CYCLIC FATIGUE IN CERAMICS - A BALANCE BETWEEN CRACK SHIELDING ACCUMULATION AND DEGRADATION

Cyclic fatigue growth rates in R-curve ceramics have been observed to depend very strongly on the maximum applied stress intensity, K-max, and only weakly on the stress intensity range, Delta K. This behavior is rationalized through measurement of crack wake shielding characteristics as a function of these fatigue parameters in a gas-pressure-sintered silicon nitride. In particular, evidence for a mechanical equilibrium between shielding accumulation by crack growth and shielding degradation by frictional wear of sliding interfaces is found for steady-state cyclic fatigue. This equilibrium gives rise to a rate law for cyclic fatigue. The data suggest that the accumulation process is the origin of the strong K-max dependence, and that the degradation process is the origin of the weak Delta K dependence, These features are shown to be related to the ''cyclic'' R-curve and to the cyclic crack opening displacement, respectively.