Anisotropic compressive behaviour of turbostratic graphite in carbon fibre

Whilst measuring fibre properties in tension is relatively simple, measurement of compressive properties, particularly in the radial direction, has remained elusive for carbon fibres due to the small diameter. Here, the compressive mechanical properties of individual carbon fibres are quantified using micropillar compression testing. Combining the measured compressive properties with a full characterisation of the microstructure has provided a deep understanding of the mechanical behaviours of turbostratic graphite structure in carbon fibres. Under compression, the turbostratic graphite sheets are prone to bend and buckle, leading to a marked reduction in elastic modulus compared to tension. Two-fold mechanical anisotropy has been found for the turbostratic graphite structure in carbon fibres, which can be exacerbated by a higher degree of crystallinity. Although the tensile strength of carbon fibres is known to be dictated by fibre defects, the intrinsic compressive strength is found to be determined by crystal size, showing the applicability of the classical Hall-Petch hardening model to carbon-based materials. Furthermore, the failure mode for carbon fibres under compression is concluded to be by separation of turbostratic graphite, irrespective of the loading direction.