Effect of Residual Stresses on the Mechanical Response of Continuous Fibre Reinforced Ceramic Matrix Composites

Abstract

Residual stresses and strains are present in composites where the constituent materials have different thermo-elastic properties. Upon cooling from the processing temperature these stresses develop as a consequence of the mutual restraint of the constituents on each other’s thermal expansion or contraction. The magnitude of the residual stresses depends on the degree of load transfer, on the amount of mutual restraint characterised by the thermal expansion mismatch, and on the elastic moduli of the constituents. In composites reinforced by continuous fibres, the residual stress state is characterised by the axial stresses in the fibre and the matrix, and by the radial stress perpendicular to the fibre-matrix interface. The latter affects the degree of load transfer between the fibres and the matrix and hence also influences the magnitude of the axial residual stresses.

When the composite is mechanically loaded, the applied stresses superimpose on the residual stresses and together they determine the mechanical response. As long as their combined effect does not trigger additional irreversible damage within the composite, the residual stress state remains unchanged. When damage accumulation (matrix cracking, interfacial debonding, fibre failure) occurs upon loading the residual stresses are relieved and their effect on the mechanical response decreases. However, for brittle matrix ceramic composites where the matrix has a lower failure strain than the fibres, this stress relief is not always possible. Indeed, when the axial residual stresses in the fibres are tensile, residual stress relief results in closure of the matrix cracks. This generates a mechanical incompatibility which prevents additional relief upon further interfacial debonding and leads to an intensification of the effect of residual stresses. The presentation gives some examples of both cases.