Nickel-alumina interfacial fracture toughness: Experiments and analysis of residual stress effects

The purpose of the present work is to account for the influence of residual stresses on the measured fracture toughness of a representative metal-ceramic system and, in conjuction with a maximum hoop stress criterion, to explain the observed increase in toughness with increasing mixity of loading. For the sandwich specimen geometry adopted in the current study, a simple argument yields a critical layer thickness below which residual stress effects are expected to be minimized. The measured fracture toughness is found to be independent of thickness for thicknesses below this threshold. For such specimens a general result is demonstrated: compared to the same loading without residual stresses present, the effect of residual stresses is to decrease the magnitude of the phase angle of the stresses which develop along the interface. It is argued that when small-scale yielding conditions hold, both the mixity and the critical hoop stress corresponding to fracture should be reported at a length which falls within the K-dominant region in the sample. In this manner, good quantitative agreement between theory and experiment is demonstrated.