Mode II constant amplitude fatigue delamination propagation tests were conducted at three different stress ratios, viz. R = 0.0, 0.5, and −1.0 using modified three-point bend test fixture and end notched flexure (ENF) test specimens of unidirectional IMA/M21 carbon fiber composite. Delamination length was measured by compliance technique. The delamination propagation rate (da/dN) plots were constructed for various stress ratios. Using an equivalent energy release rate concept, Geq, all the curves were merged into a single curve in the form of Paris equation, da/dN = C(Geq)k. The Paris constants, C and k, determined from the experimental results were subsequently used in predicting the delamination propagation life under a standard mini FALSTAFF spectrum load sequence. Experiments were also conducted under the same spectrum load sequence with various reference loads to determine the propagation lives. A reasonably good correlation was observed between the predicted and experimental mode II delamination propagation life under spectrum loads.
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