Longitudinal compressive behaviour and microstructure of PAN-based carbon fibres

The longitudinal compressive behaviour of HS and HM PAN-based carbon fibres has been examined by means of a direct compression method which can be applied to single carbon fibres. Longitudinal compressive strength was found to be similar to 30 to 50% of tensile strength depending on the modulus levels in carbon fibres used. Longitudinal compressive modulus was estimated making use of the Euler buckling formula applied to buckled samples; compressive modulus was about 50% of tensile modulus. SEM observations in-situ revealed distinct kink bands in HM fibres, which usually develop by splitting failure, this implies local crystallite buckling and the existence of needle-like pores in HM fibres. The compressive strength in HS fibres decreases with increased apparent porosity. HM fibres exhibit smaller amounts of disorder and larger crystallites, and have increased modulus; it may be deduced that large needle-like pens also exist. Therefore, a compressive failure mechanism which involves local buckling failure of large crystallites with insufficient lateral support of the large needle-like pol es may be suggested in the case of HM fibres. (C) 2001 Elsevier Science Ltd. All rights reserved.