Abstract
The stress-ratio effect on the propagation behavior of interlaminar fatigue cracks was studied with unidirectional graphite/epoxy laminates, T800H/#3631, under mixed mode (I+II) loading. Fatigue tests were conducted under the stress ratios of R=0.2 and 0.5 with several mixed-mode ratios, GI/GII, by using the mixed mode bending method. For each R value, the crack propagation rate, da/dN, was higher for higher GI/GII ratios at the same value of the maximum total energy release rate, Gmax. Under a constant value of the GI/GII ratio, both the maximum load and the load amplitude affect the crack propagation rate. For high propagation rates and high GI/GII ratios, the crack propagation rate is mainly controlled by the maximum load, while for low propagation rates and low GI/GII ratios, the contibution of the load amplitude is large. For both R values, the relation between the mode I and II maximum energy release rates, GImax and GIImax, at the fatigue threshold is approximated by the following equation: GImax/GImaxth+GIImax/GIImaxth=1, where GImaxth and GIImaxth are the threshold energy release rates for pure mode I and II loadings and decrease as the R value decreases.
