This paper investigates the roles Mode I (opening mode) and Mode II (inplane shear) strain-energy release rates (GI and GII, respectively), play in inducing delamination growth under static and fatigue loading. Double cantilever beam (DCB) specimens were used for pure Mode I tests. Cracked lap shear (CLS) specimens were used for mixed-mode tests. All specimens were fabricated using T300/5208 graphite/epoxy. Delaminations were introduced during fabrication by inserting folded Kapton films between selected plies. Delaminations were located between two 0° plies in an attempt to inhibit delamination growth across plies into adjacent interfaces. The critical Mode I strain-energy release rate, GIc, was obtained from static DCB tests. Static tests on mixed-mode CLS specimens measured the total strain-energy release rate, which was broken into GI and GII components using a geometrically nonlinear finite-element analysis. By incorporating these values of GI and GII, and GIc from static DCB test results, into an assumed failure criterion, the critical Mode II strain energy release rate (GIIc) was computed.
Fatigue-induced delamination growth was characterized by conducting constant-amplitude fatigue tests at a minimum to maximum cyclic load ratio (R) of 0.05 and a frequency (ω) of 10 Hz. During the tests, the maximum and minimum strain-energy release rates (Gmax, Gmin) and the delamination growth rate (da/dN) were monitored. Fatigue test results on DCB specimens yielded a power-law relationship between da/dN and GImax. Similar results from CLS tests provided da/dN versus Gmax relationships. The contributions of GI and GII components to mixed-mode delamination growth were assumed to be additive. Hence, the power law for a pure Mode II delamination growth was derived from CLS test results by subtracting the contribution due to GI determined from DCB tests.
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