A [90/0]s composite laminate with part-through matrix cracks in adjacent plies

A [90/0]_s orthotropic composite laminate with part-through matrix cracks is considered. Stress intensity factors are determined for the cracks using a linear-elastic analysis. These matrix cracks run along the fiber direction of the individual plies. The crack-geometry considered here is one where the matrix cracks in adjacent plies form a cross-like pattern in the plan view of the laminate. The plies are assumed bonded by thin resin-rich adhesive layers. These adhesive layers are modeled as distributed shear springs. Each ply of the laminate is modeled as a thin elastic orthotropic layer under plane stress. The laminate is subject to both tensile and shear loading. The mathematical model for the stresses and displacements in the layers reduces to a pair of Fredholm integral equations which are solved numerically. The stress intensity factors show a strong dependence on crack-sizes and nature of loading. In particular, the magnitudes of the stress intensity factors for the matrix crack in the 0° layer are increased significantly by the crack in the adjacent 90° layer.     

Matrix crack-induced delamination in composite laminates under transverse loading

Fibre-reinforced multidirectional composite laminates are observed in experiments under transverse static or low-velocity impact loading to suffer considerable delamination damage. The intensity of this damage depends on the difference in the ply angles above and below the interface. In this paper a fracture mechanics model is presented for investigating the role of matrix cracks in triggering delaminations and the influence of ply angles in adjacent plies on delamination cracking. The fracture mechanics analysis shows that for a graphite fibre-reinforced composite laminate containing a transverse intraply crack, the crack-induced largest interfacial principal tensile stress is a maximum when the difference between the ply angles across the interface is 90 °, and it attains a minimum when the difference is 40 °. When the crack tips touch the interfaces, the minimum mode II stress singularity, which is weaker than the usual square-root type, appears when the difference between the ply angles is about 45 ° for one glass fibre-reinforced laminate and three graphite fibre-reinforced laminates. These results are in agreement with the experimental observation that the largest delaminations appear at the interface across which the difference between the ply angles is the largest i.e. 90 °.     

A microdebonding study of the high-temperature oxidation embrittlement of a cross-ply glass-ceramic/SiC composite

A constant cross-section specimen with adhesively bonded tabs has been used for an investigation of the high-temperature tensile behavior of a cross-plied glass-ceramic-matrix composite consisting of CAS-II reinforced with Nicalon SiC fiber. Oxidation of the exposed interfaces along matrix cracks in 0 ° plies lowers the composite failure strain at 800 °C to the 0 ° ply matrix-cracking strain. Scanning electron microscopy and microdebonding analysis of the fracture surfaces indicate that the embrittlement process is the result of oxidation of the carbon-rich interphase as the matrix crack encounters 0 ° ply fibers, the interphase subsequently fuses with a higher bond strength and the crack grows through the fibers. Planar cracks grow inwards from the surface, covering the entire fracture surface given enough time (or sufficient strain). Degradation of the fibers does not appear to contribute to the embrittlement. Transverse plies crack at a lower strain than does the matrix in the 0 ° plies. However, it appears that oxygen does not enter 90 ° ply cracks in sufficient quantity to produce oxidation embrittlement, at least up to the 0 ° matrix-cracking strain. The strain to crack the 90 ° plies does not decrease significantly at high temperatures despite the fact that the cracks are primarily in the fiber/matrix interphase as they grow across the 90 ° plies.     

Damage mechanics of composite materials: II— a damaged-based notched strength model

A new model of the notched strength of graphite-epoxy composites has been developed. In this second paper of a four-part series, a finite element model has been used to simulate observed subcritical notch tip cracking patterns in cross-ply laminates. The model produced maps displaying tensile stress contours in the 0° ply, and it was found that all specimens failed when the maximum tensile stress in the 0° ply exceeded the strength of that ply. The strength of the 0° ply in the vicinity of the notch tip was determined independently using a Weibull statistical strength model.     

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

Stiffness reduction due to matrix cracking in a (0/90)_s glass fibre reinforced plastic (GFRP) laminate has been studied under both quasi-static and fatigue loading. The stiffness reduction is shown to be directly proportional to the density of cracks which accumulate in the transverse ply. A model for the transverse ply crack growth during fatigue gives good agreement with the experimentally determined stiffness reduction curves.     

Prediction for progression of transverse cracking in CFRP cross-ply laminates using Monte Carlo method

This study predicted transverse cracking progression in laminates including 90° plies. The refined stress field (RSF) model, which takes into account thermal residual strain for plies including transverse cracks is formulated, and the energy release rate associated with transverse cracking is calculated using this model. For comparison, the energy release rate based on the continuum damage mechanics (CDM) model is formulated. Next, transverse cracking progression in CFRP cross-ply laminates including 90° plies is predicted based on both stress and energy criteria using the Monte Carlo method. The results indicated that the RSF model and the CDM model proposed in this study can predict the experiment results for the relationship between transverse crack density and ply strain in 90° ply. The models presented in this paper can be applied to an arbitrary laminate including 90° plies.     

Numerical investigation of matrix cracking propagation in cross‐ply laminated composites subjected to three‐point bending load using concurrent multiscale model

The purpose of the present study is to analyze fiber‐matrix debonding and induced matrix cracking formation as two major micromechanical damage modes in cross‐ply composite laminates using a two‐dimensional numerical approach. To this aim, the cross‐ply laminates containing 90‐degree layers are modeled, where the fibers are arranged randomly in transverse plies. Damage modes in this numerical model are simulated by the cohesive surface method. The performed analyses reveal that in the laminates with 90‐degree layers located in the outer positions, the primary micro damage mode is micro matrix cracking which is initiated from the fiber‐matrix debonding damage mode and will be followed by matrix cracking. The main benefit of the present study in comparison to other numerical methods is proposing a virtual test method for damage analysis of different cross‐ply laminates in which, the matrix cracking formation will emerge physically in a random and antisymmetric pattern similar to the experimental observations.     

High-temperature mechanical behaviour of multidirectional Nicalon/CAS-II composite

High-temperature mechanical behaviour of Nicalon/CAS-II composite has been investigated. Oxidation of the exposed interfaces along matrix cracks at 1000 °C lowered the longitudinal unidirectional strength to the stress level at which matrix cracking began to occur. The strength of cross-plied composites was also severely reduced in 800 °C air. Transverse plies cracked prior to 0° ply matrix cracking. However, embrittlement did not occur until the matrix in the 0° plies cracked. It was established that oxidation does not take part in crack growth parallel to the fibres, except adjacent to exposed edges. Neither does oxygen enter 90° ply cracks in cross-plied composites in sufficient quantity to produce oxidation embrittlement, at least up to the 0° matrix cracking strain.The work was established while the author was at the Department of Chemical Engineering at Montana State University.     

Matrix cracking behavior of K3B/IM7 composite laminates subject to static and fatigue loading

The matrix cracking behavior of a new high-performance thermoplastic composite material, K3B/IM7, was systematically investigated. Laminates in various grouped thickness and ply stacking sequences, [0₂/90₂/0₂], [0₂/90₄/0₂], and a quasi-isotropic laminate [+45/0/−45/90]_s were tested under static and tension–tension fatigue loading. Depending on the stacking sequence of the laminates and the type of loading, various matrix cracking behavior were found. Under static loading, the matrix cracks were mainly close to the specimen edges. A few cracks were found to penetrate the specimen width, even when the load was large enough to break the specimen. However, under fatigue cyclic load, the edge initiated cracks propagated fully across the specimen width. Combined with the fatigue Paris Rule and considering the ply thickness and stacking sequence, the energy release rate method was applied to predict the relations between the loading strain amplitude and fatigue cycles for matrix cracking failure.