Effect of Specimen Thickness on Mechanical Behavior of SiC/SiC Composites

Potential composite applications in aerospace and transportation application systems have different thickness requirements. For example, space applications such as nozzle ramps or heat exchangers use very thin (less than 1 mm) structures whereas turbine blades need very thick parts greater than or equal to cm). There has been little investigation into the effect of thickness on stress-strain behavior or elevated temperature tensile properties controlled by oxidation. In this study, composites consisting of woven Hi-NicalonTM fibers, a carbon interphase, and CVI Sic matrix were fabricated with different numbers of plies to provide variable thickness. The composites ranged from a single ply (approximately 0.4 mm) to thirty-six plies (approximately 1 cm). Tensile tests were performed at room temperature with acoustic emission used to monitor matrix crack behavior. Elevated temperature tensile stress-rupture tests were performed in air. Considerably different room and elevated temperature tensile behavior was observed that will be discussed with respect to the effect of thickness on matrix crack formation, matrix crack growth and oxidation diffusion kinetics.