Mode II fatigue delamination behaviour of composite multidirectional laminates and the stress ratio effect

Shear-dominated delamination is among one of the most common damages types. Deep understanding of the mode II fatigue delamination growth (FDG) behaviour is critically important for the damage tolerance design of composite laminates. Researchers have paid much attention to mode II FDG behaviour in unidirectional laminates and its stress ratio effect. However, the FDG behaviour in the widely used multidirectional laminates with effects of fiber bridging is still lack of studies. This work investigates FDG behaviour of multidirectional laminates under three kinds of stress ratio by using end-notch flexure set-up. Corrected beam theory with effective crack length is applied for calculating strain energy release rate (SERR), which can avoid visual observation for the delamination length. The R-curve behaviour of fatigue delamination has been observed. In addition, a FDG model has been proposed to characterize the combined effects of fiber bridging and stress ratio on the mode II FDG behaviour. It is found that all fatigue data collapse into one single master curve. The proposed model is also validated by data from public literature results.