Modeling the Effect of Damage in Composite Structures: Simplified Approaches

Chapter 3Cracks

3.1 Introduction

In metals, damage, especially under cyclic loading, manifests itself as a crack. In composites, cracks appear only as a result of specific threats such as impact with sharp objects that manage to penetrate the entire laminate. Usually, damage in composites manifests itself as a combination of matrix cracks, delaminations and fibre breakage that does not form a well-defined through-the-thickness crack. As a result, cracks are relatively rare in composite structures but, nevertheless, quite important because of their effect on residual strength.

The bi-material nature of a composite laminate and the fact that it consists of layers with fibres in different directions do not promote the creation of a through-the-thickness crack. In addition, once such a crack is created, they do not allow the more ‘classical’ crack behaviour that is observed in metals. If the crack in a composite laminate grows under load, it usually does not do so in a self-similar manner. It follows a ‘jagged’ path defined and constrained by the fibre orientations in the different plies. In general, fibres act as crack stoppers. The matrix having low toughness allows the crack to grow easily until it reaches a fibre that slowdowns the crack or stops it altogether. This is the reason why matrix cracks in-between the fibres of a ply, for example, the 90° plies in a [0/90/0] laminate, are confined by the cross-fibres in adjacent plies. This is also the reason that cracks forming ...