CRITICAL STRESSES FOR EXTENSION OF FILAMENT-BRIDGED MATRIX CRACKS IN CERAMIC-MATRIX COMPOSITES - AN ASSESSMENT WITH A MODEL COMPOSITE WITH TAILORED INTERFACES

Matrix cracking was studied in a model unidirectional composite of SiC filaments in an epoxy-bonded alumina matrix, The residual clamping st ress on the filaments due to the shrinkage of the epoxy was moderated with the addition of the alumina filler, and the filament surface was coated with a releasing agent to produce unbonded frictional interface s. Uniaxial tension specimens with controlled through-cracks with brid ging filaments were fabricated by a two-step casting technique, Critic al stresses for extension of the filament-bridged cracks of various le ngths were measured in uniaxial tension using a high-sensitivity exten someter. The measured crack-length dependence of the critical stress w as in good agreement with the prediction of a stress-intensity analysi s that employed a new force-displacement law for the bridging filament s. The analysis required independent experimental evaluation of the ma trix fracture toughness, the interfacial sliding friction stress, and the residual tension in the matrix. The matrix-cracking stress for the test specimens without the deliberately introduced cracks was signifi cantly higher than the steady-state cracking stress measured for the l ong, filament-bridged cracks.