High-Temperature Behavior of Indent and Creep-Nucleated Cracks in Vitreous-Bonded Alumina

Crack behavior was studied at elevated temperatures in a commercial vitreous-bonded alumina for two types of cracks: one introduced by indentation at room temperature and the other by the creep process. Indentation cracks with relatively small initial size grew progressively longer during creep before they became blunt and arrested; however, they continued to widen throughout the creep process. Larger indentation cracks under high stress condition continued to grow until failure. The evolution of creep-nucleated cracks was so fast that they were observed only in their arrested state. Once observed, their length remained essentially constant, but they did grow in width. The crack-opening displacement rates of both types of cracks were linearly related to the creep rate as predicted by fracture mechanics for stationary cracks. All but the specimens with the largest indentation crack exhibited flaw tolerance in that they failed by the coalescence of creep-nucleated cracks instead of the growth of a single crack. The results illustrate the crack behavior in the brittle-to-ductile transition regime for ceramics that deform by grain boundary sliding.