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.