Probabilistic Design and Reliability of Silicon Carbide Ceramics

Ceramic materials generally lack ductility and toughness, and exhibit variability in properties. In order to design with ceramic materials, the variation in material properties, especially strength, has to be statistically analyzed for reliability. Conventional design can be done with calculations utilizing safety factors. However, modern design aspects include proof testing and appropriate nondestructive evaluation methodology. Possible microstructural changes which occur during proof testing may influence subsequent material behavior and must be included in the design methodology. The temperature dependence of flexural strength of two engineering structural ceramics—single-phase sintered alpha silicon carbide and two-phase fine grain reaction-bonded silicon carbide—are examined. Using Weibull statistics, the risk of rupture for various stress levels has been derived from flexural versus tensile strength relationships. Ceramic life prediction considers subcritical crack growth and strength degradation in service environments. The slow crack growth possibilities at elevated temperatures for sintered alpha silicon carbide are examined in dynamic stressing rate and stress rupture experiments. Crack arrest and crack propagation resistance during proof testing and their implications in the probabilistic design with ceramics are analyzed.