This paper reviews lifetime prediction methodologies for high-temperature structural ceramics. The methodologies consider failure from subcritical crack growth at low temperatures, 800 C to 1000 C, and creep, or creep-rupture at high temperatures. The paper discusses three methods of characterizing crack growth in silicon nitride: dynamic fatigue, static fatigue and a statistical analysis of lifetime. These techniques yield comparable results, suggesting that dynamic fatigue is the preferred procedure for characterizing crack growth. In the second part, the paper discusses a strain-based methodology for lifetime prediction. The effect of creep on acceptable engineering temperatures and stresses are considered for an allowable strain of 0.5%, and a failure probability of 0.0001. For a given stress and failure time, we show that a reduction in operating temperature of as much as 40 EC is needed when strain rather than creep rupture is considered as the failure criterion. An additional 25 EC reduction is needed to limit the failure probability to 0.0001.
International Symposium on Ceramic Materials and Components for Engines