Mechanical properties of carbon nanotube fibres: St Venant’s principle at the limit and the role of imperfections

Carbon nanotube (CNT) fibres, especially if perfect in terms of their purity and alignment,are extremely anisotropic. With their high axial strength but ready slippage between theCNTs, there is utmost difficulty in transferring uniformly any applied force. Finite elementanalysis is used to predict the stress distribution in CNT fibres loaded by grips attached totheir surface, along with the resulting tensile stress–strain curves. This study demonstratesthat, in accordance with St Venant’s principle, very considerable length-to-diameter ratios(103) are required before the stress becomes uniform across the fibre, even at low strains.It is proposed that lack of perfect orientation and presence of carbonaceous materialbetween bundles greatly enhances the stress transfer, thus increasing the load the fibrecan carry before failing by shear. It is suggested that a very high strength batch of fibres previouslyobserved experimentally had an unusually high concentration of internal particles,meaning that the pressure exerted by the grips would assist stress transfer between thelayers. We conclude that the strength of CNT fibres depends on the specific testing geometriesand that imperfections, whether by virtue of less-than-perfect orientation or ofembedded impurities, can act as major positive contributors to the observed strength.