Tensile fatigue of a 2.5D-C/SiC composite at room temperature and 900°C

The tension-tension fatigue properties were investigated for a 2.5D-C/SiC composite in warp and weft direction. The fatigue experiments were carried out at room temperature (RT) and 900°C in laboratory air. The tensile properties of the specimens survived 10⁶cycles were determined to explore the damage mechanisms. The fracture surfaces were examined by a scanning electron microscope. The composite exhibits excellent fatigue resistance at RT. The fatigue limits in both directions are about 85% of the ultimate tensile strength. The tensile strength and failure strain of the C/SiC can be enhanced for the survived composite at RT. The fatigue limits of the composite at 900°C are much lower than those at RT in both directions. Examination of the fracture surfaces revealed that the failure is closely related to the propagation of the cracks originated from the crossover of the bundles and produced within the bundles. The cracks also offered the channels for the oxygen to penetrate into the composite and are responsible for the oxidization of the carbon fibers in the composite. The oxidization of the fibers plays a critical key role in decreasing the fatigue limits at 900°C.