MODELS OF HIGH-TEMPERATURE, ENVIRONMENTALLY ASSISTED EMBRITTLEMENT INCERAMIC-MATRIX COMPOSITES

The intermediate-temperature oxidation embrittlement, or ''pest,'' eff ect found in ceramic-matrix composites (CMCs) is shown to have feature s analogous to stress corrosion cracking, The behavior involves crack growth upon oxidation of the fibers or the fiber coatings to form an o xide that weakens the fibers. It has reaction- and diffusion-controlle d regimes. The former occurs at low stresses. The latter occurs at hig her stresses, It is controlled by oxygen ingress through the matrix cr acks, There is also a crack growth threshold, Expressions for the crac k velocity above the threshold are derived as well as the stress depen dence of the rupture life.