Failure mechanisms and damage evolutionin crossply ceramic-matrix composites

Failure mechanisms were studied under the microscope in a crossply silicon carbide/glass-ceramiccomposite under axial tensile loading. Failure initiation takes place in the 90° layer. It takes the form of radial matrix cracks around the fibers, followed by interfacial cracks, which in turn coalesce into transverse macrocracks. These transverse macrocracks in the 90° layer reach a characteristic saturation crack density with a minimum crack spacing of the order of the layer thickness. Subsequently, transverse matrix cracks are generated in the 0° layer, increasing in density up to a minimum crack spacing of the order of eight fiber diameters. This stage of failure is accompanied by fiber-matrix debonding and some fiber-failures in the 0° layer. In the third stage of damage development, the macrocracks of the 90dlayer branch off and connect with the 0° layer cracks in a characteristic "delta" pattern. This is finally followed by delamination and additional cracking in the 90° layer prior to ultimate failure. The various failure mechanisms and their interactions were discussed and compared with predictions of prior experimental and analytical studies of unidirectional and crossply composites.