Evaluation of Failure Criteria for Fiber Composites Using Finite Element Micromechanics

Abstract

A micromechanical analysis of the unit cell of a unidirectional composite is performed using the finite element method. The circular fibers are assumed to be packed in a periodic square array. Assuming that the failure criteria for the fiber and matrix materials and also for the fiber-matrix interface are known, the failure envelope of the composite is developed using the microstresses computed in the unit cell analysis. This method is referred to as the Direct Micromechanics Method (DMM). The micromechanical methods were also used to simulate different tests to determine the strength coefficients in phenomenological failure criteria such as maximum stress, maximum strain and Tsai-Wu theories. The failure envelopes from the phenomenological failure criteria are compared with those of the DMM for the cases of biaxial and off-axis loading of a model unidirectional composite material. It is found that none of the phenomenological criteria compare well with the DMM in the entire range. A conservative failure envelope obtained using a combination of maximum stress and Tsai-Wu criteria seems to be the best choice for predicting the failure of unidirectional fiber composites.