Observations on the crack-enhanced creep-fracture of a polycrystalline alumina with a glassy grain-boundary phase

Abstract

An experimental study was conducted of the role of cracks in the creep-rupture behaviour of a polycrystalline alumina with glassy grain-boundary phase at stress regimes and temperatures at which failure is anticipated to occur from pre-existing flaws. Samples were tested without and with artificial flaws in the form of slots or indentation cracks. All three sample types exhibited non-linear creep. The relative rate of increase in creep rate with increasing dimension of slot-depth greatly exceeded the corresponding increase in specimen compliance, as expected for non-linear creep. An analysis of the data indicated that the observed creep behaviour was primarily controlled by crack-enhanced creep with a minor contribution from elastic creep by crack growth. Very poor correlations were found for the stress dependence of the creep rate and time-to-failure. In terms of Monkman-Grant behaviour, good correlations existed between creep rate and time-to-failure, independent of stress and size of the slots or indentation cracks. Because failure originated from pre-existing flaws, the experimental findings of this study suggest the existence of a failure mechanism referred to by the present authors as “crack-enhanced creep fracture”.