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Defect effects of carbon fiber composites under dynamic impact conditions are important to mechanical behavior design in the aerospace field. Here we report the defect effect on the impact compressive behavior of 3D braided composites at high strain rates from 550/s to 1240/s. The defect effect on damage behavior was observed by high-speed photography and digital image correlation (DIC) technology. A finite element analysis (FEA) model was developed to show the defect effect on stress distribution and thermo-mechanical behavior. The defect structure reduces the compressive strength of the composite and causes more brittle and catastrophic failure compared with the perfect composite. The defect effect on the compressive behaviors is more significant at higher strain rates. FEA results show that the defect structure causes local stress concentration, high adiabatic temperature rise, and high stress in the X-shaped shear band region, thereby accelerating composite failure. [ABSTRACT FROM AUTHOR]

Ageing conditions and braided structures will affect dynamic damage and failure mechanisms of 3D braided composites. Here we report the influence of thermo-oxidative ageing on impact compression behaviors of 3D braided composite with and without axial yarns, i.e., four directional (3D4d) and five directional (3D5d) braided composites. We found a significant difference in mechanical properties between epoxy resins and composites after ageing. The impact compressive strength of epoxy resins declined rapidly and then slowly, while the composites decreased continuously, with the increase of ageing days. The interfacial debonding is main degradation mechanism for both braided composites at the later ageing stage. Thermo-oxidative ageing accelerates the inner damage evolution which propagates from surface cracks. Compared with 3D4d braided composites, the 3D5d braided structure has higher strength retaining level after ageing. In addition, the 3D5d braided samples have higher resistance to impact deformation and lower crack propagation. Finite element analysis (FEA) results revealed that 3D5d composites showed higher load carrying capacity and less damages than 3D4d composites. [ABSTRACT FROM AUTHOR]

This paper investigates the ballistic impact on Kevlar multilayer three-dimensional angle-interlock woven fabric (3DAWF) by proposing the mesoscale geometrical model for the numerical simulation. Multilayer 3DAWF is designed to yarn level configuration by utilizing the membrane elements to reduce computational time and enhance accuracy. The general-purpose finite element code LS-DYNA is employed to predict the ballistic behavior of multilayer 3DAWF under ballistic penetration. The velocity evolution of the projectile, energy absorption mechanism, and failure morphology of multilayer 3DAWF are predicted and validated by the impact test results. It is found that the mesoscale model based on strain rate material models accurately reproduces the ballistic test results. Numerical simulations with strain rate effects in the yarn material properties have a higher precise prediction in the projectile's velocity, energy absorption mechanism, and failure morphology compared with traditional FEA. This study demonstrated the importance of the strain rate effect of material properties in simulating the ballistic impact on the fabric and dramatically improves the ballistic impact simulation's accuracy on fabric. [ABSTRACT FROM AUTHOR]

Flexible fabrics have been widely used in body armor designs. Here we report ballistic impact damage of stacked cross-plied composite fabric and cross-plied laminated panels. The ballistic impact behaviors of stacked cross-plied composite fabric and cross-plied laminated panel have been tested with fragment-simulating projectiles under the strike velocity 550–600 m/s to explore the influence of the layers combination of fabric target on ballistic impact. Two types of macroscopic anisotropy continua finite element models based on fabric targets structures are established to analyze the ballistic mechanism of stacked cross-plied composite fabric and cross-plied laminated panels. The impact damage morphologies and energy absorptions have also been compared between the tests and finite element analysis results. We have found the stacked fabric construction absorbed more energy than their counterpart cross-plied laminated panel, while the laminated panel shows better structural integrity and stability during ballistic penetration. [ABSTRACT FROM AUTHOR]

This paper reports the mode I interlaminar fracture toughness and fracture mechanisms of two-dimensional (2D) plain woven composite and three-dimensional (3D) angle-interlock woven composite. The fracture toughness behaviors were tested with double cantilever beam method at the different loading rates from 0.5 to 100 mm/min. Critical strain energy release rate was calculated to compare the difference between the 2D and the 3D woven composites. The fractographs were photographed with scanned electronic microscopy and optical microscopy to show the fracture morphologies. We found that the 3D angle-interlock woven composite has high fracture toughness than that of 2D woven composite. The binder yarns resist the crack initiation and propagation to increase the fracture toughness. While the lower in-plane stiffness of the 3D woven composites should be considered fully for designing the 3D woven composites. [ABSTRACT FROM AUTHOR]

Three-dimensional braided composite materials have been widely applied to engineering structure manufacturing. It is of a great importance to characterize the impact damage of the three-dimensional braided composite under various temperatures for optimizing the engineering structure. Here we conducted transverse impact deformation and damage of three-dimensional braided composite beams with different braiding angles at room and elevated temperatures. A split Hopkinson pressure bar with a heating device combined with high-speed camera was employed to test multiple transverse impact behaviors and to record the impact deformation developments. The results indicated that failure load, initial modulus, and energy absorption decreased with the increase of temperature, whereas the deformation increased slightly with elevated temperatures. We found that the impact brittle damages occurred earlier and the local adiabatic temperature raised higher when the temperature is lower than the glass transition temperature (Tg) of epoxy resin. While above the Tg, the impact ductile damages occurred later and the local temperature raised lower. The thermal stress distribution along the braiding yarn leads to cracks propagation in yarn direction. Part of the impact energy absorptions converted into thermal energy. In addition, the beam with larger braiding angle has high damage tolerance and crack propagation resistance. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the thermal ageing effect on the impact and post-impact behaviors of 2.5-D woven composites. A combined experimental and numerical study is carried out over a range of impact energies with the subsequent residual flexural tests on both aged (in the air for 32 days at 180℃) and pristine samples. Low-velocity impact tests are performed to assess the degradation of impact-resistant ability in terms of energy absorption mechanism and failure modes. Subsequently, three-point bending tests are conducted to determine the post-impact behaviors with respect to strain concentration, stress transfer and damage evolution process. The strain fields in the finite element model are compared with experimental results obtained by the DIC method. A larger strain concentration area could be found at higher energy levels and longer ageing time. Extended impact damage, such as fiber breakage and delamination, plays an important role in load transfer process in post-impact behavior, thus leading to the decrease of residual stress in aged samples. [ABSTRACT FROM AUTHOR]

This paper presents a multi-scale finite element model to calculate the stress field and analyze the punch shear failure of three-dimensional braided composite at high strain rates. The multi-scale model was established based on real braided structure taking the surface and corner braiding yarns into consideration. Constitutive material modeling and failure criterions were introduced into the model. Three braiding angles of 25°, 35°, and 45° were applied to reveal the relations between failure states and braided structure. The results showed that the punch shear failure states and stress distribution were greatly dependent on the strain rates and braiding angles. Nonuniform stress propagation resulted in shear bands and different formation paths were observed on the composite with different braiding angles. The ultimate failure of braided composite was determined by comprehensive action of compressive and tensile stress. In addition, the progressive damage of typical braiding yarns in different conditions was obtained from the modeling simulation. The three-dimensional braided composites with different braiding angles showed unique failure morphology. It was closely determined by the complex braided structures. [ABSTRACT FROM AUTHOR]

Transverse impact damages of 3D angle-interlock woven composites have been tested at split Hopkinson pressure bar along warp and weft directions respectively. The impact deformation and damages were photographed with a high-speed camera. A finite element analyses model was established at mesostructure level to unveil the inner yarn, resin damages, and stress distributions. There are significant differences of yarn breakages and interface damage between the two directions. From finite element analysis simulations and scanning electron microscope photographs, we found the warp yarns were in kink band deformation and shear damage, while the weft yarns were in compressive failure and had smooth fractography. The warp yarns which run through-thickness directions impede transverse impact crack propagations in resins and lead to high delamination resistances. The straight weft yarns impart high stiffness and strength to in-plane directions. [ABSTRACT FROM AUTHOR]

Fracture behaviors of three-dimensional braided composites could be designed from braided preform structures. Here, we compare the fracture behaviors of three-dimensional four direction (3D4d) and five directional (3D5d) braided composites under impact loading. A drop weight impact tester combined with a high-speed photography system was employed to characterize the progressive damages and fracture behaviors, which include crack growth and crack mouth opening displacement. The two-dimensional digital image correlation method was utilized for evaluating deformation contours and strain field. A finite element analyses model was established to reveal impact damage evolution, crack growth, stress distribution, and energy absorption at microstructure level. We found that axial yarns had great effect on the crack propagation paths. The stress at the notch bottom caused yarn breakage and then resulted in a sharply growing crack. The axial yarns in the 3D5d braided composites impede the progress of cracks under transverse impact. The 3D5d braided composites have smaller crack mouth opening displacement than that of the 3D4d composites. The braided preform is the main energy absorption component. With the same volume fraction, axial yarns exhibit the highest energy absorption. Due to the existence of axial yarns, the resistance to the fracture and crack growth of the 3D5d braided composites is better than the 3D4d braided composites. [ABSTRACT FROM AUTHOR]

Impact fracture behaviors of three-dimensional braided composites are critical to designing the braided composite parts. Here we report the impact fracture behaviors of three-dimensional braided composite U-notch beam tested on a modified split Hopkinson pressure bar. Crack mouth opening displacement, deformation process, and crack evolutions were recorded with high-speed photography camera. The digital image correlation method was used to calculate deformation contours of the braided composite. A microstructure model of the three-dimensional braided composite U-notch beam was established for analyzing damage evolution and fracture mechanisms. The histories of deformation, the load, and the crack mouth opening displacement were obtained from the impact fracture test and finite element analysis. It was found that the impact fracture resistance and morphologies were influenced by the braided structure and braided yarn orientations. The crack generated at the notch tip and then propagated along the braided angle direction rather than the perpendicular direction that often occurred for isotropic materials, such as the epoxy resin solid. The combinations of different braided angle and yarns are recommended for high impact fracture behavior design. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate yarn defects and specimen size on the impact compressive properties of 3-D angle interlock woven composites (AIWCs). The size effects on impact compressive properties were tested along in plane and out of plane directions. A new finite element model, with inherent defects in the geometrical model of yarns, was established to simulate impact compressive properties of the 3-D AIWCs. The model was further used to analyze size effects on impact compressive properties of 3-D AIWCs. We found the size effect on the 3-D AIWCs was not apparent both in experimental and numerical results; however, the random defects in yarns had a great effect on the compressive properties of 3-D AIWCs along different directions. The yarns effects will weaken the compressive stiffness and strength significantly. [ABSTRACT FROM AUTHOR]

This article reports the longitudinal compressive crashworthiness of three-dimensional four-step circular braided carbon/epoxy composite tubes at temperatures of 23, −50, and −100℃ under strain rate ranging from 340 to 760/s both experimentally and finite element analysis. The experimental results showed that the compression strength, stiffness, and specific energy absorption increased with the decrease in temperature and with the increase in strain rate. It also showed that, the compressive damage morphologies were sensitive to the change in temperature and strain rate. A coupled thermal-mechanical numerical analysis was conducted to find the thermo/mechanical coupling effect on the compressive crashworthiness of the three-dimensional composite tube. The temperature distributions in the braided preform and the resin during the impact compression were also calculated through finite element analysis. From the finite element analysis results, the inelastic heat generation was seen to be more in the preform than the matrix and its distribution and accumulation led to the damage progress along the loading direction. [ABSTRACT FROM AUTHOR]

The punch shear properties of three-dimensional carbon/epoxy braided composites were studied at quasi-static and high strain rates with finite element method at microstructure level. A microstructure model was developed to analyze the stress distribution and progressive damage of the braided composite panel with different thickness. The braiding yarns were considered as an elastic and transversely isotropic material. Ductile and shear criterion were used in finite element model to obtain the damage evolution. It was found that the braided composite exhibited high strain rate sensitivity under punch shear loading. The thickness influences the punch shear strength significantly. The braiding yarns at surface and corner parts have tensile and pullout failure modes, while at inner part have shear damage mode. [ABSTRACT FROM AUTHOR]

The axial compression loading and damage of four-step 3D circular braided composite tubes are reported in this paper by both experimental and finite element method approaches. In the axial compression experiment, the damage of the braided composite tubes was observed and the compressive load–displacement curves were acquired to analyze the compressive behaviors. In finite element analyses, three forms of repeating unit cell model were identified and used to describe the microstructure of the 3D circular braided composite tubes. The critical damage area and maximum stress theory was used as the failure criteria. From the study, it was found that the braiding parameters have a significant influence on the compression behaviors of the 3D braided composite tubes. There was a good agreement between the finite element and the experimental results. The inner and outer cells played a huge part and should be considered when calculating the overall mechanical properties of the 3D braided composite tube sample. The damage morphology was also compared and showed a good comparison between the experimental and finite element method. [ABSTRACT FROM AUTHOR]

This paper reports the three-point bending fatigue behaviors of four-step three-dimensional braided composite T-beam. The fatigue deformation and damage of three-dimensional braided composite T-beam at different stress levels and fatigue cycles have been obtained to explain the fatigue behaviors. The influence of stress levels on fatigue life is analyzed based on experimental S–N curves performed with the stress ratio (R) value of 0.1 at room temperature. Furthermore, the energy absorptions were analyzed by comparing the integral areas among different stress levels to indicate energy absorption mechanisms. In addition, the fatigue damage location distribution is found according to the damage morphologies of three-dimensional braided composite T-beam. The history of the overall stiffness degradation is found to exhibit three stages of fatigue failure: initial stage, steady stage, and final failure crack stages. The results of this investigation provide an insight into fatigue damage behavior and fatigue life of three-dimensional braided composite T-beam, and it shows that the three-dimensional braided composite T-beam is able to be used as connector. [ABSTRACT FROM AUTHOR]

This paper reports a meso-structure model of 3D-braided composites for the analysis of compressive behaviors and failure modes under quasi-static and high-strain rate compression. An idealized geometrical model of 3D-braided composite, which has the same preform meso-structure with four-step 3D-braided composite is established for finite element analyses of compressive behaviors. The compression stress–strain curves obtained from experimental along the in-plane and out-of-plane directions were used to validate the finite element analyses model. The agreement between the experimental and finite element analyses results proves the validity of the finite element analyses model. It was found that the finite element analyses model provides a way to obtain the locations of stress propagation, the 3D stress state and the progressive failure behavior. The numerical results also showed that the fiber tows in the surface and corner area of braided preform play an important role during both quasi-static and high-strain rate loading. [ABSTRACT FROM AUTHOR]

This paper reports the low-cyclic tensile responses of three-dimensional orthogonal woven composites based on the micro/meso-scale repetitive unit cells with elastic/viscoelastic models. The repetitive unit cell models for the resin impregnated fiber tows and the three-dimensional orthogonal woven composites at the fabric microstructure level were established. In the micro/meso-repetitive unit cells model, a nonlinear viscoelastic relationship with switch rule is introduced to characterize the mechanical behavior of the resin for the loading/unloading conditions. The fiber/fiber tows are characterized with linear elastic models. And damage initiation and postdamage constitutive models are also included for matrix/fiber and matrix impregnated fiber tows in the finite element analysis. The fatigue model with the mechanical parameters transferred from micro-scale to meso-scale model is numerically simulated with user-defined material subroutine and incorporated into commercial finite element software ABAQUS/Standard. The calculated results manifest the multiscale fatigue behaviors of three-dimensional orthogonal woven composites, such as the global stiffness degradation, decrease of peak stress, and even local damage behavior. The fatigue behaviors are also verified with the data in experimental and indicated that the model is an efficient strategy to predict the low-cyclic fatigue behaviors of polymer matrix composites. [ABSTRACT FROM PUBLISHER]

This paper reports bending deformation and damage of four-step 3D braided composite materials from experimental and finite element analyses approaches. In experimental, the three-point bending deformation and damage of the braided composite materials were observed and the load–deflection curves were obtained for analyzing the bending behaviors. In finite element analyses, three types of repeating unit cell geometrical models were established to characterize the microstructures of 3D braided composite materials. The critical damage area failure criteria were employed in the finite element analyses. The repeating unit cell geometrical models and the finite element analyses were verified with the experimental results. The influences of structure parameters in the repeating unit cells on the bending behaviors were analyzed. It was found that the volume fraction of surface and corner repeating unit cells were around 30%, and their elastic modulus along axis Z was about 5 times that of inner repeating unit cell. The surface and corner repeating unit cells play the important role for the entire structure under bending and cannot be neglected. [ABSTRACT FROM PUBLISHER]

This paper reports the ballistic energy absorption of three-dimensional angle-interlock composite (3DAWC) based on a ballistic experiment and a theoretical model with high strain rate constitutive equation of fiber tows. The 3DAWC panels were penetrated under a hemispherical–cylindrical steel projectile, while time testers recorded the initial velocities and residual velocities of projectile in this process. The damage modes of 3DAWC are observed and analyzed from the view of penetrated damage morphologies and experimental velocity data. In order to demonstrate the energy absorption mechanism with more accuracy, a multi-scale finite element model of 3DAWC under ballistic penetration is specially designed and established to calculate this ballistic event. The constitutive relationship of the Twaron® filament yarn in microstructural model is derived from the springs and dashpots model and compiled into user-defined material subroutine in commercial-available finite element software package LS-DYNA, which can introduce strain rate sensitivity of fiber bundles to ballistic energy absorption process. A comparison of theoretical and experimental results shows a good agreement indicating an accurate validity of the multi-scale finite element model of 3DAWC. Moreover, this model can deeply reveal the ballistic energy absorption mechanism of 3DAWC which will help to evaluate the structural tolerance of ballistic protection composite material. [ABSTRACT FROM PUBLISHER]

Quasi-static and high strain rate compressive behaviors of basalt/vinyl ester 3D orthogonal woven composites along three perpendicular yarn directions were investigated experimentally and numerically. An elastic–plastic numerical model based on the 3D fabric architecture of the 3D orthogonal woven composites was developed to analyze the compressive deformation and failure mode under different strain rate loading conditions. The stress–strain curves obtained from experimental along three directions were used to validate the finite element model. The agreement between the experimental and finite element model results proves the validity of the finite element model. From the experimental results, the strain rate sensitive and anisotropy of the compressive behaviors was found. The finite element results were employed to analyze the locations of stress propagation, the 3D stress state, and the progressive failure behavior. It was also found that this model has a similar failure shape of “broom” at one end of the composite along both of weft and warp compressive directions, while an inclined shear band was formed in through-thickness compression. This result indicates that Z-yarn contributed significantly to the in-plane responses. Additionally, the absorbed energies that induce the complete damage of the composite coupons at various strain rates were also calculated and discussed. [ABSTRACT FROM PUBLISHER]

This article reports the trapezoid tearing behaviors of uncoated and coated woven fabrics in experimental and finite element analyses approaches. In experimental, trapezoid tearing behaviors of uncoated and coated woven fabrics were tested to obtain tearing load–displacement curves and tearing failure developments. The tearing strength and damage morphologies of the two kinds of fabrics were compared to find the tearing damage differences between uncoated and coated woven fabrics during the trapezoid tearing process. In finite element analysis, microstructure geometrical models of both the uncoated and coated woven fabrics were constructed to calculate the stress distribution and tearing failure propagation. There are good agreements between the finite element analysis results and those in experimental. Moreover, the influence of the coating on the trapezoid tearing properties was discussed. The tearing failure differences of the uncoated woven fabric and the base fabric in the coated woven fabric was also compared for finding the tearing failure mechanisms of the coated fabric. Such an investigation can be extended to the design of the coated woven fabric with high tearing strength. [ABSTRACT FROM AUTHOR]

This article presents the quantitative characterization of cumulative fatigue damage behavior for the three-dimensional angle-interlock woven composite undergoing three-point bending cyclic loading. The S–N curve was obtained to demonstrate the fatigue life of the three-dimensional angle-interlock woven composite under different stress levels. The increment of cycles for each 5% interval of stress level was reported to show the difference of fatigue resistance performances of the three-dimensional angle-interlock woven composite among the high, middle, and low intervals of stress level. In addition, the Cumulative Fatigue Damage versus Number of Cycles (D–N) curve and the Deflection Index versus Number of Cycles (F–N) curve were deduced to characterize the three-stage cumulative fatigue damage. Furthermore, the damage morphologies of the three-dimensional angle-interlock woven composite after fatigue tests were photographed to compare with those in quasi-static test. The cracks initiation and propagation in the three-dimensional angle-interlock woven composite during the process of cyclic loading were summarized to find the mechanisms of fatigue damage development. [ABSTRACT FROM PUBLISHER]

The impact tensile behaviors of co-woven-knitted (CWK) composites were tested with split Hopkinson tension bar apparatus along 0°, 45°, and 90° directions under the strain rates from 1589 to 2586 s−1 and compared with that in quasi-static strain rate of 0.001 s−1. The impact tension damage mechanisms were analyzed in frequency domain with the Hibert–Huang transform (HHT) method. Specifically, the stress time histories curves of the CWK composite were transformed to find the frequency distributions for the different tension failure modes. It was found that thefailure mode of fiber breakages, fiber pull-out, resin cracks, and resin shear failurewere located at a specific frequency band. And also, the different failure modes under high strain rate loading are all located at the high frequency range. With such kind of HHT analyses, the damage mechanisms could be explored more precisely when combined with the observation results of the fractographs of the CWK composite under various strain rates. [ABSTRACT FROM PUBLISHER]

The ballistic impact damages of 3D orthogonal woven fabric (3DOWF) penetrated under a conically cylindrical rigid projectile were investigated from experimental tests and finite element simulations. A microstructure model of the 3DOWF was established and imported into finite element geometrical preprocessor. In the microstructure model, the architecture of the 3DOWF has the same spatial configurations with that of the real 3DOWF, including the spatial distributions and cross-sections of warp, weft yarns, and Z-yarns. Mechanical parameters of the yarns were obtained from high-strain rate tests which near to the impact loading condition in ballistic tests. The impact damage evolutions of the 3DOWF were simulated with the commercial finite element code ABAQUS/Explicit. From the comparisons of damage morphologies and residual velocities of the projectile after perforation between experimental and finite element simulation, it was found that the simulation can reflect the impact damage precisely. Furthermore, the stress wave propagation and damage mechanisms can be revealed from the microstructure model. Insights gained from this study will prove extremely useful in further material and architectural studies that will ultimately lead to optimization of the 3DOWF structure. [ABSTRACT FROM PUBLISHER]