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Small coupons of a three-dimensionally woven, carbon-carbon composite were axially loaded in a specially-designed SEM load stage. Notched and unnotched specimens were fatigue cycled at room temperature. Crack initiation was found to be in a sliding mode, most often near a fiber-matrix interface. The cracks would immediately turn 90 degrees to an opening mode and propagate through a matrix pocket until encountering a longitudinal fiber bundle. In most cases, the crack would then propagate along the fiber bundles, again in a sliding mode. Yarn bundle failures were not limited to the immediate region of an approaching crack, leading to the conclusion that this material is not notch sensitive. Acoustic emission was found very useful in characterizing the different stages of microstructural damage.
carbon-carbon composite, damage initiation, graphite, crack propagation, scanning electron microscopy, acoustic emission, fatigue (materials), composite materials
Research engineer, Southwest Research Institute, San Antonio, Tex.
Chairman, Illinois Institute of Technology, Chicago, Ill.