Cyclic fatigue of silicon nitride at 1000 C

The fatigue failure of a gas-pressure-sintered silicon nitride (GPSSN) has been investigated at 1000 C. Double waisted round specimens machined to a high tolerance in a well aligned servohydraulic machine were push-pull tested at 10 Hz under fully reversed loading (R=-1) and fatigue S-N curves were generated. Specimens were tested as-machined and as-machined followed by annealing. The effect of the annealing treatment was to increase the strength of the material and also to reduce the scatter of the data. The fatigue times-to-failure showed a bimodal distribution. This observation can be interpreted in terms of a rapid failure from relatively large critical machining flaws and a dynamic healing/growth behaviour of the relatively small critical machining flaws. (orig.)