Effect of thermal residual stresses on the matrix failure under transverse compression at micromechanical level - A numerical and experimental study

The influence at micromechanical scale of thermal residual stresses, originating in the cooling down associated to the curing process of fibrous composites, on inter-fibre failure under transverse compression is studied. In particular, the effect of these stresses on the appearance of the first debonds is discussed analytically; later steps of the damage mechanism are analysed by means of a single fibre model, making use of the Boundary Element Method. The results are evaluated applying Interfacial Fracture Mechanics concepts. The conclusions obtained show, at least in the case of dilute fibre packing, the effect of thermal residual stresses on the appearance and initiation of growth to be negligible, and the morphology of the damage not to be 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 derived from the numerical analysis.     

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.     

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.     

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.     

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.     

Characterization and evolution of matrix and interface related damage in [0/90]S laminates under tension. Part I: Numerical predictions

This paper considers damage development mechanisms in cross-ply laminates using an accurate numerical method that assumes a Generalized Plane Strain (GPS) state. A 2D Boundary Element Method (BEM) model is generated to investigate the two types of damage progression in a [0/90]_S laminate: transverse cracks in the 90° lamina and delamination between both laminae. The model permits the contact between the surfaces of the cracks. The study is carried out in terms of the dependence of the Energy Release Rates (ERR) of the two types of crack on their respective lengths. A special emphasis is put on the mechanisms of the joining of the two aforementioned types of crack, including the study of the distribution of the stresses along the interface between the two plies when the transverse crack is approaching this interface.     

FE macro/micro analysis of thermal residual stresses and failure behaviour under transverse tensile load of VE/CF – fibre bundle composites

This paper presents a finite element analysis of a transverse fibre bundle test (TFT) using carbon fibres embedded in a vinylester urethane hybrid matrix. The evolution of thermal residual stresses, due to the cooling phase of the curing process of the model-composite and the subsequent mechanical load transverse to the fibre direction, has been investigated. The applied displacement coupling technique allowed to transfer the boundary conditions from a global model (macro model) via an intermediate model to a micro model. As a result it could be shown that the larger fraction of the total stress build up until failure occurred was due to the implicated thermal residual stresses. The micro model offered more accurate and detailed results with regard to the stress distributions on critical locations such as the fibre/matrix interface region. Generally, the results of the global model were in good agreement with the experimental data obtained. Further, the parabolic failure criterion based on experimental data of the pure matrix was used to predict time and place of failure initiation.     

Micro-mechanical analysis of the effect of ply thickness on the transverse compressive strength of polymer composites

A micro-mechanical model is used to study the effect of ply thickness on constrained 90° plies subjected to transverse compressive loading (in situ effect). For cross-ply sublaminates with conventional, standard-thickness 90° plies, failure is dominated by fibre–matrix interface cracking and large localised plastic deformation of the matrix, forming a localised band in a plane that is not aligned with the loading direction. Ultra-thin plies show a dispersed damage mechanism, combining wedge cracking with ply fragmentation/separation. Moreover, a transverse crack suppression effect is clearly observed. To the authors’ knowledge, it is the first time an in situ effect in transverse compression has been identified. When comparing the results of the micro-mechanical model with the predictions from analytical models for the in situ effect, the same trends are obtained. These results also show that, for realistic ply thicknesses, these analytical models can be considered fairly accurate.     

Illustrations of a microdamage model for laminates under oxidizing thermal cycling

Degradations initiated near the edges of a laminate can have a significant effect on its state of degradation, even at the core. Indeed, results from the literature show that laminates which have the same stress state at the core can have completely different states of degradation, even far away from the edges. The paper discusses the influence of the edge effect on damage initiation and propagation for a specific example. A computational micromechanical approach to the degradation of laminated composites was developed recently at LMT-Cachan. This is a hybrid approach in which, depending on the scale, the mechanisms are described using continuous damage mechanics or finite fracture mechanics. Initially developed for static loading, this technique is being extended to fatigue and environmental effects. The aim of this paper is to illustrate the capability of such an approach to take into account major observations during cyclic loading in an oxidizing atmosphere, even when edge effects are significant.     

A generalized micromechanical approach for the analysis of transverse crack and induced delamination in composite laminates

The previously developed micromechanical approaches for the analysis of transverse cracking and induced delamination are limited for laminates with specific lay-ups such as cross-ply and specific loading conditions. In this paper a new micromechanical approach is developed to overcome such shortcomings. For this purpose, a unit cell in the ply level of composite laminate including transverse cracking and delamination is considered. Then, the governing equations for the stress and displacement fields of the unit cell are derived. The obtained approximate stress field is used to calculate the energy release rate for the propagation of transverse cracking and induced delamination. To show the capability of the new method, it is employed for the analyses of general laminates with [0/90]_s, [45/−45]_s, [30/−30]_s and [90/45/0/−45]_s lay-ups under combined loadings to calculate the energy release rate due to the transverse cracking and induced delamination. It is shown that the obtained energy release rates for transverse cracking and delamination initiation are in good agreement with the available results in the literature and finite element method. Furthermore, the occurrence priority of further transverse cracks and/or delamination at each damage state of the laminates will be discussed.     

Effect of interface debonding on the thermal conductivity of microencapsulated-paraffin filled epoxy matrix composites

Microcapsules containing phase change materials (microPCMs) can be filled in polymeric matrix forming smart temperature-controlling composites. The aim of this study was to investigate the effect of interface debonding on the thermal conductivity of microPCMs containing paraffin/epoxy composites. The shell thickness and average size of microPCMs were controlled by regulating the core/shell ratios and emulsion stirring rates. Test results indicated that the thermal conductivity (K_e) of all composites decreased after a thermal shock treatment. SEM and thermography measurements were applied to observe the interface behaviors of composites after a violent thermal treatment process. It was proved that the interface debonding was generated because of the mismatch of expansion coefficient between shell and epoxy. A modeling analysis of the relative thermal conductivity (K_r) indicated that the effective approach to decrease the debonding is to enhance the molecule tangling degree between shell and matrix.     

Failure response of fiber-epoxy unidirectional laminate under transverse tensile/compressive loading using finite-volume micromechanics

The transverse damage initiation and extension of a unidirectional laminated composite under transverse tensile/compressive loading are evaluated by means of Representative Volume Element (RVE) presented in this paper based on an advanced homogenization model called finite-volume direct averaging micromechanics (FVDAM) theory. Fiber, fiber-matrix interface and matrix phases are considered within the RVE in determining fiber-matrix interface debonding and matrix cracking. The simulated fracture patterns are shown to be in good agreement with experimental observations.     

Long-term bond stresses and debonding failure of FRP-strengthened RC cracked members

Most of the existing models to prevent debonding failure in beams externally strengthened by fiber reinforced polymer (FRP) laminates were developed focusing on the short term response. This paper studies the effect of concrete creep on the interfacial shear stresses and consequently, on the debonding failure load. A simplified formulation for the instantaneous and time-dependent bond stresses under sustained load is provided. Its reliability is analysed through the results obtained by a non-linear time-dependent analysis model. Previously, this model has been checked to evaluate the long-term response of existing experimental programmes in terms of deflections and strains.     

Energy-based delamination theory for biaxial loading in the presence of thermal stresses

This paper addresses the issue of using energy balance methods and crack closure concepts to predict the growth of delaminations associated with ply cracks during the progressive loading of cross-ply laminates subject to a combination of in-plane biaxial stresses and thermal residual stresses. When the effective applied stresses and the temperature are held fixed during delamination growth, and there is negligible interaction of the delamination tips with the ply cracks, very simple analytical formulae for the energy release rate can be derived for unconstrained and generalised plane strain conditions, which are exact when the ply crack separation tends to infinity.     

Effect of NiCrAlY platelets inclusion on the mechanical and thermal shock properties of glass matrix composites

NiCrAlY platelets modified glass matrix composites were prepared. Their microstructures were characterized, their Young's modulus, fracture strength in bending, Vickers hardness, and indentation toughness were measured, and their thermal shock resistance was studied using quenching-strength and indentation-quench methods. With increasing NiCrAlY content, evident enhancements of the Young's modulus and indentation toughness were obtained. The NiCrAlY alloy inclusion could exert significant influences on the retained bending strength of the samples after quench tests, from 9.6 MPa for NiCrAlY-free glass to 32.0 MPa for 30 wt.% NiCrAlY-containing composites. The indentation-quench tests showed that NiCrAlY alloy inclusion elevated the critical quenching temperatures for propagation of pre-crack, from 150 °C for NiCrAlY-free glass to 225 °C for 30 wt.% NiCrAlY-containing composites. Inclusion debonding and intersection, crack deflection and bridging were observed, and are likely the micromechanisms accounted for the improvement of fracture resistance.     

An experimental study on the effects of matrix cracking to the stiffness of glass/epoxy cross plied laminates

Three independent measurement techniques are applied to characterize glass fiber laminates. The effects of distributed fatigue damage to the stiffness related behavior of cross plied laminates are quantified. Tensile and flexural stiffness reduction is obtained from quasi-static testing. Vibration testing shows the degradation of flexural and in-plane shear stiffnesses. The reduction of the phase velocity of symmetric S₀ mode is observed from the experimental dispersion curves of Lamb waves. However, the mutual agreement of these results is less satisfactory than was earlier seen for virgin laminates. The phenomena causing the discrepancies are proposed and discussed.     

Micromechanical modelling and experimental investigation of random discontinuous glass fiber polymer–matrix composites

Videomeasurements were used to estimate the damage in chopped random glass fiber polymer–matrix composites. In order to predict the overall mechanical behaviour, voiding evolution induced by fiber debonding is incorporated into a micromechanics-based constitutive model. The comparison between the experimental data and the numerical predictions shows a very good agreement.     

The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.     

The role of carbon nanofibers on thermo-mechanical properties of polymer matrix composites and their effect on reduction of residual stresses

In this research, the effects of carbon nanofibers (CNFs) on thermo-elastic properties of carbon fiber (CF)/epoxy composite for the reduction of thermal residual stresses (TRS) using micromechanical relations were studied. In the first step, micromechanical models to calculate the coefficient of thermal expansion (CTE) and Young's modulus of CNF/epoxy and CNF/CF/epoxy nanocomposites were developed and compared with experimental results of the other researchers. The obtained results of the CTE and Young's modulus of modified Schapery and Halpin-Tsai theories have good agreement with the experimental results. In the second step, the classical lamination theory (CLT) was employed to determine the TRS for CNF/CF/epoxy laminated nanocomposites. Also, the theoretical results of the CLT were compared with experimental results. Finally, reduction of the TRS using the CLT for different lay-ups such as cross ply, angle ply, and quasi-isotropic laminates were obtained. The results demonstrated that the addition of 1% weight fraction of CNF can reduce the TRS that the most reduction occurred in the unsymmetric cross-ply laminate by up to 27%.     

Experimental and numerical studies of initial cracking in CFRP cross-ply laminates

This work is concerned with the conditions for formation of the first (initial) cracks in composite laminates with cutouts or ply drop-offs subjected to in-plane loading. We study here the crack formation on the free edge of CFRP cross-ply laminates experimentally and by numerical stress and failure analysis. The free-edge surface strains are measured by the digital image correlation (DIC) technique. The numerical analysis consists of a two-scale approach, where the macro-level analysis is performed with a three-dimensional finite-element method (3D FEM) and the micro-level analysis uses a periodic unit-cell (PUC) in the transverse plies. The constitutive assumption made for the macro-level analysis is an orthotropic linear thermo-elastic solid for the unidirectional plies with a thin isotropic viscoplastic layer between the longitudinal and transverse plies. In the PUC, the fibers are assumed linear elastic, while the matrix is modeled as an elastic–viscoplastic solid. Crack formation is assumed to occur in the matrix by the dilatation induced brittle failure mechanism for which the dilatation energy density criterion is used.