CYCLIC FATIGUE OF LONG AND SHORT CRACKS IN ALUMINA

The cyclic fatigue behaviour of long and microstructurally short crack s in a 10 mu m grain-size alumina has been investigated. This material was found to be stress sensitive, a modest drop in applied stress res ulting in a considerable lifetime enhancement. The growth of long crac ks was studied using the circular compact tension geometry and was fou nd to follow a Paris law behaviour. The crack path was entirely interg ranular in this material with long fatigue crack growth governed by th e degradation of crack-wake bridging. Short-crack growth was investiga ted using indented discs in a biaxial flexure geometry. Short cracks w ere observed to grow at lower values of applied Delta K than long crac ks, increasing with crack length as bridging of the crack wake increas ed. The fatigue crack growth of AD90 alumina was also investigated by in situ testing within the specimen chamber of an SEM. The long-crack behaviour was found to be similar to the 10 mu m grain-size alumina an d other data reported in the literature. However, the crack path follo wed a mixture of transgranular and intergranular fracture and disconti nuous in nature with frequent arrests. The crack advancement mechanism s in these two alumina materials are different and affect the short-cr ack behaviour. However, in both cases the long-crack behaviour is domi nated by crack-wake effects.