Damage evolution in cross-ply laminates under variable amplitude cyclic loadings

Predicting the initiation and propagation of multiple off-axis cracks in multidirectional laminates under cyclic loadings is essential in a stiffness-driven design approach. Even under a constant amplitude cyclic load, the multiple crack initiation represents always an inherently variable amplitude (VA) problem. Indeed, the initiation of cracks causes a stress re-distribution so that each point in a laminate is subjected to a stress state that changes continuously during the fatigue life. At present, no models or experimental evidences on the crack initiation phenomenon under VA loadings are available in the literature. Crack density prediction models usually rely on a simple linear damage accumulation rule, even if its validity has not been proved yet. In this work, two types of fatigue tests were carried out on glass/epoxy cross-ply laminates under VA two-block loadings: 1) Initially, the number of cycles in the first block was chosen low enough to prevent the initiation of transverse cracks in the first block; then the load was changed and the crack initiation phenomenon was characterized in the second block. 2) Then, two block loadings were applied on other specimens, with a high enough number of cycles in the first block to promote the initiation of multiple cracks; the crack density evolution was thus characterized in both blocks. A model recently developed by the authors was applied to the experimental data, revealing the suitability of the linear damage accumulation rule under block loadings, at least from a phenomenological point of view.