A test method for probing oxidation embrittlement in ceramic-matrix composites has been devised and demonstrated for three composites: SiC/magnesium aluminosilite, SiC/SiC, and SiC/AI,O,. The method identifies a "pest" temperature TP. The growth of an embrittled region from the surface, at Tp, has been illustrated. The embrittlement mechanism involves oxygen ingress through matrix cracks and silicate formation at the fibers, causing fiber failure. 2097

The effects of circular holes and sharp notches on the tensile strength of two Nicalon‐reinforced ceramic composites have been investigated. The influence of inelastic straining on the redistribution of stress has been elucidated through measurements of the local strains in the regions of high stress concentration, coupled with finite element simulations of the test geometries, using a nonlinear constitutive law appropriate to ceramic composites. The scale dependence of strength has been inferred from tests performed on specimens of varying size. The utility of two failure models that incorporate both the inelastic straining and the scale dependence has been assessed: one based on the point stress failure criterion and the other on weakest‐link fracture statistics. Both approaches provide a reasonably consistent description of the experimental measurements. Some of the implications and limitations associated with the failure models are discussed.

The strength and reliability of fiber-reinforced ceramicmatrix composites (CMCs) are dependent on whether conditions of local or global load sharing prevail. Global load sharing is promoted by a low interfacial sliding stress and is manifested in a zero-tangent modulus at the point of tensile failure along with random fiber failures and extensive fiber pullout. In this paper, it is demonstrated that conditions of global load sharing are not present in two commonly studied CMCs, despite the fibrous appearance of their fracture surfaces. This behavior is manifested in a volume-dependent strength, as evidenced by strength differences measured in tension and flexure (accounting for the nonlinear stress distribution in flexure). Methods of weakest-link statistics are used to relate the strengths measured in the two test configurations. Estimates for the Weibull moduli of the two systems are obtained from the experiments and compared with values obtained through Monte Carlo simulations based on a three-dimensional-lattice Green's function method. Fig. 1. Schematic showing the tensile stress-strain response of a The implications of these results on the strength of large fiber-reinforced CMC.

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