CYCLIC FATIGUE OF CERAMIC MATERIALS - INFLUENCE OF CRACK PATH AND FATIGUE MECHANISMS

Crack propagation under cyclic loading was investigated in non-transforming ceramic materials, such as silicon carbide, silicon nitride and sialon, with different microstructures produced by various processes. Cyclic fatigue crack growth was observed in the intergranular-fracture type materials and never seen in the transgranular-fracture type ones, irrespectively of the kind of materials. The occurrence of intergranular cracking causes (i) microscale-crack branching or crack-path deflection, (ii) grain-particles debonding from the matrix and (iii) production of crack surface-asperities. These have actually been observed in experiments. On the basis of these observations and other data available, the most possible fatigue mechanism for non-transforming ceramics has been proposed. It is based on the consideration that the crack resistance caused by microcrack branching or crack-path deflection during tensile loading and the crack reactivation due to the asperity-contacts during unloading occur repeatedly. These processes result in continuous crack growth under cyclic loading conditions.