Intergranular crack nuclei in polycrystalline alumina

Digital image correlation has been applied to optical images in order to detect crack nuclei in fine-grain size pure and Cr-doped aluminas. Individual intergranular crack nuclei within areas comprising tens of thousands of grains were detectable, and their crack lengths and opening strains obtained as a function of load prior to unstable fracture. Identification of the crack nuclei by this method allowed the grain boundary plane and grain orientations in the vicinity of crack nuclei to be characterised by electron microscopy. Crack nuclei were found to develop at boundaries that are predicted to have higher tensile thermal strains, caused by the orientation of the grain boundary plane relative to the adjacent grains.