抄録
In order to make it clear whether the cycle-dependent crack propagation would occur in ceramic materials under a cyclic load, the crack propagation rate was measured using compact tension specimens made of sintered Si3N4 at room temperature under ambient condition. In this study, the crack propagation rate da/dt, which ranged from 10-10m/sec to 5×10-4m/sec, was measured under static loads and cyclic loads in the form of square wave. In the case of cyclic loads, the hold time at the maximum and minimum loads was equal. The crack propagation rate da/dt was equal to 2f×da/dN, where da/dN is given in m/cycle and f, in Hz.
Crack propagation tests were carried out at 1.1Hz and at various stress ratio R=1, 0.4, 0.2, 0.1, R being the ratio of the minimum load to the maximum load. The results showed that, as R decreased, da/dt increased consistently and KImax at da/dt=2×10-10m/sec decreased. In other words, as the load amplitude became large, the effect of cycling of the load became more damaging.
Further tests were carried out at constant R (0.1) and at three levels. of f, f=0.01Hz, 0.1Hz, 1.1Hz. As f increased, da/dt increased. Even under the condition of f=0.01Hz, crack propagation was accelerated by cycling of the load, as compared with da/dt under a static load. The f-da/dt diagram was constructed from the above data in order to distinguish the cycle-dependent crack propagation from the time-dependent crack propagation. In the range where KImax was small (<3.57MPa√m), only the cycle-dependent crack propagation occurred. It is important to recognize that 3.57MPa√m is smaller than the threshold KImax under a static load. When KImax became larger than 3.57MPa√m, the time-dependent crack propagation appeared. In this region, both of the cycle-dependent crack propagation and time-dependent crack propagation existed. In the range where KImax was large (>4.34MPa√m), the time-dependent crack propagation was predominant.
