Interlaminar crack initiation and growth rate in a carbon-fibre epoxy composite under mode-I fatigue loading

In this paper, interlaminar crack initiation and propagation under mode-I loading of a composite material made of an epoxy matrix reinforced with unidirectional carbon fibres are experimentally assessed for different asymmetry ratios. In the experimental program, DCB specimens were tested and the number of cycles to failure necessary for the onset of crack growth was registered for the asymmetry ratios R = 0.2 and R = 0.5. The G–N curves under fatigue loading were determined, N being the number of cycles at which delamination commences for a given energy release rate. The ΔG–da/dN curves associated with the previously generated crack growth were also determined for different Gcr rates (90%, 80%, 70%, etc.) and two different asymmetry ratios.For the asymmetry ratios, the fatigue limits show values in the order of 50% and 70%, respectively, of the critical fracture energy resulting from the static tests employing the modified beam theory. The fatigue limit is discretionally taken as the fatigue strength corresponding to 2.5 million cycles, i.e., the limit number of cycles considered for testing. The micrographs show that the morphology of the fracture toughness under mode-I loading are quantitatively but not qualitatively influenced by the stress ratios.