VISCOELASTIC ANALYSIS OF MISMATCH STRESSES IN CERAMIC-MATRIX COMPOSITES UNDER HIGH-TEMPERATURE NEUTRON-IRRADIATION

Fiber-reinforced ceramic matrix composites, such as SiC-SiC, are propo sed for structural applications in future fusion energy systems. In a fusion nuclear reactor environment time-dependent inelastic effects ar e induced by irradiation and hence mismatch stresses are expected to c hange in time. The time evolution of the internal mismatch stresses in ceramic fiber composites under high-temperature neutron irradiation i s presented, with application to SiC-SiC composite structures. The eff ects of thermal creep, irradiation-induced creep and dimensional chang es on the build-up and relaxation of the interface pressure and residu al stresses in fibers and matrix are investigated. Residual stresses a re determined as functions of temperature and neutron exposure time. I t is shown that initial mismatch stresses are relaxed within hours bec ause of irradiation creep. It is also demonstrated that fibers with lo w density, such as Nicalon, will debond from the matrix due to excessi ve irradiation-induced densification.