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This study investigates the effects of process conditions on the inherent variabilities in fused filament fabrication (FFF) of short carbon-fiber-reinforced Nylon-6 composites, where the sources of uncertainty and their adverse effects on microstructures and Young’s modulus are quantified. Microstructural characteristics such as fiber volume fraction, void volume fraction, and their spatial distributions are first extracted via image-based data analytics, and then their uncertainties are quantified by the analysis of variance. A Monte Carlo sampling method is introduced to enrich the datasets for analyzing uncertainty propagation from micro-level (microstructures) to macro-level (mechanical property). A modified Halpin-Tsai model with the consideration of fiber and void distributions is developed to quantify the propagated uncertainties on Young’s modulus, which are further validated through quasi-static tensile tests. This study examined the process-structure-property relationship of FFF samples and quantified the underlying variations in both micro- and macro-levels.

316L stainless steel fatigue samples produced by laser powder bed fusion (L-PBF) process were stress relieved at 900 °C for 2 h to understand the effect of residual stress on fatigue strength. Two groups of fatigue samples were produced: “polished” and “machined and polished”. “Polished” samples were built to final fatigue sample geometry and then polished to avoid surface roughness effect while preserving the residual stress at the surface. The machined and polished samples were built to rod shape and machined to final fatigue sample geometry (polishing was applied later to keep the same surface roughness). The purpose of the “machined and polished” samples was to see if residual stress can be removed by removing the surface material and obtain the fatigue strength of the material in the as-built (no post-heat treatment) state for later comparison. X-ray diffraction was used to measure the surface residual stress. The results show that polished samples showed significant tensile residual stress in the as-built state. The residual stress was then removed after stress relief heat treatment. Fatigue tests were conducted on the two groups of samples under as-built and stress relieved states. Results show that the fatigue strength of “polished” samples was significantly improved by the stress relief heat treatment. The stress relief heat treatment did not change the fatigue strength of the “machined and polished” samples, which is consistent with the residual stress measurement. The study suggests that stress relief heat treatment should be performed for parts produced by L-PBF process to obtain the best fatigue property and dimensional accuracy.

The effect of the multiaxiality and proportionality of loading on the cyclic behaviour and early cracking behaviour was studied for closed die forged AZ80 Mg alloy. Several different loading paths were presented, uniaxial and biaxial with varying levels of non-proportionality. In multiaxial loading the effect of proportionality is only present on the shear response of the material and increasing levels of non-proportionality is detrimental to the fatigue life. When compared with proportional loading, a 50% reduction in life was observed under 90° out of phase non-proportional loading. The pure axial cracking behaviour is dominated by transverse cracks along the plane of maximum normal stress, whereas in pure shear the material exhibited longitudinal cracking (shear dominated) behaviour in LCF regime and helical cracking (normal stress dominated) in the HCF regime. The synergistic effects of each individual uniaxial cracking modes contribution towards the combined macroscopic cracking behaviour in multiaxial loading was investigated, and was dominated by transverse cracking when the shear strain energy density (SED) contribution to the total SED was low, and mixed cracking when the shear SED contribution was high.

The quasi-static and cyclic response of as-extruded and closed-die extruded-forged AZ80 Mg alloy forged at 250 °C and 20 mm/sec was investigated. Servo hydraulic, fully reversed, strain controlled uniaxial “push-pull”, pure torsion, and biaxial fatigue testing were also conducted on the closed-die forged alloy. Two uniaxial loading paths (pure axial and pure shear) and three different biaxial loading paths were investigated (proportional loading, and non-proportional loading at phase angles of 45° and 90°). After forging of the as-extruded billet, the yield stress in the axial direction increased by 46%, the failure elongation by 35% and the ultimate strength by 13%. The spatial variation in both the local texture, microstructure and superficial hardness throughout the cross section of the forging was investigated and discussed. The web region in the center of the forged billet exhibited a virtually fully recrystallized and refined microstructure with intense basal texture, whereas the rib regions on each side exhibited a partially recrystallized and coarser microstructure with less intense texture. The fatigue life of the forged material in the axial direction showed dramatic improvement relative to the as-extruded material at strain amplitudes which were less than 0.35%. However, at higher strain amplitudes, there are significantly higher cyclic stresses (and associated fatigue damage) in the forged material (relative to the as-extruded), resulting in a decrease in life under fully reversed strain controlled fatigue loading. For the pure axial strain path, the total amount of strain energy density (SED) observed in the stabilized response was a function of number of cycles only, and was insensitive to material condition (as-extruded or forged). However, for the forged material, of all the strain paths investigated, only the pure shear path demonstrated a significantly different relationship of total SED vs. life. Furthermore, the amount of total SED for a given number of cycles decreases when biaxial loading is introduced, though following the same general relationship as the pure axial strain path. The effect of a decrease in total SED for a given number of cycles occurred in all biaxial strain paths, with proportional being the least prominent, then 45° and 90° out of phase being progressively more pronounced respectively. Both the Smith Watson Topper (SWT) and Jahed-Varvani (JV) models were utilized to predict the fatigue life of the uniaxial and biaxial cyclic tests. Calibration of each models intrinsic parameters were done using both of the uniaxial loading paths individually, then these same parameters were used in combination with one another to predict the life in the biaxial cases. Both the SWT and JV models are capable of giving an acceptable prediction, however the JV model’s prediction is more reliable, especially when the cyclic response has pronounced asymmetry. The biaxial fatigue response is somewhat dominated by the axial component for two significant reasons; firstly, the shape of the hysteresis in the shear direction dramatically changes based on phase angle and axial strain amplitude, whereas the axial hysteresis loop remains somewhat invariant to the presence or modulation of the other loading axis. Secondly, the initial crack propagation mode is predominantly transverse to the axial direction in all strain paths except for pure shear (which exhibits longitudinal cracking). Tensile cracking dominating the biaxial failure mode provides qualitative justification of a modification factor to an existing energy based biaxial fatigue life prediction model, enabling it to predict the life of extruded and forged AZ80 Mg reliably for a variety of different biaxial strain paths.

A closed die forging process was developed to successfully forge an automotive suspension component from AZ80 Mg at a variety of different forging temperatures (300 °C, 450 °C). The properties of the forged component were compared and contrasted with other research works on forged AZ80 Mg at both an intermediate forging and full-scale component forging level. The monotonic response, as well as the stress and strain-controlled fatigue behaviours, were characterized for the forged materials. Stress, strain and energy-based fatigue data were used as a basis for comparison of the durability performance. The effects of the starting material, forging temperature, forging geometry/configuration were all studied and aided in developing a deeper understanding of the process-structure-properties relationship. In general, there is a larger improvement in the material properties due to forging with cast base material as the microstructural modification which enhances both the strength and ductility is more pronounced. In general, the optimum fatigue properties were achieved by using extruded base-material and forging using a closed-die process at higher strain rates and lower temperatures. The merits and drawbacks of various fatigue damage parameters (FDP’s) were investigated for predicting the fatigue behaviour of die-forged AZ80 Mg components, of those investigated, strain energy density (SED) proved to be the most robust method of comparison.

The material behavior of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite is investigated under both quasi-static and cyclic fatigue loading conditions in the present study. Quasi-static tensile and compressive strength data is collected, followed by a series of fatigue tests under three different cyclic loading conditions, i.e., Tension-Tension (T-T), Compression-Compression (C-C), and Tension-Compression (T-C). The experimental results show a considerable amount of variation for each loading condition, which is found to be induced by the spatial distribution of fiber orientation within the tested specimens. An analytical approach is developed to correlate the fatigue performance of the SMC material with the local modulus, which is determined by the fiber orientation distribution. Additionally, the similar failure modes are observed under different loading conditions. Based on these findings, a new form of Constant Fatigue Life (CFL) diagram is established in which the impact of fiber orientation tensor to the fatigue behavior of SMC is considered. This developed CFL diagram is examined by comparing the measured fatigue performance with prediction where local fiber orientation distribution is quantified from microscopic images.

Injection molded plastics are widely used for mass production in auto industry. However, it is challenge to produce parts that do not warp. It is helpful to have an accurate way to measure the warpage when investigating and solving warpage issues. In this paper, a new technology is introduced to measure the part warpage quickly and accurately with the help of digital image correlation (DIC) method. The polypropylene plaque were fabricated by injection molding based on An L9 (3ˆ4) Taguchi design of experiments. The warpage of plaque are characterized for all the conditions and at different time after being ejected. Different kinds of warpage results are obtained by DIC method, such as maximum deflection, deflection distribution along desired path, warpage contour on the surface. The plaque warpage increases with the time. The maximum difference can reach as high as 64.9%. The cooling time has the largest influence on the warpage, while the packing pressure has the minor effect. Nevertheless, the warpage difference due to time varying depends on melt temperature, mold temperature, cooling time and packing pressure, in which mold temperature has the highest effects.

Failure analysis is a critical process for the lifetime evaluation of high density polyethylene (HDPE) under cyclic loading since HDPE may encounter different failure modes under different service environments. However, there are only a few investigations on the failure mechanism and failure mode of HDPE in cyclic-loading tests. The objective of this paper is to understand the failure mechanism and propose an approach to predict the failure mode of HDPE under cyclic loading. On the mechanism analysis, our results demonstrated that there could be three failure modes including necking, buckling and fatigue cracking for HDPE under cyclic loading. Both necking and buckling were strongly dependent on the increase of the specimen temperature, which led to the reduction of both yield stress defining necking and tangent modulus governing buckling. On the mode prediction, the yield stress and the tangent modulus could be computed as the functions of specimen temperature which was calculated by FEM heat transfer analysis considering heat generated by inelastic deformation energy and heat transferred with surroundings. Then, necking was predicted by comparing the decreased yield stress with the applied peak stress, and buckling was predicted by comparing the declined buckling stress proportional to tangent modulus with the absolute of the applied valley stress. The proposed methodology was validated as the predicted results were well matched with the experimental data for the tests of HDPE under different cyclic loadings.

This study investigates the failure mechanisms of cross-ply laminates subjected to longitudinal compressive loading. A sequence of failure initiation and propagation process is observed based on optical microscopy images of failed specimens. Specifically, the failure process in the cross-ply laminates involves a combination of four failure mechanisms: fiber kinking, delamination, matrix cracking, and fiber/matrix splitting. We find that the kink-bands in the middle 0° plies of the cross-ply laminates most often show a wedge shape and the angle of matrix cracks in the 90° plies is slightly larger than that of pure 90° plies. The reason has been attributed to the constraining effects by the adjacent plies. The next focus of this study is to propose hybrid micro-macro models to more systematically study the failure mechanisms of the cross-ply laminates. We show that these models accurately predict the combined failure modes of the cross-ply laminates and enable us to closely investigate the interactions of different failure mechanisms. More importantly, the hybrid models achieve great computational efficiency compared to full-scale microstructural models. The combined experimental and computational analyses presented here provide a new level of understanding of the kink-band morphology and damage mechanisms in the cross-ply laminates.

The sink mark is one common defect of the injection-molded thermoplastics, which is harmful to the part appearance. A procedure for the sink mark prediction was proposed to build the finite element method (FEM) model in Abaqus. The temperature and pressure distributions were imported as boundary conditions at a different time from the injection molding simulation software Moldflow. The user subroutines UEXPAN and UMAT were coded and implemented in Abaqus to describe the pressure-volume-temperature (PVT) behaviors under actual cooling conditions and the polymer constitutive relationship. The PVT curves of the thermoplastic were obtained by the piston-die method at a cooling rate of 5 °C/min. The crystallizing behaviors were measured by a differential scanning calorimetry and adapted to modify the PVT curves at different cooling rates. The tension-compression tests at different temperatures were performed to obtain the stress-strain curves. The simulation of injection molding was conducted in the software Moldflow. The deflection tests were conducted on the coordinate measuring machine (CMM) to validate the predicted sink mark. It was found that the subroutines of UEXPAN described the volume variations under different cooling rates successfully. Furthermore, the subroutine UMAT predicted stress-strain curves successfully at different temperatures and strain rates. The predicted sink mark in Abaqus agreed well with the measured one. The sink mark depth from Abaqus reached one third of the experimental one and had obvious improvement than that from Moldflow.

In present study, the fatigue damage behavior of chopped carbon fiber chip-reinforced composite has been experimentally and numerically investigated. A large scatter is exhibited in the S-N diagram due to the random distribution of carbon fiber chips, and therefore a new analysis procedure is proposed to link the local microstructure to the fatigue behavior of the bulk material. Interrupted fatigue tests are also performed so that microstructure characterization can be conducted on the tested samples to analyze the crack initiation and propagation. Inspired from the experimental findings, a multi-scale progressive damage fatigue model is proposed to predict the fatigue behavior. The model incorporates a new stochastic chip-packing algorithm for microstructure reconstruction along with continuum damage models into the representative volume element (RVE) model in ABAQUS/Explicit. The simulation results based on the proposed model are in good agreement with the experimental data in terms of cracking modes and predicted life.

This paper presents a computational framework to predict the tensile failure of chopped carbon fiber SMC composites based on the Representative Volume Element (RVE) model. In the framework, a conforming mesh-based reconstruction algorithm is developed to generate mesostructure RVE models of SMC based on the statistical characteristics. Moreover, constitutive models are proposed for the three phases in SMC composites: fiber chip, matrix, and interface. The model predictions are validated by coupon-level experimental tests on SMC samples of various chip orientation distributions. The meso-scale failure mechanism and failure strength predicted by the RVE models are in good agreement with the experimental results. The presented study demonstrates the capability of the present modeling framework in the design of composites materials with complex micro- and meso- structure.

Failure prediction for carbon fiber reinforced polymer (CFRP) composites has been a longstanding challenge. In this study, we address this challenge by first applying a well-established computational micromechanics model based on representative volume element to predict the failure envelopes of unidirectional (UD) CFRP composites. Then, these failure envelopes are compared with the classical failure criteria. We have evaluated the performances of these failure criteria and identified the aspects for further improvement in their accuracies for the UD CFRP composites studied herein. Based on the failure mechanisms from computational analyses and the comparisons between predicted failure envelopes and classical failure criteria, a new set of homogenized failure criteria is proposed. The newly proposed failure criteria show significant improvement according to our computational and experimental results. Furthermore, we have compared the proposed failure criteria with existing experimental data and computational results available in the literature for different types of composites. Good agreements are generally observed.

Fatigue tests for unidirectional (UD) CFRP composite specimens of various sizes are conducted under different stress ratios. The results show that the specimen size has considerable influence on the transverse tension-tension fatigue performance of UD laminates. The underlying fatigue mechanisms are also investigated through SEM observations. Moreover, a fatigue life prediction method considering size effects is presented, and a good correlation is observed between the prediction and the test data. Finally, to allow for an evaluation of the effects of mean stress, constant fatigue life diagrams for 0°, 10°, and 90° UD composites are established based on four-segment anisomorphic model.

The role of the process deformation history on the mechanical performance of self-pierce riveted (SPR) joints is examined in detail. The SPR process was simulated using a non-linear finite element model coupled with a damage criterion sensitive to the stress state. Results of the FE simulation compared closely to cross-sectional images of experimental SPR joints. The FE simulation results suggest that the strain hardening due to the piercing process is the main mechanism for joint strength under quasi-static condition. Aspects of fatigue damage were modeled including crack initiation, propagation, fretting and number of cycle to failure, and compared to experimental results. Results from the study indicate that residual stresses and plastic strain may alter the crack initiation location and crack propagation plane, leading to more accurate results when included in finite element analysis of SPR joints. Including these residual stresses and plastic strains also changed the predicted fretting locations leading to better agreement with experimental results. Lastly, linear elastic fracture mechanics were performed to estimate fatigue lives based on initial crack location and angle of crack plane. The estimation of the number of cycles to failure that included residual stresses and plastic strains resulted in a better correlation to experimental data when compared to the calculations that excluded residual stresses and plastic strains.

It is important to investigate the constitutive behavior of high density polyethylene (HDPE) under cyclic loading since it is widely used for industrial applications. However, there are only a few preliminary investigations on the constitutive behavior of HDPE under cyclic loading. This work aims at investigating the deformation mechanism, developing the constitutive model especially for the cyclic-loading behavior, and proposing an efficient approach for model calibration. Firstly, the deformation mechanism was successfully explored by conducting a special designed relaxation-unloading test by which the time-independent elastic-plastic behavior was decoupled from the time-dependent overall deformation behavior. Then, a nonlinear elastic-viscoplastic constitutive model was developed based on the parallel rheological framework (PRF) composed of an elastic-plastic network in parallel with multiple nonlinear viscoelastic networks. Finally, an approach was proposed to calibrate the constitutive model by means of relaxation-unloading tests and material parameter optimizations. The agreement between the prediction and the test data successfully demonstrated that the model could be used to precisely simulate the constitutive behavior of HDPE under static, quasi-static and dynamic cyclic loading. The developed constitutive model is beneficial to the long-term durability evaluation of HDPE. The methodology for exploring the deformation mechanism and calibrating the constitutive model may also be applicable to other polymers.

Fatigue life prediction is crucial to the long-term durability evaluation of high density polyethylene (HDPE) which may experience stress-controlled fatigue or strain-controlled fatigue depending on the external loading conditions. Although abundant studies have been carried out on stress-controlled fatigue, investigations on strain-controlled fatigue are insufficient, and the relationship between these two types of fatigues is unclear. In addition, approaches for strain-controlled fatigue life prediction of HDPE are scarce. The present paper aims at revealing the relationship between strain-controlled fatigue and stress-controlled fatigue and proposing a method for fatigue life prediction. Firstly, the stress-controlled fatigue tests were performed to study the influence of stress ratio on fatigue life, and the strain-controlled fatigue tests were successfully conducted thanks to the application of anti-buckling devices. Furthermore, our results showed that the stress level (i.e. stress amplitude and stress ratio) determined the fatigue life for both stress-controlled test and strain-controlled test. More importantly, a stress-based numerical model was established to predict the fatigue life under complicated loading conditions. The numerical model was built based on the constitutive relation for the computation of the stress response, the Walker method for the evaluation of the stress ratio influence, and the damage accumulation for the calculation of the fatigue life. The proposed numerical method was validated by comparing the predicted fatigue lives with the experimental data of the strain-controlled tests.

Stress-controlled uniaxial “push-pull” fatigue testing was conducted on as-received (cast and extruded) and closed-die cast-forged and extruded-forged AZ80 Mg alloy. The as-cast material possessed random texture and somewhat symmetric cyclic responses. The extruded and forged materials possessed sharp basal texture and asymmetric cyclic responses. All materials exhibited tension/compression asymmetry in their cyclic response to varying degrees, depending on the thermomechanical processing conditions. It was discovered that the style of closed-die forging being investigated had spatially varying properties with texture orientations which varied based on the local forging directions and intensities which were dependent on the starting texture as well as the thermomechanical history. Under fatigue testing, the materials all developed some form of mean strain, with the nature and magnitude of this mean strain being dependent on primarily its texture intensity and propensity to twin in either tension or compression reversals. The type of mean strain (tensile or compressive) depends upon both the orientation and intensity of the starting texture of material. The texture induced ratcheting and resulting mean strain evolution was most pronounced in the as-cast material and had a significant impact on the fatigue life. Following forging, the material exhibited an increase in fatigue life of anywhere from 2 to 15 times for the cast then forged material and more modest yet still significant 8 times longer at stress amplitudes around 140 MPa for the extruded then forged material. The extruded forged material exhibited similar fatigue lives to that of the base material at stress amplitudes which approached the yield strength. The nature of the mean stress development and degree of fatigue life improvement depended on the processing conditions and the type of base material (cast or extruded) utilized to create the forging. Two energy based models were utilized to predict the life of the forged material, and gave a reliable life prediction for a variety of material conditions that were investigated.

This study proposed a spatiotemporal algorithm to quantitatively characterize the in-situ 3D fracture behavior of carbon-fiber-reinforced polymer (CFRP) composites at microscale . In-situ micro X-ray computed tomography (µXCT) integrated with a tensile stage was applied to capture the 3D fracture evolution of the CFRP composites, where the initiation and propagation of fracture features (e.g., fiber tip-end crack and fiber/matrix debonding) were identified. After the reconstruction of the 3D material microstructure , the proposed spatiotemporal algorithm thereafter extracted the fracture features by employing multiple image processing techniques for quantitative analysis. A similar distribution of the 3D strain obtained from the volumetric digital image correlation demonstrated the feasibility of the developed spatiotemporal algorithm. Moreover, this algorithm provided in-depth and quantitative analysis of fracture features, which provided insights into the microscale failure mechanism and thus shed light on the improvement of failure criteria for CFRP composites with complex microstructures .

This study investigates the failure mechanisms of notched unidirectional (UD) carbon fiber reinforced polymer (CFRP) laminates subjected to longitudinal compressive loading . A sequence of failure initiation and propagation is observed based on optical microscopy images of specimens during failure. The micrographs reveal that the main failure mechanism in UD laminates is fiber kinking failure. The influence of manufacturing defects and voids on fiber kinking mechanisms is also analyzed. The results show that the voids in the resin-rich area lead to kink-band splitting, while in some cases the voids cause the kink-band to deflect in a new direction depending on the location of the voids. The next focus of this study is to propose a computational micromechanics model considering local fiber waviness to study the fiber kinking failure mechanism. The computational results on kink-band formation and stress-strain response show good consistency with our experimental analysis. Finally, the failure envelopes of σ11-σ22 and σ11-τ12 subjected to combined loading conditions are obtained from the computational model, which provide essential inputs for future theoretical failure criteria.

Defects are inevitable in metal parts manufactured by any process; the size, shape and location of such defects play a critical role in determining the material’s fatigue strength. Due to the random nature of the defects’ distribution in the part, a statistical method must be employed for fatigue strength estimation. The laser powder bed fusion (L-PBF) process introduces two main types of porosity defects: keyhole pores and lack-of-fusion pores. A defect-based statistical fatigue strength model has been developed and validated for the L-PBF AlSi10Mg aluminum alloy containing keyhole defects with different size distributions. Artificial defects were also introduced for model validation. The approach is based on the modified Murakami’s formulation to address the material dependence and followed the Romano’s approach to consider the statistical behavior of the fatigue strength. The proposed model successfully predicts the fatigue strength of different keyhole porosity distributions but is unable to predict the fatigue strength of materials containing lack-of-fusion porosity, possibly due to the higher stress concentration induced by its morphology.

Stress gradient influence factors and fatigue notch factors were measured for the AlSi10Mg aluminum alloy produced by laser powder bed fusion (L-PBF) processing with respect to different stress gradients (notch geometries). Modeling the stress gradient is critical for accurate fatigue life estimations for parts containing notches or fillets. Uniaxial fatigue tests were performed to obtain the fatigue strengths at R = −1 for notched and unotched specimens. The fatigue results were then used to calibrate the stress gradient influence factor model for fatigue life estimation. Due to the small grain size produced by the L-PBF process, the stress gradient influence factor is much lower compared to the cast A319-T7 alloy (at 120 °C) at the same stress gradient.

The microstructural origins of premature fatigue failures were investigated on a variety of forged components manufactured from AZ80 and ZK60 magnesium, both at the test specimen level and the full-scale component level. Both stress and strain-controlled approaches were used to characterize the macroscopically defect-free forged material behaviour as well as with varying levels of defect intensities. The effect of thermomechanical processing defects due to forging of a industrially relevant full-scale component were characterized and quantified using a variety of techniques. The fracture initiation and early crack growth behaviour was deterministically traced back to a combination of various effects having both geometric and microstructural origins, including poor fusion during forging, entrainment of contaminants sub-surface, as well as other inhomogeneities in the thermomechanical processing history. At the test specimen level, the fracture behaviour under both stress and strain controlled uniaxial loading was characterized for forged AZ80 Mg and a structure-property relationship was developed. The fracture surface morphology was quantitatively assessed revealing key features which characterize the presence and severity of intrinsic forging defects. A significant degradation in fatigue performance was observed as a result of forging defects accelerating fracture initiation and early crack growth, up to 6 times reduction in life (relative to the defect free material) under constant amplitude fully reversed fatigue loading. At the full-scale component level, the fatigue and fracture behaviour under combined structural loading was also characterized for a number of ZK60 forged components with varying levels of intrinsic thermomechanical processing defects. A novel in-situ non-contact approach (utilizing Digital-Image Correlation) was used as a screening test to establish the presence of these intrinsic defects and reliably predict their effect on the final fracture behaviour in an accelerated manner compared to conventional methods.

The mechanical properties and damage behaviors of carbon/epoxy woven fabric composite under in-plane tension and compression are studied at the meso-scale level through experiment and simulation. An efficient representative volume element (RVE) modeling method with consistent mesh, high yarn volume fraction and realistic geometry is proposed. The material constitutive laws with plasticity, tension-compression asymmetry and damage evolution are established for the three components - yarn, matrix and interface, respectively. Significantly different mechanical properties and damage evolutions are observed depending on loading conditions and initial geometry characteristics. It shows a non-linear stress-strain curve with clear transition region and intensive damage in tension, while a quasi-linear behavior up to facture is observed in compression with little damage prior to final fracture. Moreover, compared to the constant Poisson's ratio with straining in compression, a dramatic increase in Poisson's ratio appears in tension. Simulation shows damage mechanisms including transverse damage, matrix damage and delamination, which all play critical roles in the property evolution. In particular, the rapid damage accumulation after elastic deformation destroys the strong bonds and causes the easy deformation of transverse yarns which results in the transition region and large Poisson's ratio in tension. All the mechanical behaviors and damage evolutions are well captured and explained with the current RVE model.

In the present study, the quasi-static and fatigue behavior of chopped glass fiber reinforced sheet molding compound composite was investigated. Considering the stochastic meso-structure induced by processing, fiber orientation within specimens was analyzed based on the sectional images obtained by X-ray Computerized Tomography (XCT). Quasi-static tensile and compressive tests were conducted, in which the strain distribution was recorded by Digital Image Correlation (DIC) system. The fatigue behavior of material was evaluated under tension–tension (R = 0.1) and tension–compression (R = -1) loading. Interrupted fatigue test was conducted to investigate the damage evolution on the side surface of specimen using optical microscopy. A modeling method based on Representative Volume Element (RVE) was modified and applied to predict the fatigue behavior of glass fiber SMC. The simulation result showed a good consistency with experimental results on both prediction of the fatigue life and the damage mechanisms under cyclic loading.

Computational analyses of emerging materials such as fibrous composites rely on multiscale simulations. To account for the inherent uncertainty in these materials, such simulations must be integrated with statistical uncertainty quantification (UQ) and propagation (UP) methods. However, limited advancement has been made in this regard due to the significant computational costs and complexities in modeling spatially correlated structural variations coupled at different scales. In this work, a nonintrusive approach is proposed for multiscale UQ and UP to address these limitations. We introduce the top-down sampling method that allows to model nonstationary and continuous spatial variations of uncertainty sources by creating nested random fields (RFs) where the hyperparameters of an ensemble of RFs is characterized by yet another RF. We employ multiresponse Gaussian RFs in top-down sampling and leverage statistical techniques to address the considerable computational costs of multiscale simulations. We apply our approach to quantify the uncertainty in a cured woven prepreg due to spatial variations of fiber volume fraction and misalignment, fiber and matrix modulus, and yarns' architecture (i.e., angle, height, and spacing). Our results indicate that, even in linear analysis, the effect of uncertainty sources on material's response could be significant.

Woven fiber composites have been increasingly employed as light-weight materials in aerospace, construction, and transportation industries due to their superior properties. These materials possess a hierarchical structure that necessitates the use of multiscale simulations in their modeling. To account for the inherent uncertainty in materials, such simulations must be integrated with statistical uncertainty quantification (UQ) and propagation (UP) methods. However, limited advancement has been made in this regard due to the significant computational costs and complexities in modeling spatially correlated structural variations coupled at different scales. In this work, a non-intrusive approach is proposed for multiscale UQ and UP to address these limitations. We introduce the top-down sampling method that allows to model non-stationary and continuous (but not differentiable) spatial variations of uncertainty sources by creating nested random fields (RFs) where the hyperparameters of an ensemble of RFs is characterized by yet another RF. We employ multi-response Gaussian RFs in top-down sampling and leverage statistical techniques (such as metamodeling and dimensionality reduction) to address the considerable computational costs of multiscale simulations. We apply our approach to quantify the uncertainty in a cured woven composite due to spatial variations of yarn angle, fiber volume fraction, and fiber misalignment angle. Our results indicate that, even in linear analysis, the effect of uncertainty sources on the material’s response could be significant.

A multi-scale computational analysis based on representative volume element (RVE) modeling and molecular dynamics (MD) simulations is developed to investigate the microscopic failure mechanisms of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites. The average properties of the 200-nm thickness interphase region between fiber and matrix are characterized through MD simulations and an analytical gradient model. The results demonstrate that the interphase region has higher Young’s modulus and strength, compared to the bulk matrix. This stiffened interphase region influences the composite response significantly. Specifically, the traditional two-phase model with zero-thickness interface fails to capture the stress-strain behavior compared to the experimental data. However, by adding the interphase region to a modified RVE model, the accuracy of simulation results will be improved significantly. Furthermore, a coupled experimental-computational micromechanics approach is adopted to calibrate and validate the cohesive parameters of the interface. By including the cohesive interface, our modified RVE model accurately captures the failure strength of the composites. Finally, different failure mechanisms for specimens are investigated using our multi-scale computational framework. The results show that the failure modes of UD CFRP composites are very complex and multiple failure mechanisms co-exist depending on the loading conditions, agreeing well with our experimental analyses.

Predicting the mechanical behavior of the chopped carbon fiber Sheet Molding Compound (SMC) due to spatial variations in local material properties is critical for the structural performance analysis but is computationally challenging. Such spatial variations are induced by the material flow in the compression molding process. In this work, a new multiscale SMC modeling framework and the associated computational techniques are developed to provide accurate and efficient predictions of SMC mechanical performance. The proposed multiscale modeling framework contains three modules. First, a stochastic algorithm for 3D chip-packing reconstruction is developed to efficiently generate the SMC mesoscale Representative Volume Element (RVE) model for Finite Element Analysis (FEA). A new fiber orientation tensor recovery function is embedded in the reconstruction algorithm to match reconstructions with the target characteristics of fiber orientation distribution. Second, a metamodeling module is established to improve the computational efficiency by creating the surrogates of mesoscale analyses. Third, the macroscale behaviors are predicted by an efficient multiscale model, in which the spatially varying material properties are obtained based on the local fiber orientation tensors. Our approach is validated through experiments at both meso- and macro-scales, such as tensile tests assisted by Digital Image Correlation (DIC) and mesostructure imaging.

Mixed-mode fatigue crack growth behavior of an automotive adhesive bonding system (AA5754O- A951-BM4601) has been investigated using a compound compact mixed-mode specimen. The test was performed under load control with 0.1 R ratio and 3Hz frequency. A long-distance moving microscope was employed to record the real-time length of the fatigue crack in the adhesive layer during testing. The strain energy release rate components of the crack were analyzed by finite element method. The pure-mode fatigue crack growth test results show that a given value of G I results in a fatigue crack growth rate over an order of magnitude higher than for an equal value of G II. The mixed- mode tests results indicate that under a given loading angle, the fatigue crack growth behavior is also influenced by the test load due to the changing shear mode ratio when the crack grows. The fatigue crack growth data were then processed by a generalized Paris relation formula and employed in the direct fatigue life prediction of single lap shear joints with the same bonding system. The life prediction shows good results in the low-cycle region but slightly conservative results in the high- cycle region when compared to coupon fatigue tests.

In this article, experimental tests under static tensile loadings and tension-tension cyclic loadings were conducted for T300/924 unidirectional laminated composites at different porosity levels. On the basis of the experimental tests, a physical-based residual stiffness model for porous CFRP composites was put forward. The present model describes the deterioration of composites under cyclic loading in perspective of the initiation and propagation of cracks in the matrix, and is capable of capturing the effect of voids on fatigue behaviors of the composites. Lastly, the stiffness degradations of laminates with different void contents under various stress levels were predicted, and the predicted stiffness reduction as well as fatigue life of the material agreed well with the experimental data.