A strength-based wearout model for predicting the life of composite structures

A model to predict the residual strength and life of polymeric composite structures subjected to spectrum fatigue loadings is described. The model is based on the fundamental assumptions that the structure undergoes proportional loading, that the residual strength is a monotonically decreasing function of the number of fatigue cycles, and that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary load history may be represented by two parameter Weibull functions. The model also incorporates a "cycle mix factor" to account for the drastic reduction of fatigue life that may be caused by a large number of changes in the stress amplitude of the loading. The model's predictions are compared to experimentally determined fatigue life distributions for uniaxial loadings of a number of laminates comprised of different materials and layups. Constant-amplitude, two-stress level, and spectrum fatigue loadings, including the FALSTAFF (Fighter Aircraft Loading STAndard For Fatigue) spectrum, are considered. The theoretical fatigue life distributions are shown to correlate well with the experimental results. Moreover, excellent correlation of theory and experiment is obtained for an "average fatigue life" that is based on the 63.2% probability of failure.