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 res idual stresses on the measured fracture toughness of a representative metal-ceramic system and, in conjuction with a maximum hoop stress cri terion, to explain the observed increase in toughness with increasing mixity of loading. For the sandwich specimen geometry adopted in the c urrent study, a simple argument yields a critical layer thickness belo w which residual stress effects are expected to be minimized. The meas ured fracture toughness is found to be independent of thickness for th icknesses 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 b e reported at a length which falls within the K-dominant region in the sample. In this manner, good quantitative agreement between theory an d experiment is demonstrated.