Microstructure effects on transverse cracking in composite laminae by DEM

A particle discrete element method (DEM) was employed to simulate transverse cracking in laminated fiber reinforced composites. The microstructure of the laminates was modeled by a DEM model using different mechanical constitutive laws and materials parameters for different constituents, i.e. fiber, matrix and fiber/matrix interface. Rectangular, hexagonal and random fiber distributions were simulated to study the effect of fiber distribution on the transverse cracking. The initiation and dynamic propagation of transverse cracking and interfacial debonding were all captured by the DEM simulation, which showed similar patterns to those observed from experiments. The effect of fiber volume fraction was also studied for laminae with randomly distributed fibers. It was found that the distribution and volume fraction of fibers affected not only the transverse cracking path, but also the behavior of matrix plastic deformation and fiber/matrix interface yielding in the material.     

A multi-scale finite element approach for modelling damage progression in woven composite structures

A significant challenge in the numerical modelling of composite structures with a multi-axis fibre architecture is the reproducibility of the textile mechanics [1]. A numerical analysis procedure for woven composite structures using a multi-scale finite element approach has been developed, and is presented in this paper. The approach is demonstrated for a flat two-dimensional woven glass/epoxy laminate. Digital microscopy is used to estimate tow cross-section and path, and quantify the amount of variation of these parameters. This data is used to generate both a meso-scale model of a single unit cell as well as a macro-scale model of the complete structure. Numerical results from the proposed approach are compared to experimental stress-strain data, which show good agreement in the lower strain range.     

2D and 3D imaging of fatigue failure mechanisms of 3D woven composites

A detailed investigation of the failure mechanisms for angle-interlocked (AI) and modified layer-to-layer (MLL) three dimensional (3D) woven composites under tension–tension (T–T) fatigue loading has been conducted using surface optical microscopy, cross-sectional SEM imaging, and non-destructive X-ray computed tomography (CT). X-ray microCT has revealed how cracks including surface matrix cracks, transverse matrix cracks, fibre/matrix interfacial debonding or delamination develop, and has delineated the complex 3D morphology of these cracks in relation to fibre architecture. For both weaves examined, transverse cracks soon become uniformly distributed in the weft yarns. A higher crack density was found in the AI composite than the MLL composite. Transverse cracking initiates in the fibre rich regions of weft yarns rather than the resin rich regions. Delaminations in the failed MLL specimen were more extensive than the AI specimen. It is suggested that for the MLL composite that debonding between the binder yarns and surrounding material is the predominant damage mechanism.     

Feasibility and limitations of damage identification in composite materials using acoustic emission

One of the current challenges in health monitoring of composite materials is the use of acoustic emission to identify damage modes. Many classification procedures have been reported in the literature but none of them clearly state limitations to their applicability, making it difficult to quantify them in different testing conditions. In the present paper, a method is described to characterize energy attenuation and how it affects AE signals features based solely on AE signals recorded during mechanical tests. Limitations to damage identification based on AE signals features can therefore be defined. The method is demonstrated on AE signals recorded during tensile tests on four different layups of carbon fiber reinforced polymer composites using signals frequency centroids to describe AE sources.     

Single-fibre Broutman test: fibre–matrix interface transverse debonding

The single-fibre Broutman test was used to study the fibre–matrix interface debonding behaviour when subjected to a transverse tensile stress. During testing, damage was detected using both visual observation under polarized light and acoustic emission (AE) monitoring. Separation of failure mechanisms, based on AE events, was performed using time domain parameters (amplitude and event width) and fast Fourier transform (FFT) frequency spectra of the AE waveforms. The latter can be considered as a fingerprint allowing to discriminate fibre failure, matrix cracking, fibre–matrix interface debonding, friction and ‘parasite noise’. Stresses in the specimens were evaluated using a two-dimensional finite element model (FEM) and monochromatic photoelasticity was used to verify the simulated stress distribution.Two failure mechanisms appeared to be in competition in the Broutman test: fibre failure under compressive stresses and fibre–matrix interface debonding under transverse tensile stresses. For systems in which the interfacial adhesion is not so ‘good’, like glass fibre–polyester systems for instance, fibre–matrix debonding was observed, and the progression of the debonding front with the interfacial transverse stress was recorded. Thermal stresses are also discussed, and a FEM simulation shows that they encourage fibre failure under compressive stresses.     

Damage detection in holed composite laminates using an embedded FBG sensor

This paper discusses damage detection in a holed CFRP laminate under static and cyclic loading using an embedded fiber Bragg grating (FBG) sensor. In order to detect the damage extension in the laminate, the change in the spectrum shape was measured using an embedded FBG sensor and was compared with that obtained by numerical simulation. The shape of the reflection spectrum did not change during the cyclic load test; however, it did change with increased strain in the static load test, due to damage around the hole. To clarify this difference, the polished surface of the cross section of the specimen was analyzed. Debonding was observed between the optical fiber and matrix during the cyclic load test. These results lead us to conclude that fatigue damage around a hole in a composite laminate may not be detected with an FBG sensor due to the debondings.     

Micromechanical modelling of the transverse damage behaviour in fibre reinforced composites

A micromechanics damage model is presented which examines the effect of fibre-matrix debonding and thermal residual stress on the transverse damage behaviour of a unidirectional carbon fibre reinforced epoxy composite. It is found that for a weak fibre-matrix interface, the presence of thermal residual stress can induce damage prior to mechanical loading. However, for a strong fibre-matrix interface the presence of thermal residual stress is effective in suppressing fibre-matrix debonding and improving overall transverse strength by approximately 7%. The micromechanical model is subjected to a multiple loading cycle (i.e. tension-compression-tension), where it is shown to provide novel insight into the microscopic damage accumulation that forms prior to ultimate failure, clearly highlighting the different roles that fibre-matrix debonding and matrix plasticity play in forming the macroscopic response of the composite. Such information is vital to the development of accurate continuum damage models, which often smear these effects using non-physical material parameters.     

Evaluation of long-term durability in high temperature resistant CFRP laminates under thermal fatigue loading

Thermal fatigue tests were conducted on high temperature resistant carbon fiber reinforced plastics cross-ply laminates to evaluate microscopic damage progress which affects macroscopic mechanical behavior of the laminates. Materials system used were thermoplastic polyetheretherketone based, AS4/PEEK and thermoset bismaleimide based, G40-800/5260. Several types of laminate configuration were used to clarify the effect of ply thickness on microscopic damage progress. Microscopic damages were observed using optical microscopy and soft X-ray radiography. Energy release rate associated with transverse cracking was calculated using variational analysis. The modified Paris law was used to predict transverse cracking. From comparison to mechanical fatigue test results, it is clarified that transverse crack accumulation rate was larger under thermal fatigue loading at same energy release rate range due to the dependence of the fracture toughness on temperature.     

Measurement of the in situ transverse tensile strength of composite plies by means of the real time monitoring of microcracking

Failure of a ply due to transverse loading is one of the mechanisms that was taken into account in physically-based failure criteria, used in composites design. However, experimental data are scarce and the measurement techniques used in the past are time consuming and involve a lot of specimen handling during testing. While some physical information is currently well consolidated (such as the dependence of the strength on ply thickness, or in situ strength), there still remain relevant open questions. This work presents a methodology, which does not interfere with the tensile test, to detect transverse cracks by optical means. Four different configurations of CFRP are considered. The results show that the in situ strength depends on the thickness of the ply and the orientation of the adjacent layers. In the case of thick transverse plies, the strength is controlled by full-width transverse cracks whereas, in thin plies cracking parallel to the specimen’s mid-plane occurs before transverse matrix cracking.     

Evolution of damage during the fatigue of 3D woven glass-fibre reinforced composites subjected to tension–tension loading observed by time-lapse X-ray tomography

The development of fatigue damage in a glass fibre modified layer-to-layer three dimensional (3D) woven composite has been followed by time-lapse X-ray computed tomography (CT). The damage was found to be distributed regularly throughout the composite according to the repeating unit, even at large fractions of the total life. This suggests that the through-thickness constraint provides a high level of stress redistribution and damage tolerance. The different types of damage have been segmented, allowing a quantitative analysis of damage evolution as a function of the number of fatigue cycles. Transverse cracks were found to initiate within the weft after just 0.1% of life, followed soon after (by 1% of life) by longitudinal debonding cracks. The number and extent of these multiplied steadily over the fatigue life, whereas the spacing of transverse cracks along with weft/binder debonding saturated at 60% of life and damage in the resin pockets occurred only just before final failure.     

In-situ damage sensing of woven composites using carbon nanotube conductive networks

We report an in situ analysis of the microstructure of woven composites using carbon nanotube (CNT)-based conductive networks. Two types of specimens with stacking sequences of (0/90)_s (on-axis) and (22/85/−85/−22) (off-axis) were manufactured using ultra-high-molecular-weight polyethylene fibers and a CNT-dispersed epoxy matrix via vacuum-assisted resin transfer molding. The changes in the electrical resistance of the woven composites in response to uniaxial loading corresponded to the changes in the gradient of the stress–strain curves, which is indicative of the initiation and accumulation of microscopic cracking and delamination. The electrical resistance of the woven composites increased due to both elongation and microscopic damage; interestingly, however, it decreased beyond a certain strain level. In situ X-ray computed tomography and biaxial loading tests reveal that this transition is due to yarn compaction and Poisson’s contraction, which are manifest in textile composites.     

Monotonic and cyclic short beam shear response of 3D woven composites

Monotonic, multi-step and cyclic short beam shear tests were conducted on 2D and 3D woven composites. The test results were used to determine the effect of z-yarns on the inter-laminar shear strength as well as the multi-loading behavior. The presence of z-yarns was found to affect not only the inter-laminar shear strength of the composite but also the behavior of the composite beyond the elastic limit. Microscopic examination of the damaged specimens revealed large delamination cracks in 2D woven composites while delamination cracks were hindered by z-yarns in 3D composites. This crack arrest phenomena resulted in a reduction in inter-laminar crack lengths and a higher distribution of the micro-cracks throughout the 3D composite. The multi-step and cyclic loading tests are found to be useful in the monitoring of specimen behavior during short beam shear testing. The induced damage was quantified in terms of the loss of strength and stiffness during each loading cycle. It was found that while the 2D composites have higher damage resistance, the 3D composites have a higher damage tolerance.     

Modelling impact damage in marine composite panels

In order to establish the survivability of a composite structure under a dynamic load, it is important to be able to predict the damage incurred and the effect of the contact force. The damage caused by low velocity impact is often hidden and for thicker plates the transverse stresses are significant in promoting delamination. In this study, woven vinyl-ester composite plates up to 1.37 m long are numerically modelled with a simple, gradually damaging three-dimensional material model and the results are compared with full-scale tests. The model is based on damage mechanics principles using cyclic test data to obtain modulus reduction with damage. Delamination is modelled with a mixed-mode traction-separation law using cohesive elements. The nonlinear elastic orthotropic material model for the woven plies is written in a VUMAT in Abaqus/Explicit and the effects of varying some of the modelling parameters are briefly discussed through the examination of the contact force.     

On modelling of damage evolution in textile composites on meso-level via property degradation approach

The continuous damage mechanics (CDM) approach is a popular tool for modelling of damage evolution in textile composites on the meso-level. It is based on the assumption that a material with defects can be replaced by a fictitious material with no defects but with degraded elastic constants. In such way the presence of defects is only reflected in the material elastic properties and damage evolution is recorded through the loss of these properties. The CDM approach incorporated into finite element analysis often predicts unphysically wide damage zones and in some cases failure across yarns – findings that are not supported by experimental data. The current work is geared toward identifying the source of inconsistencies between experiment and modelling by revisiting basic assumptions of CDM. A test problem is proposed to illustrate a break down of the CDM approach where a single crack-like defect in a yarn is modelled as an inhomogeneity with elastic constants reduced according to Murakami–Ohno model. It is shown that CDM in combination with local stress analysis of failure may predict a different direction of damage evolution as well as an incorrect failure mode in comparison with the crack problem. We also investigate whether the Murakami–Ohno model adopted for calculation of properties of a fictitious inhomogeneity contributes to the unphysical results. For this we compare contributions of a crack and an inhomogeneity into material elastic response. A new property degradation procedure is suggested (referred here as an effective elastic response model) where the size of an inhomogeneity and properties of the surrounding material are taken into account.     

Effect of thermal residual stresses on matrix failure under transverse tension at micromechanical level: A numerical and experimental analysis

In the present work the influence at micromechanical scale of thermal residual stresses, originated in the cooling down associated to the curing process of fibrous composites, on inter-fibre failure under transverse tension is studied. In particular, the effect of the presence of thermal residual stresses on the appearance of the first debonds is discussed analytically, whereas later steps of the mechanism of damage, i.e. the growth of interface cracks and their kinking towards the matrix, are analysed by means of a single fibre model and making use of the Boundary Element Method (BEM). The results are evaluated applying Interfacial Fracture Mechanics concepts. The conclusions obtained predict, at least in the case of dilute fibre packing, a protective effect of thermal residual stresses against failure initiation, the morphology of the damage not being significantly affected in comparison with the case in which these stresses are not considered. Experimental tests are carried out, the results agreeing with the conclusions of the numerical analysis.     

A computational strategy for the analysis of damage in composite pipes

The aim of this work is the determination of the influence of manufacturing defects on the behavior of composite pipes. We present an efficient numerical tool to solve this problem which relies on a strategy dedicated to pipes. Since most of the harmful manufacturing defects are located at the ends of the pipes, a complete 3D calculation is not necessary and only the ends are analyzed finely. The center of the pipe is modeled as an elastic beam and the ends as 3D nonlinear zones. The connection between the beam model and the end models is carried out using the Saint-Venant solution of the elastic problem, so that no spurious effects occur near the connection zone. This Saint-Venant solution is built from the exact beam theory. The nonlinear end problem is solved by a traditional method (secant Newton), but each elastic stage benefits from a particular treatment. In order for each 3D elastic problem to be solved efficiently, it is uncoupled into a set of dedicated 2D problems using a Fourier series expansion. Transfer between the 2D and 3D states is achieved using fast Fourier transform.     

Assessment of fatigue damage evolution in woven composite materials using infra-red techniques

Thermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and two ply, 2 × 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclic loading with maximum stresses of 10%, 15% and 20% of the ultimate failure stress. The development of fatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimens were subsequently inspected using optical microscopy to evaluate the location and extent of cracks. Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated that the lighter weight fabric is more resilient to damage progression. A signature pattern is identified in the TSA phase data that indicates the onset and presence of fatigue damage in the composite material.     

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally.     

Shock loading response of sandwich panels with 3-D woven E-glass composite skins and stitched foam core

Sandwich composite are used in numerous structural applications, with demonstrated weight savings over conventional metals and solid composite materials. The increasing use of sandwich composites in defense structures, particularly those which may be exposed to shock loading, demands for a thorough understanding of their response to suc highly transient loadings. In order to fully utilize their potential in such extreme conditions, design optimization of the skin and core materials are desirable. The present study is performed for a novel type of sandwich material, TRANSONITE^® made by pultrusion of 3-D woven 3WEAVE^® E-glass fiber composites skin preforms integrally stitched to polyisocyanurate TRYMER^TM 200L foam core. The effect of core stitching density on the transient response of three simply supported sandwich panels loaded in a shock tube is experimentally studied in this work. The experimental program is focused on recording dynamic transient response by high-speed camera and post-mortem evaluation of imparted damage. The obtained experimental results reveal new important features of the transient deformation, damage initiation and progression and final failure of sandwich composites with unstitched and stitched foam cores. The theoretical study includes full 3-D dynamic transient analysis of displacement, strain and stress fields under experimentally recorded surface shock pressure, performed with the use of 3-D MOSAIC analysis approach. The obtained theoretical and experimental results for the transient central deflections in unstitched and two stitched foam core sandwiches are mutually compared. The comparison results reveal large discrepancies in the case of unstitched sandwich, much smaller discrepancies in the case of intermediate stitching density, and excellent agreement between theoretical and experimental results for the sandwich with the highest stitching density. The general conclusion is that further comprehensive experimental and theoretical studies are required in order to get a thorough understanding of a very complex behavior of composite sandwiches under shock wave loading.