Structure-property relationships in carbon fibres

The tensile modulus of carbon fibres is related to the preferred orientation of the layer planes, and it is easier to achieve high values with carbon fibres from mesophase pitch than from polyacrylonitrile (PAN). Tensile strength has been related to the flaw population, but with improved processing conditions, fundamental structural organisation is more important. Carbon fibres fail in both tension and compression by the rupture of layer planes in misoriented crystallites and the propagation of a crack beyond the critical size. PAN-based carbon fibres have a complex structure of interlinked layer planes which hinder crack propagation unless voids are present with extensive walls. In many cases crystalline walls are formed by catalytic graphitisation due to contaminant particles; it is these shells rather than the voids themselves which lead to premature failure. Mesophase-pitch-based carbon fibres crystallise so that the layer planes are arranged in sheets; this facilitates crack propagation and leads to lower strain-to-failure than for PAN-based fibres of similar modulus.