Fracture Phenomena in Advanced Fiber Composite Materials

The extension of linear elastic fracture mechanics (LEFM) from metallic alloys to advanced fiber composite laminates is considered. LEFM is shown to be applicable to both isotropic and anisotropic homogeneous continua; the applicability of LEFM to advanced fiber composites is thus dependent on the validity of a homogeneous continuum model for such materials. An experimental program to determine the validity of such a model for graphite/epoxy laminates is reviewed. Such laminates are found to have an apparent fracture toughness, from which it is inferred that a homogeneous material model is appropriate for the particular specimen geometry and composite laminates considered. Strain energy release rates are calculated from the experimentally determined fracture toughnesses of the test laminates. These strain energy release rates fall into two distinct groups, corresponding to fracture across fibers and fracture between fibers. The latter case is further investigated. It is concluded that matrix fracture is governed by the tensile stress normal to the crack path.