Fracture of metal/ceramic laminates—I. Transition from single to multiple cracking

Abstract

The transition from single crack extension to multiple laminate cracking near the crack tip has been studied over a range of metal/ceramic thickness ratios ( t m/ t c), in three metal/ceramic laminate systems: Al/alumina, Cu/alumina and Ni/alumina. Multiple cracking occurs above a critical value of t m/ t c, with the critical value increasing with decreasing ratio of equivalent constrained metal yield stress to mean ceramic fracture stress, σ e/ σ c. These results are shown to be inconsistent with the predictions of a linear elastic fracture mechanics (LEFM) model, which considers a cracked laminate as a bridged monolithic crack, for the lower metal yield strength Cu/alumina and Al/alumina laminates. A modified model considers a limiting case where the stress distribution in the ceramic layers in the crack wake is determined by shear-lag below a critical value of the ratio σ e/ σ c. This combined model is shown to predict more accurately the transition from single to multiple cracking in the three laminate types and also to predict the ceramic layer crack spacing in the multiple cracked specimens.