Prediction of fatigue damage and life for composite laminates under service loading spectra

A residual stiffness degradation model, which has been proved to be capable of estimating the cumulative damage in composite laminates under constant amplitude loading, is used to investigate the fatigue damage of composite laminates subjected to service loading spectra. Baseline stiffness reduction data under spectrum loadings ave incorporated in this model to predict the statistical distributions of the residual stiffness as well as the residual stiffness of an individual specimen. An empirical failure criterion is proposed and fatigue life distributions for composite laminates subjected to different levels (or severities) of spectrum loadings are theoretically derived. Experimental tests were conducted on [0 degrees/90 degrees/+/-45 degrees](2s) graphite/epoxy laminates to generate statistically meaningful data for the verification of the theoretical approaches presented. Experimental test results demonstrate that the predictive capabilities of the proposed theoretical methods are quite satisfactory. (C) 1996 Elsevier Science Limited