Microcracking of Ceramic Matrix Composites at Elevated Temperature

摘要

Ceramic matrix composites (CMC) are potentially designed for use in the high temperature environment because of their high strength and noncatastrophic failure characteristics. For ceramics at elevated temperature, the materials exhibit time-dependent deformation. The phenomenon is further accelerated by an increase in the stress or the temperature. Reinforcement by incorporating high-strength fibers is one of the several approaches to significantly improving the creep fracture resistance of the CMC. Experimental studies in mechanical behavior of CMC (1-4) have shown that failure is preceded by matrix cracking at high temperatures. Further, the creeping matrix causes stress transfer from the matrix to the fiber. However, there is a lack of any investigation in understanding of the relation between the matrix cracking at the micro level and the overall mechanical behavior of CMC at elevated temperature. In this research effort, the crack tip fields for a matrix crack are considered. Using generalized expansions at the crack tip in each region and matching the stresses and displacements across the interface in an asymptotic sense, a series asymptotic solution is constructed for the stresses and strain rates near the crack tip. It is found that the stress singularities, to the leading order, are the same in each material, oscillatory higher-order terms exist in both regions, and stress higher-order term with the order of O(r(exp 0) ) appears in the elastic material. The stress exponents and the angular distributions for singular terms and higher order terms are obtained for different creep exponents. A full agreement between asymptotic solutions and the full-field finite element solutions has been obtained.