A new criterion for composite material mixed mode fracture analysis

Abstract

The strain energy density factor or S criterion has been widely used and our theoretical, numerical and experimental analysis indicates that it can be successfully applied to explain the cracking behavior of a homogeneous, isotropic material. In this paper we have investigated the application of a variety of energy based fracture criterion for fiber composites. A new hypothesis called the Z criterion is proposed for the prediction of crack initiation conditions and propagation direction. This new criterion suggests that for composite materials, mode I crack initiation is controlled by the dilatational strain energy density factor, mode II crack initiation is controlled by the distortional strain energy density factor and for mixed mode crack initiation, by the damage energy density factor (Z factor). Analytical equations have been derived to express the crack tip stress fields as well as the total, dilatational and distortional strain energy density factors and the Z factor for a composite material in which the crack direction is arbitrary with respect to the unidirectional fibers.