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.
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