Matrix Crack Propagation Research Articles

Compression-after-impact response of woven fiber-reinforced composites

This manuscript investigates compression-after-impact failure in woven fiber-reinforced composites. Compression failure of composite structures previously damaged by an impact event is due to the propagation of impact-induced damage mechanisms such as interlaminar debonding, constituent (i.e., matrix and fiber) microcracking, sublaminate buckling, as well as the interactions between these mechanisms. The failure mechanisms within each ply are idealized based on a reduced order multiscale computational model, in which, the damage propagation in the matrix and fibers upon compression is explicitly modeled. Delamination along the ply interfaces is idealized using a cohesive surface model. The initial impact-induced damage within the microconstituents and interfaces are inferred from experimental observations. A suite of numerical simulations is conducted to understand the sublaminate buckling, propagation of delamination and constituent damage upon compression loading. The numerical investigations suggest extensive propagation of delamination with mode transition preceding sublaminate buckling. Initiation and propagation of matrix and fiber cracking, observed upon sublaminate buckling, is the cause of ultimate shear failure.

Damage Evolution of Laminated Composites Using Continuum Damage Mechanics Incorporate with Interface Element

In this paper, 3D continuum damage mechanics (CDM) incorporated with layer-wise theory and interface element is employed to investigate the progressive damage inside and between the laminate's layers under quasi-static axial loading. For this purpose, a finite element program is developed. To simulate the delamination, a quadratic interface element is used which is compatible with the 8-node numerical layers of layer-wise theory. Matrix cracking and delamination initiation and propagation of [302/-302]s angle-ply laminate are investigated and the obtained results are compared with the available experimental evidence.

In situ observation of interfacial debonding process induced by matrix crack propagation in two-dimensional unidirectional model composites

Interfacial debonding behavior is studied for unidirectional fiber reinforced composites from both experimental and analytical viewpoints. A new type of two-dimensional unidirectional model composite is prepared using 10 boron fibers and transparent epoxy resin with two levels of interfacial strength. In situ observation of the internal mesoscopic fracture process is carried out using the single edge notched specimen under static loading. The matrix crack propagation, the interfacial debonding growth and the interaction between them are directly observed in detail. As a result, the interfacial debonding is clearly accelerated in specimens with weakly bonded fibers in comparison with those with strongly bonded fibers. Secondary, three-dimensional finite element analysis is carried out in order to reproduce the interfacial debonding behavior. The experimentally observed relation between the mesoscopic fracture process and the applied load is given as the boundary condition. We successfully evaluate the mode II interfacial debonding toughness and the effect of interfacial frictional shear stress on the apparent mode II energy release rate separately by employing the present model composite in combination with the finite element analysis. The true mode II interfacial debonding toughness for weaker interface is about 0.4 times as high as that for a stronger interface. The effect of the interfacial frictional shear stress on the apparent mode II energy release rate for the weak interface is about 0.07 times as high as that for the strong interface. The interfacial frictional shear stress and the coefficient of friction for weak interface are calculated as 0.25 and 0.4 times as high as those for strong interface, respectively.

Compressive deformation behaviour of coarse SiC particle reinforced composite: Effect of age-hardening and SiC content

Compressive deformation of 2014 Al-alloy–SiCp composite has been compared with that of 2014 Al-alloy in as-cast and peak aged condition. In as-cast condition, the strength, elastic modulus and strain-hardening exponent of the alloy were improved significantly at the expense of its ductility due to reinforcement of SiC particles. After peak aging, the strength of individual material increased, but the elastic modulus and work hardening rate remained almost unchanged. It was interestingly noted that in peak aged condition composites exhibited less strength as compared to that of the alloy. However, the strength, the elastic modulus and the strain-hardening exponent of the composite increased with increase in SiC content irrespective of the processing condition. These have been explained in the perspective of matrix flow and crack propagation depending on interface characteristics, matrix strength and particle shearing. In cast condition, the matrix strength is low enough and the particles are quite stronger than the as-cast matrix. As a result, the particles are capable to resist the flow of the matrix, and the crack propagates primarily through the interdendritic region of the matrix. But in heat-treated condition, the matrix strength increased significantly. It results in intensifying the stress intensity above a critical value at the sharp tip of the flaws within the particles causing particle failure prior to the matrix failure.

Effect of water on the fatigue behaviour of a pa66/glass fibers composite material

The aim of this study is to evaluate the effect of the humidity on the long term behaviour of glass fiber reinforced thermoplastic in fatigue. Two sets of samples were studied, one set contained 0.2 wt% of water, the second 3.5 wt%. The fatigue tests are performed at a 10 Hz frequency, at room temperature and two various relative humidity ratios, 50% RH and 96% RH. The S–N curve of dried samples (0.2%) is above the one of humid samples (3.5%), the endurance limit at 107 cycles for dried samples is equal to 40 MPa against 35 MPa for the second set. For a given strain, the fatigue life is higher for humid samples because the induced stress is much lower due to the plasticizing effect of water. Though the tests are carried out at room temperature (23 °C), the sample temperature at the surface reaches values higher than Tg and whatever the applied strain, the matrix is in a rubbery state when the fracture occurs. On the basis of S.E.M. examinations, the following scenario is proposed: crack initiation at the fiber end, crack propagation along the fiber sides going with debonding, then crack propagation in the rubbery matrix.

Modeling interfacial debonding and matrix cracking in fiber reinforced composites by the extended Voronoi cell FEM

This paper introduces an extended Voronoi cell finite element model (X-VCFEM) for modeling the initiation and propagation of interfacial debonding and matrix cracking in fiber reinforced composite materials. Bilinear and linear cohesive zone models are added for representing interfacial debonding and matrix crack propagation, respectively. A series of criteria based on cohesive zone models are proposed for assessing the direction of damage development, which includes the crack propagation in matrix and its deflection behavior at an interface. Comparisons of X-VCFEM simulations with reference results validate the effectiveness of this new model. The capability of predicting the development of microcracks in composites is of great importance to the design and evaluation of structure. Effect of stereographic features such as size and shape of heterogeneities on damage evolution is also discussed.

Analysis of composite skin/stiffener debounding and failure under uniaxial loading

In this paper, damage mechanisms in the composite bounded skin/stiffener constructions under monotonic tension loading are investigated. The approach uses experiments to detect the failure mechanisms, two and three-dimensional stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for cracks and delamination growth. The laminates strength and damage mechanisms obtained from both experimental and finite elements analysis are presented for several laminates lay-up configurations. Observations on the performed experiments show matrix crack initiation and propagation in the skin and near the flange tip, causing the flange to almost fully debounded from the skin in some cases, interlaminar debounding and fiber breakage up to the failure of the components. The finite elements analysis is also show that the matrix cracks are initiated in the first skin layer for most of the cases. With increasing the applied load the matrix cracks are propagated through the thickness to reach the next layer and causes delamination between the two layers. With increasing the applied load this delamination is propagated up to the occurrence of unstable delamination growth or the first fiber breakage known as the final failure of the component. The obtained experimental failure loads are compared with those calculated by the finite elements analysis.

Numerical simulation on failure process in brittle and heterogeneous matrix filled with randomly distributed particles

Based on an essential assumption of meso-heterogeneity of material, the macro characteristic of composite reinforced with particles, the crack initiation, propagation and the failure process in composite were studied by using a numerical code. The composite is subjected to a uniaxial tension, and stiff or soft particles are distributed at random manner but without overlapping or contacting. The effect of reinforcement particle properties on the fracture process and mechanism of composite with brittle matrix, furthermore, the influence of the particle volumetric fraction is also investigated. Numerical results present the different failure mode and re-produce the crack initiation, propagation and coalescence in brittle and heterogeneous matrix. The mechanism of such failure was also elucidated.

Viscoplastic finite element analysis of matrix crack propagation in model continuous-carbon fibre/epoxy composites

Abstract Non-linear finite element (FE) analysis was used to investigate the effect of matrix viscoplasticity in a continuous carbon fibre/epoxy composite subject to tensile loading. During fibre fracture, the sudden increase in the strain rate near the fracture location reduces matrix ductility which results in catastrophic failure involving rapid matrix crack-growth. By applying the minimum strain energy density criterion to the FE results, the semi-cone angle of a matrix crack was predicted to be in the range 32°–36°, twice the value for the corresponding short fibre system. The FE results were compared with experimental results obtained from laser Raman spectroscopy (LRS), and it was found that the fibre stress profiles are characteristic of conical matrix crack geometries. Good correlation was observed with the experimental results confirming matrix cracking as the primary mode of failure for this composite system. The energy balance model was used to determine the available strain energy to propagate the crack.

Through-Thickness Connection of Matrix Cracks in Laminate Composites for Propellant Tank

Matrix crack accumulation in multiple plies of composite laminates is experimentally investigated to assess the through-thickness connection of matrix cracks, which is closely related to propellant leakage problems of composite tanks. Cracking patterns in [0/θ 2 /90] s laminates reveal that layup angles have a significant effect on crack induction mechanisms in contiguous plies. Matrix crack propagation behaviors in the fiber direction of θ ply induced by 90-ply cracks are investigated in terms of energy release rates using parametric-oriented finite element analysis based on an oblique coordinate system. The effect of cryogenic conditions on crack induction in contiguous plies is also studied, and discussions on through-thickness connection of matrix cracks in composite laminates are provided.

Comparison of damage development in C/epoxy laminates during isothermal ageing or thermal cycling

Abstract The aim of this study is to characterize and compare damage processes in carbon/epoxy laminates submitted to either isothermal ageing or thermal cycling in neutral (vacuum or nitrogen) and oxidative (air) atmospheres. During a thermal cycling test performed in an oxidative atmosphere like air, there is a coupling effect between matrix oxidation, occurring at the highest temperatures of the cycle, and matrix cracking due to thermo-mechanical ply stresses induced by the prevented differential expansions of the plies. In order to separate those two damage mechanisms, a model of oxidation is used to determine the experimental conditions of an isothermal test ‘equivalent’ to a thermal cycling one, in terms of mass loss due to oxidation. After having checked this equivalence, a quantitative analysis of the damages induced by the two types of tests, is carried out. It is shown that the oxidation of a laminate in isothermal conditions results in damages, which concern only the skin of the sample. On the contrary, the coupling of such damage mechanisms with cyclic stresses in thermal cycling accelerates the damage processes and especially the matrix crack propagation from the surfaces to the core of the laminate.

Hygrothermal aging behaviour of VARTMed three-dimensional braided carbon-epoxy composites under external stresses

The hygrothermal aging of three-dimensional braided carbon fibre-epoxy resin composites prepared by vacuum assisted resin transfer moulding (VARTM) was investigated. Emphasis was given to the effect of external stresses, both tensile and compressive modes at stress levels of 10 and 20% ultimate tensile strength (UTS). Unstressed, tensile-stressed and compressive-stressed specimens were immersed in distilled water at 37 °C. Specimens were characterized by examining weight changes and evaluating variations of flexural and shear properties due to moisture ingress. It was concluded that the application of external stresses did not change the mechanism of moisture sorption. However, external stresses exerted apparent effects on moisture uptake, equilibrium moisture content and mechanical degradation. Moisture sorption was promoted and resisted, respectively under the actions of tensile and compressive stresses. The degree of mechanical deterioration of the composites depended on the level of moisture content. A mechanism which attributed the external stresses effects to changes of matrix strain, propagation of matrix cracks, and moisture transport at interfaces was proposed. Moreover, a comparison between the 3D and unidirectional composites was made to assess the influence of fibre structure.

광섬유 브래그 격자 센서를 이용한 복합재 구조물의 변형률 및 파손신호 동시 측정

For the simultaneous measurement of strain and damage signal a fiber Bragg grating sensor system with a dual demodulator was proposed. The dual demodulator is composed of a demodulator using a tunable Fabry-Perot filter measuring the low-frequency signal with large magnitude such as strain and the other using a passive Mach-Zehnder interferometer detecting the high-frequency signal with small amplitude such as impact or damage signal. Using the proposed fiber Bragg grating sensor system, both the strain and damage signals of a cross-ply laminated composite beam under tensile loading were simultaneously measured. The strain and damage signals detected by single fiber Bragg grating sensor showed that sudden strain shifts were accompanied with vibration at a maximum frequency of several hundreds of kilohertz at the instant of matrix crack propagation in the 90 degree layer in composite beam.ጊ缀Ѐ㘰〻Ԁ䭃䑎䴀

Effects of carbon black on the fatigue life, critical J-value and fracture morphology and a new estimated equation for natural rubber

This study investigated the fatigue lives and mechanical properties of the carbon black filled natural rubber for the vibration-proof parts of the railway vehicle and automobile. The carbon blacks were one of the sources of crack nucleation and crack propagation in the rubber matrix, like the cementite and the maganese sulfide in iron matrix. Different kinds of carbon blacks resulted in different fatigue lives, critical J-values, and fracture morphologies. It was noticed that the critical J-value remained almost the same regardless of the length of a pre-crack. In addition, different kinds of carbon blacks generated different fracture morphologies, and microscopic and macroscopic roughnesses. The critical J-value has linear relations to the roughness, and it seemed related to the size distribution of carbon black particles. By reviewing all the experimental data, we found the factors that were related to the fatigue lives, and the logarithmic value of the fatigue life could be linearly expressed by the combination of the critical J-value and the macroscopic roughness. We also proposed a new estimative equation of fatigue life.

A criterion for modelling initiation and propagation of matrix cracking and delamination in cross-ply laminates

A variational approach is used to model the behaviour of composite cross-ply laminates damaged by transverse, longitudinal cracking and delamination. An energetic criterion is proposed. It is based on the strain energy release rate associated with each of the three damage modes. The first part of this paper is concerned with the modelling of the transverse and longitudinal cracking. In the second part, a model for studying delamination damage is presented. The numerical results show that these models provide a consistent level of accuracy for a variety of thin laminate material systems and configurations, with various combinations of delaminations and matrix cracks. In this paper several numerical simulations meant to describe initiation for each damage mode are proposed. The estimation of damage modes contributions is achieved for two thin laminates in order to predict the evolution of damage mode transition.

Effects of inter-fibre spacing and matrix cracks on stress amplification factors in carbon-fibre/epoxy matrix composites. Part I: planar array of fibres

When the loading on a composite is sufficient to cause fracture of an individual fibre, the resulting stress amplification in the adjacent intact fibres may be large enough to cause failure of these fibres. In this work, 3D elasto-plastic finite element analysis was used to investigate the effect of inter-fibre spacing on the stress amplification factor in a composite comprising a planar array of fibres. A Progressional Approach was used in the FE analysis to simulate the constituent non-linear processes associated with the generation of thermal residual stresses from fabrication, the fibre fracture event and the subsequent initiation and propagation of conical matrix cracks induced with incremental tensile loading. As the inter-fibre spacing increases, the effect of fibre fracture on the stress distribution in the neighbouring intact fibres is reduced, whereas the effect on the matrix material is increased, thereby inducing localised yielding. The presence of a conical-shaped matrix crack was found to increase both the stress amplification factor and the positively affected length in neighbouring fibres. For a large inter-fibre spacing, a longer matrix crack is required to obtain good agreement with LRS measurements of fibre stress.

Impact Damage Resistance of CFRP Laminate with Epoxy-Resin Surface Layer

We investigated the effect of an epoxy surface layer to improve the impact damage resistance in carbon-fiber-reinforced-plastic (CFRP) laminate. The specimens were CFRP laminates covered on the impact face or the back face with a highly viscous epoxy resin. The thickness of the epoxy layer was 1.0mm or 2.0mm. Projectiles were launched from an air gun and impacted onto the laminates. The projectiles were made of silicone rubber or aluminum alloy. C-scan images obtained with a scanning acoustic microscope after the tests revealed that the damage mechanism for the CFRP laminates is independent of the surface-layer material and the material properties of the projectile. When we compared the contact area, the crack length on the back face and the delamination areas on each specimen, we found that the epoxy surface layer improved the impact damage resistance of the CFRP laminate. The epoxy surface layer on the impact face decreased the impact force transferred to the laminate, and that layer on the back face suppressed the generation and propagation of fiber-directional matrix cracking on the back face during impact.

Multi-scale study of tensile properties and large deformation mechanisms of polyethylene terephthalate/polypropylene knitted composites

This paper is concerned with large deformation tensile properties and energy absorption mechanisms of thermoplastic weft-knitted composites, fabricated by compression molding co-knitted preforms consisted of polyester (polyethylene terephthalate) and polypropylene continuous filament yarns. The large deformation mechanisms have been studied by observing morphological changes during quasi-static tensile loading in meso- and micro-scales. In-situ SEM observations have identified critical points including brittle matrix crack initiation from imperfect composite surfaces, fibre/matrix debonding, serious loop distortion along the loading directions and fibre straining during the large deformation until composite fracture. The energy absorption mechanisms have been identified as localized matrix compression and extension, matrix crack propagation, localized loop distortion, and fibre straining during the deformation process. Effects of fabric structure, fibre volume fraction and plasma treatment on fibre surface have been also studied.

Green's function vs. shear-lag models of damage and failure in fiber composites

A Green's function method (GFM) for simulating fiber damage evolution and tensile strength in fiber-reinforced composites is compared in detail to the predictions of the standard shear-lag model (SLM) widely used in the literature. The GFM extracts the in-plane stress concentration factors describing how a broken fiber redistributes in-plane load to surrounding unbroken fibers from more-detailed micromechanical models, such as finite-element models, and uses this limited information to then calculate the propagation of fiber damage up to composite failure. The GFM only approximately includes the three-dimensional nature of the stress transfer and uses an approximate superposition method, but reduces the computational problem significantly. Here, elastic and elastic/plastic 3D SLM are used to provide the stress-transfer input to the GFM, and then predictions for composite behavior from the GFM are compared directly to those from the SLM. For exactly the same starting configuration of stochastic fibers, the GFM predicts (i) evolution of the fiber damage and the formation of critical clusters that are nearly identical to, (ii) composite tensile strengths within 2% of, (iii) a Weibull modulus for the composite strength essentially equal to, that of the SLM, all while requiring over an order of magnitude less computational time for modest-size composites. The approximations made in the GFM are found to have little effect on composite properties. These results support the use of the GFM approach with stress transfer input from accurate detailed finite element studies rather than from approximate SLM. Furthermore, the GFM is well-suited for tackling a wide range of problems that cannot easily be studied using the SLM; e.g. bending deformation and failure, matrix crack propagation, fatigue crack growth, and other situations in which the fiber stress distribution is nonuniform even in the absence of any fiber damage.

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.