The applicability of fracture mechanics to conventional isotropic materials has been well demonstrated. For fiber-reinforced materials, however, fracture mechanics investigations have met with mixed results. For collinear crack extension of a crack on a plane of symmetry in orthotropic materials, fracture behavior has been successfully predicted using linear elastic fracture mechanics concepts; however, extrapolation to other fiber orientation combinations has been less successful. This is because fracture in anisotropic materials is more complex than in isotropic materials and is governed by additional parameters such as fiber orientation, lamination order and the constitutive relations that describe the mechanical responses of the fiber, the matrix and the interface. This paper reviews the fundamentals of fracture mechanics for isotropic materials and discusses its extension to orthotropic materials. This is followed by a discussion of the variety of failure modes observed in composites and a review of the predominant fracture mechanics theories that attempt to predict fracture using semi-empirical parameters. The main aim of this paper is to present a survey of the field in its current state and to demonstrate the approach taken by each investigator. The paper concludes with two examples where linear elastic fracture mechanics concepts have been satisfactorily employed for composite materials.

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