Role of Cracks in the Creep Deformation of Polycrystalline Structural Ceramics
Final rept. 1 May 88-30 Apr 91,
VIRGINIA POLYTECHNIC INST AND STATE UNIV BLACKSBURG DEPT OF MATERIALS ENGINEERING
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A study was conducted concerning the role of cracks in the creep deformation and fracture behavior of polycrystalline structural ceramics. Grain boundary cavitation during creep in a course-grained alumina was found to cause a decrease in Youngs modulus by as much as a factor of five, indicative of the contribution from elastic creep by crack growth and crack-enhanced creep to the total creep deformation. Multiple crack formation in a fine-grained alumina was found to increase the strain-rate sensitivity of the failure stress, as the result of strain-rate dependent decreases in Youngs modulus. For alumina with glassy grain boundary phase, the one-to-one correlation between creep rate and time-to-failure presented evidence for crack enhanced creep fracture. Silicon carbides whisker-reinforcement of aluminum oxides was found to suppress cavitation, coupled with an increase in the stress exponent. A micromechanical analysis of this effect indicated that creep deformation of SiC whisker- reinforced alumina appears to be governed by stress-dependent sliding at whisker-matrix interfaces.
- Ceramics, Refractories and Glass