Micromechanical view of failure of the matrix in fibrous composite materials

Many criteria have historically been proposed to predict failures in composites. Some of them were quasi-mimetically taken from metallic materials failure criteria and others were inspired by observation of the appearance of the failure of the composites. Prediction of the failure of the matrix (also called inter-fibre failure) has traditionally been assigned to a certain interaction (typically quadratic) between the components of the stress vector associated to the plane of failure. In this paper, a revision of these proposals is first of all carried out in order to examine in greater depth the implications of some of them. Then, a micromechanical study is conducted considering, based on failure observations, that the mechanism of failure starts with a crack running between the fibre and the matrix. The objective of this micromechanical analysis is to elucidate whether the assumption that the stress vector associated to a plane controls the failure of the plane is physically based. The numerical analysis is performed using the Boundary Element Method, allowing contact between the debonded surfaces of fibre and matrix. Different combinations of loads are applied (perpendicular and parallel to the plane of failure) to check their influence in the energy release rate, which is the fracture parameter evaluated. The results obtained prove numerically that stresses not associated to the macromechanical plane of failure play an important role in the micromechanism of failure of fibrous composites. This fact has been experimentally checked by means of the performance of a series of two dimensional loading tests. (C) 2003 Elsevier Science Ltd. All rights reserved.