Effect of matrix cracking and interface sliding on the thermal expansion of fibre-reinforced composites

The effect of matrix cracking on the thermal expansion behaviour of brittle, unidirectional fibre-reinforced composites is studied. Sliding along the fibre-matrix interface accompanying matrix cracking has a major effect on the change in thermal expansion. This problem is also addressed, both with and without friction. For the most common composite systems, whose fibres have a smaller coefficient of thermal expansion than that of the matrix, matrix cracking and interface sliding result in a reduction of the thermal expansion of the composite. A cylindrical cell model of a composite with uniformly spaced matrix cracks is invoked for analysis. Shear-lag approximations, enhanced by selected finite element solutions to the cell model, provide estimates of the functional dependence of thermal expansion on constituent properties, matrix crack density and extent of sliding. Hysteresis behaviour during thermal cycling is analysed accounting for reverse frictional sliding along debonded portions of the fibre-matrix interface. A nondimensional parameter, E mΔαΔT/τ (where E m is the matrix modulus, Δα the thermal expansion mismatch between fibre and matrix, ΔT the amplitude of the temperature cycle and τ the frictional resistance to sliding), is identified which governs the extent to which sliding reduces the effective expansion coefficient of the composite.