FAILURE MECHANISMS AND DAMAGE EVOLUTION IN CROSSPLY CERAMIC-MATRIX COMPOSITES

Failure mechanisms were studied under the microscope in a crossply sil icon carbide/glass-ceramic composite under axial tensile loading. Fail ure initiation takes place in the 90 degrees layer. It takes the form of radial matrix cracks around the fibers, followed by interfacial cra cks, which in turn coalesce into transverse macrocracks. These transve rse macrocracks in the 90 degrees layer reach a characteristic saturat ion crack density with a minimum crack spacing of the order of the lay er thickness. Subsequently, transverse matrix cracks are generated in the 0 degrees layer, increasing in density up to a minimum crack spaci ng of the order of eight fiber diameters. This stage of failure is acc ompanied by fiber-matrix debonding and some fiber-failures in the 0 de grees layer. In the third stage of damage development, the macrocracks of the 90 degrees layer branch off and connect with the 0 degrees lay er cracks in a characteristic ''delta'' pattern. This is finally follo wed by delamination and additional cracking in the 90 degrees layer pr ior to ultimate failure. The various failure mechanisms and their inte ractions were discussed and compared with predictions of prior experim ental and analytical studies of unidirectional and crossply composites .