Towards a Rational Failure Criterion for Unidirectional Composite Laminae

A failure-mechanism-based micromechanical theory has been proposed for the development of a failure envelope for unidirectional composite plies. A representative volume element of the laminate under local loading is micromechanically modeled to predict experimentally determined strengths, and this model is then used to predict points on the failure envelope in the neighborhood of the experimental points. The NISA finite element software has been used to determine the stresses in the representative volume element. From these microstresses, the strength of the lamina is predicted. A correction factor is used to match the prediction of the present model with the experimentally determined strength so that the model can be expected to provide accurate prediction of the strength in the neighborhood of the experimental points. A procedure for the construction of the failure envelope in the stress space has been outlined and the results are compared with the some of the standard and widely used failure criteria in the composite industry. Comparison of results with the Tsai–Wu criterion shows that there are significant differences, particularly in the third quadrant, when the ply is under biaxial compressive loading; Comparison with the maximum stress criterion indicates better correlation. The present failure-mechanism-based micromechanical approach opens a new possibility of constructing reliable failure envelopes for biaxial loading applications using the standard uniaxial test data.