School of Engineering and Materials Science
Research Student Awards
PhD Thesis: High temperature uniaxial fatigue of oxide ceramics
Author: WILLIAMS, Kelly
Supervisor(s): Mike Reece
Three oxide ceramics, mullite, zircon and zircon mullite, all containing an amorphous phase have been tested at elevated temperatures under uniaxial static and cyclic loading conditions. For each material the final crack length was independent of load ratio when temperature and maximum stress were held constant, implying that failure was controlled by a critical stress intensity factor. Values of the critical stress intensity factor were estimated and although there was a large amount of scatter in the results, the mean values were in good agreement with those obtained by testing single edge notched specimens. Under any particular test condition increasing the maximum stress decreased the time to failure. Lifetimes under cyclic loading were longer than under static loads. Cyclic loads with a compression phase had longer lifetimes than tensile cyclic loads. Increasing the load ratio was found to reduce the fatigue resistance (fatigue parameter). Increasing the test temperature decreased the time to failure and decreased the fatigue parameter. Comparing the different materials showed that the material with the highest volume fraction of glass had the lowest lifetimes, and the material with the longest lifetimes had the smallest volume fraction of glass. High magnification examination of the fracture surfaces revealed glass bridging ligaments. Attempts were made to model the effect of these on the fatigue lifetimes. The model used was extremely sensitive to the viscosity of the glass phase. The model did predict the trends observed for the individual effects of load ratio and temperature on times to failure. However it was unable to predict the combined effect of load ratio and temperature, and a possible explanation for this is given. The fatigue data was also used to provide estimates of the Weibull modulus at high temperatures.