Growth of internal delaminations under cyclic compression in composite plates

Abstract

The growth of internal delaminations in composite plates subjected to cyclic compression is investigated. Due to the compressive loading, these structures undergo repeated buckling-unloading of the delaminated layer with a resulting reduction of the interlayer resistance. An important characteristic of the problem is that the state of stress near the delamination tip is of mixed mode, I and II. Equations describing the growth of the delaminations under cyclic loads are obtained on the basis of a combined delamination buckling-post-buckling and fracture mechanics model. The latter is based on a mode-dependent critical fracture energy concept and is expressed in terms of the spread in the energy release rate in the pre- and post-buckling state. It is shown that such a model allows for the accumulation of microdamage at the delamination front. The growth laws developed in this manner are integrated numerically, in order to produce the delamination growth vs number of cycles curves. Furthermore, the investigation includes the possibility of unstable delamination growth. The study does not impose any restrictive assumptions regarding the delamination thickness and plate length (as opposed to the usual thin film assumptions). The results show that for a given value of delamination thickness h, the fatigue delamination growth is strongly affected by the relative location of the delamination through the plate thickness T, the fatigue growth being slower for a smaller value of h T (delaminations located closer to the surface). These theoretical predictions are confirmed by experimental results that are obtained for the growth of delaminations in graphite-epoxy unidirectional specimens under cyclic constant amplitude compressive loading. The test data, which were obtained for several different locations of the delamination through the thickness (hence different degrees of mode mixity), and different applied maximum compressive displacement, seem to be well-correlated with the theory.