The purpose of this study was to gain a better understanding of the parameters affecting crack growth direction in unidirectional comµosite materials. To achieve this, the effect of anisotropy and biaxial loading on the direction of crack growth in unidirectional off-axis composite materials were investigated. Specific emphasis was placed on defining the crack tip stress field and finding a consistent criterion for predicting the direction of crack growth.

Two models are presented to predict the crack tip stress field, an anisotropic elasticity solution and a singular isoparametric finite element formulation. After defining the crack tip stress field, three crack extension direction criteria, the Normal Stress Ratio, the Tensor Polynomial and the Strain Energy Density Criterion, were applied to predict the direction of crack extension.

The theoretically predicted crack extension directions were then compared with experimental results. After comparison, it was determined that only the Normal Stress Ratio Criterion correctly predicts the direction of crack extension.