Damage Mechanisms In Composites Research Articles

Characterization of the damage mechanism in CFRP composites under mode I based on comprehensive analysis of AE signals

This study utilises several methods centred on acoustic emission (AE) waveform characteristics to conduct in-depth analysis of the damage mechanism of composite materials under mode I. Combined with the peak frequency statistics and continuous wavelet transform (CWT) of AE signals, three damage modes of laminated composites during loading are identified. The cumulative energy of three damage modes separated by variational modal decomposition (VMD) is used to characterize their damage evolution process. Based on the findings, delamination and fiber–matrix debonding are proven to be the dominant damage mechanisms for mode I. The comprehensive analysis results from AE's accumulated energy, peak frequency distribution, CWT, VMD, and sentry function can accurately detect the onset time of delamination in laminates. The equivalent crack length is calculated by the load value determined at the initial time of delamination, and the strain energy release rate calculated by the load value and crack length at this time is used as the interlaminar fracture toughness value of mode I. This research method will provide a new idea for the damage tolerance analysis of mode I delamination in laminated structures.

Compression behavior and microscopic damage mechanism of waste rock-tailings matrix composites: Experiments and models

Waste rock-tailings matrix composites used for goaf filling are a significant approach to promote solid waste recycling in mines and achieve safe and environmentally friendly production. In this study, the effects of waste rock content (WRC) and size (WRS) on the compressive mechanical properties of cemented rock-tailings backfill (CRTB) were investigated by experiment. Additionally, PFC3D numerical simulation software is utilized to visualize the micro-crack extension mechanism and microscopic damage characteristics of CRTB. The results indicate that irregularly shaped waste rock exhibits bridging and interlocking effects within CRTB, effectively increasing the uniaxial compressive strength and elastic modulus. However, excessive WRC and WRS lead to a looser skeleton structure in CRTB, thus compromising its load-bearing capacity. The optimum WRC and WRS are 30% and 4–6 mm, respectively. The incorporation of irregularly shaped waste rock effectively suppresses the Poisson effect, and as the WRC increases, CRTB failure gradually transitions from tensile failure to complete shear failure. Numerical simulation results demonstrate that irregularly shaped waste rock redirects and seals crack expansion, resulting in a distinct X-shaped distribution of the main crack after CRTB damage. The findings of this research provide valuable guidance for the efficient application of waste rock-tailings matrix composites in mines.

Prediction of temperature dependent effective moduli of metal particle composites with debonding damage

Debonding is one of the most commonly observed damage mechanisms in metal particle composites, which poses additional difficulties for material characterization and performance evaluation. In this paper, the equivalent modulus of debonded particles in tension and compression was derived in accordance with the debonding configurations, respectively. The debonded particles were then incorporated into the Mori-Tanaka method as a third phase with reduced modulus but perfectly bonded to the matrix. Finally, a micromechanical model of the temperature dependent effective moduli of metal particle composites was proposed, taking into account the degradation of the matrix modulus and the evolution of percentage of debonded particles at elevated temperatures. The model predictions were in high agreement with the experimental and simulation results, demonstrating the predictive ability of the micromechanical model. This study gives a highly practical forecasting toolkit for composite modulus evaluation over a wide temperature range. In addition, parametric analyses were carried out to investigate the effects of debonding configuration, temperature, and percentage of debonded particles, which contributes to further understanding of the debonding mechanism, leading to rational material design.

Study on the Low-Velocity Impact Response and Damage Mechanisms of Thermoplastic Composites.

A comparative experimental and numerical study of the impact behaviour of carbon-fiber-reinforced thermoplastic (TP) and thermoset (TS) composites has been carried out. On the one hand, low velocity impact (LVI) tests were performed on TP and TS composites with different lay-up sequences at different energy levels, and the damage modes and microscopic damage mechanisms after impact were investigated using macroscale inspection, C-scan inspection, and X-ray-computed tomography. The comparative results show that the initial damage valve force under LVI depends not only on the material, but also on the layup sequence. The initial valve force of the P2 soft layer with lower stiffness is about 11% lower than that of the P1 quasi-isotropic layer under the same material, while the initial valve force of thermoplastic composites is about 28% lower than that of thermoset composites under the same stacking order. Under the same stacking order and impact energy level, the damage area and depth of TP composites are smaller than those of TS composites; while under the same material and impact energy level, the indentation depth of P2 plies is greater than that of P1 plies, and the damage area of P2 plies is smaller than that of P1 plies, but the change of thermoplastic composites is not as obvious as that of thermoset composites. This indicates that TP composites have a higher initial damage threshold energy and impact resistance at the same lay-up order, while increasing the lay-up ratio of the same material by 45° improves the impact resistance of the structure. In addition, a damage model based on continuum damage mechanics (CDM) was developed to predict different damage modes of thermoplastic composites during low velocity impact, and the analytical results were compared with the experimental results. At an impact energy of 4.45 J/mm, the error of the initial damage valve force is 5.26% and the error of the maximum impact force is 4.36%. The simulated impact energy and impact velocity curves agree with the experimental results, indicating that the finite element model has good reliability.

Bottom-up stochastic multiscale model for the mechanical behavior of multidirectional composite laminates with microvoids

This study for the first time develops a coherent and efficient bottom-up stochastic multiscale modeling approach, which aims at revealing the impact of microvoids on the mechanical properties and damage mechanisms of multidirectional (MD) carbon fiber-reinforced polymer (CFRP) composites. To this end, void defects within the composites are characterized using X-ray micro-computed tomography to construct a high-fidelity representative defective volume (RDV). Subsequently, the influence of voids on the microscale failure modes and macroscopic mechanical properties are analyzed by using micromechanics-based finite element (FE) method. Finally, a stochastic multiscale analysis method is developed to predict the mechanical response of MD CFRP laminates and validated by tensile and three-point bending experiments. The results indicate that the proposed framework can accurately predict the mechanical behavior and damage mechanisms of MD CFRP composites. This framework is believed to be a promising method to provide valuable guidance for reliability design and optimization of CFRP composites.

Investigation on the mechanical properties and damage mechanisms of plain-woven composites at elevated temperatures through a newly-designed testing system

In this paper, the mechanical properties and damage mechanisms of T800/epoxy plain-woven composites at elevated temperatures are investigated by using a newly-designed testing system, including improved digital image correlation (DIC) equipment, modified fixture and dedicated environmental chamber. The molecular structure, glass transition temperature (Tg) and thermal stability of the composites and epoxy resin are separately characterized. The tensile and compressive experiments of plain-woven composites are conducted at 20 °C, 60 °C, 100 °C and 150 °C. The full-field strain distributions of plain-woven composites at elevated temperatures can be accurately measured by the newly-designed testing system. The experimental results indicate that the modulus, failure strengths and damage mechanisms of plain-woven composites are highly temperature dependent. In general, the tensile and compressive moduli decrease slightly with increasing temperature, and the temperature exhibits a more pronounced weakening effect on compressive strength. When the temperature exceeds Tg, the resin attached to fiber yarns is gelatinous. High temperatures severely degrade the interlayer performance of plain-woven composites, resulting in severe delamination accumulation. Particularly, a modified Gibson model is proposed to predict the failure strengths of plain-woven composites at elevated temperatures, which is in high agreement with the experimental results.

Different effects of interfacial properties on the tensile and compressive damage mechanisms of 3D woven composites: Multiscale damage model and numerical comparative study

A multiscale damage model is proposed based on a micro-mesoscale bridging model and a multiscale kinking model, which can predict the damage development processes of 3D woven composites (3DWCs) under different loading conditions using only few material parameters. At the microscale, the various mesoscopic damage modes of yarns, including tensile fiber breakage, compressive fiber kinking and transverse inter-fiber crack, can be simply divided into matrix- and fiber-dominated damage. Using this multiscale damage model, a numerical comparative study is conducted to investigate the effect of interfacial properties on the mechanical performances and damage mechanism of 3DWCs. The numerical results indicate that the interfacial properties exhibit different effects on the tensile and compressive strengths and damage mechanisms of 3DWCs. The interaction of different damage modes controls the overall trend of the tensile strength of 3DWCs. There is an optimum point for the tensile strength of 3DWCs at different interfacial fracture toughness. Beyond the optimum point, the tensile strength of 3DWCs is negatively correlated with interfacial properties. Unlike the tensile loading case, the better the interfacial properties, the later fiber kinking occurs, resulting in higher compressive strength of 3DWCs.

A novel design of three‐dimensional/unidirectional hybrid composites to balance in‐plane and interlaminar mechanical properties

AbstractThe working condition of the rail transit components such as bogies is complicated, so the high comprehensive performance of composite parts is desperately required. However, existing pure three‐dimensional braided or laminated composites have been unable to satisfy the cooperative demand of in‐plane and interlaminar properties. In this paper, a novel design of three‐dimensional braided/unidirectional (3D/UD) hybrid composite is proposed to balance the in‐plane (tensile, compression, bending), interlaminar (interlaminar shear, low‐speed impact) properties of the composite. The effects of different 3D/UD mixing ratio and layup sequence on the mechanical properties and damage mechanism of hybrid composites were studied by the parametric analysis of the experimental test results, and the comprehensive mechanical properties of the hybrid structure were optimized by COPRAS evaluation method. The results show that the larger the proportion of unidirectional layer, the better the mechanical property in‐plane, but the worse interlayer property. In addition, the change of layup sequence induces a complex nonlinear relationship between 3D/UD mixing ratio and mechanical properties. Combined with microscopic failure characterization results, it is found that the sandwich structure and the addition of uniformly distributed interfacial layer can improve the mechanical properties of the hybrid composite. The optimization results show that the sandwich structure hybrid composite UD6/3D/UD6 with the best balance between in‐plane and interlaminar properties was selected by COPRAS. Compared with pure 3D composites, the comprehensive mechanical properties of UD6/3D/UD6 are increased by 53.9%, which provides a reference for the structural design of hybrid composites in the future.Highlights A novel design of 3D/UD hybrid composite is proposed to balance the in‐plane and interlaminar properties. The influence regularity of 3D/UD mixing ratio and layup sequence on properties of hybrid composites were obtained. The mixing failure mechanism of interlayer and in‐plane was revealed. The optimal structure UD6/3D/UD6 under complex working conditions was obtained by COPRAS.

Influence of realistic microscopic fiber misalignments on compressive damage mechanisms of 3D angle-interlock woven composites: In-situ measurements and numerical simulations

Stochastic fiber misalignments have been proven to have a significant impact on the compressive strength of traditional unidirectional laminates. However, these inherent manufacturing defects are rarely considered when dealing with 3D woven composites, due to the difficulty in characterizing the misalignment angles of individual fibers within all yarns. The current work provides the first in-situ measurements of fiber misalignments in 3D angle-interlock woven composites (3DAWCs) at the microscale, where the continuous fibers inside the warp, weft, and binder yarns are systematically examined using a high-resolution optical microscope. Statistical analysis reveals that the microscopic fiber misalignments follow a normal distribution, and the magnitude of the misalignment angle increases slightly with the degree of yarn fluctuation. Through a developed user subroutine, SDVINI, the obtained misalignment distributions are initialized in the corresponding yarns that belong to the constructed high-fidelity model. In comparison to the idealized one, the model introducing realistic fiber misalignments can effectively correct the overestimated predictions, and exhibits better agreement with the performed experiments. Furthermore, the influence of fiber misalignments on compressive damage mechanisms, particularly kinking failure, is also parametrically studied by incorporating different misalignment distributions.

Experimental and numerical investigation on tension–tension fatigue behavior of three-dimensional five-directional hybrid braided composites

This paper presents an experimental and numerical investigation on tension–tension fatigue behavior of hybrid carbon/glass three-dimensional five-directional (3D5D) braided composites. Fatigue tests were carried out under the stress ratio of 0.1. The experimental results show that the fatigue strength significantly decreases as the fatigue life increases and the stiffness gradually decreases as the loading cycles increase. The fatigue life at the stress level within 35 %–50 % ultimate tensile strength (UTS) is within 4 × 103–6 × 105 cycles. The stiffness degradation before failure increases from 39 % to 58 % as the fatigue stress decreases from 50 % UTS to 35 % UTS. Under tension–tension fatigue loading, matrix cracking inside the yarns and interfacial debonding between the yarns and the matrix firstly occur and propagate, then matrix cracking appears in the resin-rich regions and numerous fiber breakage happens until final failure. The fatigue performance and damage mechanisms of braided composites are affected by fiber hybrid mode and braiding methods. Additionally, a mesoscale representative volume element (RVE) model with periodic boundary conditions and a progressive fatigue damage model was developed for the 3D5D hybrid braided composites. The numerical simulation results of fatigue life, stiffness degradation and damage progression have good agreement with experiments, demonstrating the validity of the developed model.

Multiscale progressive damage model for plain woven composites

In this paper, a novel multiscale progressive damage model is proposed to study the complex mechanical behavior and damage mechanism of plain woven composites. In the macroscale, composites are treated as full-scale model with three-dimensional solid elements. The mechanical response of element integration points depends on the mesoscale constitutive relationship, including orthotropic undulating yarn and isotropic matrix. Based on the microscopic representative volume element (RVE), the stress amplification factor (SAF) database is introduced to characterize the microscale stress state of the fiber and matrix. SAF is used to realize the stress correlation calculation from the yarn level to the micro level, and a progressive damage model based on the micro-mechanics of failure (MMF) theory is developed. Quasi-static tension experiments are carried out to validate the accuracy of the model, and the influence of meso‑scale waviness and non-waviness constitutive models on the simulation results is compared. The results indicate the waviness constitutive model predicts are more consistent with the experimental results, and the prediction deviations about modulus, failure strength and failure strain are around 5%. The damage initiation of matrix in the yarn and pure matrix occurs successively, and finally the fiber failure is the main cause of catastrophic damage. Furthermore, the proposed multiscale method can be further used to predict the strength and damage of full-size woven composites under complicated loading.

Characterization and modeling of the damage mechanisms in ductile steel metal-matrix composites: Application to virtual forming

The aim of this work is to investigate the damage mechanisms and stiffness loss in Fe-TiB2 metal-matrix composites during plastic deformation. First, experimental results of interrupted tensile tests are performed to quantify the evolution of damage, using SEM observations, as well as the decrease of Young’s modulus as a function of the tensile strain. The experimental results are then used to calibrate a two-step homogenization model for metal-matrix composites in which the nucleation and growth of voids modify incrementally the overall elastic properties. The model is finally applied to the numerical prediction of stiffness loss in a problem of metal forming based on Nakazima tests. Overall, the stiffness loss predicted before the onset of coalescence is moderate and its distribution is homogeneous, emphasizing that Fe-TiB2 metal-matrix composites could be used in applications requiring metal forming.

Investigation of the effects of hybridization types and temperature on the quasi‐static indentation properties of glass/flax hybrid composites for various test speeds

AbstractHybridizing man‐made fiber composites with natural fibers has been chosen for numerous structural or semi‐structural applications over the last several decades. Even though several studies have been conducted to determine the mechanical properties of inter‐ply hybrid composites, the effects of hybridization on the out‐of‐plane mechanical properties of hybrid composites still need to be clarified. The mechanical properties of hybrid composites at low temperatures are another significant matter to be elucidated. In this study, E‐glass and flax yarns were used to produce woven fabrics to obtain intra‐ and inter‐ply hybrid composites. They were then subjected to the quasi‐static indentation test at −30°C, 25°C, and 60°C. It was concluded that, according to the hybridization type, an improvement or decrease can be obtained in the mechanical properties of hybrid composites. The contact force and energy absorption properties of pure glass composite could be increased at 8.88% and 3.87% proportions respectively, thanks to the intra‐ply hybrid composite‐1 in the study. Furthermore, the through‐thickness properties and damage mechanisms of hybrid composites were profoundly affected by the temperature.

Experimental investigation on shear damage evolution of 3D woven composites using in situ computed tomography and multi-scale digital image correlation

A comprehensive experimental method combining microscope observation, multi-scale digital image correlation (DIC) and in situ computed tomography (CT) is proposed to reveal the complex damage evolution of 3D woven composites under short beam shear (SBS) loading. The surface damage process and the final damage mode of the SBS specimen are observed with a microscope, and the cause of fiber kinking is explained. The global and local strain distributions on the specimen surface are obtained by multi-scale DIC, which explain the causes of weft yarn transverse cracks and interface debonding. Finally, the in situ CT experiment is carried out to identify the reasons for the nonlinear segments of the load–displacement curve and the decline of load bearing capacity of the specimen in the late stage of the experiment. The information obtained by different observation methods is complementary, which is conducive to understanding the complex damage mechanism of 3D woven composites.

Effect of Yarn Reduction Types on the Mechanical Response and Damage Mechanism of 3D Woven Composites Subjected to Low-velocity Impact

Effect of Yarn Reduction Types on the Mechanical Response and Damage Mechanism of 3D Woven Composites Subjected to Low-velocity Impact

Fiber architecture effect on tensile and compressive damage mechanisms of 3D angle-interlock woven composites

Due to the intricate fiber architecture of 3D textile composites, understanding their failure mechanisms presents a challenge. Here we experimentally studied the quasi-static tensile and compressive properties of 3D angle-interlock woven composites (3D AWC) under both warp-loading (AWC-warp) and weft-loading (AWC-weft). Damage morphologies and mechanisms were examined using optical microscopy and X-ray computed tomography. In order to demonstrate the effect of fiber architecture, the results were combined and compared with that of unidirectional (UD) composites with the same total fiber volume fraction (FVF). The fiber architecture effects in 3D AWC can be manifested as the on-axis FVF dependency and fiber undulation influence. When on-axis load-bearing fiber is undulating-free, the difference in mechanical properties is primarily determined by the on-axis FVF. However, the presence of fiber undulation can further weaken the mechanical properties. Moreover, for AWC-warp specimens, fiber undulation has a significant impact on the modes and characteristics of damage, resulting in fiber breakages and kinks that primarily occur in the connection region of warp yarns.

High pressure and long-term gas diffusion coupled damage of composites through a multi-physical phase field framework

In extreme service environments, composites are often exposed to high pressure and prolonged chemical penetration, leading to irreversible material damage and structural failure. To address this multi-field coupling problem that has long been a concern, we develop a theoretical framework for fully coupled stress distribution, gas diffusion, heat conduction, and damage evolution of composites to reproduce complex failure modes and disclose the damage mechanisms of composites in complicated environments. To verify the theoretical feasibility, the susceptibility of material fracture properties to hydrogen content is well revealed, and a typical problem, i.e., damage of multi-layered composite hydrogen storage vessels in distinct working stages, is investigated and discussed insightfully. We highlight that (i) the fracture phase field theory is rederived under the thermodynamic consistent framework; (ii) the proposed computational model can accurately capture the heat conduction response under the coupled effects of chemical diffusion and crack propagation; (iii) through a detailed analysis of the energy evolution, the model allows precise yield of the dominant mechanisms in the diverse damage stages of the materials. This study can reproduce the different damage patterns of composite structures under the action of multi-physical fields in extreme environments, thus providing a predictive computational approach to prevent disastrous failures.

Transverse indentation response and residual axial compressive characteristics of metal-composites hybrid tubes by deep learning-based acoustic emission and micro-CT

This work investigated transverse indentation response and residual axial compressive characteristics of Al-CFRP hybrid tubes with different configurations. The failure mechanism and damage evolution of the hybrid tubes under transverse indentation load were investigated by acoustic emission and micro-CT technologies. To gain an insight into the damage mechanism of CFRP composites, a structural health monitoring system based on image-based acoustic emission waveform and deep learning was developed. It showed that acoustic emission response of internal damage was corresponding to the damage visualization analyses of micro-CT. The proposed damage classification model provided a reliable path for extreme damage mode recognition and damage monitoring of composite structure. Under transverse indentation load, delamination and fiber breakage were the main failure modes of H-I hybrid tube (i.e. the CFRP tube internally filled with aluminum tube). For H-II hybrid tube (i.e. the aluminum tube internally filled with CFRP tube), the interfacial delamination of Al/CFRP tended to aggravate its failure behavior. The quasi-static axial compressive properties of intact and pre-indentation tubes were further compared to evaluate the effects of indentation damage on residual crashworthiness. The transverse indentation resulted in the change of the failure mode. Compared with H-I hybrid tube, H-II hybrid tube showed more significant property degradation.

Damage mechanisms of bismaleimide matrix composites under transverse loading via quasi-static indentation

Abstract With the application of carbon fiber reinforced plastics (CFRPs), the damage behaviors and mechanisms are desirable to understand. In this work, the Bismaleimide (BMI) matrix composites were employed to study the damage mechanism under transverse loading through quasi-static indentation, where optical microscopy, ultrasonic C-scan, and scanning electron microscope were subjected to examine the damaged regions. The results show that cracks first take place in a resin-rich interlaminar region before the loading reached to a threshold value. Then, the cracks would develop into intralaminar cracks in the fiber bundle region consisting of the shear and bending cracks. Subsequently, the cracks randomly extend to adjacent plies and result in delamination. Finally, the back surface of composites presents a remarkable ply splitting and fiber breakage under increasing loading. Furthermore, conical-shaped indentation patterns in cross-sectional and the undamaged zone under the indenter suggest the BMI matrix composites possess potential applications in engineering materials with a superior stress-releasing structure.

Identifying damage mechanisms of composites by acoustic emission and supervised machine learning

Acoustic emission (AE) is a well-established technique for in-situ damage analysis of composite materials. The main challenge, however, is to be able to correlate the measured AE signals with their respective damage mechanism sources. Hence, an innovative approach to classify AE signals based on supervised machine learning is presented in this work. At first, the constituents of a composite (fiber, matrix and interface) are characterized separately and fingerprint information regarding the characteristic AE features of each damage mechanism is gathered. This dataset is then used to train a model based on the k-nearest neighbors algorithm. Model accuracy is calculated to be 88%. Subsequently, AE signals measured during tensile tests of commercial composites are classified by the trained model. The analysis provides important information regarding location, time, frequency and intensity of each damage mechanism. Matrix cracking and fiber debonding are the most frequent damage mechanisms representing around 40% and 20% of the measured AE hits. Nevertheless, fiber breakage is the mechanism that dissipates the most AE energy (40%) for the studied composite. Furthermore, the presented method can also be applied together with other techniques like computer tomography, delivering a powerful approach to understand different multi-phase materials.