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One of the oldest and most firmly established concepts associated with the fracture of uniform homogeneous materials is the idea that strain energy release rate can be used to characterize the “driving force” for flaw growth, that is, the tendency for propagation. For linear elastic treatments of self-similar crack propagation the concept can be shown to be equivalent to the single parameter field stress intensity representation. The inhomogeneity and nonuniformity of composite materials and the consequent complexity of damage makes a direct extension of that concept to damage development in those materials difficult, if not impossible. Progress in that direction has been hampered by difficulty in defining the stress state in a damaged composite, and by inadequate laboratory information concerning the precise physical nature of the damage state.
In the present work these two difficulties were approached, respectively, by using a three-dimensional finite element scheme to generate internal stress field information and by making extensive scanning electron microscope studies of damage in two composite laminates. The paper reports the calculation of energy release rates for debonding, delamination and fiber breakage, and subsequent predictions of damage modes for two material systems. Correlation with experimental observations is good. The results also identify the influence of such parameters as fiber-matrix stiffness ratio on fracture mode, and the prediction of most probable debond sites.
composite materials, failure modes, strain energy, fracture mechanics, fatigue (materials), crack propagation
Senior Research Engineer, Babcock and Wilcox, Lynchburg Research Center, Lynchburg, Va.
Chairman, Virginia Polytechnic Institute and State University, Blacksburg, Va.