Microcracking in resins and matrices; Progress towards understanding the relative influence of carbon fibre properties and sizing

Matrix micro-cracking in carbon fibre composites is a problematic defect that often occurs at the interface between the fibre and the resin. When a composite material is placed under a load, micro-cracking may propagate through the matrix with these cracks joining up to form macroscopic cracks resulting in a deterioration of mechanical properties. A number of studies have focussed on the effect of interfacial adhesion on the formation of micro-cracks in laminates, leading to the reasoning that the fibre sizing is important in determining whether micro-cracks are present. Other studies have noted that the micro-cracking is mostly prevalent in composite materials that have been reinforced with high modulus fibres and not intermediate and standard modulus fibres. It has been postulated that this is due to the structural characteristics of the fibres leading to a greater difference in the coefficient of thermal expansion (CTE) between the fibre and the matrix. However the linear CTE was not related to the micro and nanoscale properties of the fibre and the critical property, the radial CTE of the fibre has not yet been reliably measured. The relative importance of the roles of carbon fibre sizing and modulus in the formation of micro-cracks remains unclear. In this paper, the structure and morphology of three different grades of carbon fibres; high modulus (M46J), intermediate modulus (IM7) and low modulus (BS24) were analysed using electron microscopy and SAXS-WAXS experiments leading to a greater understanding of the relationship of carbon fibre modulus to microstructure.