Residual stiffness in cross-ply laminates subjected to cyclic loading

A new approach has been developed to evaluate the residual stiffness of cross-ply laminates under tensile fatigue loading. In the present research, a stiffness degradation model is generalized by using a new fatigue life measure and shear-lag analyses to improve correlation at low fatigue cycles and high stress states. The developed model is integrated into a progressive approach to predict the transverse stiffness degradation of 30° unidirectional plies made of AS4/3501-6 carbon/epoxy under both constant and variable amplitude cyclic tensile loading. Also, the stress history in 0° and 90° plies and transverse stiffness degradation in 90° plies of a cross-ply laminate are predicted by using the developed model. The generalized model, in contrast to original one, is capable of simulating stiffness reduction under high stress states where failure occurs in transverse plies of a cross-ply laminate during the first cycles of the fatigue loading. Predicted results by the model show a good correlation with the experimental data.