Fracture Micromechanics as Influenced by Environment in Textile Reinforced Ceramic Matrix Composites

摘要

The goal of this research was a basic understanding of the factors that contribute to the fracture toughness of fabric reinforced ceramic matrix composites (CMCs). A technique was developed for measuring the fracture toughness of fabrics. The mechanical properties of matrix materials were measured using a Nanoindenter, and found to be low compared to the properties expected from matrix composition. Crack tip micromechanics, i.e., crack opening displacements and strains near the crack tip, were measured both for fabrics and from CMCs. It was found that fiber bundles move relative to each other during fracture, and that voids in the material accommodate that motion. The ability of fiber bundles to move is believed to be an important factor in developing a high fracture toughness in these materials. A finite element model was developed to simulate fabric fracture and the effects of an including matrix to form a composite. Exposure to air and Argon at 1100 deg C for 146 hrs. resulted in a decrease in fracture toughness of several composites evaluated. Micromechanics evaluation found no differences that could be attributed to the gas environment, so it was concluded that fiber strength degradation was the cause of lower fracture toughness.