Abstract

Carbon nanotube (CNT) fibres, especially if perfect in terms of purity and alignment, are highly anisotropic. With their very large axial strengths and stiffnesses, but ready slippage between the CNTs, there is utmost difficulty in transferring an applied load uniformly throughout the fibre. Finite element simulations show that, in accordance with St Venant’s principle, very considerable length-to-diameter ratios (> 103) are required before the stress becomes uniform, even at low strains. We discuss several different models for predicting the strength of yarn-like CNT fibers, and their implications for improving macroscopic fibre properties. It is proposed that lack of perfect orientation and presence of carbonaceous material between bundles greatly enhances the stress transfer, thus increasing the load carried by fibre before failing in shear. We conclude that the measured strength of CNT fibres depends very much on the specific testing geometries and that imperfections, whether by virtue of less-than-perfect orientation or of embedded impurities, are actually major positive contributors to the observed strength.