Mixed mode delamination growth of multidirectional composite laminates under fatigue loading

Fatigue delamination growth of multidirectional laminates with 45°/−45° fracture interface was experimentally studied. Delamination tests under three mixed I/II mode ratios were carried out via mixed mode bending test apparatus. Significant R-curve effects on the fatigue crack growth rates and thresholds due to fiber bridging and crack branching were observed, which make the traditional Paris Law unsuitable. In this paper, a formerly developed modified Paris Law with the normalized strain energy release rate as the fracture governing parameter is investigated again. The results from the multidirectional delamination tests fit the modified Paris Law very well, and hence have validated it by experiments. Moreover, the thresholds normalized to delamination resistance were utilized to evaluate the critical fatigue crack growth conditions. By the combination of test data for both 0°/5° and 90°/90° laminates, an interface-independent normalized threshold was proposed, which was found linearly decreasing with mode mixture ratios.