Volume 35, Issue 1 (January 2007)
Time-Dependent Reliability of Thermally Shocked SiC Tubes
In real world structural applications, applied thermal and mechanical loads vary with time such as in thermal shock, startup, shut down, and dynamic and vibrational settings. In addition, when a given component is subjected to transient environmental and or thermal conditions, the material properties also vary with time. The objective of this paper is to demonstrate an analytical methodology capable of predicting the transient reliability of components subjected to time-dependent thermomechanical loads taking into account the change in material response with time. Weibull parameters, slow crack growth parameters, as well as the thermal and elastic material properties are allowed to vary with time. In this approach, progressive damage due to slow crack growth is accounted for. This transient reliability capability was added to the NASA CARES/Life (ceramic analysis and reliability evaluation of structures/Life) code, which was also updated to interface with commercially available finite element analysis programs executed for transient load histories. The methodology was then used to predict the transient failure probability of thermally shocked sintered alpha silicon carbide SiC tubes quenched down from 1000°C in order to demonstrate the viability of this approach and the CARES/Life code. The failure probability was computed to be 92 % while the measured failure probability was found to be up to 35 %. Sensitivity analysis showed that the computed failure probability is very sensitive to the magnitude of the axial and hoop thermoelastic stress distributions, with 10 % reduction in the stress magnitudes yielding a computed failure probability of 55 % matching the experimentally measured value.