Crack propagation behavior of sintered silicon nitride under cyclic load of high frequency and high stress ratio.

In order to clarify the effect of vibratory stress of high stress ratio on the crack propagation of sintered silicon nitride, the crack propagation rate was measured on its compact tension specimen. The main results obtained are as follows.(1) Under the cyclic load with a stress ratio smaller than 0.92, the crack propagation rate was accelerated by the cyclic load compared to the crack propagation rate under the static load. The cyclic load with a stress ratio larger than 0.92 produced the same crack propagation rate as that under static load.(2) In case of KImax smaller than KISCC, the crack propagated mainly cycle-dependently. Therefore, care must be taken in case of the fatigue design of ceramics under the vibrational stress. On the other hand, the crack propagated mainly time-dependently when KImax was larger than KISCC.(3) Debris induced crack-closure was observed under the cyclic load even with a large stress ratio of 0.8.(4) In contrast to the two different behaviors of metal under cyclic SCC and under dynamic SCC, the crack propagation behavior of sintered silicon nitride under a cyclic load of high stress ratio and high frequency was same as that under a cyclic load of low stress ratio and low frequency.