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, Δ 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 Δ dependence. These features are shown to be related to the "cyclic" R -curve and to the cyclic crack opening displacement, respectively.