CYCLIC FATIGUE IN MONOLITHIC ALUMINA - MECHANISMS FOR CRACK ADVANCE PROMOTED BY FRICTIONAL WEAR OF GRAIN BRIDGES

The microstructural basis of cyclic fatigue-crack propagation in monol ithic alumina has been investigated experimentally and theoretically. A true cyclic fatigue effect has been verified, distinct from environm entally assisted slow crack growth (static fatigue). Microstructures w ith smaller grain sizes were found to promote faster crack-growth rate s; growth rates were also increased at higher load ratios (i.e. ratio of minimum to maximum applied loads). Using in situ crack-path analysi s performed on a tensile loading stage mounted in the scanning electro n microscope, grain bridging was observed to be the primary source of toughening by crack-tip shielding. In fact, crack advance under cyclic fatigue appeared to result from a decrease in the shielding capacity of these bridges commensurate with oscillatory loading. It is proposed that the primary source of this degradation is frictional wear at the boundaries of the bridging grains, consistent with recently proposed bridging/degradation models, and as seen via fractographic and in situ analyses; specifically, load versus crack-opening-displacement hyster esis loops can be measured and related to the irreversible energy loss es corresponding to this phenomenon.