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This article introduces an iterative model to predict the fatigue behavior of fiber reinforced laminated composites under general loading conditions. The model simulates the damage progress and subsequent material property degradation of composite laminates and predicts their fatigue life by using a three-stage procedure. In the first stage, state of stresses in the whole component is specified. Then, multi-axial stresses of each element are utilized to damage assessment by using a newly developed damage estimation technique. Considering each element as an orthotropic material, stiffness degradation due to the induced damage is calculated based on two sets of material property degradation rules. For this aim, a generalized approach is used for gradual and sudden material property degradation. In order to evaluate the model, a macro program is developed that simulates the fatigue behavior of unidirectional and multidirectional composite laminates. The results of the model are consistent with available experimental data. Using this program, some fatigue life predictions are also accomplished for laminates with arbitrary stacking sequences.

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The theory of photoelasticity is used to study analytically the effects of T-stress on the fringe patterns around the crack tip in mode II crack specimens. The locus of an isochromatic fringe determined by taking into account the T-stress is compared with the locus of a fringe with no T-stress. It is shown for mode II cracks that in the presence of T-stress, the fringe loops are neither symmetric nor continuous. Asymmetric and discontinuous fringe patterns predicted in this paper are consistent with the experimental results observed previously in photoelasticity tests.