The full-cell cracking mode in unidirectional brittle-matrix composites

Abstract

In this work we consider the issues associated with predicting the initiation of matrix cracking within a unidirectional brittle-matrix composite (BMC). The analysis is accomplished as a case study for a restrictive, though important, class of composites consisting of silicon carbide fibres and a glass-ceramic matrix. A newly-derived axisymmetric variational model is employed to predict the stress field and energy release rates in a composite having a single damaged cell (or randomly located non-interacting cells) in which an annular matrix crack is introduced. Propagation of this crack to the fibre/matrix interface generates what we call the full-cell cracking mode. Strength and toughness properties of the matrix material are assumed to infer the potential for growth of the annular crack as well as its influence on the interface stresses and plausible scenarios governing the behaviour of secondary flaws. In turn, the synergistic influence of the latter flaws on the original annular crack is predicted. Since fibre spacing seems to be of paramount importance in real composites, a model of a composite having a non-uniform fibre distribution is also developed. The work includes a study of crack deflection at the fibre/matrix interface and comments are made regarding the perceived need for poor bonding in these materials. Also noted are the key parameters that need to be determined by experiment to quantify the failure prediction.