Transverse Crack Initiation Research Articles

Initiation and growth of transverse and longitudinal cracks in composite cross-ply laminates

The aim of this study is to show the influence of transverse and longitudinal cracks on the distribution of the stress field and on the stiffness loss and Poisson ratio reduction of a cross-ply laminate subjected to monotonic or fatigue tests. Two analytical models are proposed, which rely on different distributions of the stress field in the thickness direction of the laminate. These models are based on a variational approach. A second type of approach is proposed, a three-dimensional finite element study. The three models are meant to predict the evolution of mechanical properties of [0m/90n]s cross-ply laminates with (m+n=8) in order to better understand the damage evolution.

Statistical analysis of the progression of transverse cracking and delamination in cross-ply laminates

The paper presents a statistical analysis of the transverse cracking and delamination process induced in cross-ply laminates subjected to tensile loading. First, the paper considers the initiation and progression of transverse cracking. This progression is evaluated by an iterative procedure combining a generalized model for evaluating the stress field in a cracked laminate with a statistical distribution of strengths in the 90° layer. Comparisons between the results obtained by this procedure and the experimental results of the literature show that for very low crack densities the progression of transverse cracking is governed by a strength distribution associated with weakness areas resulting from the presence of particular defects in laminates. Next, for higher crack densities the strength distribution in the 90° layer can be described by a pseudo-normal distribution associated with the other defects. Finally, the paper considers the development of delamination which is initiated from the crack tips in the case of some laminates.

A new technique for the ultrasonic detection of internal transverse cracks in carbon-fibre/bismaleimide composite laminates

Through-transmission ultrasonic C-scan imaging with inclined focusing transducers in confocal configuration has been used for the detection of partial and internal cracks in a cross-ply, [0/90/0] carbon/epoxy laminate. A number of partial cracks (intersecting only one edge of a coupon) as well as completely internal cracks were found ultrasonically after the sample was subjected to static loading. The validation of the result by cutting the sample along two lines in the 0° direction and checking the presence of the internal cracks with an optical microscope shows a good correlation between the ultrasonic image and the optical data. Experimental results show that in the IM7/5250-4 composite, which contains a toughened BMI resin, initiation of transverse cracks under static loading occurs with the formation of partial cracks not necessarily originating at the edge of the sample.

Micromechanically Based Constitutive Models for Damage Evolution in Composite Laminates

The structure of micromechanically based constitutive models for composite laminates developing transverse matrix cracks, local delaminations, fibre/matrix debonding and fibre fractures is discussed. Nondimensional damage variables describing these four damage modes are introduced and the principal form of a damage dependent constitutive law is formulated. As a special case transverse matrix cracking is investigated in further detail. A previously developed model for stiffness changes resulting from transverse matrix cracking is presented, and criteria for initiation and growth of transverse cracks are discussed. In particular, limitations of the linear elastic fracture mechanics approach are investigated and a cohesive zone model capturing crack bridging effects is suggested.

Surface topography and wear mechanisms in polyamide 66 and its composites

A study has been made of the tribological behaviour of polyamide 66 (PA66) running against itself, in unlubricated, non-conformal and rolling-sliding contact. Tests were conducted over a wide range of loads and slip ratios using a twin-disc test rig. The wear and friction behaviour of unreinforced PA66 is dominated mainly by three major features: a critical slip ratio under a fixed load and running speed, macro-transverse cracks and a layer of film on the contact surface. Both the wear and friction properties of unreinforced PA66 can be improved considerably by filling with 20wt% PTFE, and the tribological mechanisms are changed significantly. This reinforcement prevents both the initiation and propagation of transverse cracks on the contact surfaces which occurred in the unreinforced material. It also decreases both the wear rate and the friction coefficient substantially. The 30wt% short glass-fibre reinforced PA66 also suppresses the transverse cracks from initiation on the surfaces. A thin film on the contact surfaces plays a dominant role in reducing wear and friction of the composite and in suppressing the transverse cracks. These results offer the prospect of enhanced applicability of PA66 in engineering components.

Micromechanically Based Constitutive Models for Damage Evolution in Composite Laminates

The structure of micromechanically based constitutive models for composite laminates developing transverse matrix cracks, local delaminations, fibre/matrix debonding and fibre fractures is discussed. Nondimensional damage variables describing these four damage modes are introduced and the principal form of a damage dependent constitutive law is formulated. As a special case transverse matrix cracking is investigated in further detail. A previously developed model for stiffness changes resulting from transverse matrix cracking is presented, and criteria for initiation and growth of transverse cracks are discussed. In particular, limitations of the linear elastic fracture mechanics approach are investigated and a cohesive zone model capturing crack bridging effects is suggested.

Steady-state cracking and edge effects in thermo-mechanical transverse cracking of cross-ply laminates

Composites for space applications such as advanced satellites will require the use of new and/or improved materials so that more stringent dimensional stability requirements can be met. In this work, we study the effect of ply-level initiation of damage under loading and processing stresses and imposed thermal loading. In particular, included in the study is the prediction and demonstration of a condition of steady-state cracking and the temperature change and loading level necessary to create this damage state, thus leading to the possibility of a predetermined loading range over which layer cracking will not propagate. We also consider the influence of the free-edges on the initiation of layer transverse cracking and potential microcracking that may develop at the fiber/matrix interface. The condition of constrained edge cracking is demonstrated where cracks develop but do not propagate across the laminate. To accomplish these studies unidirectional composites and various cross-ply laminates of IM7/5250-4 (graphite/bismaleimide) were produced and characterized.

Impact-induced delamination of composites: a 2D simulation

The delamination process in thin composite plates subjected to low-velocity impact is simulated using a specially developed 2D cohesive/volumetric finite element scheme. Cohesive elements are introduced along the boundaries of the inner layers and inside the transverse plies to simulate the spontaneous initiation and propagation of transverse matrix cracks and delamination fronts. The analysis is performed within the framework of the finite deformation theory of elasticity to account for the nonlinear stiffening of the thin composite plate and the large rotations which accompany the fracture process. The simulation is dynamic and uses an explicit time stepping scheme. Comparison with existing experiments performed on graphite/epoxy laminates indicates that the cohesive/volumetric finite element scheme is able to capture the complex mechanisms leading to the delamination, including the initial micro-cracking of the matrix, the appearance of critical transverse matrix cracks and the rapid propagation of delamination cracks initiated at the intersections between the critical matrix cracks and the adjacent plies.

Transverse Ply Cracking in Cross-Ply CFRP Laminates—Initiation or Propagation Controlled?

The initiation and propagation stages of transverse ply cracking in crossply CFRP laminates under quasi-static loading have been studied using laminates with a range of transverse ply thicknesses. Specimens of two types have been tested-coupons with polished edges and coupons with defects (notches) introduced into the transverse ply prior to testing. Detailed observations of crack propagation across the transverse ply thickness and width have been made, along with measurements of crack density and residual properties. In laminates with thin transverse plies (less than 0.25 mm for the materials tested here), fully formed transverse cracks are observed at about the same strain in undamaged and notched laminates. In laminates with thicker transverse plies (greater than 0.25 mm), the notched laminates show fully formed transverse cracks at lower strains than the undamaged laminates. The experimental results have illustrated the situations (laminate properties, presence of flaws) in which fracture mechanics models for transverse cracking available in the literature can be applied. In particular, it is shown that the fracture mechanics models which assume implicitly the presence of initial defects spanning the thickness of the ply may underestimate the failure strain of laminates with thick transverse plies. The effect of composite intralaminar toughness is also discussed.

Advanced waveform-based acoustic emission detection of matrix cracking in composites : 59252 Prosser, W.H.; Jackson, K.E.; Kellas, S.; Smith, B.T.; McKeon, J.; Friedman, A. Materials Evaluation, Vol. 53, No.9, pp. 1052–1058 (Sep. 1995)

An advanced, waveform based acoustic emission system was used to study the initiation of transverse matrix cracking in cross-ply graphite/epoxy composites. The acoustic emission signals were detected with broad band, high fidelity sensors, and digitized for analysis. Plate wave propagation analysis was used to discriminate noise signals from those generated by cracks. The noise signals were confirmed to have originated in the specimen grip region by a new, highly accurate form of location analysis which was independent f threshold setting. Six different specimen thicknesses ([0{sub n}, 90{sub n}, 0{sub n}], n = 1 to 6) were tested under stroke controlled, quasi-static tensile loading. The presence and location of the cracks were confirmed post test by microscopy. Back scatter ultrasonics, penetrant enhanced X-ray techniques, and in limited cases, destructive sectioning and microscopy were also used to determine the length of the cracks. The average absolute value of the difference between the microscopy determined crack location and the acoustic emission crack location was 3.2 mm (0.125 in.) for a nominal sensor gage length of 152 mm (6 in.). For all cracks, the location of the crack initiation site was at one of the edges of the specimen.

Microscopic fatigue failure process in interleaved and toughness-improved CFRP cross-ply laminates

Microscopic damage progress in CFRP cross-ply laminates under tensile fatigue loading was measured by the replica technique. Material systems were interleaved CFRP, T800H/3631-FM300 and toughness-improved CFRP, T800H/3900-2, with selectively toughened interlaminar layers. The damage parameter, the transverse crack density, was presented. A Paris law model was proposed, which related the cyclic strain range and the number of cycles at the transverse crack initiation. Based on the experimental data, a variational stress analysis combined with the modified Paris law was conducted to model the transverse crack multiplication.

2D and 3D numerical models of transverse cracking in cross-ply laminates

Two numerical models, 2D and 3D, have been developed in order to study the behaviour of cross-ply laminates during multiplication of transverse cracks. The displacement and stress fields have been evaluated by the finite-element method in an elementary cell which is characteristic of regularly spaced cracks. The longitudinal stiffness reduction has then been deduced as a function of the transverse crack multiplication. In the case of quasi-static tensile tests, the 2D model has been used to estimate the initiation and development of transverse cracking. The different numerical results obtained have been compared with experimental results.

Microscopic fatigue damage progress in CFRP cross-ply laminates

Damage progress in toughened-type carbon fibre-reinforced plastic (CFRP) cross-ply laminates under tensile fatigue loading was measured using the replica technique. The laminate configuration was [0/90 m/0], where m = 4, 8 and 12. The damage parameters, transverse crack density and delamination ratio, were determined. A power-law model was proposed, relating the cyclic strain range and the number of cycles at transverse crack initiation. Based on experimental data, a simple shear-lag analysis combined with the modified Paris law was conducted to model the transverse crack multiplication. An extension of the shearlag analysis for laminates containing delaminations initiating from the tips of the transverse cracks was used to conduct a modified Paris law analysis for delamination growth.

Simulation of 90� ply fatigue crack growth along the width of cross-ply carbon-epoxy coupons

We study the mechanism of fatigue cracking of the matrix of cross-ply carbon-epoxy laminates. Primary attention is given to the study of the influence of the specimen width on the evolution of damage. On the basis of shear lag analysis, we determine the strain energy release rate in the processes of initiation and growth of transverse fatigue cracks. We also present results of experimental research on the evolution of the edge crack density per ply, the average length of the cracks, and the crack propagation rate under transverse fatigue cracking. It is shown that these characteristics are independent of the specimen width. At the same time, as soon as the edge crack density reaches its saturation value, the average crack growth rate becomes constant. All the experimental results are in good agreement with results obtained by using the theoretical model.

PROGRESSIVE MICROCRACKING IN UNIDIRECTIONAL BRITTLE MATRIX COMPOSITES

Abstract In this work we investigate the initiation of microcracking in a unidirectional NicalonTMfibre, glass-ceramic (CAS) matrix composite under flexural loading. We find that damage develops and grows in these composites as a continual initiation of new transverse cracks in a more-or-less random manner, as well as growth of existing cracks, within the matrix. Initial cracks are usually of limited extent, being stopped when they encounter the nearest fibres, in a mode idealized by the so-called full-cell cracking model. Each new crack that initiates under increasing load yields another data point, provided the precracked state of stress and crack geometry at the initiation site can be determined. The latter conditions are satisfied since the zone of influence of each crack can be determined analytically and the full-cell cracking mode geometry receives at least partial validation. Analysis is accomplished by use of an axisymmetric micro mechanical model based on Reissner's variational principle in whic...

Tensile damage development in fiber-reinforced ceramic matrix composites

The mechanisms that control damage development and tensile failure of cross-plied Nicalon-fiber-reinforced glass and glass ceramic matrix composites were investigated using an edge replication technique. Four different systems were studied to determine the effect of interfacial bond strength and residual stress on the failure process. Composites with weak interfacial bonds were more resistant to matrix microcracking. Residual stresses, due to fiber-matrix and ply-ply thermal expansion mismatch, were found to have a strong influence on the damage development process; initiation of transverse ply cracks was delayed to higher strains (greater than 0.1%) in systems with residual compressive stresses in the transverse plies. Crack spacing measurements showed a saturation crack density was achieved with increasing load in both the longitudinal and transverse of all four composites. The experimental findings were used to modify for ceramic matrix composites the Laws and Dvorak model of composite cracking.

The behaviour of cross-ply hybrid matrix composite laminates: Part 1: Experimental results

Two types of cross-ply laminate have been made from prepreg: (a) hybrid matrix laminates consisting of longitudinal plies of glass fibres in epoxy resin and transverse plies of glass fibres in epoxy resin/urethane elastomer blend; and (b) uniform matrix laminates with the same resin in both the longitudinal and transverse plies. The presence of the urethane in the transverse plies increases the applied strains necessary for the initiation and development of transverse cracking during the extension of both hybrid matrix and uniform matrix laminates. The effect is greater with increasing amounts of urethane. The cracking data, stress/strain behaviour, acoustic emission response and ply thickness effects on crack development are discussed in the light of existing theories concerning transverse cracking.

A simplified analysis of transverse ply cracking in cross-ply laminates

This paper presents a method of analyzing transverse crack initiation and multiplication in symmetric cross-ply laminates. The method is based on the concept of a through-the-thickness inherent flaw and the energy balance principle. With a second-order polynomial assumed for the crack opening displacement, the perturbed stress field due to the presence of ply cracks is determined from the equilibrium conditions. The energy released as a result of ply cracking is then calculated and used to predict the increase in crack density. Based on an experimental correlation of the analytical result, a resistance curve is proposed to be used as a measure of the resistance to crack multiplication. The resistance to crack multiplication is shown to increase with the increasing crack density.

Erosion Behavior of Polystyrene

Abstract Erosion tests of polystyrene were made by impingement of glass beads at various attack angles and impact velocities. Two erosion damage peaks near the angles of 20 and 50°, which have never been observed on other materials, were found. In the erosion process near the peak of 50°, a characteristic ring-like damage pattern on the surface was recognized. To explain the experimental results, a new erosion mechanism based on the initiation and propagation of transverse cracks beneath the surface at the tip of longitudinal cracks was introduced. And by this mechanism, the erosion rate could be given by R = ρhS*/Mo where h, S*, and Mo are the factors describing the characteristics of the ring-like damage pattern and ρ is the density of material. The theoretical values agreed quite well with the experimental ones.

Initiation and growth of transverse cracks and edge delamination in composite laminates. Part 1 An energy method

Initiation and growth of transverse cracks and edge delamination in composite laminates. Part 1 An energy method