Yield and ultimate strengths of fibre composite laminates

A modified maximum strain failure criterion is proposed to predict the failure of the laminae of fibre-reinforced composites when the laminate is subjected to combined loading. The criterion includes two levels of failure. The lower level predicts the onset of stiffness degradation or ‘yield’ in the lamina. Beyond this level the lamina response is non-linear until the upper lever is reached, which corresponds to ‘ultimate’ failure. After the ultimate failure of a ply in a laminate in a certain mode it is assumed that the failed layer unloads gradually in the same mode only. The theoretical predictions of the present analysis are compared with predictions of other failure theories as well as with experimental data obtained from tests on composite tubular specimens.