Fracture Toughness Testing of a Ceramic Matrix Composite at Elevated Temperatures

摘要

The need for a load fixture and technique for determining the fracture toughness in fiber reinforced ceramic composites at elevated temperatures was identified. A three point bend load fixture capable of withstanding temperatures up to 1000 degrees Fahrenheit was designed and constructed. A test procedure for finding compliance and critical load at room and elevated temperatures was developed. Specimens were tested at varying crack lengths at room temperature, 600, and 1000 degrees Fahrenheit. Postmortem examination was done on specimens for each temperature. Specimens tested at 600 F had smoother Mode II fracture surfaces than the room temperature specimens including a more brittle fracture and supporting the experimental results. The specimens tested at 1000 F had a rougher Mode II fracture surface with fiber pull-out and small matrix fragments attached to the fibers indicating a more ductile failure and therefore increasing the fracture toughness. A color change from black to dark gray and the presence of crystals on the Mode I fracture surface of the 1000 F specimens indicated a chemical change in the specimens. The apparent chemical change and significantly higher fracture toughness in the material at elevated temperatures. Keywords: Military theses; Fracture mechanics; Crack propagation. (KT/JES)