MECHANISMS OF CYCLIC FATIGUE-CRACK PROPAGATION IN A FINE-GRAINED ALUMINA CERAMIC - THE ROLE OF CRACK CLOSURE

Cyclic fatigue-crack growth and resistance-curve behavior have been st udied in a fine-grained (similar to 1 mu m), high-purity alumina. Spec ific emphasis is given to the mechanisms associated with crack growth that are controlled by the maximum (K-max) and the alternating (Delta K), stress intensities and to the role of crack-face interference (cra ck closure), which is known to be an important crack-tip shielding mec hanism in metal fatigue. Significant levels of subcritical crack growt h were detected above a threshold stress intensity of similar to 60% o f the fracture toughness (K-c) in the alumina, with growth rates displ aying a far larger dependence on K-max compared to Delta K. The role o f crack closure was examined using constant-K-max experiments, where t he minimum stress intensity (K-min) was maintained either above or bel ow the stress intensity for crack closure (K-cl). Where K-min < K-cl, growth rates were found to exhibit a lower dependence on Delta K, whic h was rationalized in terms of the frictional wear model for crack gro wth in grain-bridging ceramics. It is concluded that crack closure, as conventionally defined, has little relevance as a crack-tip shielding mechanism during fatigue-crack growth in grain-bridging ceramics, due to the low dependence of growth rates on Delta K compared to K-max.