Fracture of metal/ceramic laminates - II. Crack growth resistance and toughness

The propagation of cracks has been studied through Al/alumina, Cu/alumina and Ni/alumina laminates, with a range of metal and ceramic thicknesses. Two specimen orientations were chosen to propagate a crack either through the metal and ceramic layers in parallel or through the layers sequentially and perpendicular to the laminate. In both orientations a rising T-curve (toughness) was found with increasing crack length with a very steeply rising T-curve at large crack extensions. These observations can be explained by a large-scale bridging model with a crack closure traction imposed by the plastically deforming metal ligaments in the wake of the propagating crack. A simple model is derived which predicts the performance of the Ni/alumina laminates well. It does not predict the behaviour of the lower metal yield strength Cu/alumina and Al/alumina laminates so well, although it does account for the general trend of their behaviour.