Fatigue of composite materials under spectrum loading

A method to predict fatigue of composites under spectrum loading is proposed. It is based on a recently developed approach for constant amplitude loading where the cycles to failure are determined using the cycle-by-cycle probability of failure under a given loading. Failure under spectrum loading occurs when the residual strength equals the applied maximum (or minimum) stress. Closed-form expressions are obtained and compared to Miner’s rule. It is shown that the present method results in a damage law which, depending on the case, can sum to more or less than unity unlike Miner’s rule which always sums up to unity. Comparison of the predictions with test results show very good agreement with minor exceptions that are discussed. The present method does not need any semi-empirical curve-fitting. All the parameters used in the model can be derived from the characteristics of the static strength distribution of the structure.