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A new strain energy based failure model is developed for fibrous composite laminates under multi-axial loadings, taking into account the effect of hydrostatic stress. A failure mode dependent exponential stiffness reduction model is used to predict material response beyond the initial failure. Predicted mechanical responses and failure envelopes are presented for the 12 benchmark test cases of Part A of the Second World Wide Failure Exercise. The cases cover a wide variety of isotropic, unidirectional and multidirectional laminates under combined in-plane, out-of-plane and triaxial loadings. Both stress-strain curves and the complete failure envelopes were successfully predicted. In some instances, the failure envelopes were open. The predictions together with suitable adjustment of certain parameters are compared with test data in Part B of the Second World Wide Failure Exercise, to be published in Journal of Composite Materials.

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An analytical model to predict the constitutive response, failure initiation and post-initial failure behavior of fibrous composites under triaxial loading has been developed based on a strain energy formulation. This paper highlights the means of developing several key parameters in the model and extends the model to include the effects of in-plane thermal residual stresses accumulated during the curing of composite laminates. The derivations of a set of consistent model input parameters for several composite material systems are described. A series of comparative stress-strain curves and failure envelopes are provided comparing model predictions to published experimental data. The strain energy based failure model is found to provide good agreement with the experimental observations studied in this work.