Fatigue damage evolution in glass/epoxy cross-ply laminates under spectrum loadings

Fatigue tests were carried out on a glass/epoxy cross-ply laminate under constant amplitude, two-stage repeated blocks and three different types of spectrum loadings. The damage evolution, in terms of transverse crack initiation, propagation and crack density evolution, was analysed. It was found that the linear damage accumulation rule, previously validated for long block loadings, is not suitable for a generic spectrum. Indeed, it leads to an underestimation of the number of cracks and their propagation rate. The results are discussed to identify the main spectrum features influencing the fatigue damage evolution. A theoretical framework is derived to interpret the results and predicting the crack density evolution under spectrum loadings.