The morphology of thermal cracks in brittle materials

The geometry of indentation cracks propagated by quenching has been studied as a function of the severity of the quench. Practical results are obtained for alumina, silicon carbide whisker reinforced alumina and high speed steel. On quenching the crack growth is higher in the surface direction compared to the depth direction and an algorithm is suggested for calculating the crack depth from the measured surface crack length. The fracture mode in alumina differs between the indentation and the quench part of the crack and it is suggested that the stress intensity at the crack tip is one important parameter that determines the fracture path. When the stress intensity is close to the fracture toughness of the material, the fracture path is predominately intergranular while the fracture path is transgranular for higher levels of stress intensity. In the case of reinforced alumina the fracture path is transgranular through the matrix independent of the stress intensity.