Assessing the validity of micro-pillar compression for determining strength and stiffness of carbon fibres

The longitudinal compressive mechanical behaviour of polyacrylonitrile (PAN)-precursor T300 carbon fibres was assessed using micro-pillar compression testing, with direct comparison to published data on entire fibre compression. Micro-pillars, fabricated via focused ion beam (FIB) milling, exhibited compressive modulus, strength, and failure strain values closely matching those of whole fibres, thereby validating this microscale technique for accurate stiffness and strength measurements. A progressive reduction in stiffness with increasing compressive strain — indicative of non-linear elasticity — was directly observed and quantified under compression for the first time. Although the failure modes of micro-pillars differed from those of intact fibres, the results support the hypothesis of a mechanically homogeneous fibre microstructure and suggest the presence of a stabilising outer sheath that delays failure initiation. These findings reinforce the methodological basis for small-scale mechanical testing of carbon fibres and carry implications for multiscale modelling and the prediction of compressive strength in unidirectional composite plies.