Mesostructures and the Tensile Strength and Fatigue Endurance of Aligned Fibre Composites

Abstract

Mesostructures are structures which are middle rank in size, and usually unintentional. They provide a direct link between production processing and final properties of a composite. In this study an orientation mesostructure, i.e. fibre waviness, was artificially introduced into aligned carbon fibre composites using optical fibres (80–250 μm in diameter) oriented at right angles to the carbon. Although this caused severe curvature of the fibres, it was over very short lengths (a few microns) and appeared not to be the cause of the reduction in tensile strength of these composites. Instead it was concluded that the fibre crowding effect in the cross section of the optical fibre caused fibre contacts, and hence fibre damage, which weakened the composite. The tensile strengths of the composites without the optical fibres were higher than values quoted by the manufacturers, but were still less than theoretical values, probably due to stress concentrations in the end tabs. Better tensile testing methods therefore appear to be needed before embarking on a more detailed examination of the effect of mesostructures on tensile strength of unidirectional composites. Fibre waviness has also been suggested as a possible initiator of damage during the fatigue of aligned fibre composites. In this work, the presence of the optical fibres induced “ply cutting”. This could have resulted from the fibres in the regions of high fibre volume fraction rubbing against each other. The largest optical fibre caused, in addition, massive splitting of the specimen, in agreement with a mechanism previously proposed for the early stages of fatigue failure of these composites.