The strength-limiting flaws that develop in carbon fibres from impurity particles in the polyacrylonitrile (PAN) precursor fibres have been studied using a fibre-spinning apparatus installed in a clean room. Carbon fibres prepared from PAN fibres spun from a filtered spinning solution in the clean room had tensile strengths of about 3 GPa after heat treatments to both 1400 and 2500 °C. Flaws due to impurities reduced the strengths of the carbon fibres, particularly after heat treatments to 2500 °C, and caused the strengths to decrease as the length tested was increased. Possible mechanisms are discussed for the formation of flaws and a failure criterion, based on the strain energy developed in a misaligned crystallite, is used to predict carbon fibre strengths. The ranges of strength predicted from the model show satisfactory agreement with the ranges of strength obtained experimentally for fibres heat-treated to various temperatures.
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Reynolds et al. (1980) studied this question.