Nucieation and Growth of Cracks in Vitreous‐Bonded Aluminum Oxide at Elevated Temperatures

The nucleation and growth of cracks was studied at elevated temperatures on a grade of vitreous‐bonded aluminum oxide that contained ∼8 vol% glass at the grain boundaries. Cracks were observed to nucleate within the vitreous phase, close to the tensile surface of the flexural test specimens used in these experiments. Crack nucleation occurred at a strain of ∼0.08% to 0.12% which corresponded to a crack nucleation time of ∼35% of the time to failure by creep rupture. Once nucleated, cracks propagated along grain boundaries, as long as the stress for crack propagation was maintained. The crack velocity for cracks that were nucleated by the creep process was found to be linearly proportional to the apparent stress intensity factor, whereas for cracks that were nucleated by indentation, the crack velocity was proportional to the fourth power of the apparent stress intensity factor.