. The mixed invariant of stress tensor and tensor of the fourth rank characterized the medium was used as an additional argument converting linear law to nonlinear one. Approximate solutions of boundary problems for non-linear equations are used.
A New Micromechanics Theory for Homogenization and Dehomogenization of Heterogeneous Materials
To avoid the difficulty of applying periodic boundary conditions and deal with aperiodic heterogeneous materials, a new micromechanics theory is developed based on the mechanics of structure genome. This new theory starts with expressing the kinematics including both displacements and strains of the original heterogeneous material in terms of those of the equivalent homogeneous material and fluctuating functions with the kinematic equivalency enforced through integral constraints of the fluctuating functions. Then the principle of minimum total potential energy can be used along with the variational asymptotic method to formulate the variational statement for the micromechanics theory. As this theory does not require boundary conditions, one is free to choose the analysis domain of arbitrary shape and they need not be volumes with periodic boundaries. This theory can also handle periodic materials by enforcing the periodicity of the fluctuating functions. To demonstrate the application of this new theory, we will compare the results of different micromechanics approaches for both periodic and aperiodic materials.
Rapid Consolidation and Curing of Advanced Composites with Electron Beam Irradiation
The ‘holy grail’ of advanced thermoset composites manufacturing would combine resin infusion (mixed in situ), low-cost consolidation, and snap curing into one step. As a first step, a hybrid process that unites vacuum infusion processing (VIP), resin curing by electron beam (EB) irradiation and high pressure consolidation with specialized elastomeric tooling (SET) – referred to as VIP-EB-SET – has been proposed. To demonstrate feasibility, a VIP-EP-SET mold set was designed to allow a flat four-ply carbon fiber preform to be infused with epoxy, consolidated with an elastomer-faced SET tool to 690 kPa and EB-cured using a 3 MeV source. Test specimens were made by drawing resin from a container through a tube into fiber preform-filled mold cavity until full wet-out was achieved, compressing the mold set with specified force in a hydraulic press and then bolting together to maintain that force, and finally exposing to a EB accelerator to cure in 10’s of seconds. Although the thick weave reinforcement fabric and part locations proved troublesome, three parts were successfully made for each combination of two different infusion resins, one with a cationic and the other with a free radical chemistry, and two different consolidation pressures, 91 kPa (full vacuum) and 690 kPa (typical autoclave pressure). Two other parts were also made by wet layup for comparison purposes. Preliminary part quality metrics measured included thickness, maximum flexural stress and flexural secant modulus. As expected, high pressure consolidation with a snap cure system dramatically reduced final part thickness. Interesting, consolidation resulted in significant improvement for part strength and stiffness for the cationic resin but not so for the free radical resin. Although feasibility of the VIP-EB-SET process has clearly been demonstrated, future work will involve more quality metric measurements for existing parts, understanding the role of resin chemistry in this new process, and designing and testing for a 3D part shape of commercial importance.
An Integrated Modeling Approach for Composite Processing
Advanced composite material manufacturing often comprises of preform preheating, non-isothermal resin flow and cure. In the multi-step process, a previous step normally dictates the initial conditions of its immediate subsequent step, with each step alone typically involving highly coupled multi-physics. The inter-step dependency and complex multi-physics coupling pose a challenge to modeling, which is essential for composite process optimization, and process-to-mechanical performance effect evaluation. This paper introduces an integrated predictive modeling approach for modeling the vacuum assisted resin transfer molding process including preheating, resin infusion and cure. Using this approach, models are developed for thick non-crimp fabric laminates, in which non-isothermal resin flow is found to be important. The part sizes are designed to be representative of typical engineering applications. Predictions of processing parameters such as temperature, flow-front, and cure exotherm are validated with experimental results. The proposed approach provides a practical route to new resin system development and composite process optimization to increase the rate of success for desired mold filling and structural integrity, with minimum residual stress from the exothermic resin cure process.
Electrospun Carbon Nanofibers from Polyacrylonitrile Solution: Influence of Relative Humidity on Morphology and Mechanical Properties
Effect of relative humidity (RH) on morphology, fiber diameter distribution and tensile properties of the as-spun, stabilized and carbonized nanofibers has been investigated. The nanofibers are produced from 10% polyacrylonitrile (PAN) in N, Ndimethylformamide (DMF) solution using a homemade electrospinning system. The morphologies of the as-spun nanofibers are varied by changing the relative humidity (RH) from 22% to 60% while keeping all other electrospinning parameters constant. The as-spun nanofibers are subsequently stabilized at 260°C in air for 180 min and carbonized at 1000°C in nitrogen for 60 min. The nanofibers are characterized by scanning electron microscope (SEM) while tensile properties are measured by dynamic mechanical analyzer (DMA). It has been found that the RH affects fiber diameter, surface morphology, and tensile strength significantly of as spun, stabilized and carbonized nanofibers.
Influence of Skin-core Effects on Residual Stress/Strain Distribution in Thick CFRTP Laminates
Carbon fiber reinforced thermo-plastic (CFRTP) has several advantages such as fast-cycle molding and high toughness. Hence CFRTP is recently expected to be applied to primary structures of aircrafts. When we manufacture a thick CFRTP product, it is difficult to suppress temperature distribution in the through-thickness direction under practical high-cycle processing condition. Since the thermoplastic resin does not show a curing reaction, the start time of solidification differs between surface and central parts of the laminate. This difference leads to the residual stress/strain and material property distribution. This phenomenon is called “skin-core effectsâ€. However, the mechanism of skin-core effects has not been investigated experimentally. Therefore, this study evaluates the residual stress/strain distribution due to the skin-core effects in thick CFRTP laminates. We evaluated residual strain distribution in CF/PPS thick uni-directional (UD) and cross-ply laminates by embedded fiber Bragg grating (FBG) sensors. They were placed near the surface and central of the laminate. In the test, specimens were cooled by three methods which resulted in different temperature and cooling speed distributions. Residual strain depending on the cooling method was successfully characterized. This trend was especially-pronounced in UD laminates. It could be estimated that in the rapidly cooled specimen tensile (compressive) stress existed in the central (surface) part after molding. Furthermore, the residual strain of UD specimens changed after the subsequent annealing when the crystallinity did not reach the maximal level. Finally, the effects of residual stress distribution on the mechanical property were evaluated by tensile tests of cross-ply thick laminates. The density of the transverse cracks in the 90-degree layers was non-uniform in the specimen affected by the skin-core effects. It was confirmed that premature cracks occurred due to the residual tensile stress at the central part.
A Model for Electromagnetic Induction of Composites
The electromagnetic and thermal response of composites subjected to magnetic fields is simulated by solving Maxwell and heat transfer equations simultaneously. The developed analysis accounts for the anisotropic nature of the electrical and thermal properties in three dimensions. A finite element code is developed to predict the response of composite structures subjected to transient magnetic fields. The analysis has been validated against a closed form solution and applied to simulate the induction heating process of composite cylinders. The developed analysis can be applied to simulate composite manufacturing processes such as induction cure.
Force Attenuation of Aluminum Foam Core and Stainless Honeycomb Armored Sandwich Specimens Subject to Dynamic Loading
Porous sandwich core specimens are finding increased use in impact limiting applications such as armor protection due to their ability to attenuate transmitted forces via energy dissipative core crushing. For this research, dynamic testing was performed using a Hopkinson bar and gas gun and the transmitted force response of aluminum foam and stainless steel honeycomb sandwich specimens were captured at various stages of core crush. The initial contact force did not change significantly with velocity, but instead, remained relatively constant for a given core density. Increasing core density did increase initial contact force (stiffer response), but also lowered peak force recorded at core densification due to the amount of energy that could be dissipated through the sandwich specimen. Examining two representative coupon specimens (one 0.5 g/cc aluminum foam core and one 6.35 mm cell-size stainless steel honeycomb core), finite element analysis based on an isotropic foam material model in Abaqus/Explicit was able to simulate the transmitted force time histories and appeared similar to the actual dynamic force data. The same material model when applied to a larger 610 x 610 mm panel and subject to a wide area dynamic load experienced similar stress concentrations and response as actual panel tests. The implication of this investigation is that the force attenuation and dynamic behavior of small coupon specimens can be used to validate and predict the behavior of larger panels under dynamic loading.
Mesoscale Simulations of Ballistic Impact: From Woven Fabrics to Unidirectional Composites
Following a careful step-by-step validation, simulations of impact on woven fabrics of Kevlar, PBO, and Dyneema were performed. The procedure consisted on, firstly, validating the impact on a single yarn and checking that the transverse wave velocity and the critical velocity matched the theoretical and experimental values observed using an ultra-high-speed camera. Secondly, a single woven layer was created using the “validatedâ€ yarns. The pyramid that forms during normal impact on the single layer propagates similarly to the transverse wave of a single yarn (but at a different velocity). This pyramid propagation velocity was also compared and validated with the tests and is a characteristic of the fabric material. Finally, using the single layer from the second step, a lay-up with multiple layers was impacted and the position of the apex and base of the pyramid created on the back was measured in the simulations and tests producing a validated multi-layer model. An additional validation performed on the multi-layer model was to compare the ballistic limit of simulations and tests. The multi-layer fabric model was further explored by adding matrix to create a woven composite. Various meshes were created, always respecting the total areal density, matrix volume fraction, and porosity. The model selected was able to reproduce both the apex and pyramid motion in the composite and provide good estimates of the ballistic limits for thick targets. Unidirectional composites have a very different architecture, i.e. they are made of individual fibers embedded in a matrix in a 0/90 lay-up. An FEA model cannot simulate all the individual fibers (microscale) and their interactions because of the high computational cost. Instead, the strategy adopted was to use a mesoscale model where the fibers of a layer are bundled in a flat yarn. The results and validation of this model as well as future challenges will also be presented in this paper.
Practical and Robust Bonded Joint Analysis Methodology: Application to Durable Redundant Joints
Adhesively bonded joints offer weight savings over mechanical joint designs. However, a disadvantage of bonded joints is that their structural performance is highly sensitive to manufacturing parameters. This poses a challenge when assessing joint strength. Since failure is most likely to initiate near the free edges of the joint, significant effort has been expended to better predict the stress field near those regions. This paper recommends that the design be shown to be robust by assuming flaws in regions of the joint where non-destructive inspection (NDI) is ineffective. This requires characterization of the joint’s fracture toughness using coupon level tests, which should use the same manufacturing method as service hardware. The fracture toughness is then used to predict the joint strength and to adjust the design as needed, while keeping the limitations of NDI in mind. This methodology is illustrated with a durable redundant joint and a secondary structure bonded to a primary structure. Known limitations of NDI are used to place flaws in joint locations where they may not be detected, and the capability of the joint is estimated. While, theoretically, a joint design may be defect tolerant, it is important to recognize that the structure may still not be reliable because of the potential for weak bonds, a separate phenomenon. A methodology composed of proof testing combined with pre- and post-proof NDI will be discussed as an effective workmanship screen for mitigating concerns with weak bonds.
Improving the Damage Tolerance of Durable Redundant Joints
A previous analysis and test program demonstrated that the Durable Redundant Joint (DRJ), a patented concept that features multiple adhesive load paths, exhibited superior performance compared to a conventional joint design. This paper describes follow-on investigations on design improvements to the DRJ to further understand its damage tolerance characteristics. Before embarking on this particular study, it was necessary to further investigate the failure sequence of the DRJ. One short flaw and one large flaw were intentionally manufactured between the doubler termination and the facesheet. Surprisingly, fracture analysis predicted the shorter flaw to be more critical than the larger flaw, and it was confirmed with examination of the coupons. Using the insight gained from these studies, three design attributes are investigated relative to increasing damage tolerance characteristics of the DRJ: (1) Insert stiffness, (2) Relative length between the doubler and the insert, and (3) The use of tapers at the ends of doublers. The paper will include findings from all these ongoing studies.
Failure Predictions of Out-of-Autoclave Sandwich Joints with Delaminations under Flexure Loads
An analysis and a test program was conducted to investigate the damage tolerance of composite sandwich joints. The joints contained a single circular delamination between the face-sheet and the doubler. The coupons were fabricated through out-of-autoclave (OOA) processes, a technology NASA is investigating for joining large composite sections. The four-point bend flexure test was used to induce compression loading into the side of the joint where the delamination was placed. The compression side was chosen since it tends to be one of the most critical loads in launch vehicles. Autoclave cure was used to manufacture the composite sandwich sections, while the doubler was co-bonded onto the sandwich face-sheet using an OOA process after sandwich panels were cured. A building block approach was adopted to characterize the mechanical properties of the joint material, including the fracture toughness between the doubler and face-sheet. Twelve four-point-bend samples were tested, six in the sandwich core ribbon orientation and six in sandwich core cross-ribbon direction. Analysis predicted failure initiation and propagation at the pre-delaminated location, consistent with experimental observations. Fracture analyses methods predicted failure loads in close agreement with tests. This investigation demonstrated a strength reduction of 10 percent due to a flaw of significant size compared to the width of the sample. Therefore, concerns of bonding an OOA material to an in-autoclave material was mitigated for the geometries, materials, and load configurations considered.
Plasma Coating of Glass Fibers to Improve the Interfacial Shear Strength in GF/Polyester Composites
The fiber coating method is essential for improving the interfacial adhesion in glass fiber reinforced polyester composites. In our study, plasma polymer interlayers of pure tetravinylsilane monomer and monomer in a mixture with oxygen gas (0-71%) were used to surface modify glass fibers at an effective power of 2.5 W. We demonstrate that elemental composition and chemical structure of plasma polymer can be influenced by an amount of oxygen atoms (0-19 at.%) partly incorporated into plasma polymer network and partly forming side hydroxyl and carbonyl groups eliminating cross-linking of the network. Thus, the chemical properties result in reduced mechanical properties at enhanced oxygen concentration. The adhesion at the interlayer/glass interface can be optimized as the result of sufficient concentration of Si-O-C bonding species and adequate crosslinking of the network. The vinyl groups at the interlayer surface are responsible for chemical bonding to the polyester resin and their concentration was approximately constant. The polar groups (hydroxyl, carbonyl) at the interlayer surface are favorable for wetting the surface by the resin. A fiber bundle (unsized fibers) coated by plasma coatings was embedded in polyester resin and cured to fabricate composite specimen that was cut and polished using conventional metallographic techniques to form composite cross-sections. The microindentation test (Interfacial Testing System) was carried out on the individually selected glass fibers (GF) on a cross-section of GF/polyester composite to determine the interfacial shear strength. The shear strength for plasma coated fibers depends on oxygen amount in mixture and varied from 0 to 32% above the strength of industrially sized fibers. Nonlinear finite element analysis was used to correctly interpret the microindentation data. The plasma polymer interlayers with high shear yield strength (0.6-1.5 GPa) may be effectively applied to improve the interfacial shear strength in GF/polyester composites.
An Assessment of the SFFT in Hierarchical Composite Materials
The science of materials has witnessed a notorious development of composite materials using nano-scale reinforcements such as nano-tubes, nano-flakes, or nano-fibers, all with promising exceptional mechanical properties. However, when these composites have been used to replace advanced fiber reinforced composites materials in certain applications, the results have been poor or definitively insufficient. An approach proposed to take advantages of the nano-scale material outstanding properties is their use in hierarchical composite materials, that is, in multi-scale composites materials. The use of the single-fiber fragmentation test to measure the IFFS is analyzed in this paper considering the presence of the nano-reinforcement material in the hierarchical composite material. A parametric study of the stress distribution for a monofilament carbon fiber whose surface is modified with carbon nanotubes (CNT) and embedded in an epoxy resin is first discussed. The analysis at two different scales, nano- and micro-scales and the stress interactions between them was necessary. For this purpose, concepts of the mechanics of a heterogeneous media, specifically the Mori-Tanaka Methods was used for the nano-scale interactions. The finite element method was used for the micro-scale. The linear elastic results reveal that, when the CNT are present in the carbon fiber-surface, contribute to a better stress distribution along the fiber-matrix interface once the fiberfragmentation process is started. However, the presence of the CNT may contribute to the appearance of large stress concentrations in the surrounding matrix, especially when the CNTs are randomly oriented. An attempt to establish relationships between the stress-transfer behavior and the physico-chemical interactions is made.
Determination of Key Mechanical Properties for Simulating Delamination Damage in Impacted Bonded Joints of Composites
Non-linear dynamic FE simulations of impact event and ensuing progressive delamination and disbond failure, were performed for lap joints of composites (woven fabric reinforced phenolic) having four different overlap widths. The joints were impacted transversely using a hemispherical impactor with different velocities in the low velocity impact range. These simulations were used to explain the experimentally observed dependence of damage on overlap width and impact velocity using the concept of characteristic overlap width. The experimental observations of damage mechanisms were carried out using a combination of techniques including, through transmission photography, ultrasonic c-scan and x-ray micro tomography (XMT). Progressive damage simulations like the one described in this study require significant input material properties to run and generally little details are provided in literature about the experimental procedure to determine these values. Thus in this paper, determination of key material parameters for progressive delamination modelling in single lap joints of woven glass fabric reinforced phenolic composites, has been discussed thoroughly.
Study of Low Velocity Impact Damage in CFRP Laminates Using 3D Computed Tomography
In this work low velocity impact damage in IM7/977-3 carbon fiber reinforced polymer (CFRP) composites has been investigated using 3D computed tomography (CT). 32-ply IM7/977-3 symmetric cross-ply composites were impacted at different impact energy levels and with different impactors (flat-ended cylindrical and hemispherical) using an Instron 8200 Dynatup drop-weight impact machine. Impact energy was chosen to produce slight damage, which was characterized by the small crack on the front, impacted side and small visible delamination on the back, nonimpacted side of the specimens, and barely visible damage, which was characterized by slight indentation on the impacted side, but no visible delamination on the back side of the specimens. The difference in the impact energy to produce these types of damage was about 30%. Internal impact damage was assessed using ZEISS METROTOM 1500 CT scanning system. CT images were reconstructed using VGStudio MAX. To determine the extent of the damage zone, impacted 6 in by 6 in composite specimens were scanned first. As the relatively large specimen size did not allow for detailed evaluation of damage (internal cracks and delaminations were not visible), smaller 45 mm by 45 mm specimens, with enclosed impact damage zones, were cut out and analyzed. The CT imaging results showed that volume of the impact damage zone had a positive correlation with impact energy, maximum load and maximum deflection, but that relationship was generally weak. Absence of noticeable correlation between damage volume and impact energy was unexpected as the difference in the impact energy was up to 30%.
Structural CNT Composites Part II: Assessment of CNT Yarns as Reinforcement for Composite Overwrapped Pressure Vessels
Carbon nanotubes (CNTs) are one-dimensional nanomaterials with outstanding electrical and thermal conductivities and mechanical properties. This combination of properties offers routes to enable lightweight structural aerospace components. Recent advances in the manufacturing of CNTs have made bulk forms such as yarns, tapes and sheets available in commercial quantities to permit the evaluation of these materials for aerospace use, where the superior tensile properties of CNT composites can be exploited in tension dominated applications such as composite overwrapped pressure vessels (COPVs). To investigate their utility in this application, aluminum rings were overwrapped with thermoset/CNT yarn composite and their mechanical properties measured. CNT composite overwrap characteristics such as processing method, CNT/resin ratio, and applied tension during CNT yarn winding were varied to determine their effects on the mechanical performance of the CNT composite overwrapped Al rings (CCOARs). Mechanical properties of the CCOARs were measured under static and cyclic loads at room, elevated, and cryogenic temperatures to evaluate their mechanical performance relative to bare Al rings. At room temperature, the breaking load of CCOARs with a 10.8% additional weight due to the CNT yarn/thermoset overwrap increased by over 200% compared to the bare Al ring. The quality of the wound CNT composites was also investigated using x-ray computed tomography.
Structural CNT Composites Part I: Developing a Carbon Nanotube Filament Winder
Carbon nanotube (CNT) based materials promise advances in the production of high strength and multifunctional components for aerospace and other applications. Specifically, in tension dominated applications, the latest CNT based filaments are yielding composite properties comparable to or exceeding composites from more established fibers such as Kevlar and carbon fiber. However, for the properties of these materials to be fully realized at the component level, suitable manufacturing processes have to be developed. These materials handle differently from conventional fibers, with different wetting characteristics and behavior under load. The limited availability of bulk forms also requires that the equipment be scaled down accordingly to tailor the process development approach to material availability. Here, the development of hardware and software for filament winding of carbon nanotube based tapes and yarns is described. This hardware features precision guidance of the CNT material and control of the winding tension over a wide range in an open architecture that allows for effective process control and troubleshooting during winding. Use of the filament winder to develop CNT based Composite Overwrapped Pressure Vessels (COPVs) shall also be discussed.
Effect of MWCNTs on Creep of Epoxy for CFRP Deployable Composites
Deployable aerospace joints are proposed to be made of thin laminated carbon fiber reinforced polymer (CFRP) composites. To enable efficient deployment, a method to dissipate the elastic strain energy that develops in the CFRP deployable composites is necessary. A three layer tape spring, incorporating two outer plain weave carbon fiber laminas loaded off-axis and one middle unidirectional carbon fiber lamina loaded onaxis, has been suggested. The proposed tape spring will enable damping of the deployment process by enabling creep/stress-relaxation in the plain weave carbon fiber laminas. Studies show that creep behavior of the tape spring is governed by creep of epoxy used in fabrication of the CFRP deployable composites. In this work, we suggest that creep of epoxy used for CFRP deployable composites can be controlled using Multi-walled Carbon Nanotubes (MWCNTs). Three contents of MWCNTs including 0.0 wt.%, 0.5 wt.% and 1.0 wt.% of the total weight of epoxy are examined. DMA testing of neat epoxy and epoxy-MWCNTs nanocomposites incorporating surface functionalized MWCNTs is reported to extract the creep master curves for epoxy incorporating three different weight content of MWCNTs. Laplace transform is used to identify Prony series coefficients for stressrelaxation curves of epoxy with three different weight content of MWCNTs. A finite element model incorporating the viscoelastic behavior of plain weave lamina was developed and used to simulate the stress-relaxation behavior of the tape spring. It is apparent that incorporating functionalized MWCNTs resulted in altering the mechanical performance of epoxy. In particular, the use of MWCNTs results in increasing stress-relaxation of the plain weave lamina during stowage time period. The observed change in creep of epoxy incorporating MWCNTs would improve strain energy dissipation during the stowage period and thus better control of CFRP tape spring deployment compared with that produced with neat epoxy without MWCNTs.
An Enriched Shell Element for Delamination Simulation in Composite Laminates
A formulation is presented for an enriched shell finite element capable of delamination simulation in composite laminates. The element uses an adaptive splitting approach for damage characterization that allows for straightforward low-fidelity model creation and a numerically efficient solution. The Floating Node Method is used in conjunction with the Virtual Crack Closure Technique to predict delamination growth and represent it discretely at an arbitrary ply interface. The enriched element is verified for Mode I delamination simulation using numerical benchmark data. After determining important mesh configuration guidelines for the vicinity of the delamination front in the model, a good correlation was found between the enriched shell element model results and the benchmark data set.
Challenges in Experimental Validation of Composite Damage Progression Models
While attempting to benchmark composite progressive damage analysis tools, the Air Force Research Laboratory experienced challenges in obtaining some of the material properties needed for input to the models. Specifically, the standard tests conducted to obtain the transverse tension strength (ASTM 3039) and longitudinal compression strength (ASTM 3410) resulted in unacceptable failure modes. Alternate testing approaches were investigated which produced a consistent set of results and recommendations for future model calibrations.
Composite Bonded and Bolted Joint Analysis Toolkit Based on Discrete Crack Network
Both bolted and bonded joints have been used extensively in the integration and repair of composite structures. The load transfer provided by either a bolted or a bonded joint often drives the design under monotonic and fatigue loading. A composite bolted joint is a complex mechanical system that includes a plastically deformed bolt, damaged base and adjoint laminated plate, contact surfaces, and pretension in the bolt. An adhesively bonded composite structure of arbitrary configuration is also a complex mechanical system that includes local plastic deformation in an adhesive layer, intra damage and its associated stiffness degradation within a composite ply and/or within the adhesive, interface damage from debonding and delamination, contact between these delaminated interfaces, spatial distribution and evolution of the fabrication and environment variation induced residual stress, and the impact of temperature and moisture distribution within the bonded structure on the material properties and modeling parameters. Accurate evaluation of the 3D stress state from the joint system has a direct impact on the damage initiation and the resulting stress re-distribution. In order to accurately capture the damage evolution without remeshing and their associated energy dissipation, an extended finite element based approach for static and fatigue failure prediction of both bolted and bonded joints is developed for Abaqus via its user-defined element. To facilitate the model generation, property definition, contact and cohesive interface definition, and display of damage progression associated with matrix cracking and delamination, a composite bonded and bolted analysis toolkit for Abaqus (CB2ATA) is developed to simulate the discrete damage evolution under static and cyclic loading. A suite of verification and validation examples are use to demonstrate its applicability in the prediction of the first failure mode, post-peak softening, and fatigue damage accumulation of bonded and bolted composite structures.
Buckling Characteristics of Composite Sandwich Plates with Corrugated Core
Buckling characteristics of composite sandwich plates with corrugated core under transverse loading is studied using finite element method. The sandwich construction constitutes of a repetitive corrugated core and two face panels. The corrugated core has either a triangular shape or a rectangular shape. The material for both face and core panels has the same laminate construction, which is either a [±α]S or a [0/α]S. The fiber orientation angle α in the laminates is varied from 0 to 90o. The study first considers the local deformations of the face panels and the web panels. It then determines the buckling modes and critical buckling loads of the sandwich structures. The effect of fiber orientation angle on the critical buckling loads of the sandwich plates with two different core shapes is demonstrated.
Evaluation of Adhesively Bonded Repair and Scarf-Type Infused Repair of Impact Damaged Fiber Reinforced Composites
The high specific stiffness and strength offered by composite materials has allowed significant adoption of these materials in aeronautic, automotive, naval, defense industries and construction sectors to substitute conventional materials such as steel, aluminum and other alloy materials. In transport sectors, specifically in aeronautical, automotive and marine, in which low velocity impacts are frequent, composite repair technologies have attracted much attention. Recently, various repair techniques have been developed to rehabilitate the damaged composite structures. Experimental and numerical studies to investigate the influence of different parameters on the strain-stress distribution, failure mode, ultimate strength, and stress concentration factor of repaired composite structures have been recently reported. In this paper two types of repairing methods were considered; adhesively bonded and infused scarf-type patches. The performance of both repair techniques applied to GFRP laminates was assessed under three-point bending test. The specimens were eight balanced plain weave S-glass layers/epoxy resin manufactured using infusion technique. In infused patches, two types of resins, namely, pristine and nano-modified resins were used to investigate their effects on the flexural strength and failure modes of the repaired composite specimens. Results indicate that depending on the type of repair and nano-filler used the flexural properties of repaired composite specimen exhibited significant improvements. The choice of the type of repair would vary on the desired application along with accessibility to the damaged area. Overall, repair of composites allow considerable economic benefits over the replacement of an entire structural component.
Nonlinear Analysis and Preliminary Testing Results of a Hybrid Wing Body Center Section Test Article
A large test article was recently designed, analyzed, fabricated, and successfully tested up to the representative design ultimate loads to demonstrate that stiffened composite panels with through-the-thickness reinforcement are a viable option for the next generation large transport category aircraft, including non-conventional configurations such as the hybrid wing body. This paper focuses on finite element analysis and test data correlation of the hybrid wing body center section test article under mechanical, pressure and combined load conditions. Good agreement between predictive nonlinear finite element analysis and test data is found. Results indicate that a geometrically nonlinear analysis is needed to accurately capture the behavior of the non-circular pressurized and highly-stressed structure when the design approach permits local buckling.
Extending Service Life and Detecting Degradation of Poly(p-phenylene-2,6-benzobisoxazole) (PBO) Fiber
Poly(p-phenylene-2,6-benzobisoxazole) (PBO) fiber has a high strength, a high modulus, and is light weight, which makes it a leading candidate for soft body armor and fiber reinforced plastic composites. The use of PBO fiber in protective applications is limited by its mechanical degradation during service. Battelle has developed a supercritical (sc)CO2 chemical diffusion post processing method to treat PBO fiber that decreases the rate of mechanical properties degradation and increases its useable service life. The tensile data show that this treatment yields a 14 % increase for tenacity over untreated fibers after 120 days of accelerated aging (70 ºC, 90 %RH) when using 3- glycidoxypropyldimethoxymethylsilane (GPDMS) epoxy silane as the chemical infused into PBO fibers. The epoxy silane chosen demonstrated an incremental increase in fiber life; with optimization, greater gains could be realized. Additionally, fluorescent spectroscopy was performed on methanol rinses of aged PBO fibers as an analytical method for determining extent of fiber degradation. Initial testing has indicated that the intensity of fluorescence peak emission have a correlation to the degradation level of the PBO fiber. There is potential for an analytical technique to be developed to test fibers from deployed soft armor to determine if the material still possesses ballistic protection, or if it should be removed from service.
Ballistic Modeling of S-2 Glass/SC-15 Epoxy Composites Using Mesoscale Models
Traditional continuum models can typically be used to model the elastic behavior of composite structures, but they often break down once nonlinear deformation and damage begin to occur. Accurate modeling of these behaviors, however, is required for accurate predictions of the composite’s response to ballistic impact. Thus, many current efforts have focused on mesoscale modeling to better capture the relevant deformation and damage mechanisms that can occur within composites. New mesoscale models describing S-2 glass/SC-15 epoxy resin plainweave and 3D-weave composites were recently created. Because the yarns, matrix, and interfaces are individually defined, the models can capture damage due to yarn breakage and yarn/matrix delamination despite the use of simple material models for each component. The mesoscale models were previously validated against quasistatic and medium-rate mechanical tests that subjected composite specimens to a larger variety of tensile and shear loads than are typically used during typical model validations found in the literature. In this study, simulation results using the plainweave mesoscale model are compared to ballistic data and photographs from tested composite panels available in the literature. The mesoscale models can accurately predict the ballistic limit of S-2 glass composite panels and damage extents after impact.
Comparison Between Numerical and Experimental Results on Ballistic Strength of Multi-layer Fabric Body Armors
In this paper, a fiber level micromechanics model is utilized to simulate real scale ballistic tests against multi-layer fabric soft body armors. The bottle-neck obstacle for micro-scale simulation of real scale ballistic tests of fabric body armors is computer capacity limitation. In order to improve efficiency, hybrid digital element meshes are adopted. Two hybrid element mesh concepts are investigated. The first concept is similar to the concept employed in the finite element approach, called the area based hybrid mesh concept. In its application, yarns are discretized into fine digital fibers in the vicinity of the ballistic impact center and into coarse digital fibers in regions far from the impact center. The second hybrid mesh concept is called the yarn based hybrid mesh. In its application, only principal yarns and yarns near principal yarns are discretized into fine digital fibers because high stress only develops within principal yarns after ballistic impact. Other yarns are discretized into coarse digital fibers. Numerical results evidence the yarn based hybrid mesh as being much more efficient than the area based hybrid mesh. Because only a few principal yarns resist load in a typical ballistic impact, the yarn based hybrid mesh technique could improve simulation efficiency up to 90-95% without sacrificing accuracy. This would enable simulation of the perforation process of real scale body armor using a fiber level micromechanics model. In this investigation, the yarn based mesh is applied to simulate real scale ballistic tests of soft body armor systems composed of 4 to 28 piles of 2-D plain woven fabrics. Ballistic limits, V50, derived by numerical simulations are compared to real scale standard ballistic results.
Modeling Transverse Impact on Multi-Layer UHMWPE Soft Ballistic Armor Pack (SBAP)
High performance UHMWPE soft ballistic sub-laminates ([0/90]ô€¯¡, SBSL) are stacked to build a soft ballistic armor pack (SBAP) which can defeat hand gun projectiles. Transverse impact on a single layer soft ballistic sub-laminate is modeled with shell elements and is solved using LS-DYNA composite material model MAT54. The finite element model is validated using 1D fiber and 2D membrane theories. The validated FE models are then used to study the transverse impact and perforation behavior of a multi-layer soft ballistic armor pack (SBAP). Perforation mechanics of a SBAP consisting of 32 layers of [0/90] SK76/polyurethane soft ballistic sublaminates (SBSL) is presented.
Phenylethynyl-Terminated Polyimide (LaRC PETI-5), Exfoliated Graphite Nanoplatelets (xGnP), and Their Composites: An Overview
In order to understand the characterization, properties, and processing of phenylethynyl-terminated imide (LaRC PETI-5) oligomers and polymers as a hightemperature sizing material for carbon fiber composites, the imidization and thermal curing behaviors based on phenylethynyl end-group reaction were explored. The role of LaRC PETI-5 as a high-temperature interphase sizing material to enhance the interfacial adhesion between the carbon fiber and the BMI matrix as well as the dynamic mechanical properties of carbon/BMI composites was discussed. Real-time thermal expansion behavior of intercalated graphite flake was preliminarily studied. The effects of exfoliated graphite nanoplatelets (xGnP) on the properties of LaRC PETI-5 matrix composites were also investigated. The dynamic mechanical and thermal properties, the fracture surface morphology, and the electrical resistivity of phenylethynyl-terminated polyimide composites reinforced with xGnP were emphasized. The effect of carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN)-functionalized xGnP on the impact toughness, tensile properties, thermal stability, fracture surfaces and electrical surface resistivity of xGnP/vinyl ester composites has been stressed.
Polylactic Acid Based Blends and Biocomposites with Improved Performance
In an effort to improve the toughness of PLA, multiphase blends of PLA, ethylene-methyl acrylate-glycidyl methacrylate (EMAGMA) terpolymer and a partially biobased thermoplastic copolyester elastomer, Hytrel (HYT) were fabricated through reactive melt blending. High impact strength with non break behavior was achieved at an optimized blending ratio of 70 wt% PLA, 20 wt% EMAGMA and 10 wt% HYT. A unique morphological structure with partial encapsulation of GMA and HYT minor phases was observed for the PLA/GMA/HYT (70/20/10) blend revealing the importance of particular blend morphology and composition in enhancing the toughness. In order to achieve concurrent improvements in impact strength and heat deflection temperature (HDT) of PLA, the super-toughened blend was combined with natural fibers and nucleating agents to fabricate PLA biocomposites. Mechanical and thermal properties assessment revealed a 4 fold increase in impact strength compared to neat PLA and the HDT of PLA biocomposites increased to 88°C from 55°C noted for neat PLA and PLA blends. With the addition of 1 wt% Lak-301 as nucleating agent at mold temperature of 110°C PLA biocomposites exhibited dramatic reduction in crystallization half time to 1.3 min with crystallinity content of 42%.
Advanced Biorefinery Towards a New Innovation in Biocomposites Uses: How Sustainable this Journey is!
The thermochemical and biochemical conversion of biomass are striving towards developing materials, chemicals and fuels (bioproducts). Again, the well-accepted biorefinery approach has been envisioned for a sustainable growth of such emerging biobased industries. One of the major challenges for these industries is the disposal and management of their downstream products generated during the biomass conversion. Such coproducts and byproducts from biofuel industries currently being undervalued and show tremendous potential in value-added biocomposite uses. World Economic Forum estimates that the biorefinery sector across the whole biomass valuechain would generate $340 billion per year by 2020. The advanced biorefinery approach is targeted to recover the value from every coproduct of biomass conversion, ensuring the sustainability. “There is no such thing as waste – these are coproducts that look for an opportunity in new value-added industrial usesâ€ – is defined here as “advanced biorefineryâ€. This article highlights the recent advances in the utilization of coproducts from biofuel and pyrolysis industries for biocomposite manufacturing.
Exfoliated Graphite Nanoplatelets (xGnP) Based Nanocomposites: Its Past, Present, and Future
The research of Exfoliated Graphite Nanoplatelets (xGnP™) originally started back in 1998 in Drzal group at Michigan State University. The original idea was to mimic the reaction of clay nanocomposite, which had been developed from late 80s to early 90s. However, the difference in chemistry soon made it clear that totally different strategy was necessary to exfoliate graphite to make very thin nanomaterials. Later this research was expanded to investigate the properties and fabrication processes of graphene and graphene family nanomaterials (GFN). Since then, a variety of xGnP with different characteristics have been developed and the detailed characteristics and properties of these materials have been analyzed and investigated. The size of these xGnP varies from sub-micron to 100um, the thickness from a few nm to 20nm, and the surface area from 50 m2/g to over 700 m2/g. Following the development of these nanomaterials, many xGnP/polymer nanocomposites have been fabricated and investigated. These nanocomposites include thermoplastics, thermosets, multi-phase hybrid systems, SMC, papers, inks, coatings, and thermal interface materials. These composites often exhibited multi-functionality, which include mechanical, electrical, thermal, barrier, abrasion resistance, and lubricity. These materials have been expected to be used in many applications such as electronics, automotive, biosensors, and batteries. Depending on the requirements of each application, the properties of the xGnP based nanocomposites can be tailored. Based on these research and development efforts, some of the composite materials have already been used in commercial applications. This paper will look back the history of xGnP and xGnP based composite related research from the beginning and summarizes the results. Then, the current development works and future possibilities will be examined.
A Numerical Study on the Mechanical Behavior of Composite Suture Joint Structure
The mechanical strength for composite joint structure is of great importance for its application in aerospace engineering. The traditional bolt connection introduces extra stress concentration due to the drilling of hole, and will result in earlier failure of the composites structure. Based on nature suture joint mechanics, a composite suture joint is introduced to improve the loading capability of composite joint structures. In this work, finite element models for various designs of suture joint connection are developed and analyzed to study the interaction between geometric parameters (tooth angle, tooth density and complexity etc.) and structure performance. Cohesive element model is applied to model the interface failure of the suture joint. The progressive damage behavior of the suture joint structure is studied. The numerical results show the capability of this suture joint structure in reducing stress concentration factor and improving the structural damage tolerance. Through this work, we will achieve understandings of the correlation between mechanical strength, damage mechanism and suture geometries, which is of great important for the design and optimization of novel high performance composite joint structures.
Development of Reversible Bonded Joints Using Nano-Ferromagnetic Particles
In this study, an efficient, simple, inexpensive and reversible adhesive bonding technology was developed to improved life cycle and recycling solutions for vehicle applications. For the purpose of remotely heating and melting the adhesive by subjecting the modified adhesive to an external electromagnetic fields, varying concentrations of commercially available nano-ferromagnetic particles were embedded in thermoplastic hot-melt adhesives using extrusion mixing process to investigate the thermal behavior and interaction with the electromagnetic field for this novel material. Furthermore, advanced deformation and temperature profile measurement techniques were used to monitor the temperature profiles of the substrates and nano-ferromagnetic particles modified adhesive film while subjected to the electromagnetic fields using infrared (IR) thermal camera and Fiber Bragg-grating (FBG) sensors. Results indicate that depending on the amount of nano-ferromagnetic particles and electromagnetic field exposure time, rapid temperature rise at targeted location (adhesive film) was observed, while the substrate/surrounding materials exhibited minimal temperature change. Overall, reversible adhesives such as the ones explored in this work show great promise in a wide range of multi-material joining applications.
Failure Analysis of Adhesively Bonded GFRP Single Lap Joints After Cyclic Environmental Loading
This experimental study provides an insight into failure mode analysis of adhesively bonded composite single lap joint that are tested in tensile shear after they have been subjected to cyclic heat and high relative humidity conditions. Glass fiber reinforced polymer (GFRP) composite is used with a liquid type adhesive. Changes in static load transfer capacity is examined in light of the failure analysis of fracture surfaces. Fracture surfaces of the bonded area is divided into different regions, and they are examined with both Scanning Electron Microscope and Energy Dispersive Spectrometry, as well as the cross-section of the adherend by Optical Metallography. Test data show that the ultimate failure load of GFRP/GFRP single lap joint is tripled by heat cyclic at high relative humidity, as compared to the baseline value at ambient condition. Detailed failure analysis, discussions, and conclusions are presented.
End-to-End FE Based Homogenization of Woven Composites
Whereas prediction of elastic properties of woven is a standard in the industry, prediction of their non-linear behavior remains a challenge. Accurate prediction of the non-linear behavior is the key for a successful modeling of woven composites solicited by off-axis loading. Woven composites are represented by interlacing yarns impregnated by a resin matrix. Yarns are made of a resin matrix reinforced by continuous fibers. Homogenization of woven composites therefore requires two levels of homogenization, the one of the yarn and the one of the ply. Finite element (FE) based homogenization at the ply level can be combined with mean-field homogenization at the yarn to predict the mechanical behavior of a single ply. The main difficulty of this approach lies in the generation of representative volume element (RVE) of a single ply. An end-to-end FE based homogenization of woven composites was developed in Digimat®. The developed set of tools allows running finite element simulations on single ply woven RVE with the use of mean-field homogenization at the yarn level. Generated RVEs are meshed with voxels to avoid meshing troubles in resin rich pockets between yarns or close to yarn crossings. The local fiber orientation used in the yarn homogenization is function of the yarn curvature. A fully analytical framework based on mean-field homogenization has also been developed. The particularity of this framework is that it takes yarn curvature into account. The developed tools will be presented and the FE based and mean-field homogenization predictions of linear and non-linear properties will be compared to experimental measurements on plain weave and 5HS carbon epoxy woven composites.
Multi-Scale Modeling of Failure of Continuous Carbon Fiber Composites for Virtual Allowables
Aerospace and automotive industries are increasingly using continuous carbon fiber composite to answer lightweighting challenges and meet high performance requirements. Such composite materials combine a high strength and stiffness with low density, making them competitive compared to classical metal based solutions. But this unique combination of material properties comes along with increased complexity across several scales, requiring extensive material characterization. Starting at the constituent level, matrix and fiber behavior drive the ply response, which typically shows anisotropy and non-linearity. Failure mechanisms are various (matrix cracking, fiber breakage, fiber-matrix debonding, delamination). Different ply stacking sequences must be investigated in the design process. And various laminate tests must be performed, such as unnotched tension/compression and open-hole tension/compression. Across all scales variability must be considered, such as constituent property variability or fiber volume fraction variability. This overall complexity results in extensive experimental coupon testing, expensive from both a time and cost point of view. Hence there is a strong need for a simulation solution which predicts composite coupon properties, replacing physical testing by virtual testing. This paper describes the technology behind the simulation software recently developed by e-Xstream engineering aiming at generating virtual allowables. The software offers micromechanics and continuum damage to describe the complex composite material behavior combined with a non-linear finite element analysis generator and solver to efficiently predict notched and unnotched coupon testing results and cover any desired test matrix. Variability can be applied to micro-level parameters such as constituent properties or fiber amount in order to generate A or B-basis allowables, thereby significantly reducing the amount of required physical tests and allowing a deeper understanding of the material parameters driving the composite strength.
The Effects of Extrudate Swell, Nozzle Shape, and the Nozzle Convergence Zone on Fiber Orientation in Fused Deposition Modeling Nozzle Flow
Fused Deposition Modeling (FDM) is a rapidly growing Additive Manufacturing (AM) technology that allows for low cost and intricate three-dimensional parts to be created. FDM parts are created in layers by extruding a bead of polymer melt from a filament fed nozzle onto a platform. Several recent developments aimed at improving the mechanical properties of FDM components have focused on adding structural chopped fibers to the FDM filament feedstock. It is understood that the discrete fibers will orient in response to the melt flow within the nozzle. In addition the fluid expansion effects of extrudate swell at the nozzle exit impacts the melt velocity gradients and therefore the underlying fiber orientation. The local varying fiber orientation directly impacts the mechanical properties of the part, making the fiber orientation prediction during processing a necessary portion of FDM composite part design. A computational approach is presented for determining fiber orientation in the nozzle flow domain taking into consideration the effects of the convergence zone in the FDM extrusion nozzle, the fluid expansion caused by extrudate swell, and nozzle shape on the fiber orientation in the extruded polymer. The polymer melt is modeled as a Newtonian fluid in Creeping flow and includes extrudate swell expansion. Fiber orientation is calculated using the Advani-Tucker (1987) orientation tensors with the Fast Exact Closure from Montgomery-Smith, et. al. (2009) assuming the Folgar- Tucker (1984) isotropic rotary diffusion model for fiber-fiber interactions. The results show that the shear flow prior to the nozzle section causes high alignment in the direction of the flow as is expected. This alignment state is further increased within the nozzle convergence zone. The expansion flow in the die swell region causes a noticeable decrease in fiber alignment in the direction of the flow and an increase in fiber alignment transverse to the flow direction.
The Integration of Composite Constituent-Level Failure Models During Composite Size Optimization
A number of structural optimization methods have been applied over the years to aid in the design process for structural components. The design criteria are often based on stress, strain, or displacement allowables which can be easily calculated with any finite element analysis model. For composites, the design criteria are often based on more detailed failure models at the constituent level which attempt to evaluate interlaminar shear and tension, matrix crazing and shear yielding, and fiber fracture. This paper looks at methods to be able to incorporate advanced composite failure models, including commercial codes and in-house proprietary models, into the optimization process to further improve the optimization results. A few case studies will be reviewed with a discussion on the challenges and potential solutions to incorporating these methods in a simple and efficient manner.
Prediction of Fracture Properties in Nanographene Reinforced Polymers Using Atomistic Simulations
This paper is directed towards understanding damage initiation and failure progression in advanced nanostructured composite materials using molecular dynamics. The critical value of the J-integral (JI) at crack initiation is related to the fracture toughness of the material, where the subscript I denotes the fracture mode (I=1, 2, 3). Therefore, the J-integral could be used as a suitable metric for estimating the crack driving force as well as the fracture toughness of the material as the crack begins to initiate. However, for the conventional macroscale definition of the J-integral to be valid at the nanoscale in terms of the continuum stress and displacement fields and their spatial derivatives requires the construction of local continuum fields from discrete atomistic data, and using these data in the conventional contour integral expression for atomistic J-integral. One such methodology is proposed by Hardy that allows for the local averaging necessary to obtain the definition of free energy, deformation gradient, and Piola-Kirchoff stress as fields (and divergence of fields) and not just as total system averages. Further, the conventional isothermal definition of J-integral does not take into account the entropic contribution to the free energy, and consequently, may lead to significant over- estimation of the J-integral at the atomistic level, especially at finite temperatures. As a case study, the feasibility of computing the dynamic atomistic J-integral over the molecular dynamics (MD) domain at finite is evaluated for a graphene nano-platelet and the values are compared with results from linear elastic fracture mechanics (LEFM) for isothermal crack initiation at 0 K and at 300 K. Good agreement is observed between the atomistic J and the LEFM results at 0 K, with predictable discrepancies at 300 K due to entropic effects, which have been discussed in detail in the paper. Issues regarding concurrent coupling of MD with continuum domain are also discussed in this paper.
Modeling of Fracture Behavior in Polymer Composite Using Concurrent Multi-Scale Coupling Approach
Embedded statistical coupling method (ESCM) was developed to provide computational efficiency, to decrease coupling complexities, and to avoid discretization of the continuum model to atomic scale resolution in concurrent multi-scale modeling. ESCM scheme is relatively easy to implement with conventional FEM code and has been tested in standard solid lattice structures. However, this method encounters difficulties when being implemented into amorphous materials like polymers, due to the fact that they lack specific ordered lattice structure and atoms may not be covalently bonded with each other, which are the requirements of common coupling schemes. Therefore, a new approach needs to be developed to solve this problem. In this work, details of a modified ESCM approach for atomistic-continuum coupling developed to perform simulations of crack growth in polymers is presented. The presence of the continuum domain surrounding the MD region prevents stress wave reflections from the external boundary impinging back on the crack tip. In this approach, generalized interpolation material point method (GIMP), which is a meshless particle-in-cell method based on Arbitrary Euler-Lagrange (ALE) scheme and has been proven to have good performance in large deformation problems, is used as the continuum domain. It is concurrently coupled with molecular dynamics (MD), a widely used method in atomistic simulations, using a so-called handshake region. Anchor points, the equilibrium positions of the constrained particles, which are designed to transmit displacements and forces between nanoscale and macroscale model, are defined in the handshake region. A concurrently coupled GIMP-MD simulation of nanoscale crack propagation inside a polymer is performed to verify this new coupling approach, and provide a better understanding of the fracture mechanisms at nanoscale to predict the macro-scale fracture toughness of polymer system. Results are presented for concurrently coupled crack propagation in a di-functional cross-linked thermoset polymer, EPON 862.
Nanowire Stiffening of Woven Composites Towards Enhancing Interlaminar Fracture Toughness
The objective of this paper is to explore the feasibility of using Zinc Oxide nanowires as potential interlaminar stiffeners textile polymer based composites. A comparison between the fracture toughness of a regular carbon fiber composite and a nanowire stiffened composite is performed to evaluate whether the ZnO nanowires enhance or decrease the fracture toughness. In this paper, both mode I and mode II fracture toughness of the interfaces within carbon fiber woven polymer matrix composites are investigated. Different methods have been proposed to calculate the fracture toughness experimentally and computationally. The interlaminar mode I (GIC) and mode II (GIIC) fracture toughness of a woven carbon/epoxy composite was measured using double cantilever beam (DCB) and end notched flexure (ENF) test, respectively. Eight woven carbon laminate specimens were tested for each method, where in mode I each specimen had 8 layers and in mode II each specimen had 20 layers of dry fabric of carbon fiber. A 0.002 in thick Teflon sheets were inserted in the mid-plane of the composite to simulate a crack. Reinforcement of ZnO nanowires were added to four specimens of mode I and mode II each. The fracture toughness of all the specimens were calculated. The values of GIC and GIIC were compared between the specimens with and without ZnO nanowires. The initiation and propagation of the crack was recorded, and every specimen showed a stable crack growth during the tests. The results showed the fracture toughness of the specimens with ZnO nanowires increased 43% in comparison to the specimens without ZnO nanowires in mode I. Also, the fracture toughness of the specimens with ZnO nanowires increased 28% in comparison to the specimens without ZnO nanowires in mode II.
Magnetically Recoverable Antibacterial Silver-Doped Magnetite/Zinc Oxide Core/Shell Composite Nanosheets for Photocatalytic Removal of Pollutants
Silver-doped core/shell magnetite/zinc oxide composite perforated nanosheets have been synthesized and characterized by high resolution scanning electron and transmission electron microscopies, energy dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), powder X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometry, solid state electrochemical impedance spectroscopy and UV-visible diffuse reflectance, photoluminescence and time correlated single photon counting (TCSPC) lifetime spectroscopies. All the employed techniques reveal the implantation of nanoparticulate magnetite in silver-doped zinc oxide nanosheets. The length and breadth of the nanosheets run to a few microns. The observed lattice fringes indicate the core/shell structure and silver-doping. The SAED confirms the presence of magnetite, zinc oxide and metallic silver in the nanocomposite. The EDS shows the presence of the constituent elements in the nanocomposite. The XRD and Raman spectrum are characteristic of the Ag-doped zinc oxide shell. The nanocomposite displays moderate absorption of visible light due to the magnetite core and sharp UV-A light absorption because of the zinc oxide shell. The emission spectrum of the nanocomposite is similar to that of pristine zinc oxide which confirms the zinc oxide shell. The TCSPC lifetime spectroscopy shows that the lifetime of charge carriers in zinc oxide shell is not influenced by the magnetite core. The charge transfer resistance of the nanocomposite is less than that of pristine zinc oxide but larger than that of magnetite. The magnetic hysteresis loop reveals the magnetic properties of the nanocomposite. The prepared core/shell composite nanosheets possess photocatalytic activity and mineralize dye efficiently. The synthesized nanocomposite is a bactericide and disinfects E. coli efficiently. Thus the synthesized core/shell nanosheets are two-in-one material - removes organic pollutants without requiring any chemical and disinfects bacteria even in dark.
Certification of Discontinuous Composite Material Forms for Aircraft Structures: Stiffness and Strength Predictions
The overall goal of this study is to help establish methods to certify prepreg-based discontinuous fiber composite (DFC) aircraft parts based on numerical analysis supported by a modest experimental database. Towards that end a stochastic (Monte Carlo-type) finite-element modeling approach has been developed. During a typical analysis the DFC structure of interest is divided into regions called Random Laminate Volume Elements (RLVEs). Nominal in-plane RLVE dimensions are 19 mm x 19 mm (0.75 x 0.75 in). A unique randomly generated nonsymmetric stacking sequence is assigned to each RLVE. All finite elements present within a given RLVE are assigned the same non-symmetric stacking sequence. Experimentally-observed variations in the stiffness of a DFC part are then simulated by performing many FE analyses, where a new random stacking sequence is generated for each RLVE during each analysis. Fracture predictions are obtained by increasing the load until a ply (i.e., a “chipâ€) is predicted to fail in an element somewhere in the structure. Stiffness’s of the failed ply are reduced, and loading is increased until the next ply failure is predicted to occur, etc. Final structural failure of the part is declared when all plies within a single element have failed. A typical analysis predicts that ply failures (i.e., “damageâ€) will accumulate at many regions within a DFC part. That is, damage is not predicted to occur just at stress risers (e.g., at/near holes), but instead damage is predicted to evolve in a distributed manner throughout a typical DFC structure, in qualitative agreement with experimental measurements. Analyses of un-notched tension and compression coupon specimens are discussed in this paper, and predicted B-basis and B-Max measures of modulus and B-basis strengths are presented.
Limit State Design of Composite Structures Based on Characteristic States in Damage Evolution
Limit state design (LSD) of composite structures is a common practice in industry where the strategy is to keep the loads on a structure such that the material in the structure does not exceed a selected limiting state. In composite laminates, one can identify certain states in damage evolution that can be used as limiting states depending on the structural design function. For instance, if the structure is to have no cracks at all, then the initiation of first crack in any of the plies of the laminate can serve as the limiting state. If the structure is stiffness critical, but can tolerate a certain loss of stiffness, then the limiting state corresponding to the maximum attainable crack densities in all plies of the laminate is a useful limiting state. This state is known as the characteristic damage state (CDS) and has been a focus of many studies. For strength-critical structures, the load bearing capacity of the laminate is a design variable, and its limiting values are given by an unstable progression of damage and total failure. In this work, we firstly address the CDS under static and fatigue loading for symmetric laminates containing plies in transverse as well as other off-axis orientations. Following previous studies of transverse cracking in cross ply laminates, we use attainment of a compressive stress between cracks as a criterion for the limiting state of cracking also for other off-axis cracks. We develop a beam-on-elastic-foundation model that analytically calculates the minimum crack spacing of transverse cracks in cross ply laminates and in combination with a three-dimensional finite element (3-D FE) analysis also calculates this spacing for other off-axis orientations. The model predictions agree well with available test data. For fatigue loading, it has been observed that the crack saturation may vary with the loading history. We discuss the mechanism of such a CDS in the plies. Finally, the potential application of the studied CDS for LDS is proposed.
Experimental Study of the Meso-Scale Heterogeneous Deformation Response of Polymer Composites
Meso-scale deformation response of an orthogonally woven carbon fiber reinforced composite is investigated using digital image correlation. The evolution of low and high strain domains within the material’s structure are experimentally captured and analyzed. The meso-scale analyses are performed by studying the evolution of local normal and shear strain components for different off-axis specimens. Using the experimentally measured strains at meso-scale, the local stressstrain response is also quantified. Global tensile response and the bulk failure mechanisms present in the examined material are explained through the results obtained at meso-scales. The local deformation, irrespective of the fiber orientation, is highly heterogeneous, while the degree of inhomogeneity is found to be more remarkable in the case of ±45o off-axis specimen. It is revealed that the large magnitude of localized normal strain developed within the regions of soft polymer, confined with transverse fiber bundles, will eventually lead to fiber pull-out and formation of a rather flat fracture surfaces in specimens with loading direction parallel to the principal fiber directions. On the other hand, domains containing large magnitudes of shear strain formed within the polymer-rich regions of the off-axis ±45o specimen were shown to result in a shear type failure associated with a rough conical fracture surface.
Damage Analysis of Laminated Composites with SAMCEF: Validation on Industrial Applications
In this paper the solution available in the LMS Samtech Samcef finite element code for damage analysis of composites is presented. Laminates made up of unidirectional (UD) plies are considered. Static and fatigue analyses are conducted. The damage models for inter- and intra-laminar damages are discussed and the parameter identification procedure is described. Comparison between tests and simulations at different levels of the pyramid of tests (building block approach) demonstrate the reliability of the numerical solution.
Allocation of Samples Between Exploration and Replication for Open-Hole-Tension Test
It is possible to test structural elements for a matrix of dimensions, stacking sequences and processing parameters, and then to interpolate or extrapolate using surrogate models to predict the failure at untested points. Extrapolation suffers larger uncertainty than interpolation especially when failure mode changes beyond the window of experiments. With a given budget for tests, there is a tradeoff between exploration of different configurations and replication to minimize the adverse effects of noise. For simulated tests, exploration has been shown to be more effective based on the assumption that noise in data is normally distributed and uncorrelated. This paper studies experimental data from a series of Open-Hole-Tension tests to check whether the conclusion stands with real experimental data. The tests are intended to investigate the impact of the ratio between coupon width and hole diameter on failure strength of a composite panel. The failure mode and strength are evaluated at four structural configurations. We first examine the statistical distribution of failure strengths and correlation among them from same pre-preg batch. Experimental results show that the noise distribution is closer to Gaussian than to Weibull or uniform distributions. The failure strengths from same batch tend to show systematic bias. The surrogate model selection is justified based on the experimental data, and the performance of two sample allocation strategies, exploration or replication, is compared. Sample selections emphasizing exploration proved to be more favorable for prediction accuracy.
Transverse Mechanical Properties of Unidirectional Hybrid Fiber Composites
Much experimental characterization and theoretical modeling on the mechanical properties of hybrid fiber composites in the fiber direction can be found in the literature. For the direction transverse to the fibers, theoretical models for the elastic properties of hybrid composites are available, but no experimental data could be found by the authors. Therefore, the objectives of the current investigation are to manufacture unidirectional carbon and E-glass hybrid fiber composites of various fiber proportions by a filament winding process, to characterize the elastic modulus and flexural strength of beam specimens tested transversely to the fibers, and to evaluate several analytical models for the transverse elastic modulus. The nominal compositions tested included 100% carbon, 75% carbon and 25% glass, 50% carbon and 50% glass with two differently shaped regions of carbon and glass fiber, 25% carbon and 75% glass, and 100% glass. The modulus increased monotonically with an increasing glass-to-carbon ratio. The tension-governed flexural strength was higher in the all-glass composite than the allcarbon composite, but the hybrids all had nearly the same strength halfway between the non-hybrids. A modified iso-stress model had the best correlation with the experimental modulus data and a modified Halpin Tsai model was second best. The conventional iso-stress model consistently under-predicted the experimental moduli, as expected for a lower bound model.
Using Full-Field Measurement Capability of Digital Image Correlation to Assess 3D Properties of Composites
The ability to accurately determine the basic material properties including 3D stress-strain constitutive relations, fully characterizing material behavior under mechanical loads, is essential for understanding complex deformation and failure mechanisms of structures manufactured from highly anisotropic polymeric composite materials. Non-contact full-field deformation measurement techniques, such as digital image correlation (DIC) allowing for assessment of all surface strain components on the entire specimen surface, enable simpler experimental setups and material specimen designs for more efficient and accurate material characterization. This work presents a method which uses the full-field measurement capability of DIC for a simultaneous assessment of transverse tensile behavior and non-linear shear stress-strain relations for composites in all three principal material planes. The method, which employs a small rectangular plate torsion specimen, takes full advantage of the full-field measurement and advances our ability to measure 3D material properties in composite materials in a single experiment. Small rectangular plate torsion (plate-twist) specimens are placed in a custom test fixture and loaded in a servo-hydraulic load frame at a constant crosshead displacement rate until failure. DIC data including the in-plane as well as out-of-plane strain components is monitored simultaneously in the three principal material planes until failure using a system of 6 high-resolution cameras. An iterative FEM updating method is developed to solve the inverse problem of deriving the material constitutive properties using the DIC full-field strain information. The basis of the inverse method is the minimization of the weighted regularized least square error between DIC-measured strains and FEM-predicted strains by adjusting the parameters of the material constitutive model. Results include stress-strain curves in all three principal material planes and transverse tensile properties simultaneously captured for a practical carbon/epoxy tape material system. Non-linearity in the transverse tensile response is discussed.
Carbon Fiber Textile Composite Solutions for High Speed Armor Piercing Ammunition
In this study, considering the following military ammunition of 7.62 × 51 mm AP WC (AP8 type) fired at 930 m/s, following the STANAG 4569 norm [1], a full textile composite solution has been considered to both meet the mass reduction and impact resistant requirements. Among the different existing fibrous reinforcement, preimpregnated fabrics of high resistance carbon yarns, with lower tensile modulus but higher relative elongation value, have been chosen and stacked together to obtain the sufficient number of layers to stop the bullet. In order to choose this number of layers, the initial proposed model of Chocron et al. [2] have been first considered and improved by introducing two main parameters as; a linear model of the fiber elongation value allowing to select the time Tmax, whom elongation is maximum, and the selection of different projectile diameter values, noted Dp. Different configurations of fibrous behavior corresponding to the different Tmax and Dp have been considered to determine the different number of layers of carbon fabrics to stack and cure together per block of 50 fabric layers for industrial production considerations. It has been observed, during the post-mortem analysis, integrity of the hard-steel core of the 7.62 × 51 mm AP WC (AP8 type) ammunition for all the ballistic tests corresponding to a lack of erosion during the impact, avoiding reaching the expected surface mass of 100 kg/m2. However, the impact behavior of this 100% textile composite solution seems to cope with this highly hardness projectile.
Processing and Mechanical Properties Investigation of Epoxy-Impregnated Graphene Paper
One-layer (0.15 mm thick) and 15-layer (5.43 mm thick) graphene composites have been fabricated through the impregnation of epoxy resin into porous graphene nanoplatelet (GnP) paper. Three different flake sizes of GnPs were used for the 1-layer graphene composites: 5 μm, mixed 5 μm and 25 μm, and 25 μm. The largest graphene flakes showed the best material properties in terms of tensile strength and elongation at break. Both tensile and DMA test results demonstrated that the 15-layer graphene composite had better material properties than the 1- layer graphene composites. The storage modulus of 15-layer graphene composite was 170% higher than that of neat epoxy. The transverse surface of 15-layer graphene composite exhibited a higher elastic modulus but lower hardness than the top surface. Ballistic limit test results showed that the combination of polyurea (PU) and a 15-layer graphene composite coating can increase the ballistic limit of TC-128 steel plate slightly.
Nano-Modified Hybrid Recycled Composite: An Alternative for Ballistic Shields
A critical issue on ballistic materials, in addition to the impact resistance is weight. One class of fibers that combines a good weight/impact resistant ratio is aramid fibers. The usage of aramid fibers into bulletproof vests takes the advantage of such good properties. However, the high cost of these fibers is a disadvantage and these vests cannot be discarded without proper treatment, thus a recycling procedure is urgent. This paper investigates new applications for aramid fabrics from discarded bulletproof vests and its application as ballistic shield. The new ballistic composite material is a hybrid laminate, where recycled aramid fibers are consolidated by epoxy resin and a shear thickening fluid (STF) reinforcement. The aramid fibers from bulletproof vests have an areal density of approximately 300 g/m2 (12.6 oz./yd2) and were intercalated to form a symmetric and balanced laminate. The epoxy system is an amine-cured bisphenol-A-diglycidyl ether epoxy resin from Barracudatec Inc., while the nano-reinforcements are colloidal silica and powder of organically modified montmorillonite nanoclay (Cloisite 30B). As this research deals with two different levels of analysis, i.e. the first one on nanoscale and the second one on macro scale, the experimental investigation must be divided in two different time frames. At nanoscale, the nanostructures are investigated by SEM and FTIR. At macro scale, ballistic tests are based on the NIJ Standard–0101.04. Two types of ammunition are considered, i.e. 9 mm Full Metal Jacketed (FMJ) and 357 Magnum Jacketed Soft Point (JSP). The results demonstrated a significant enhancement (close to 25%) in ballistic penetration resistance due to the addition of shear thickening fluid to the recycled fabric, without any loss in material flexibility. Moreover, the new system can be applied to shield and patrol cars protection system at low cost with the same efficiency of the new aramid fibers.
Dynamic Compressive Response of Polymeric Foams Subjected to Direct Impact Laoding
Dynamic compressive response of rigid closed-cell polymeric foams under high strain rate direct impact loading conditions is investigated. The foam specimens are subjected to direct impact loading, while their deformation response is observed and quantified using high speed photography in conjunction with 3D digital image correlation. A modified single stage shock tube apparatus with an aluminum projectile are used to generate the direct impact loading. The load applied on the specimen is measured using the load-cells at the other end of the specimen. The major challenge in high strain experiments, i.e. achieving equilibrium at the early stages of the deformation, has been compensated by performing an inverse analysis and accounting for the effect of inertia stresses into the analysis. The full-field acceleration distribution has first been calculated over the entire regions of interest. Then the inertia stresses are determined considering a one-dimensional equation of motion, and superimposed to the stress magnitude measured from the load-cells. The effects of material compressibility and the local change of density have also been included in the analysis. For this purpose, a mathematical model based on the principle of conservation of mass is proposed which enables the calculation of the local material density as a function of initial density, local plastic strain and local plastic Poisson’s ratio. Finally, the dynamic stress-strain responses of the foam specimens are presented as a function of foam density. The failure mechanisms as a function of foam density and loading rate are discussed.
Synthesis of Bipolar Plates for Fuel Cells Based on Exfoliated Graphene Nanoplatelets Filled Polymer Nanocomposites
This research explored the potential of using exfoliated graphene nanoplatelets, GNP, as the conductive filler to construct highly conductive polymeric nanocomposites to replace conventional metallic and graphite bipolar plates in the polymer electrolyte membrane (PEM) fuel cells. PEM fuel cell is a relatively new but fast developing power system which is considered to be one of the most promising power sources for stationary and transportation application in the future. Polyphenylene sulfide (PPS) was selected as the polymer matrix because of its high thermal and chemical tolerance. Solid state ball milling (SSBM) followed by compression molding was then applied to fabricate PPS/GNP nanocomposites. Results showed that PPS/GNP nanocomposites made by this method exhibit excellent mechanical and gas barrier properties but unsatisfied electrical conductivity. However, it was found that the electrical conductivity of these nanocomposites could be substantially enhanced if we combine GNP with second minor conductive filler for a positive synergistic effect. Meanwhile, the processing time of SSBM was also discovered as a critical factor in determining the electrical conductivity of the resulting GNP nanocomposites.
Damping Behavior and Mechanical Property of Graphene Nanoplatelet Paper
Graphene nanoplatelet (GNP)-polyetherimide (PEI) papers, in which GNP loadings were controlled up to nearly 100 wt%, were prepared by filtration and hot-press. The impact of GNP content on electrical conductivity, damping behavior and mechanical properties was investigated. While tensile tests showed that there were generally some decreases in tensile strength and strain at break, dynamic mechanical analysis (DMA) has shown that compared with either the pristine GNP paper or the neat PEI film, the storage modulus increased 7 to 20 times and the capacity to dissipate damping energy in terms of loss modulus was substantially improved by up to 30 times. The tan δ value gained more than 5 times, indicating that a higher fraction of loaded energy was converted to heat. Comparative studies showed that the PEI with relatively smaller molecular weight and large GNPs exhibited better damping effects. These results may suggest a new potential application of GNP or graphene reinforced polymer-based nanocomposite specifically applicable in energy- absorption upon dynamic loading. Besides, all GNP-PEI papers in this study exhibited very good electrical conductivity.
The Effect of Processing on the Electrical Properties of Exfoliated Graphite Nanoplatelet/Polylactic Acid Nanocomposite Films
The potential for engineering/tailoring properties, specifically electrical properties, of polylactic acid (PLA) nanocomposite films is examined by comprehensively studying process-structure-property relationships. This work focuses on investigating the effect of composite processing methods and conditions on the electrical response of exfoliated graphite nanoplatelet (GNP) / PLA films. GNP are incorporated into the PLA matrix either via polymer dissolution / solution casting or via melt compounding and fiber melt spinning / compression molding to produce films of ~160 μm thickness. In order to investigate the effect of polymer crystallinity on the electrical response of the films, the melt compounded / compression molded films are either cooled rapidly (~16°C/min and denoted fast cooled or FC) or cooled slowly (~0.4°C/min and denoted slow cooled or SC) to alter the matrix crystallinity. Crystallinity and crystal structure of the composite films are investigated using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Mechanical properties are examined in tensile mode using a standard tension setup. Electrical properties of the films are examined using impedance spectroscopy (IS). The SC films percolated between 1 and 5wt% of GNP, followed by the solution cast films which percolated between 5 and 8wt% of GNP, and finally the FC films had the highest percolation between 12 and 15wt% of GNP. Furthermore, the percolation of the solution cast films can be lowered to below 5wt% of GNP by minimizing voids present in the films which can be done via compression. The differences in percolation can be attributed to differences in compounding technique and the polymer matrix crystallinity.
Polymer Nanocomposite for Strain Resilient Solder
Commercial off the shelf (COTS) electronics generally are not specifically designed to perform in extremely transient high impact scenarios. In these specific scenarios, the interconnect materials in electronic packaging undergoes through high strain rate (high-g) inducing severe degradation of the materials properties, dramatically limiting the device performance. Traditional interconnect materials (e.g. Pb-Sn solder) suffer from limited performance life under high strain rate and gradient. In this study, we have developed a high performance high strain rate resilient compliant solder. In order to develop the polymer solder to be strain resilient in its normal application temperature, the polymer system should have the glass transition temperature (Tg) significantly lower than the operating temperature. In this research, a series of the epoxy polymers were successfully made and the Tg of the polymer can be tuned and controlled by tailoring the concentration and the composition of the epoxy resins and the curing agent. Although the cross-linked elastomeric polymers are generally flexible and shock absorbing, they are electrical insulators. It was reported that the dispersion of the carbon nanotubes into the polymeric matrix can significantly improve the electrical conductivity to 10-5 - 103 S/m. However, this is still far away from the desired electrical conductivity for the solder materials replacement. Here, we report that the 20 wt. % MWCN/Epon 828/D2000 nanocomposite was successfully fabricated. Its morphology was investigated by SEM and TEM nanotubes were well connected to form a network. Initial measurement via van der pauw showed that the electrical conductivity of the nanocomposite could potentially reach to 2 x 104 S/m. The combined electrical-mechanical testing based on the test of the Split Hopkinson Pressure Bar (SHPB) showed excellent durability and small changes in electrical conductivity of this CNT/epoxy nanocomposite during the dynamic SHPB test.
Assessment of Composite Manufacturing Defects—Current Approaches and Future Challenges
Approaches used for assessing and dispositioning manufacturing discrepancies and defects in composite structures are discussed. An overview of methods is provided for commonly observed anomalies (porosity, wrinkles, delaminations and disbonds), and includes discussion of anomaly morphology, effects upon structural stiffness and strength, and the use of analytical and empirical methods for substantiating defect acceptance (and the limitations thereof). The effects of anomalies such as porosity and wrinkling are most often assessed through empirical testing, or through the use of semi-empirical methods which correlate lower-level lamina/laminate properties to higher level test results. Analytical methods such as the Virtual Crack Closure Technique (VCCT) have found use in finite element-based assessment of delaminations and disbonds, due to their ability to model discrete geometry and consider complex, combined load and deformation states.
Effect of Manufacturing on the Dielectric Properties of Composite Materials
From golf clubs to airplanes, in today’s world there is a great need for lighter, stronger, and more durable high performance composite structures. Partially cured epoxy with fiber reinforcement, commonly referred to as prepreg material, is routinely used in the manufacturing of these composite materials. In general the laminae can be formed from multiple prepreg by the lay-up method either manually or by machine. These stacked materials then go through a highly controlled cure cycle consisting of a combination of pressure and temperature to allow the thermoset epoxy enough time to fully cure. Defects or air voids are inherent in composite structures and depend greatly on the cure cycle used and the manufacturing process. Normally, the defects are distributed and have an important influence on the final material performance. Local defects have different electrical properties, e.g., dielectric constant, than the surrounding fiber reinforcement and matrix materials. In this type of complex, three phase material system, polarization in an electric field is typically observed as an interfacial effect due to the build-up of electric charge on the constituent material interfaces and interfacial polarization induced by distributed internal defects. For decades Broadband Dielectric Spectroscopy, BbDS, has been a well-known and established method of determining polymer matrix cure kinetics. Likewise, BbDS can be used to extract material-level information, including morphological changes caused by microdefect generation and configuration. This paper presents an experimental investigation based on measuring dielectric properties of carbon fiber reinforced composites (CFRP) manufactured with different cure cycles. We will show how the relationship between the mechanical strength of the composite structure is uniquely linked with changes in the bulk dielectric response that reflect the internal defect state of the as-manufactured material.
Investigation of the Fiber Reinforced Laminates Stiffness Reduction Due to Internal Defects and Matrix Cracks
Internal defects and matrix cracks in the polymer matrix of fiber reinforced composites are often treated the same. The analytical and numerical damage modeling techniques apply the same degradation coefficients in the transverse and shear direction independent of the damage source. While the transverse stiffness reduction is similar for cracks and internal defects, the shear modulus degradation is different. A modified Iosipescu coupon was used in this study to investigate the transverse and shear modulus reduction due to cracks and internal defects. Transverse cracks had a greater effect on the transverse modulus than the shear modulus. It was shown that the difference in the transverse and shear direction was due to the roughness of the crack surfaces.
Stress and Failure Analysis of Trailing Edge Joints in Wind Turbine Blades Containing Manufacturing Defects
Trailing edge joints in wind turbine blades are potential source of failure. To gain an understanding of this failure, bonded joints with nonparallel adherends are analyzed in this paper. The stress states and consequent failure conditions in the bondlines of the joints are examined. A computational parametric study is conducted to investigate the effects of geometry and material properties on the location of critical stress states and potential failure modes. The finite element computations in the parametric study are supported by an analytical model, which uses the beam-on-elastic-foundation assumption in the classical Euler beam theory. The failure analysis of the joints is then applied to the case of defects resulting from manufacturing.
Tensile Properties of Cenosphere/HDPE Syntactic Foams Manufactured Using an Industrial Scale Injection Molding Technque
Composite materials having hollow fillers in a matrix material are known as syntactic foams. Such lightweight composites are used in a variety of applications ranging from sports, transportation, aviation and marine structures due to high dimensional stability, compressive strength and low moisture absorption. The present work is focused on using an industrial scale injection molding technique for fabricating syntactic foams and characterizing the fabricated syntactic foams for tensile properties. Use of such techniques can help in developing foams with high quality for bulk applications. Hollow fly ash cenospheres are used as the filler particles due to their low cost. Use of these industrial waste particles in beneficial applications can also help in their disposal. The present work is aimed at characterizing syntactic foams for tensile properties and investigating the effect of filler content. The syntactic foam density can be changed by changing cenospheres content. Cenospheres in 20 and 60 wt.% are used in syntactic foams in the present work. ASTM D 638-10 standard for the tensile properties of unreinforced and reinforced plastics is followed for the tensile testing.
Stiffness and Strength Models for Rigid PVC Structural Foams
Light-weight polymeric foams are frequently used as core materials in composite sandwich construction in which the core material properties will influence the overall performance of the sandwich structure. Foam core mechanical properties usually depend on a number of factors, including foam density, cell microstructure, and properties of the polymer matrix. Although properties of the foam matrix polymer are mainly determined by the base (parent) polymer, they are also affected by other factors such as foam processing conditions. With the large number of material and microstructure parameters that influence the foam properties, detailed modeling the mechanical behavior of polymeric foams could be quite involved, especially if the foam behavior is anisotropic. This paper describes an effort to model the elastic stiffness and strength of rigid PVC closed-cell foams. The rigid PVC foam is modeled as a transversely isotropic material with properties in the foam rise direction different from those in the planar (plane of isotropy) directions. An approximate theory is developed to predict in-plane and out-of-plane stiffnesses of PVC foams. Strength equations are also developed for predicting tensile, compressive, and shear properties of PVC foams. The validity of the stiffness and strength predictions for PVC foams is first demonstrated with the test results on DIAB H80 foam obtained from a systematic in-house test program. Comparison between stiffness and strength predictions with the material property data provided by the foam manufacturer for other density PVC foams are also carried out. Reasonable agreements are obtained which support the applicability and effectiveness of the proposed stiffness and strength models for assessing properties of PVC foams. Comparison of property predictions obtained in this paper with those from published models on mechanical properties is also discussed.
Test Method Development, Deformation and Failure Strength of Rigid PVC Structural Foams
A study has been conducted to develop a proper test method to evaluate deformation behavior and failure strength of a rigid PVC structural foam material (Divinycell H80) with a nominal density of 80kg/m3. Examination of the foam microstructure revealed that the cell aspect ratio is near unity in the in-plane directions but greater than one in the out-of-plane (rise) direction. To investigate the effect of test specimen geometry on compression properties, foam specimens with both straight-sides and reduced-gage sections were designed, fabricated and tested. The effect of aspect ratio in the gage section of specimen on foam properties was examined. To determine foam tensile properties, specimens with different crosssectional dimensions in the gage section were used. Foam elastic properties, nonlinear constitutive relationships, strength and failure modes were all obtained in different loading conditions, in both in-plane and out-of-plane directions. Test results reveal transversely isotropic characteristics of the foam material. To evaluate foam shear properties a miniaturized shear test method, similar to the ASTM standard test, was developed. Shear loading and unloading experiments were carried out to identify the causes of large shear deformation observed in the tests. The damage and failure modes of the foam material under different loading modes were also examined, both macroscopically and microscopically.
Characterization and Simulation of Divinycell H80 Closed-Cell Foam
Modeling the fracture response of complex sandwich structures often requires an elastic-plastic model of the core to accurately predict progressive damage modes. To address the failure modes occurring in foam core materials, fracture criteria should be selected to show the applicability to be consistent with non-linear finite element analyses and the elastic-plastic response. However, this may depend on some factors including the choice of an appropriate foam yield surface, applied load path and geometric shapes of the test coupons. In this study, experiments are conducted to measure the deformation behavior of Divinycell H80 foam under compression and tension loading with a variety of sample geometries. Each test specimen was modelled using Abaqus® with the actual test dimensions and boundary conditions. The description of the elastic-plastic behavior was simulated using material parameters and hardening curve to model the entire failure of foam. Further, additional failure criteria were involved in the modelling of dog-bone and two different shapes of butterfly shear specimens. The validations show a good agreement between experiments and the selected fracture criteria involved in numerical simulations in compression specimens. For the dog-bone specimens, it was clear that the modeling combination of failure criteria and elastic-plastic material model have the ability to track the stability and fast occurrence of cracks, since the crack only propagates normal to the load axis. For modelling the 45° and 90° butterfly shear specimens, the cracks propagated with nonlinear path matched with actual crack direction in the testing. However, in the force/displacement curves, both specimen models showed a gradual decrease, as opposed to the sudden failure in the actual tests.
Multi-Scale Characterization of an Adhesive Bondline with Fabrication Induced Defects
A multiscale characterization of an adhesive bondline is performed to investigate the fabrication defects dependent failure mechanisms of an I-beam under four point bending. To obtain a realistic representative volume of the bondline at its microscopic level, a 3D all-hexahedral mesh generator is used to create the unit cell model, which is capable of considering multiple material domains. The unit cell model contains the geometric information of glass beads, matrix of adhesive material and voids that are based on their micro-structural descriptions from the sectional 2D X-ray scanned images. The adhesive layer is modeled by cohesive interaction in the macro- beam model. The cohesive properties at the macroscopic level are extracted based on the average stress displacement curve to reserve the equivalent energy dissipation. Using the fabrication defects dependent cohesive properties, the numerical simulation predicts a local-buckling driven failure, which is consistent with the static test observation. Based on the shear dominant bondline failure pattern observed under the fatigue test, a parametric study of cohesive properties is performed to investigate the root cause of shear failure under fatigue which is different from the failure mode under the static test.
Remove Homogeneous Layer Assumption from Lamination Theories
Composite multilayer structures are often modeled using lamination theories. All lamination theories inherently assume the composite laminate is made of homogeneous layers characterized with lamina constants, which will restrict the lamination theories from capturing the real microstructural details of the composite material, and thus create artificial layer boundaries and result in many phenomena predicted by lamination theories occurred at wrong locations. Another drawback of lamination theories is that the lamina constants can not be rigorously defined. Recently a new theory called Mechanics of Structure Genome (MSG) has been developed without the homogeneous layer assumption. The variational asymptotic method (VAM) has been used to minimize the loss of information between the original 3D heterogeneous structures and the final equivalent plate model without invoking any ad hoc kinematic assumptions. To analyze multilayer plates/shells, MSG decouples the original 3D problem into a plate analysis and a constitutive modeling over a Structure Genome (SG), where SG is the mathematical smallest building block of the multilayer plate/shell structure. Through the analysis of SG, a constitutive relation between the generalized 2D strains and stress resultants is obtained for the plate analysis which can be solved at the same computational cost as Classical Laminate Theory (CLT). Dehomogenization can also be conducted to obtain the displacements, strains and stresses in the original heterogeneous structure. Several examples will be used to disclose the aforementioned flaws of lamination theories and the advantage of MSG. It is numerically demonstrated that while using the same plate elements in ABAQUS, MSG has an excellent accuracy compared to 3D FEA in terms of all the displacements and in-plane stress components, as well as a fair agreement of the transverse normal stress.
Loss of Accuracy Using Smeared Properties in Composite Beam Modeling
Composite structures, particularly composite laminates, usually consist of many layers with different lay-up angles. The anisotropic and heterogeneous features render 3D finite element analysis (FEA) computationally expensive in terms of the computational time and the computing power. Smeared properties are intensively used in the design and analysis of composite laminates. Although this approach has the advantage of saving computational time and cost of modeling significantly in comparison to 3D FEA, the prediction of the structural responses may not be accurate, particularly the pointwise stress distribution. Therefore, it is important to quantify the loss of accuracy introduced by using smeared properties. In this paper, several different benchmark problems are carefully investigated in order to exemplify the effect of the smeared properties on the global behavior and pointwise stress distribution of the composite beam. A new theory called the Mechanics of Structure Genome has been recently developed to model composites structures. SwiftComp is a code implementing the theory of multiscale structural modeling based on the concept of Structure Genome (SG).We will show in this paper that using smeared properties will introduce errors to both global structural behavior and local stress distributions, while using SwiftComp enables efficient and high fidelity analysis of composite beams with the same set of benchmark examples by comparing to the detailed 3D FEA using a commercial finite element software.
Experimental and Theoretical Evaluation of Stiffness Properties of Fused Deposition Modeling Parts
An experimental and theoretical investigation of mechanical properties of the ULTEM 9085 FDM printed unidirectional parts was performed. The mechanical property prediction was performed based on three-dimensional homogenization methodology and did not involve any empirical coefficients. The only calibrated parameter was the insitu elastic modulus of the FDM roads. The influence of the shape of the air gap between the roads was investigated. Best agreement for the predicted and measured transverse modulus was achieved for the equilateral shape of the gap. Good agreement also was observed for shear modulus.
Whole Field 3D Internal Strain Distribution in a Composite Beam Under Three-Point Bent
In this paper we present a new technique that can map the whole field 3D information of internal deformation of composites. The technique, called Digital Volumetric Speckle Photography (DVSP) is an extension of the 2D version developed by the senior author and his colleagues in 1993 called Digital Speckle Photography (DSP) or Computer Aided Speckle Interferometry (CASI). DVSP requires the use of an X-ray Micro CT to record a volume image of the object. Appropriate algorithm using 3D FFT is developed to process the data. As a demonstration we apply the technique to mapping the internal strain field of a composite beam under three-point bending load. Strain distribution in three transverse sections and two longitudinal sections of the beam are fully mapped. It should be noted that the technique can calculate the strain field in any section of the specimen and as many sections as needed. It is a truly whole field technique.
Role of Microstructure and Sea Water Confinement on Damage Evolution of Vinyl Ester Based Carbon Fiber Marine Composites
A fundamental study on the damage evolution of composite materials being considered by US Navy made of T700 based carbon fiber fabric (12k tows and plane weave) and vinyl ester resin (Derakane 510A) using VARTM process is presented utilizing high-resolution X-ray computed tomography, 3-D Digital Image Correlation integrated with mechanical testing systems. Panels of approximately 1.5 m x 1.5 m are manufactured using VARTM and coupon samples are obtained from various orientations. The deformation behavior of matrix dominated samples having [±45]2S orientation with the fiber dominated samples of [0/90]2S typically show large changes in damage evolution with increased deformation. A time aged (dry) and saturated sample are then subjected to controlled rate of strain loading monotonically with several unload-reload loops to evaluate the accumulated plastic strain (damage) as a function of stress amplitude. Damage is quantified using surface strain variations along the gage length of a deforming sample using a high resolution three-dimensional digital imaging correlation system. The surface deformation data obtained from 3DDIC is then considered synergistically with the variation of microstructural features obtained from high-resolution X-ray micro-tomography. The experimental techniques developed for this study are then utilized to evaluate the effect of sea environment on the damage behavior of marine composites for tensile monotonic loading conditions and fatigue loading. The motivation of the current study is to identify failure modes applicable for naval carbon fiber reinforced polymer composites and for developing future material systems with a targeted microstructure that can minimize accumulation of damage from repeated loading, blast, and impact conditions under harsh naval environment. Additive manufacturing techniques can realize the prospect of printing carbon fiber reinforced composites with target microstructure.
Equivalent Mechanical and Thermal Properties of Composite Materials in Lamina, Laminate and Structure Levels
Structures made of fibrous composite materials are heterogeneous in nature due to different geometrical orientations of fibers in a laminate. To analyze these structures, it is extremely difficult and time consuming to retain their true structural representation. To overcome this difficulty, homogenized material properties can be evaluated for different levels, so- called equivalent property, which is usually adopted. In this homogenization approach, macroscopic homogeneity and their characteristics of structural response need to be considered. In this study, equivalent properties of a composite in lamina, laminate and structural levels are defined as the properties of a homogeneous orthotropic composite to represent a material’s anisotropic heterogeneous behavior. A set of equations for evaluating equivalent mechanical and thermal properties in lamina and laminates as well as structures is developed. The developed model is applied to angled lamina and various laminates. For a structural case, an I-Beam is analyzed utilizing the concept of narrow beam. Results indicate that the modified method is a significant improvement over the conventional method and can give accurate results in most composite laminate analyses.
Acoustic/Elastic/Mechanical (AEM) metamaterials exhibit negative effective mass density when the lattice system consists of mass-in-mass microstructural units. AEM metamateirals fall within the family of composites, since the constituents are the microstructural units embedded within the composite matrix. It is found out that the effective mass density of AEM metacomposite becomes frequency dependent and displays negativity for frequencies near the resonant frequency of the internal resonators. The effect of a negative mass property simply implies that stress wave propagation is prohibited; this leads to structural applications like vibration control, impact protection and shock wave mitigation. Under impact loading, internal resonators in the AEM metacomposite are revealed to effectively reduce the displacement and velocity of the overall structure, and attenuate the stress wave propagation over a specifically-designed range of frequency where the negative effective mass density is exhibited. In this paper, the transient and dynamic behavior of AEM metacomposites is analyzed. Firstly, the concept of AEM metamaterials is introduced by presenting the unique property of negative effective mass density through simple analytical mass-in-mass models. Next, the impact wave attenuation capability of the metacomposites is demonstrated using computational simulation. In particular, this study investigates the mechanical energy incurred by the internal resonators of an AEM metacomposite when the structure is subjected to an impact pulse load. Both strain energy and kinetic energy, as well as the total mechanical energy are considered. Results revealed that the mechanical energy consideration agrees well with the kinematic characteristics of the resonators in the system.
A Bridge Between Finite Element and Shear Lag Analyses for Fiber Pull Out
The strength of fiber-reinforced composites is often dependent on the strength of the fiber-matrix interface bond. Thermal, chemical, and other means have been used to modify the surface of fibers, resulting in increased fiber-matrix interface bond strength. One potential mechanism for improving interfacial strength is through increased surface roughness. It is desirable to develop physics-based models capable of predicting the effect of surface treatment on interfacial bond strength. It is anticipated that experimental, numerical, and analytical efforts will be needed to contribute toward this endeavor. A shear-lag approach has been used to model the transfer of load from fiber to matrix in fiber pull out, microbond, and axisymmetric macrobond tests. However, the shear lag parameter, β, must be fitted to experimental results to use this approach. Finite element analyses could be useful in interpreting experimental results, and predicting the effect of surface roughness on load transfer. However, shear lag models do not capture the singularity that is present along the fiber-matrix interface at the free surface of the matrix, meaning that finite element and shear lag analyses do not agree near the location that fiber-matrix debond is most likely to begin. In this paper, a numerical approach is presented that allows the shear lag parameter, β, to be extracted from finite element results. This allows a bridge between numerical and analytical approaches that does not currently exist. Axisymmetric finite element analyses of fiber pull out and axisymmetric macrobond configurations are discussed in light of this approach. The effect of the different boundary conditions in these two test configurations are considered for a range of ratios for matrix and fiber Young’s moduli. It is anticipated that this approach will be essential in future research efforts to simulate the effect of fiber surface texture on pull out strength.
A Novel Composite Tube Design with Bend-Twist Coupling
A novel design of a composite circular tube possessing bend-twist coupling at the structural level is presented. When subjected to a bending moment the tube undergoes twisting in addition to bending. Conversely a torque results in bending in addition to rotation. This coupling causes the shear center of the cross section to be away from the center of the tube. An analytical model is proposed and closed-form equations for shear center distance are derived for both straight and tapered composite tubes. The results are verified by performing finite element analysis. Special tubes were fabricated and tested to measure the shear center distance. The agreement in results is excellent. It is found that the location of the shear center is independent of the shaft radius but proportional to the length of the tube.
Use of Electrochemical Impedance Spectroscopy to Monitor Delamination and Moisture Uptake in CFRP-Reinforced Concrete Structures
Electrochemical impedance spectroscopy (EIS) electrodes have been used to inspect carbon fiber-reinforced polymer (CFRP)-reinforced concrete structures exposed to a variety of laboratory test conditions (salt fog, alternate immersion in fresh water, alternate immersion in salt water, 100% relative humidity, 50% relative humidity, elevated temperatures, and below-freezing temperatures). Other specimens were used to investigate the detection of delamination between the CFRP and the concrete using a wedge test. External electrodes, coupled with the embedded rebar, provided the best results. Equivalent circuit modeling was used to analyze the impedance spectra. Several circuit parameters, especially the capacitance and constant phase element (CPE) magnitude, correlated very well with both bonded area/ delamination and moisture content. The different relationships between the different circuit parameters and moisture uptake or bonded CFRP-concrete area should allow these two material characteristics to be separately determined. Differences in the different exposure conditions were seen and explained. Coupled with a commercially available portable potentiostat, these measurements can be made in the field to facilitate inspection of concrete structures reinforced with CFRP, such as bridges.
Functionalization of Graphene and Its Reinforced Nanocomposite
Research at Michigan State University has shown that it is possible to exfoliate natural graphite into stacks of graphene nanoplatelets having thicknesses of less than 10nm and diameters of any size from sub-micron to 15 microns or greater. Since graphene is one of the stiffest materials found in nature (Young’s Modulus = 1060 GPa), and also has excellent electrical and thermal conductivity, the addition of graphene nanoplatelets to polymers should have a beneficial effect on the resulting composite physical properties. In order to get good dispersion of graphene nanoplatelets (GnP) in the matrix and good adhesion between the GnP and the matrix, functionalization of the GnP is one of the most effective approaches. We have investigated functionalization of GnP with DGEBA and maleic anhydride, which was then used to fabricate GnP/vinyl ester nanocomposites. The thermal, mechanical and electrical properties of these nanocomposites have been evaluated. Morphology results showed that functionalization improved the dispersion of GnP in the matrix. Dispersion of GnP is an important factor to control mechanical properties of its reinforced nanocomposite. DGEBA-MA functionalized GnP reduced the electrical conductivity of it reinforced nanocomposite, however, DGEBA-MA significantly improved the flexural strength and unnotched Izod impact strength. In addition, DGEBA-MA functionalized GnP also reduced the thermal conductivity.
Barrier and Mechanical Property Enhancement of High Density Polyethylene Through the Addition of Graphene Nanoplatelets
Processing methods to disperse Graphene nanoplatelets (GnP) in high density polyethylene (HDPE) are being investigated in an attempt to manufacture a material with mechanical and barrier properties suitable for fabrication of vehicle fuel tanks. Previously reported results indicated that through a simple melt mixing process, the oxygen transmission through a 7.5% wt. GnP in HDPE composite was reduced by 55% compared to the neat composite, however there was also a 50% loss in impact resistance. Alternative methods to melt extrusion were investigated to improve the dispersion and alignment of GnP. Microlayer co-extrusion (MCE) of the GnP powder and HDPE, in which the melt nanocomposite is stretched into thin films, stacked and consolidated, resulted in higher relative improvements in barrier properties, however was still limited by dispersion issues. A 30% wt. GnP in HDPE masterbatch was produced and then processed through the MCE process. This actually resulted in a decrease in properties due to platelet curling and size reduction. A solution mixing approach was employed in which HDPE and GnP were mixed in heated xylene to improve the dispersion, but the advantage was lost when it was re-processed with melt extrusion. A low molecular weight hydrocarbon coating of the GnP platelets was applied prior to processing in melt extrusion to improve the dispersion but resulted in only a small improvement in the impact resistance, had a negative impact on the flexural properties, and did not improving the barrier properties. An elastomeric coating of the GnP resulted in recovering up to 34% of the lost impact resistance but the flexural modulus and strength were lowered and there was a negligible effect on barrier properties at a 5% wt. concentration. Alternative thin film methods are now currently being investigated
Optimized Fiber-Reinforced Polymer Composites for High-Performance Applications: Toughening of Aromatic Epoxy Polymers via Aliphatic Epoxy Copolymers
The importance of carbon-fiber reinforced epoxy composites is increasing with the current efforts to increase efficiency through weigh-reduction (lightweighting). The brittle nature of the epoxy material that is often used for the matrix is an issue that needs to be addressed. Many different approaches of toughening the epoxy polymer matrix have been established, however these are usually associated with a corresponding decreases in other mechanical properties. This paper discusses an approach to increasing the toughness of carbon-fiber reinforced aromatic epoxy composites by looking at two areas of the composite: first, the fiber/matrix interface; second, the polymer matrix itself. By using a di-functional aliphatic epoxy polymer in these two area, a toughening of the composite should be possible without a reduction in other mechanical properties. The preparation of the fiber surface via a 90 second ozone treatment under UV light has been shown to increase the surface concentration of oxygen, leading to better fiber/matrix adhesion. A 60% increase in interfacial shear strength (IFSS) was seen for the UVO-treated over the as-received fiber. An additional increase of 7% in IFSS was measured when an aromatic epoxy sizing was applied to the UVO treated fiber. By contrast the application of an aliphatic epoxy sizing yielded a substantial increase in IFSS of 32% over the UVO-treated fiber. To address the toughness of the matrix, the application of low concentrations (1wt%) of aliphatic copolymers has been shown to substantially increase (~60%) the impact toughness of the epoxy matrix material, without reducing other mechanical properties. In cases where the IFSS was at low, the aliphatically toughened matrix increased the IFSS by about 10%. By contrast at high IFSS values a 10% reduction of IFSS was seen. In between, the aliphatically toughened matrix showed similar IFSS to the neat matrix.
Tailorable Adhesives for Multi-material Joining, Facile Repair and Re-assembly
Joining of materials and components is inevitable as it allows versatility in assembly and repair along with reduction in time and cost of manufacturing. This work uses ‘active adhesives’ that inherit all the benefits of bonded joints, such as lightweight, eliminate holes and associated stress-concentrations, and overcomes the shortcomings of disassembly and repair. Thermoplastic adhesives modified by the incorporation of electrically conductive graphene nanoplatelets at a concentration above the percolation point provide a unique synergy of mechanical, thermal and electrical properties. While the choice of the thermoplastic is governed by the desired application the addition of the graphene nanoplatelets allows energy to be deposited primarily in the adhesive. The percolated network of graphene particles in the adhesive at less than 2% can quickly couple to high frequency radiation (microwave, or MW) via non-contact methods and increase the adhesive temperature to above the required processing temperatures. The adhesive melts and flows over the adherends and upon cooling forms a structural adhesive bond. Furthermore, the process can be used to disassemble the adhesive joint if repair or reworking is required. In this work, the concept of aforementioned active adhesives was studied using varying graphene contents in nylon-6 adhesive. Activation and re-assembly was studied on in-plane (lap-shear) joints. Structural behavior of re-assembled joints was found to be similar to thermally bonded joints. Overall, active adhesives such as the ones attempted in this work have a great potential in a wide range of applications wherein in-situ repair, re-assembly and recyclability are essential.
Creep Behavior of Insulated Concrete Sandwich Panels
Insulated concrete sandwich panels has been widely used because of their advantages of light weight and energy efficiency. More recently, research has been conducted to study their applications as roof/floor panels, where long term creep behavior is an important design concern. This paper presents a combined experimental and analytical study on the creep behavior of insulated concrete sandwich panels under bending. Four concrete panels were tested for creep loading. One was a conventional solid reinforced concrete slab and this was used as the benchmark panel. The other three sandwich panels had top and bottom concrete wythes with various thicknesses and a 3â€ insulation layer in the middle. The wythes were connected with FRP segmental shear connectors. There were also steel reinforcing bars in both the longitudinal and transverse direction. The tests were conducted with a static load of approximately 13.3 kN (3,000 lbs), which corresponded to the linear-elastic range of the panel’s load-deflection curve. The duration of the test ranged from 150 days to 250 days for different panels. It can be concluded that one sandwich panel showed better long-term deflection results than the solid panel. Equations from American Concrete Institute (ACI) Building code and Finite Element (FE) method were used to analyse the panels. Good correlations can be observed between the FE and test results.
Effective Width of Insulated Sandwich Panels with Flexible FRP Shear Connectors Considering Partial Degree of Composite Action
Insulated sandwich panels consist of two layers of wythe separated by a foam insulation. Many different types of connectors have been used in the past to connect the two wythes, including steel ties, wire trusses, bent wires, truss-shaped connectors, and solid concrete zones. In recent year, fiber-reinforced polymer (FRP) materials, in both truss and mesh configurations, have begun to be incorporated as shear connectors, as they have a thermal conductivity about 14% that of steel, and can significantly reduce thermal bridging. Until now, there is no effective guidelines for the design of these panels. Generally, they are treated as rectangular beams, which is not reasonable since the longitudinal stress over the wythe section is non-uniform due to the in-plane shear flexibility of the wythe, which is called shear lag effect. Effective flange width has been used to describe the shear lag effect for a deck-ongirder composite beam system, reducing a three dimensional behavior of the composite beam system to the analysis of a T-beam section with a reduced width of deck. This paper extends the concept of effective flange width to insulated concrete sandwich panels. A shear lag model is firstly developed to study the sandwich panel system, where partial Degree of Composite Action (DCA) due to flexible FRP shear connector is considered. The analytical model is then verified through close correlations between finite element and analytical results for a concrete sandwich panel with FRP shear connectors. Next, a parametric study is conducted using the analytical model to study the effects of deck stiffness and aspect ratio on effective flange width. Finally, a simplified method is proposed to calculate effective width.
A Study on Failure Behavior of Unidirectional Carbon Fiber Reinforced Thermosetting and Thermoplastic Composite
In this study, the morphology of fracture of unidirectional (UD) carbon fiber reinforced plastics (CFRPs) has been studied to categorize the fracture characteristics for several types of loading. Polyamide 6 (PA6) and epoxy resin as thermoplastic and thermosetting matrix were used to fabricate UD CFRPs plates through compression molding process. Specimens were tested in tensile and bending tests, both in the axial and the transverse direction. Optical microscopy and scanning electron microscopy were used for the fracture observation. Fracture features characteristic of each failure mode were identified.
Effect of Water on the Mechanical Properties of Fiber Reinforced Polymers
As part of an ongoing product design effort, the effects of exposure to water on the mechanical performance of several thermoset polymer materials are evaluated. A series of tests were performed on a suite of thermoset materials, some with, and some without, fiber reinforcement. Aging of the materials in water is performed at elevated temperatures to accelerate any damage mechanisms. As control conditions, some samples are aged in air at the same temperatures, sealed to prevent wicking of liquid through the fibers, placed through the curing process for the epoxy sealant, and conditioned at elevated temperature air for one week prior to exposure to elevated temperature water. Trends observed from several materials are compared. Factors that are evaluated include the presence of fiber-reinforcement, a process of coating edges of samples to avoid wicking of liquid into the material, and the rate at which environmental effects occur. In addition, general conclusions are drawn regarding the development of test matrices to evaluate suitability of polymer-based materials for conditions that include exposure to water.
Understanding the Damage of Polymer Matrix Composites by Integrating Chemical, Morphological and Mechanical Properties
Detailed physical and mechanical characterization of the matrix as well as the interphases of polymer matrix composites can lead to a more complete understanding of failure mechanisms in polymer matrix composite (PMC). This study illustrates mechanical damage of polymers in both the bulk, as well as around the interphase region through integrated computation & experimentation approach. We have developed a quantum mechanics-molecular dynamics framework, which has enabled the prediction of bond scission under load, creation of intermittent free radicals, and exploration of the potential energy surface for possible secondary reactions immediately following bond scission. In parallel, we have conducted experiments with epoxy systems with varying molecular weight and cross-linker density at different load conditions to benchmark the simulation findings of the chemical species present on fracture surfaces of the polymer. In order to evaluate experimentally molecular level effects of mechanical load in epoxy-systems, detail characterizations were conducted combing spectroscopy (X-ray photoelectron spectroscopy, FT-Infrared spectroscopy), microscopy (HRTEM, AFM-IR, SEM), X-ray diffraction (SAXS) and mechanical testing (3-point bending). Similarly, the nanoscopic nature of interphases of PMCs in terms of topography, chemical mapping/bonding, fractography, and modulus are also studied in order to find a bridge between nanoscopic, microscopic and macroscopic mechanical properties.
Mechanical Properties of Graphene Nanoplatelet/Carbon Fiber/Epoxy Hybrid Composites: Multiscale Modeling and Experiments
Because of the relatively high specific mechanical properties of carbon fiber/epoxy composite materials, they are often used as structural components in aerospace applications. Graphene nanoplatelets (GNPs) can be added to the epoxy matrix to improve the overall mechanical properties of the composite. The resulting GNP/carbon fiber/epoxy hybrid composites have been studied using multiscale modeling to determine the influence of GNP volume fraction, epoxy crosslink density, and GNP dispersion on the mechanical performance. The hierarchical multiscale modeling approach developed herein includes Molecular Dynamics (MD) and micromechanical modeling, and it is validated with experimental testing of the same hybrid composite material system. The results indicate that the multiscale modeling approach is accurate and provides physical insight into the composite mechanical behavior. Also, the results quantify the substantial impact of GNP volume fraction and dispersion on the transverse mechanical properties of the hybrid composite, while the effect on the axial properties is shown to be insignificant.
Fracture of Carbon Nanotube—Amorphous Carbon Composites: Molecular Modeling
Carbon nanotubes (CNTs) are promising candidates for use as reinforcements in next generation structural composite materials because of their extremely high specific stiffness and strength. They cannot, however, be viewed as simple replacements for carbon fibers because there are key differences between these materials in areas such as handling, processing, and matrix design. It is impossible to know for certain that CNT composites will represent a significant advance over carbon fiber composites before these various factors have been optimized, which is an extremely costly and time intensive process. This work attempts to place an upper bound on CNT composite mechanical properties by performing molecular dynamics simulations on idealized model systems with a reactive forcefield that permits modeling of both elastic deformations and fracture. Amorphous carbon (AC) was chosen for the matrix material in this work because of its structural simplicity and physical compatibility with the CNT fillers. It is also much stiffer and stronger than typical engineering polymer matrices. Three different arrangements of CNTs in the simulation cell have been investigated: a single-wall nanotube (SWNT) array, a multi-wall nanotube (MWNT) array, and a SWNT bundle system. The SWNT and MWNT array systems are clearly idealizations, but the SWNT bundle system is a step closer to real systems in which individual tubes aggregate into large assemblies. The effect of chemical crosslinking on composite properties is modeled by adding bonds between the CNTs and AC. The balance between weakening the CNTs and improving fiber-matrix load transfer is explored by systematically varying the extent of crosslinking. It is, of course, impossible to capture the full range of deformation and fracture processes that occur in real materials with even the largest atomistic molecular dynamics simulations. With this limitation in mind, the simulation results reported here provide a plausible upper limit on achievable CNT composite properties and yield some insight on the influence of processing conditions on the mechanical properties of CNT composites.
Computational Design of Carbon Enriched Ceramics for Improved Strength and Toughness
Silicon Carbide (SiC) is a strong and hard engineering material frequently considered for abrasives, rotating disks, bearing, high temperature coatings etc. Poor fracture toughness due to brittleness is one of the limitations that keeps SiC from applying in widespread structural applications. In this study mechanical properties of a new type SiC-based “C enrichedâ€ ceramics where certain Si atoms are substituted by C atoms have been studied using MD simulation. Five different systems with different fraction of “Câ€ enrichments, namely 10%, 20%, 30%, 40% and 50% have been investigated. After equilibrating all types of enriched systems as well as the control SiC system, we studied their equilibrium densities, free energy profiles and internal morphologies as a function of “Câ€ enrichment amount. The energy profiles suggest that all “Câ€ systems should be thermodynamically viable because total configuration energies for all systems were minimized and remained stable over a long period of time. The densities of different “Câ€ enriched system drop from 3.25 gm/cm3 to 3.05 gm/cm3 for “Câ€ enrichment upto 20%. For higher than 20% “Câ€ enrichments, densities then increase monotonically. We explore the microstructures by measuring the average coordination number and radial distribution functions of different systems. Both these studies confirm that the newly designed materials have local microstructure change. We then evaluated tensile and shear response of these newly developed materials and found the mechanical properties depend on amount of carbon enrichment. Our study suggests that “Câ€ enrichment has strong influence on both tensile and shear properties of ceramics with optimum results attained at enrichment between 20% and 30%.
Viscoelastic Response of Microtubule—Tau Proteins Assembly During Axonal Stretch: Combined Atomistic and Continuum Predictions
It is believed that damage during traumatic brain injury is a multiscale process that begins from the axonal substructures. The axonal cytoskeleton can be viewed as a nanofiber reinforced composite structure where nanoscale microtubules are arranged in staggered arrays and cross-linked by tau proteins. The unique viscoelastic nature of axons is thought to govern the damage during traumatic brain injury. Recent studies suggest that dynamic loading of axons may lead to microtubule or tau protein failure, depending on the rate of loading. In particular, it has been found that the viscoelastic behavior of tau proteins leads to mechanical breaking of microtubules at high strain rates. At small strain rates, large extension of tau without failure allows for reversible sliding of microtubules without any damage. Despite experimental and computational evidences suggesting that microtubule and tau protein are both viscoelastic, microtubules have been considered as elastic material in these studies for simplicity. Here, inspired by the earlier works, we have developed a modified shear-lag model to predict axonal damage under dynamic loading conditions. In our model, we have considered both microtubule and tau protein as viscoelastic materials. We have assumed that the viscoelastic response of these two materials can be described by the two-parameter Kelvin model. We have employed full atomistic computations to determine the stiffness and viscosity of microtubules and tau proteins. We have then studied the effect of strain rate on the viscoelastic response of microstubule-tau protein assembly. We have attempted to determine a phase diagram in terms of loading rate and microtubule length to isolate the two possible axonal deformation modes, namely microtubule failure due to excessive stretch and reversible microtubule sliding due to tau protein stretch.
WSFE-based User-Defined Elements in ABAQUS for Modeling 2D Laminated Composites with Complex Features
Wavelet spectral finite element (WSFE) model is much smaller than conventional finite element and it’s a highly efficient method for wave propagation analysis. Since the wavenumber solutions are not available for complex features (delamination, crack, impact damage and etc.), modeling of these features using WSFE is practically not possible. In this paper, WSFE-based elements are defined and implemented in ABAQUS through UEL subroutine. WSFE works in the frequency domain, so most of the variables are complex values. Implementing complex values in ABAQUS is not feasible. All of the complex values are decoupled in to real and imaginary parts and presented to ABAQUS as real values. Thus the number of nodes is doubled, two nodes to represent real parts and two nodes to represent imaginary parts in a beam structure. Impact damage in a plate is modeled reducing the stiffness in the damaged area by 50%. Three elements are used to model the entire area. The plate is excited at the tip by a tone-burst and results are obtained at the same point.
Computationally Efficient Solution of the High-Fidelity Generalized Method of Cells Micromechanics Relations
The High-Fidelity Generalized Method of Cells (HFGMC) is a powerful technique for simulating composite materials and is based on Aboudi’s method of cells micromechanics theories. Unlike the original generalized method of cells, the HFGMC uses a higher order approximation for the subcell displacement field. Although this allows for a more accurate determination of the subcell stress/ strain fields, the solution to the simultaneous set of equations can become computationally burdensome. In order to overcome expensive computational costs associated with solving large systems of equations, order-reduction techniques have been developed to approximate the solution with an acceptable error. These techniques are widely used in the computational fluid dynamics community and are increasingly being implemented for solving solid mechanics problems involving the finite element method. In this study, the HFGMC global system of equations for doubly-periodic RUCs was reduced in size through the use of Proper Orthogonal Decomposition (POD) with Galerkin projection. Orderreduced models were then implemented within an ABAQUS UMAT and used to perform multiscale analyses. A number of cases were presented that show the computational feasibility of using order-reduction techniques to solve the HFGMC set of simultaneous equations. By simulating composite materials in a more computationally efficient manner, a pathway forward is presented for performing multiscale analyses of composite structures consistent with the Airframe Digital Twin concept.
Effect of Notch-Induced Strain Gradients on the Applicability of Multiscale Approaches for Woven Composites
Under load and structural features such as notches and fasteners act as stress concentrators and failure nucleation points, which can significantly reduce the strength and durability of the material. The focus of this work is on evaluating a computationally efficient multiscale approach for modeling woven composites in the presence of a strain gradient. The multiscale modelling approach is utilized to evaluate fiber or matrix failure. Then a series of homogenization steps are used to compute the effect of constituent failure on redistribution of strain at the structural level. The key feature of both the localization and homogenization steps is the representative volume element (RVE). But the critical assumption of an RVE is an average homogeneous state of strain across the RVE. Traditional multiscale methods that rely on an RVE are not readily extensible to models in which average strains vary appreciably across the RVE, such as can occur with woven composites in the presence of notches. In this study, we quantify the strain gradient present in open hole tension (OHT) specimens comprised of woven composites and assess the impact of these gradients on the RVEbased modeling approach. This is accomplished by conducting progressive failure simulations of a unit cell in the presence of a strain gradient and comparing with progressive failure simulations of the same unit cell under the same average homogeneous strain field. We find that the critical strain gradient is much smaller than anticipated and requires additional mesoscale studies to determine.
Mechanical Performance of Fiber-reinforced Polymer Composites Under Concurrent Hygro-thermo-mechanical Loading
Fiber-reinforced polymer composites are used in a variety of engineering structures, where they may be subject to service conditions with concurrent static and fatigue mechanical loading, thermal fluctuations, moisture in water or service fluids. Predicting the durability of such materials in service conditions is critical to ensure that the appropriate condition-based maintenance routines are carried out, with consequences on structural safety. In this work, sandwich composite samples are manufactured with unidirectional T700/vinylester skins and polyvinyl chloride (PVC) foam core, using a conventional out-of-autoclave Vacuum Assisted Resin Transfer Molding. The skin orientation selected for this study is 90 deg., to trigger a polymerdriven response. Polymeric foam samples and sandwich composite samples were tested under concurrent hygrothermo-mechanical loading in a custom-made testing apparatus, where displacements are obtained from resistance changes, which can be monitored continuously by digital multimeters. Specimens are immersed in deionized water at room temperature while being concurrently loaded in three-point bending, at set percentages of their dry ultimate static load. After immersion, the specimens are tested to failure. A multi-scale model incorporating the time-dependent responses of the polymers in the sandwich skins and core is used to investigate the experimental results of this study. The multi-scale model consists of a micromechanics model for the fiber-reinforced polymeric skins and layered beam elements, in order to predict the deformations of sandwich composites under coupled mechanical and non-mechanical effects.
Seawater Effects on Transverse Tensile Properties of a Carbon/Vinylester Composite
The effect of seawater immersion on the transverse tensile modulus and strength of a carbon/vinylester composite has been analyzed using micromechanics. Unit cell modeling was conducted on a representative volume element of a transverse section of the composite using finite element analysis (FEA). Predictions from FEA and simple micromechanics models were compared to experimental values of transverse tensile modulus and strength measured at dry and seawater saturated conditions. The FEA over predicted modulus, and under predicted strength both at dry and seawater saturated conditions. The measured modulus only slightly exceeds the lower bound estimate for both the dry and seawater saturated composite which indicates inadequate load transfer between fiber and matrix. The conservative FEA strength predictions are most likely due to the assumption of a regular square packing geometry of the fibers. Predictions of strength of the composite using the Cooper-Kelly micromechanical model were in relatively close agreement with test results. Below is a set of instructions for preparing your paper in a format suitable for the intended media for these proceedings. It is most important that you follow these guidelines as closely as possible and adhere to the margins (discussed below) for overall consistency and format relationship to all other papers in the intended proceedings. Print or type text as close to the margins as possible without going beyond these margins.
Dynamic Response of Composite Plates Moving in Water
This study investigated dynamic responses of composite plates travelling inside a tow tank while the speed of the plate increased from zero to various steady-state values and remained at the steady state values. Both experimental and numerical studies were undertaken. For the experimental part, strain responses of the plate were measured using strain gages. The resultant drag forces were also measured. In order to determine the effect of the composite plate with fluid-structure interaction, an aluminum plate of the same size was also tested and its results were compared to that of the composite plate. The composite plates were also tested under various setting angles relative to the towing direction. Some experimental data were compared to numerical results to validate the computational model. After validation, various cases were examined numerically which could not be tested physically in the tow tank. The drag forces and maximum stresses/strains were much greater during the transient period of the increasing speed than at the steady state speed. Additionally, the hydrodynamic loading was greater on the polymer composite plate than on the aluminum plate of the same size. Therefore, design and analysis of composite structures moving in water should consider the drag force during the transient motion including fluid-structure interaction.
The Effect of Single Fiber Elastic Modulus Distribution and Modulus Strain Dependency on Carbon Fiber Tow Behavior
The relation of individual fiber properties to their assemblies, i.e. fiber bundles or tows, is critical to the predictive performance of fiber reinforced polymer composites. Statistical modelling of fiber bundles based on the mechanical behavior of single fibers has demonstrated that a large distribution in single fiber failure strength within a tow will lower the tow failure stress, despite equivalency in mean fiber failure stress. Using very precise mechanical testing, it has been shown by the authors that the elastic properties of single PAN carbon fibers within a small fiber tow (12k) have significant variation as well, which has only recently been quantified using a novel nano-tensile testing system. In this paper, the tensile behavior of a fiber tow is simulated using the property distributions obtained from single carbon fiber tensile testing, in particular the statistical distribution of axial modulus and the dependency of axial modulus on strain amplitude. Considering identical fiber failure statistics, a 15% reduction in tow failure stress was found for a standard deviation in modulus equal to ±8%, a typical value experimentally observed by the authors for high performance carbon fibers. Furthermore, carbon fiber modulus dependency on strain amplitude reduces tow strength if non-uniform fiber loading is considered, indicating that manufacturing process control of fiber alignment and residual stress is critical to improving the overall strength of a polymer reinforced carbon fiber composite.
Prediction of Capillary Pressure for Resin Flow within Fiber Unit Cells
The flow of resin into fiber tows is driven by the applied pressure gradient and the capillary pressure, which is dependent on the static contact angle between the fibers and resin as well as fiber diameter and arrangement. Commonly, only two fibers are used to calculate the average capillary pressure, which does not take into account the effect of neighboring fibers in a closely packed tow. This paper introduces a novel method to calculate the average capillary pressure of resin moving through a unit cell containing five fibers. Both numerical and analytical solutions are presented, validated, and compared. The role of selected parameters on average capillary pressure is investigated. This work should prove useful in predicting the average capillary pressure of resin moving between fibers and the results can be used to address filling of fiber tows.
Permeability of Glass Fabric Reinforced Vinyl Ester Composite by VARIM Process
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 its simplicity and cost-effectiveness. Understanding permeability of the composite is essential in controlling resin infusion for efficient manufacturing process development. Deformation of the vacuum bagging in the VARIM results in ever-changing permeability during infusion. The objectives of this research are to: (1) identify unique features of the apparent permeability during VARIM of glass-fabric laminate composite, (2) investigate the effects of process variables and manufacturing variations on the apparent permeability, and (3) determine the effective permeability value for use in VARIM simulations. In the experimental program, a series of glass-fabric panels were placed in a variety of laminate lay-ups and infused with vinyl ester resin and a selected low-viscosity fluid (corn oil). The process was monitored with embedded thermocouples, a video camera, a digital viscometer and an LVDT to monitor flow front motion and dimensional stability. Two methods were used to evaluate the VARIM composite apparent permeability. In the first method, permeability and part quality of the composite panels were determined by vinyl ester resin flow-front tracking and burn-out tests. In the second method, the apparent permeability of corn-oil infused composite panels was determined as a function of mass flow-rate and panel thickness. Resin fluid motion and other manufacturing parameters were found to govern the apparent permeability in the first method. With an improved lay-up design the second method was used and the resulting composite dimensional stability was found to be more significant in pre-infusion compaction tests than that during infusion. Distinct features were obtained in both effective and apparent permeabilities. Infusion times in the experiment are compared favorably with the results obtained from FLUENT 16.0’s volume of fluid method, using inputs from the second experimental method.
Effects of Processing Conditions on Mechanical Properties of Quartz/BMI Laminates
Bismaleimide (BMI) resin with Quartz (AQ581) fiber reinforcement is a hightemperature composite material that is frequently used in aerospace structures, such as engine cowlings and radomes. Variations in laminate fabrication conditions, such as prepreg conditioning and applied cure pressure, can have a significant effect on microstructural features of fiber-reinforced composites, such as fiber volume fraction and void content, and consequently influence mechanical properties of composites. In this paper, flexural properties of Quartz/BMI laminates manufactured by prepregs conditioned at different relative humidity, to generate process-induced microvoids, and subsequently cured at different pressures are presented. Flexural test specimens were prepared in accordance with the testing standard ASTM-D790 and were not subjected to any degradation prior to flexural testing. Prepregs were conditioned in an environmental chamber at 2%, 40%, 70%, or 99% relative humidity at room temperature for a period of 24 hours prior to laminate fabrication. To alter the fiber volume fraction, the laminates were manufactured at cure pressures of 68.9 kPa, 206.8 kPa, 344.7 kPa, or 482.6 kPa via a hot-press. The laminates are shown to have different levels of microvoids and fiber volume fractions based on the processing conditions, which are also observed to affect the mechanical properties of Quartz/BMI. Flexure stiffness increased from 23 GPa to 29 GPa, or approximately 25%, for quartz/BMI laminates as the laminate fabrication pressure increased. An increase in flexure strength from 455.4 MPa to 555.9 MPa was observed for increasing laminate fabrication pressure. As the relative humidity level of prepreg conditioning increased, a decrease in flexure strength of approximately 50 MPa to 100 MPa was observed for the quartz/BMI laminates.
Ultrasonic Energy Assisted Synthesis of Zinc Oxide Nanoparticles
Zinc Oxide (ZnO) nanoparticles have various applications in a wide variety of fields such as hydrogen fuel cell technology, cancer treatment, antibacterial treatment, and energy storage applications. This study highlights the synthesis of ZnO noanoparticles utilizing potassium hydroxide and zinc chloride solutions as precursors in presence of ultrasonic energy. It is expected that the nucleation and growth rates will be affected differently due to the application of ultrasonic energy. The investigation focuses on the application of ultrasonic energy and the relationships of particle size and morphology of ZnO nanoparticles with respect to process variables. It is found that ZnO nanoparticles with different morphologies can be achieved by varying the intensity of ultrasonic energy, test temperature, and precursor injection method.
Internal Strain Monitoring of Vacuum Bag Only Process Using Optical Fiber Sensors
Vacuum bag only (VBO) cure is one of out-of-autoclave (OOA) methods that are attractive alternatives to the conventional autoclave-based cure method. Though prepreg sheets are used for the VBO method, only vacuum pressure is applied. Thus, products can be fabricated without any size constraints, and the manufacturing cost is significantly reduced. However, low applied pressure leads to higher void content in VBO products than that in autoclave cured ones, degrading their quality. Several researchers investigated void emergence and fiber impregnation. Although important insights have been obtained in these works, no researches addressed the residual strain which also affects part quality. This study measured strain development during the VBO cure using fiber Bragg grating (FBG) sensors which is one of the optical fiber sensors. Specimens were fabricated with two cure processes: one was the manufacturer’s recommended cure cycle (MRCC) and the other was a modified cure cycle that was proposed in this study to reduce the residual strain. Cross sectional observation was also conducted to evaluate the void content. The results revealed that the residual strain is 1300 με smaller in the modified cure than the MRCC even though the void contents are the same. It implies that the internal strain state should be considered in addition to the void content to achieve high quality parts. Then, a tensile test was performed to evaluate the effect of the residual strain on the strength. Cross ply specimens fabricated with the two cure cycles were tested. The modified cured specimen showed 450 με higher strength than the MRCC cured specimen, showing the effectiveness of the modified cure cycle.
Modeling Arterial Walls as Composite Structures, and Their Interaction with Pulsatile Blood Flow
We study the interaction between an incompressible, viscous fluid and a composite structure, which consists of a thin and a thick elastic layer. The thin layer is modeled using the linearly elastic shell model, while the thick layer is modeled using the equations of linear elasticity. The system is coupled via the kinematic and dynamic coupling conditions. We present an unconditionally stable, loosely-coupled numerical algorithm and show a comparison of the results obtained using the proposed algorithm to the ones obtained using a single layered structure model. This work reveals a new physical property of FSI problems involving thin interfaces with mass: the inertia of the thin fluid-structure interface regularizes solutions of the full FSI problem.
Two-component Dissipative Particle Dynamics Model of Red Blood Cell Membranes
By modeling the lipid bilayer and the cytoskeleton as two distinct layers, we developed a two-layer whole cell dissipative particle dynamics (DPD) model of erythrocyte membranes to investigate quasi-static, thermal and dynamic problems. First, we validated our two-layer DPD model by simulating the membrane thermal fluctuations of healthy and malaria-infected erythrocytes. We found that the bilayer-skeletal interaction plays a significant role and the two-layer DPD model matches the experiment better than the one-layer DPD model. In addition, by simulating the bilayer-skeletal detachment in a flow channel experiment, we predicted the interaction strength at the molecular level.
Thermal Properties of Mycology Materials
Natural composite materials continue to gain momentum as alternatives to synthetic composite materials for lower-load applications. Mycology materials, in addition to being more carbon neutral, show good potential as a natural composite option due to their benefits of non-toxicity and ease of disposal by composting, reducing environmental impact. The cellular structure of mycology materials make them an attractive option for insulation applications, and as they can be faced with either natural or synthetic laminate materials, they have potential for use in composite insulation panels. Characterization of mycology materials’ thermal insulation properties is needed to provide information for their development into such applications. The following thermal insulation properties of mycology materials will be characterized using several experimental methods carried out according to American Society for Testing and Materials (ASTM) standards. The ability of the material to be used for internal and external applications will be investigated by considering the effect of environmental degradation on the material, using ASTM C488−05 Standard Test Method for Conducting Exterior Exposure Tests of Finishes for Thermal Insulation. Current commercialized applications of mycology materials include packaging and furniture, where the product comes in close contact with users, and potential insulation applications need information on the material’s odor-release behavior at elevated temperatures. This will be determined using ASTM C1304−08 Standard Test Method for Assessing the Odor Emission of Thermal Insulation Materials. Determining the maximum use temperature is key when selecting the material for suitable applications, and will be determined using ASTM C447–03 Standard Practice for Estimating the Maximum Use Temperature of Thermal Insulations. Finally, the thermal resistance R value of the material, commonly used to describe the efficiency of insulating materials, will be determined using ASTM C1044–12 Standard Practice for Using a Guarded-Hot-Plate Apparatus.
Tribological Response of Cenosphere/Epoxy Syntactic Foams
Fly ash cenospheres are generated in thermal power plants as waste materials. Use of these waste materials in beneficial applications is desired. The present work deals with developing syntactic foams with fly ash cenospheres filled in epoxy resin matrix. Such syntactic foams can be used as core materials in sandwich composites. The fabricated syntactic foams are evaluated for dry sliding wear behavior. Sliding velocity and filler content were the study parameters, effects of which were analyzed on wear rate. Specimens were prepared with 10, 30 and 50 wt. % cenospheres for the study. Sliding against hardened ground steel on a pin-on disc wear testing machine was conducted. The experimental results and subsequent analysis showed that the addition of cenospheres as filler material in epoxy matrix considerably increases the wear resistance of the composites. The craters on the specimen surface due to presence of cenospheres play important role in the wear process.
Influence of Exothermic and Endothermic Chemical Foaming Agents on the Physical and Mechanical Properties of the Polypropylene-Based Thermoplastic Structures
Using a chemical foaming agent (CFA) along with a thermoplastic polymer in a foam extrusion process has a significant effect on the density and thickness of the final product so that highly light-weight and low-cost structures could be achieved. In this experimental study we have investigated the thermal and chemical behavior of two different types of CFA and their effect on the foam quality and physical/mechanical properties of the final product in an extrusion foaming process. A polypropylene-based thermoplastic (PP) sheet was produced via a plastic extrusion machine with 12″ width and 4mm thickness. An exothermic foaming agent Azodicarbonamide (EV AZ-3.0) was added into the PP pellets in 5 wt%. They were completely mixed and melted inside the extruder to decompose and liberate gas. The melt temperature and pressure must be high enough to guarantee a total decomposition of the foaming agent and make the generated gas dissolved inside the polymer melt until it exits from the die opening. The same trial was conducted with an endothermic CFA named Styrene-Ethylene/Butylene-Styrene (PN-40E). Our thermal TGA analysis and physical tests demonstrate that each of the foaming agents has its own unique specifications which could be utilized based on the operating temperature and pressure. However, using the exothermic CFA is recommended for the PP-based thermoplastic foams as it can build up the product with low weight and high specific stiffness.
Fixed Stiffness Weight and Cost Tradeoff of Hybrid Sandwich Structures
Producing a light structure with relatively affordable cost has always been a challenging task for designers. Using a hybrid material approach provides an expanded methodology to combine materials having different costs and properties (for example, combining fibers with high cost and high stiffness fibers such as carbon with low cost, less stiffness such as glass). Hence, a comparative approach is required for the evaluation of design solutions in terms of weight and cost. In this study, a methodology for a combined weight and cost optimization for sandwich plate with hybrid composite facesheets and foam core is presented. The weight and cost of the sandwich plate considered are the objective functions with respect to the equivalent constraint based on the bending stiffness of the sandwich plate. The sandwich plate considered consisted of thin hybrid composite facesheets, symmetric with respect to the mid-plane of the sandwich plate. The facesheets considered consisted of longitudinal carbon and glass fiber reinforced polymer. Two different densities (with two difference cost) of polyisocyanurate closed-cell foam core were studied. Singleobjective and multi-objective optimization techniques were performed to obtain the optimum weight and optimum cost of the sandwich plate. An Interior-Point Algorithm was used to perform the single and multi-objective optimization. For the multiobjective cases, a weighting method was used to solve the problem by combining the objective functions of weight and the cost into one objective and Pareto trade-off curves were constructed. The weight of the adhesive bond material was neglected and only the cost of the fibers and the core materials was considered in this study. A key finding is that design curves can be created showing that hybrid solutions can be preferred when both cost and weight are considered simultaneously.
Applying Reactive Molecular Dynamics to Predict and Compare the Mechanical Response of Di-, Tri-, and Tetra-functional Resin Epoxies
The influence of monomer functionality on the mechanical properties of epoxies is studied using Molecular Dynamics (MD) with the Reax Force Field (ReaxFF). From straining simulations, the Young’s modulus, yield point, and Poisson’s ratio are calculated and analyzed. The results demonstrate an increase in stiffness and yield strength with increasing resin functionality. Experimental comparisons show reasonable agreement, and therefore, this technique is confirmed to be a useful tool for understanding the structure-property relationships of epoxies.
Computational Design and Mechanical Behavior of Carbon Nanotube Reinforced LDPE/Nylon 6 Hybrid Polymer Nanocomposites
We report mechanisms to enhance the strength, modulus, and fracture strain of linear low density polyethylene (LDPE) polymer through hybridizing it with nylon 6 as a minor phase and simultaneously reinforcing it with single-walled carbon nanotubes (SWCNTs). Loading of nylon 6 and SWCNTs into LDPE was 25.0 -75% wt% (to understand the effect of minor phase) and 2.0 wt%, respectively. Hybridized polymer nanocomposites were virtually processed using successive simulations including Self-avoiding random walk (SARW) to build polymer network and high temperature molecular dynamics simulation to enable melt mixing. After developing thermodynamically stable hybrid polymer nanocomposite systems, we performed virtual tensile tests using molecular dynamics. The molecular dynamics simulation studies reveal that the interface between polymers and nanotube plays critical role. It is found in study that hybridized LDPE with nylon 6 increase the interfacial strength as well takes more energy to pull out the SWCNT from polymer.
A Computational Approach for Linking Molecular Dynamics to Finite Element Simulation of Polymer Chains in Polyethylene Fibers
In this paper, an approach for transferring the complex interactions of polyethylene fibrils from molecular dynamics (MD) simulations to finite element (FE) modeling is developed. Simulations of intermolecular interactions using an all-atom MD model with the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential are used to calibrate the force descriptions of surfacebased interactions in an equivalently configured finite element simulation. The anisotropic material description of the finite element fibril is derived from MD simulations of crystalline polyethylene subjected to mechanical tension, compression, and shear in all directions. Fibril pull-out simulations are conducted in both MD and FE domains to determine the inter-fibril, surface-based Lennard- Jones interaction parameters for the continuum fibril model. This approach will help elucidate the complex interaction forces that exist at the atomistic level and ultimately enable scale-bridging towards the micro- and macro-fibril length scales.
Quantitative NDE of Composite Structures at NASA
The use of composite materials continues to increase in the aerospace community due to the potential benefits of reduced weight, increased strength, and manufacturability. Ongoing work at NASA involves the use of the large-scale composite structures for spacecraft (payload shrouds, cryotanks, crew modules, etc). NASA is also working to enable the use and certification of composites in aircraft structures through the Advanced Composites Project (ACP). The rapid, in situ characterization of a wide range of the composite materials and structures has become a critical concern for the industry. In many applications it is necessary to monitor changes in these materials over a long time. The quantitative characterization of composite defects such as fiber waviness, reduced bond strength, delamination damage, and microcracking are of particular interest. The research approaches of NASA’s Nondestructive Evaluation Sciences Branch include investigation of conventional, guided wave, and phase sensitive ultrasonic methods, infrared thermography and x-ray computed tomography techniques. The use of simulation tools for optimizing and developing these methods is also an active area of research. This paper will focus on current research activities related to large area NDE for rapidly characterizing aerospace composites.
Time Reversal Microwave Methods for Detecting Disbonds in Adhesively Bonded Metal-composite Structures
Composites are being increasingly used to replace metals, partially or completely, in aerospace, shipping and automotive industries because of their light weight, corrosion resistance, and mechanical strength. However, the quality of adhesively bonded joints can be severely compromised if there are manufacturing defects such as disbonds or impact damage when in use. These factors can significantly affect the strength of resulting bonded joints and their performance. Hence there is need for detecting these anomalies during manufacturing and maintenance. Time reversal focusing is based on the property of the wave solution to focus back at the source when reversed in time and back propagated. This property has been used in ultrasonic NDT for detecting defects in test objects that act as sources of scattered fields. This paper presents a model based feasibility study that applies time reversal principles to microwave NDE data. Microwave NDE is well suited for dielectric materials, including composites because of the ability of electromagnetic waves to interact with these materials. The scattered field depends on dielectric properties of the medium, hence providing information about the structural integrity of these materials. In this paper microwave NDE is applied to geometry comprising adhesively bonded metal composite structure. Two dimensional Finite Difference Time Domain (FDTD) method is developed for both forward and back propagation of electromagnetic wave field propagation in a test geometry that comprises three layers, namely, composite, epoxy and metal. A Gaussian modulated pulsed source is used to illuminate from the composite side and the reflected field from the metal is measured by a receiver array. The measured microwave fields are time reversed and propagated backwards using the FDTD model. The maximum energy of the back propagated field highlights the defect location. Simulation studies exhibits the feasibility of this approach to detect and characterize disbonds in metal-composite joints.
Guided Wave Structural Health Monitoring for Impact Damage Detection and Characterization
Laminated composite structures are susceptible to impact damage mechanisms such as disbonds and delaminations that significantly weaken the structure. Due to the unpredictable and sudden nature of impacts, traditional scheduled maintenances might fail to detect such damage in a timely fashion. Guided wave structural health monitoring (GWSHM) uses a network of sensors that are permanently surface-mounted or embedded in the structure, allowing for continuous monitoring, and potentially make it possible to detect an impact damage the moment it happens. In order for a GWSHM system to be reliable, it is important to investigate the ability of guided waves to detect impact damage with high probability and with a low false alarm rate. In this work, a glass fiber composite coupon with plain-weave S-glass/SC-15 epoxy is instrumented with lead-zirconate-titanate (PZT) piezoelectric thin films that are used for the generation and detection of guided waves. Baseline data is then recorded for each transducer path in the bonded GWSHM transducer network. The composite coupon is then hit by drop weight impactor with varying energies. The correlation between impact energy, observed structural damage, and the change in the guided wave signals is investigated. Based on those observations, multiple signal features based on baseline comparison are then investigated, including time domain and frequency domain signal features.
Quantitative Description of Damage Evolution and Property Degradation of Fiber Reinforced Composite Materials
Fiber reinforced composite materials are materials are increasingly being used in many applications. However, characterization and prediction of their long term behavior is still an active area of research. Composite inherently heterogeneous and go through a complex process of material state changes. Their functional life depends upon characterizing and predicting evolution of local details (e.g. distributed damage initiation, accumulation, and interaction) which affect global property (strength and stiffness) degradation and eventual failure. The lack of understanding of those local changes often results in empiricism, limiting innovation in the use of composite materials. Importantly, the intermediate, precatastrophic stage of damage development when isolated defect sites accumulate and begin to form incipient fracture paths is especially difficult to characterize. This proposed research aims at improving material level understanding of the formation of fracture path. In this paper, broadband dielectric spectroscopy (BbDS) has been utilized to capture material state changes due to degradation in composite materials. This multi-physical response of composite materials from BbDS provides important features which relate to degradation behavior and associated loss of properties (strength and stiffness) due different applied conditions. In this study, multiphysical description of material state change has been discussed in two example cases: i) stages of incipient fracture path formation due to cyclic bending, and ii) stiffness evolution under tensile fatigue loading. A 3D X-ray microscope has been used to visualize and hence validate incipient fracture path formation. Validated results from these test cases show that degradation process, and evolution of properties can be characterized with a high sensitivity using multi-physical response of composite material. These results can potentially help enhance predictive modeling. Details of experimental methods, and results are included in the paper.
3D Progressive Damage Modeling for Laminated Composite Based on Crack Band Theory and Continuum Damage Mechanics
A simple continuum damage mechanics (CDM) based 3D progressive damage analysis (PDA) tool for laminated composites was developed and implemented as a user defined material subroutine to link with a commercially available explicit finite element code. This PDA tool uses linear lamina properties from standard tests, predicts damage initiation with an easy-to-implement Hashin-Rotem failure criteria, and in the damage evolution phase, evaluates the degradation of material properties based on the crack band theory and traction-separation cohesive laws. It follows Matzenmiller et al.’s formulation to incorporate the degrading material properties into the damaged stiffness matrix. Since nonlinear shear and matrix stress-strain relations are not implemented, correction factors are used for slowing the reduction of the damaged shear stiffness terms to reflect the effect of these nonlinearities on the laminate strength predictions. This CDM based PDA tool is implemented as a user defined material (VUMAT) to link with the Abaqus/Explicit code. Strength predictions obtained, using this VUMAT, are correlated with test data for a set of notched specimens under tension and compression loads.
On Nonlocal Mechanics Based Ultrasonic Methods for the Detection of Inception of Damage in Composites
Inception of damage quantification during fatigue loading has been a challenge in the field of structural health monitoring (SHM). Interaction between intrinsic damage state and ultrasonic wave signals of composite material is not fully understood. Fiber reinforced composite materials unveils damage precursor in the form of micro crack in matrix and fiber fracture which leads to catastrophic failure of the structure. Here, in this paper tensile-tensile fatigue tests were performed on two different types of ASTM standard composite specimen. For carbon fiber reinforced composite (CFRP) specimens at a regular interval of 10,000 cycles and for glass fiber reinforced composite (GFRP) interrogation of material state during fatigue tests were performed at the interval of 1,000 cycles, until micro cracks are developed. Quantitative Ultrasonic Microscopy is performed during the same intervals. For the Carbon-Carbon composites, the non-local material properties degraded continuously during fatigue loading due to onset of matrix cracking and fiber breakage while for glass fiber reinforced composite specimen material degradation was not detected conclusively. Here an attempt has been made to measure material properties degradation in the form of damage entropy at each pixel point of the selected area by using the scanning acoustic microscopy. The average damage entropy material properties are calculated, after each 10,000 cycles for CFRP and at 1,000 cycles for GFRP. “Damage Entropyâ€ is used as a measure of damage accumulation over number of cycles and are plotted until damage incubates or incepts. Tensile test specimens were instrumented with high frequency-piezoelectric sensors. Interaction of guided lamb wave with local stiffness reduction has also been studied to extract information of damage initiation by intermittently collecting sensor data and analyzing them by using advance signal processing techniques. Optical microscopy was also performed to validate our results of damage inception. Based on the above study, possibility of correlating off-board ultrasonic technique and on-board Lamb wave technique are demonstrated for the detection of precursor to damage in composites.
Experimental and Numerical Analysis of Triaxially Braided Composites Utilizing a Modified Subcell Modeling Approach
A combined experimental and analytical approach was performed for characterizing and modeling triaxially braided composites with a modified subcell modeling strategy. Tensile coupon tests were conducted on a [0º/60º/-60º] braided composite at angles of 0°, 30°, 45°, 60° and 90° relative to the axial tow of the braid. It was found that measured coupon strength varied significantly with the angle of the applied load and each coupon direction exhibited unique final failures. The subcell modeling approach implemented into the finite element software LS-DYNA was used to simulate the various tensile coupon test angles. The modeling approach was successful in predicting both the coupon strength and reported failure mode for the 0°, 30° and 60° loading directions. The model over-predicted the strength in the 90° direction; however, the experimental results show a strong influence of free edge effects on damage initiation and failure. In the absence of these local free edge effects, the subcell modeling approach showed promise as a viable and computationally efficient analysis tool for triaxially braided composite structures. Future work will focus on validation of the approach for predicting the impact response of the braided composite against flat panel impact tests.
Modeling of 3D Woven Composites with Realistic Geometry for Accurate Prediction of Kinking under Compressive Loads
The accuracy of modeling dictates the reliability of predictive analysis for 3D woven composites (3DWC). Past research is based on analysis of ideal geometry with specified nonlinearity. Co-relating the actual non-linearity arising from imperfection in the specimen with that of the model becomes an iterative process. In this study, digital element approach is implemented for creating the composite geometry. This technique simulates the individual fibers and the interactions between them, thus allowing the user to create a realistic RUC with random nonlinear geometry induced during manufacturing stage of 3DWCs. Thus response and strength analysis captures the unique weaving signature and provides better predictive models without the necessity to run iterative analysis. This approach can be extended to predict tensile and compressive failure of 3DWC with more accuracy without having to seed imperfections for failure initiation.
Simulation of the Manufacturing Process of Braided Composite Tubes for Realistic Microstructure Representation
In order to create representative numerical models of textile composite structures and their underlying microstructure, the braiding process of a composite tube made from a triaxial braided textile fabric is simulated. The tows are treated as a collection of representative fibers. The kinematics of the braiding process are modeled completely and the typical cross sectional shape of the tows emerge automatically. Then a procedure is outlined on how to convert the results of the braiding simulation to a solid mesh model that can be used for further structural investigations. The plausibility of the resulting model of the microstructure of a composite tube is shown through an example of a simple tension test.
Biochar and Its Size Effects on Polyamide 6/Biochar Composites
Biochar was used as a filler material for polyamide 6 (PA6) composite and the effect of particle size on the composite properties were analysed. Large particles, when prepared by manual (median particle size ~ 84 μm) and mechanical grinding (~ 41 μm), caused a reduction in tensile, flexural and impact strength, but did not strongly affect the Young’s or flexural modulus. However, using strongly reduced particle sizes after ball milling the biochar for 6 (~ 3.8 μm), 12 (~ 3.2 μm) and 24 (~ 2 μm) hours respectively leads to improved mechanical properties. Atomic Fore Microscopy (AFM) based Quantitative Nanomechanical Property Mapping (QNMP) revealed improved bonding of the ball milled particles with the polymer due the increased particle surface area. The addition of biochar also causes an increase in heat deflection temperature (HDT) and crystallinity compared to the neat polymer.
The Opportunities of Biofuel Coproducts in Value-Added Biocomposite Applications
Nowadays, renewable energy resources are gaining more attraction since the future demand for energy and the depleting petroleum resources will soon become an issue to the rapidly-growing global population. In this regard, bioethanol has expanded hugely during the last 10-15 years, especially in the form of corn-based ethanol. In a corn-based ethanol plant, dried distillers’ grain with solubles (DDGS) is also produced as the coproduct as much as ethanol on a weight basis. The surplus amounts of the produced distillers’ grain are beyond its tradition animal feed usage so that the accumulation can become a concern for environmental sustainability of the so-called first-generation bioethanol industry. Moreover, the application of distillers’ grains in new markets is another aspect to be thought of in order to help economic sustainability of the industry. In the present work, the capabilities of DDGS are evaluated for using as a filler and/or reinforcement in producing green biocomposites with a number of bioplastics. The bioplastics used here are partially originated from renewable resources with end-life biodegradability in compost conditions. The focus of this research is to design and engineer material formulations with balanced performance. Extrusion and injection molding techniques have been employed to produce the biocomposites and the performance of the produced materials was evaluated through mechanical, thermomechanical and physical properties. The discussions are augmented with scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). With this approach, biocomposites of DDGS and bioplastics were successfully designed which exhibit acceptable performance such as tensile strength of more than 20 MPa, flexural modulus of more than 1 GPa, impact strength of more than 100 J/m and melt flow index of more than 5 g/10min.
Evaluation of Mechanical and Interfacial Properties of Individual Lignin Carbon Fiber and Composites
Use of lignin as a precursor, instead of PAN, for manufacturing carbon fiber shows promise in developing renewable and low-cost carbon fiber composites. The lignin used in this research was a highly pure form based on the extraction process that was implemented. Suitable procedures were developed to melt spun lignin fibers which were subsequently stabilized and carbonized. The resulting carbon fibers were characterized to determine mechanical and interfacial properties of single carbon fibers with future scale-up to fiber reinforced epoxy composite testing. Using VARTM, initial lignin carbon fiber composites were manufactured by the authors. Mechanical testing of single carbon fibers was performed using a highly precise nanouniversal testing machine and the results were interpreted using a Weibull statistical distribution. The tensile testing on single fibers resulted in a Weibull shape parameter of 3.66 and scale parameter of 630 MPa respectively. The interfacial properties were determined using a Single Fiber Fragmentation Testing and a custom testing apparatus. Average interfacial shear strength values on the order of 31 MPa were obtained with an average size of 100 μm delamination zone. X-Ray Diffraction (XRD) was used to determine the structural characteristics. The structure of the lignin carbon fiber developed to date had little to no crystalline regions with minimal alignment. Initial trials associated with the mechanical testing of these lignin-based carbon fiber composites prepared using VARTM were considered and are in development. Mechanical testing will integrate digital image correlation (DIC) for highly accurate strain mapping on specimen for future testing. This top-to-bottom approach will evaluate lignin at every step of the process in realizing a dream to make a lignin and bio-epoxy based renewable carbon fiber composite.
Influence of Multiwalled Carbon Nanotube on Mixed Mode Fracture of Sandwich Composite
Mixed mode fracture toughness of glass-epoxy (G/E) polyvinyl chloride (PVC) core sandwich composite with and without multiwalled carbon nanotubes (MWCNT) are investigated through experimental mixed mode bending (MMB) methods. Results demonstrate improvement in mixed mode fracture toughness (G) values of samples with a certain percentage of MWCNT at 0.8 mode ratio (M.R.). Mixing of hardener in sonicated MWCNT-epoxy resin system and vacuum assisted resin infusion (VRI) technology of this study is an easy and cost effective methodology in comparison to previously adopted other methods limited to laminated composites. The study identifies the optimum weight percentage of MWCNT addition in resin system for maximum performance gain in mixed mode fracture toughness. The results are supported by high resolution transmission electron microscope (HRTEM) analysis and fracture micrograph of field emission scanning electron microscope (FESEM) investigation.
Facesheet to Core Interface Characterization of Sandwich Structures
An important failure mode in sandwich structures is the debonding between the core and facesheet, which can destroy the load capacity of the structure. This work addresses the critical interfacial modes and studied the effects of thickness variation of the core material. The single cantilever beam (SCB) geometry is adopted for conducting experiments after optimizing the thicknesses of the core and facesheet by minimizing the difference in the bending stiffness matrix between the upper facesheet and the lower facesheet/core combination. Two samples of single cantilever beam, having two different core material thicknesses, were tested. The experimental results showed that the critical energy release rate could be influenced by core thickness variations. Furthermore, the cohesive zone method and elastic-plastic core material model in conjunction with fracture criteria were used to model the entire structure failure response. The validation results predicted load/extension curves in full agreement with actual tests for both specimen SCB geometries. The model also had the ability to predict the crack initiation in the core materials which occurred under the interface zone as in the actual test. In addition, the mixed-mode ratios through the interface area were analyzed as function of crack length.
Partially Reacted Substructures Method for Thermoset Epoxies Studied Using Molecular Dynamics Simulations
We employed the “partially reacted substructure (PRS)â€ method to enhance the material properties of the diglycidyl ether of bisphenol A (DGEBA) epoxy thermosets. This method involves first partially curing DGEBA with a small amount of poly(oxypropylene)diamine (POPDA) and then adding DGEBA and diethyltoluenediamine (DETDA) to continue to a fully cured sample. PRS-induced DGEBA-DETDA/POPDA 2:1 epoxy-amine stoichiometric samples have consistently higher Tg’s and glassy state densities than samples created using one-step cure of a blend of DGEBA with both DETDA and POPDA. PRS-induced samples were found to be twice as tough as non-PRS samples. Because the compositions are identical, differences observed must arise from differences in the network isomers formed by the two curing protocols and the resulting differences in packing and intermolecular interactions. Molecular dynamics (MD) simulations were carried out to better understand these phenomena. The density and glass transition temperature in the simulations are in quantitative agreement with the experimental results. One reason for this Tg increase in PRS-induced samples may be related to the pendant methyl groups on POPDA, as illustrated using radial distribution functions (RDF’s) computed from the MD simulation trajectories. Methyl-methyl RDF’s for non-PRS samples did not differ significantly from those of the prepolymerized liquid, but the methyl-methyl RDF’s for the PRS-samples showed stronger methyl-methyl correlation at all distances, signifying better intra- and intermolecular POPDA packing. This results in lower POP backbone flexibility in the PRS samples and consequently higher Tg’s relative to the non-PRS samples. Further simulations aim to ultimately focus on the ductility enhancement conferred by PRS.
Effect of Cross-Linker Length of EPON 828 Resin Properties Using Molecular Dynamics Simulation
Using molecular dynamics (MD) simulations, we study the effect of the length of Jeffamine cross-linker molecules on the thermo-mechanical properties of the Epon 828 epoxy resins. Cross-linked networks are created from stoichiometric mixtures of Epon and Jeffamine using a single-step cross-linking algorithm. We determine the glass transition temperature by gradual cooling from the rubbery state to below room temperature. Then, the networks are subjected to mechanical deformation to determine their mechanical properties, such as modulus and strength. Simulation results show that length of the cross-linker affects the thermomechanical properties of the epoxy. Epoxy with shorter cross-linker molecules gives better properties.
A Computational Molecular Dynamic Study on Epoxy-Based Network: Thermo-Mechanical Properties
A computational molecular dynamics model for determining the thermomechanical properties of EPON 862 and DETDA systems was developed using OPLS all atoms force field. The approach for building heavily cross-linked epoxy-based network is defined and presented. The simulations allowed thermal properties such as glass transition temperature and coefficient of thermal expansion; as well as elastic properties of the materials to be estimated. The results exhibited a good agreement with both available experimental data and simulated results in literature.
Computational Insights into High Strain Rate Self-stiffening Mechanism in Nacre
It has long been identified that nacre, natural body armor, has extraordinary mechanical properties such as high strength and eminent toughness. What’s more, under dynamic strain rate (~103 s-1), both modulus and strength are surprisingly increased comparing with quasi-static strain rate (~10-3 s-1). However, the mechanism of this unusual high strain rate self-stiffening phenomenon in the ceramic based natural armor remains ambiguous. In this study, a slab model is built to describe stacking fault and twin formation processes when nacre is deformed. Ab initial calculation is carried out using density function theory (DFT) under generalized gradient approximation (GGA). From this model, the surface energy is calculated and compared with experimental data to validate the accuracy of our calculation. The stacking fault energy, twin formation energy along with generalized planar fault energy are calculated to explain the formation tendency of stacking fault and twinning. The findings have uncovered the mechanism of nacre’s self-stiffening properties and provided a new route for designing strong and tough ceramic materials.
Vibration Characteristics and Damage Detection of Composite Structures with Anisotropic Damage Using Unified Particle Swarm Optimization Technique
Composite structures may undergo several types of damages such as delamination, fiber breakage, etc. due to the defects in the manufacturing process or fatigue loading during service which in turn affects the effective performance of the structure. The damage in the composite is anisotropic in nature whose extent is distributed in several orthogonal directions and the extent of damage in any orthogonal direction is independent of the other. The anisotropic damage is introduced into the composite using concept of reduction in stiffness due to reduction in effective load bearing area which is represented by damage parameters or variables. Vibration based damage detection is one of an effective scheme to detect damages due to its simplicity in implementation and ability of acquiring both the global and the local damage information of the structure. From the literature, it has been revealed that the frequency response function (FRF) based damage detection is more accurate compared to frequency based damage detection since it provide much more information on damage for a particular frequency range. A numerical procedure is developed in MATLAB to detect and quantify damages in composite structures using vibration response parameters with the help of unified particle swarm optimization (UPSO) algorithm. From the numerical and experimental results it is inferred that the presence of damage in composite structure affect the vibration characteristics such as frequency, mode shapes and FRF curves as well. Moreover, the damage parameters show strong orthogonality on vibration characteristics. It has been observed that, damage in the direction of fiber results in steeper deterioration of natural frequencies. The outcome shows that as the intensity of damage or the area of damage increases, the natural frequency deteriorates irrespective of ply orientation. The proposed methodology is able to detect and quantify damages accurately for various single and multiple damage scenarios.
Ultrasonic and X-ray Inspection of a High Performance Carbon Fiber Composite for Automotive Applications
Ultrasonic and radiographic inspections can be used to rapidly qualify carbon composites in automotive production environments as well as detect in-service damage. These evaluations have been demonstrated on high quality automotive thermoset composites with woven, unidirectional, and random-chopped carbon fibers and with a variety of layups including unidirectional, 0/90, and quasi-isotropic. The performance on flat plaques and flanged beams as well as adhesively-bonded assemblies has been assessed. High resolution ultrasonic imaging both with raster scanners and phased arrays can detect voids and foreign matter including the inter-ply location. Using ultrasonic 3-d datasets, the orientation of individual plies can be determined within areas as small as 30-mm. Three dimensional ultrasonic images have been compared with bi-directional SEM images of the same area. Ultrasonic imaging is especially useful for detecting non-visible collision damage. The differences found from impact, flex, and crush damage have been determined. 3-d renderings of the damage have been made. Low-energy radiography has even higher in-plane resolution and can detect features such as voids below 0.1-mm in size in regions as big as 30-cm. The ability of low-energy computed tomography to resolve ply information in component size regions has also been evaluated.
Non-Destructive X-ray Computed Microtomography Inspection of Composites Containing Manufacturing Defects
Honeycomb sandwich constructions are used extensively in the aerospace industry to increase stiffness of composite structures without adding significant weight. As part of the manufacturing process, immersion ultrasonic non-destructive inspection (NDI) is typically conducted in order to validate compliance of components. Ultrasonic NDI is relatively cost effective. Attenuation variations from baselines (known good components) indicate non-compliance of manufactured components, but the source of attenuation is often unavailable from ultrasonic inspection. Examples of sources of attenuation of ultrasonic signals include porosity in the bondline or laminate, delaminations between the core and facesheets, and foreign objects that entered the component during manufacturing. In order to improve manufacturing processes and reduce the costs associated with non-compliant components, a relatively new (to composites manufacturing) NDI technique, X-ray computed microtomography (CμT) is used. The advantages and limitations of CμT are discussed via investigation of noncompliant components. Ultrasonic NDI results of non-compliant cored composite aircraft components are presented. Regions of non-compliance are further investigated via CμT. The CμT investigation reveals unanticipated defects, blistering within the honeycomb core cell walls, at the locations that ultrasonic NDI indicated as non-compliant. As part of the CμT process, 3-dimensional digital volumes of the components are created. From these volumes, virtual cross-section cuts are created, including the creation of videos showing the ability to make “infiniteâ€ section cuts, which allow defects within the component to be investigated. Finally, results of destructive investigation are presented to confirm the blistering observed via the CμT investigation.
Quantitative Acoustics Approach for Damage Detection in Hard Armor Protective Inserts
The tap-test has long been a benchmark in non-destructive inspection of material systems, including the investigation of delamination effects in composites. In this research, tap-test related impact force variations in the (human) audible acoustic range (up to 20 kHz) are associated with damaged and undamaged hard armor protective inserts using an experimental approach. Damaged and undamaged states are determined a priori by Army quality inspections and further investigated by x-ray computed tomography (XCT), which are used to validate the presented approach. Digital signal processing of impact-echo type acoustics data is conducted to determine metrics that may differentiate between the damaged and undamaged states while providing hardware/software solutions to robustly identify damage locations. Funding for this effort is provided by the U.S. Army Program Executive Office – Soldier.
Three-Dimensional Analysis of Damage Propagation in Unidirectional Polymer Composites Using X-ray Computed Tomography and Digital Volume Correlation
X-ray micro-computed tomography (μCT) provides valuable insights into the structures within materials. When combined with digital volume correlation (DVC), the response to applied load can be measured in the form of the three-dimensional displacement and strain fields within materials. This study has applied X-ray μCT and DVC to investigate the propagation of damage in a unidirectional IM7/5250-4 composite under compression loading. Volumetric images were acquired from the composite specimen before and after loading using X-ray μCT. The images were analyzed using DVC to measure displacement field, map crack formation, growth and subsequent displacement of deformed specimen. Initial results demonstrate that volumetric displacement of the composite material using the naturally occurring pattern can be obtained successfully using DVC, making it a useful tool for quantitatively investigating the damage behavior of composite specimens. Several compression steps will be applied to the composite specimens under displacement control. Other selected polymer matrix composites will also be investigated.
Tortuosity of Fracture Surfaces in Composite Microstructures
Fiber reinforced composites are commonly used as structural materials in the aerospace industry. Due to safety requirements on the industry, materials are rigorously tested, requiring both schedule and financial cost to the design and manufacturing timeline. Thus, a computational suite allowing for “virtual testingâ€ is preferred, which could allow for part of the testing to be performed via computer simulation. One barrier to this paradigm shift is the accurate prediction of damage evolution within these materials. In this contribution, we focus on predicting the fracture surface at the scale of the microstructure. The dependence of the tortuosity of the fracture surface through microstructures of various sizes and volume fractions is explored by determining a cleaving crack path requiring minimal fracture energy for each microstructure. A mathematical graph theory approach is used to calculate the minimal energy crack path.
Mechanical Properties and Initial Fracture Behavior of Multilayer Laminated Intra-Hybrid Woven Fabric Composite
In this paper, carbon/glass intra-hybrid woven fabric as reinforcement and vinyl ester resin as matrix, CF/GF hybrid composite was manufactured through hand lay-up method. Then, the investigation on the mechanical property and initial fracture behavior was carried out. Initial fracture stress or knee point stress was the main factor of initial damage which discussed with AE characteristics during mechanical test. Optical microscope observation was also carried out to observe the fracture of materials before and after testing. Besides, model of the woven fabric was built by finite element analysis method (FEM) and then the fracture mechanism was simulated by the software Marc. By combining the experimental results and finite element analysis, stress-strain curve and initial behavior of fracture progress in resin hybrid composite materials were simulated and predicted.
Dynamic Yarn Pull-Out from In- and Out-of-Plane Directions
In this study, the mechanical response of a single yarn pull-out from single layers of Kevlar® and Twaron® fabric under out-of plane loading at both quasi-static and dynamic rates was experimentally investigated. In order to perform the dynamic experiments, a pendulum impact setup was designed and constructed to pull out a single yarn dynamically. The pull-out load was measured directly by a load cell and the movement of the fabric was measured to portray the load-displacement history. The effects of transverse pressure, different weave direction, and loading rates were also investigated.
Discussed is the development of an analysis for studying the deformation behavior of Belleville springs fabricated of fiber-reinforced composite materials. The springs are subjected to a compressive loading, and owing to the shallow conical geometry of a Belleville spring, snap-though behavior can be exhibited and thus geometric nonlinearities are considered. Considering cost and ease of fabrication, off-the-shelf textile materials along with standard resin transfer molding techniques are strong candidates for the production of composite Belleville springs. However, as off-theshelf textile materials typically employ fibers at fixed angles relative to some reference direction, e.g., 0° /90°, or 0°±45°, or 0°/±60°, using such textiles in a conical geometry results in overall material properties that vary with circumferential direction. This could complicate the response, potentially preventing an axisymmetric response even though the loading is axisymmetric, or at least complicate the analysis of the response. These issues are addressed in the paper, and since modeling of a textile composite Belleville spring involves material properties that vary spatially, in addition to geometric nonlinearities, approximate techniques are used. Specifically, a Rayleigh- Ritz approach is used to approximate the mechanical behavior. Discussed are the material property variations, load-deformation behavior, and strains and stresses as a function of circumferential location. For the particular case considered, a four-layer [(0/90)/(+45/-45)]S glass-epoxy textile laminate, the kinematic response is, for all intents and purposes, axisymmetric. The stress response, however, is not.
Progressive Damage and Failure Responses of Hybrid 3D Textile Composites Subjected to Dynamic Flexural Loading
The dynamic flexural response of a hybrid 3D textile composites (H3DTC) was examined through a three-point bend test. The H3DTC contains three different types of constituent fibers, including carbon, glass, and Kevlar that are integrally woven into a single preform. Tests were carried out using a drop tower facility, which can provide different impact velocities by varying the height of the weight that is dropped onto the specimen. High-speed cameras were utilized to record the deformation history and identify the modes of failure. The experimental results show that the development of kink bands in the tows undergoing compressive straining, limits the strength of the H3DTC in flexure, while the final catastrophic failure is due to tow rupture occurring in regions subjected to tensile straining. A global-local multiscale finite element (FE) model was developed to predict the progressive damage and failure response of the H3DTC subjected to low velocity impact. The composite was homogenized as an orthotropic solid at the macroscale, while the damage and failure was incorporated through a mesoscale FE model, which is a collection of repeat unit cells (RUCs) that are composed of fiber tows embedded in a surrounding matrix. A novel micromechanics model was implemented at the fiber-matrix level to compute the fiber tow constitutive responses at the mesoscale. The smeared crack approach (SCA) was employed to model the observed failure modes, including matrix cracking, tow kinking, and tow rupture. The computational results agree well with the experiment. Therefore, the proposed multiscale model can be used as a validated computational tool to understand the effect of textile architecture and constituent properties on the progressive damage and failure responses of 3D textile composites.