Effect of loading parameters on fatigue damage development in notched composite laminates

The long term mechanical fatigue of notched AS4/3501-6 graphite/epoxy quasi-isotropic laminates is investigated to determine the influence of loading parameters on damage growth during compression-dominated fatigue loading. The effects of load type, load level, load sequence, and the effect of load spectrum modification were evaluated in this study. Fatigue damage accumulation in form of splitting and delamination growth around the hole was monitored using X-ray radiography. Experimental results indicate that growth of split-induced delamination in constant amplitude tension-compression (T-C) fatigue occurred mostly between and along the splits in a narrow band. In compression-compression (C-C) loading the mode of damage growth changed from splining to widthwise delamination extension and to fiber failure in load bearing plies depending on the fatigue stress level. The resulting residual compressive strength was strongly influenced by the type of damage. Thus, it was difficult to identify a unique damage parameter that could be correlated to changes in compressive strength. Effects of modification of the compression dominated spectrum loading indicate that omission of low load levels can significantly reduce testing time without large influence on damage propagation.