Finite Fracture Mechanics of Matrix Microcracking in Composites

The first form of damage in composite laminates is often matrix microcracks that are defined as intralaminar or ply cracks across the thickness of the ply and parallel to the fibers in that ply (see reviews in [1,2]). Tensile loading, fatigue loading, environment, and thermal cycling can all lead to microcrack formation. Microcracks can form in any ply that has a significant component of the applied load transverse to the fibers in that ply. Microcracks lead to degradation in properties of the laminate including changes in effective moduli, Poisson ratios, and thermal expansion coefficients [3]. Although these changes are sometimes small, microcracks can nucleate other forms of damage. For examples, microcracks can lead to delaminations, cause fiber breaks, and provide pathways for entry of corrosive liquids. An important issue in design of composite laminates is to be able to predict the initiation and development of microcracking damage following complex loading conditions. This chapter describes a fracture mechanics approach to the microcracking problem.