Delamination Fatigue Crack Growth in Unidirectional Graphite/Epoxy Laminates

Delamination fatigue crack growth experiments were carried out on unidirectional T300/914 C Graphite/Epoxy laminates. Both mode I and mixed mode (mode I and mode II in combination) situations were investigated for the stress ratios R = 0.1, 0.3 and 0.5. Double Cantilever Beam (DCB) specimens were used for the pure mode I tests and Cracked Lap Shear (CLS) specimens were used for the mixed mode tests. The initial delaminations were produced by inserting 0.03 mm thick PTFE films during laminate layup. The cyclic loading of the specimens was carried out at a frequency of 5 Hz and with a constant stress ratio maintained throughout the test. It was found that the mode I cyclic crack growth rate yielded a power law relationship between da/dN and the cyclic variation in the energy release rate ΔGI. This relation was virtually independent of the stress ratio, at least for 0.1 < R < 0.5. The mixed mode results showed a similar power law relation between da/dN and Δ(G I + GII). Here a small stress ratio dependence was observed. The data were fitted to two alternative power law crack growth relations. Knowing the ratio of GIlGII and with the assumption that the mode I and mode II contributions of crack growth were additive, it was possible to formulate a crack growth relationship of: