Fracture Mechanics Analysis of Composite Materials

Abstract

The present chapter presents an analysis of the effects of cracks and delaminations on the strength of laminated fiber composites using the principles of fracture mechanics. Study of the various failure mechanisms (fiber breaks, matrix cracks, and interface debonds) during a progressively increasing load is not considered in this chapter. First, the strength of composites with through-thickness cracks is studied by fracture mechanics approach. Using effective modulus theory the heterogeneous, anisotropic fiber composite material is replaced by a homogeneous anisotropic elastic material. The equations for the stress components for an anisotropic elastic material are presented and the stain energy release rate is determined. Next, the problem of interlaminar cracking or delamination of laminated composites is studied. If the laminated structure carries bending loads delamination may cause severe loss of its bending strength and stiffness. The same happens in compression since the composite may buckle due to reduction of its bending stiffness. Delamination can occur under all three basic modes of crack growth, that is, under mode I, mode II and mode III or under combinations thereof. In this chapter delamination is studied under mode I, mode II, mode III and mixed-mode I and II loading.