High-temperature mechanical behaviour of multidirectional Nicalon/CAS-II composite

High-temperature mechanical behaviour of Nicalon/CAS-II composite has been investigated. Oxidation of the exposed interfaces along matrix cracks at 1000 °C lowered the longitudinal unidirectional strength to the stress level at which matrix cracking began to occur. The strength of cross-plied composites was also severely reduced in 800 °C air. Transverse plies cracked prior to 0° ply matrix cracking. However, embrittlement did not occur until the matrix in the 0° plies cracked. It was established that oxidation does not take part in crack growth parallel to the fibres, except adjacent to exposed edges. Neither does oxygen enter 90° ply cracks in cross-plied composites in sufficient quantity to produce oxidation embrittlement, at least up to the 0° matrix cracking strain.