The influence of the interphase and associated interfaces on the deflection of matrix cracks in ceramic matrix composites

A model is proposed to determine the influence of an interphase on the deflection of a matrix crack in ceramic matrix composites. Then, a finite element analysis is performed for a microcomposite geometry with an annular crack which initiates in the matrix and propagates in the interphase. It is applied to a SiC/C/SiC microcomposite with a pyrocarbon interphase. Criteria for penetration and deflection of the matrix crack are expressed in terms of toughness of the interphase and of the various interfaces (matrix/interphase and interphase/fibre interfaces). The predictions are found to agree with crack deflections observed in practical SiC/SiC composites and with the available interphase toughness data. Results also suggest that the real crack deflection mechanism involves debonding ahead of the propagating matrix crack. (C) 2000 Elsevier Science Ltd. All rights reserved.