Micromechanics of crack bridging in sapphire epoxy composites

Fracture-mechanics analyses of matrix-cracking stress and fracture toughnes s of brittle-matrix composites require knowledge of the crack-closure tract ions applied by the bridging fibers. The closure fraction is expressed as a relationship between a distributed closure stress (p) and a the local crac k-opening displacement (u). The p/u relationship is typically derived rom a micromechanics analysis of stress transfer from the matrix to the fiber in the vicinity of the matrix crack. This paper describes measurements of bri dging stresses and crack openings in a model composite made of a brittle ep oxy matrix and sapphire filaments coated with release agents to prevent int erfacial bonding. Stresses in the crack-bridging sapphire filaments were de termined by measuring the shift of the fluorescence lines of trace chromium impurity with a laser microprobe. The measured p/u data were compared with micromechanical models applicable to composites with unbonded, frictional interfaces and found to be consistent with models that predict a lower-boun d bridging stress at the crack tip equal to the far-field fiber stress. The implications of this finding for the prediction of the matrix-cracking str ess of brittle-matrix composites are discussed ill the paper. (C) 1998 Else vier Science Ltd. All rights reserved.