Fatigue delamination growth under cyclic compression in unidirectional composites

Fatigue delamination growth under cyclic compression in composite plates is investigated. Experimental work in this regard is performed on two material systems namely graphite/epoxy and glass/epoxy. The delaminations are implanted at several different locations through the thickness (hence different degrees of mode mixity) and different levels of cyclic displacement are applied for fatigue testing. Due to this compressive loading these structures undergo repeated buckling/unloading of the delaminated layer with a resulting reduction of the interlayer resistance. A proposed model for delamination growth is formulated on the basis of a combined delamination buckling and fracture mechanics analysis. The state of stress near the delamination tip is of mixed mode and hence, a mode dependent critical energy release rate concept is used. The growth laws developed in this manner are integrated numerically in order to produce the delamination growth vs number of cycles curve. This study does not impose any restrictions regarding the delamination thickness or the plate length. The experimental results show that the delaminations that would not grow statically, indeed grow after a sufficient number of cycles and the fatigue growth rate is slower for the delaminations of smaller h/T values (delaminations closer to surface). The experimental results obtained for the growth of delamination in graphite/epoxy and glass/epoxy unidirectional specimens under cyclic constant amplitude correlate adequately with the proposed growth laws. The detailed experimental frame work, results and their correlation with the proposed growth laws, important conclusions and recommendations for future work on the subject are presented.