Observations of inelastic deformation during high-temperature fatigue and failure of a polycrystalline silicon nitride

The effects of rate-dependent inelastic deformation were observed during tensile static, dynamic, and cyclic fatigue testing of a hot-isotatically pressed, monolithic, polycrystalline silicon nitride in ambient air at temperatures of 1150, 1260 and 1370°C. Constant stresses in static fatigue ranged from 50 to 300 MPa. Stress rates in dynamic fatigue ranged from 10^-4 to 10¹ MPa/s. Waveforms in cyclic fatigue included trapezoid, triangle and sine at a stress ratio, R, of 0.1, frequencies of 0.1 and 10 Hz, and maximum stresses ranging from 75 to 325 MPa. At 1150°C, all fatigue results showed a similar slow crack growth failure mechanism with no separate cyclic fatigue mechanism. However, at 1260 and 1370°C the failure mechanism was multi-faceted. For both temperatures, the failure in static fatigue was dominated by the accumulation of diffusion-controlled creep cavities. In dynamic fatigue, the inelastic deformation exhibited by non-linear stress-strain curves supported the conclusion of failure by slow crack growth alone at high stress rates and by a mixture of slow crack growth and creep damage at low stress rates. Under cyclic loading, 'enhanced' times to failure, attributed to viscous rate effects, occurred regardless of waveform or frequency. The stress rate was related to the stress at the onset to non-linearity, σ₀, indicating a quasi-endurance limit below which cyclic fatigue had little 'enhancing' effect. Creep compliance relations indicated non-linear viscoelastic (or viscoplastic) behavior, leading to speculation on the role of this behavior in producing the 'enhanced' cyclic fatigue resistance.