Prediction of the Failure Probability of High Strength Ceramics Subject to Thermal Shock Loading

Failure of monolithic ceramics can be predicted using the Weibull theory. From the point of view of experimental verification the theory is well established for multi-axial mechanical loading as well as fatigue loading at constant temperatures. In view of high temperature applications failure due to thermal creep and sub-critical crack growth is also included within several commercial available design tools for ceramic components. However, these tools are not verified to reliably predict failure caused by special loading conditions under thermal excursions such as thermal shock conditions or off-normal overloads, where components like gas turbines are locally loaded by steep temperature gradients and steep stress gradients within a short time span. A thermal shock experiment was performed in order to verify the Weibull theory in a case with a rather involved load history. Notched rectangular bars were heated up to constant temperature of 1450°C and subsequently exposed to a rapid cooling. The results of thermo-mechanical analyses using FEM served as an input file for the STAU code which allows in post-processing to predict the failure probability. The predicted failure probability agreed very well with the findings of the experiment provided that the pronounced stress gradient in the notch root were correctly taken into consideration.