Induced anisotropy of chiral carbon nanotubes under combined tension-twisting

The twist-induced anisotropic behavior of chiral carbon nanotubes (CNTs), namely the (6,3) CNT, under combined tension-twisting is presented and discussed in this paper. CNT chirality triggers anisotropic responses that depend predominantly on the direction of twisting. Both the level of axial tension and twist-induced anisotropy play a key role in the stiffness and strength of the chiral CNT. Molecular dynamics (MD) simulations of (6,3) chiral CNT under pure tension, pure twisting and combined tension-twisting are performed. The anisotropy induced by the twisting direction was shown to be remarkable: the shear modulus for direct twisting is 25% higher than that for inverse twisting whereas the buckling torque for inverse twisting is 40% higher than that for direct twisting. In the post-buckling regime, the ovalization of the CNT is higher for inverse twisting than for direct twisting and we show that ovalization leads to a decrease of post-buckling torsional stiffness. The post-buckling torsional stiffness for direct twisting was much higher than for inverse twisting. We show that the twist-induced anisotropic behavior of the chiral CNT is much more evident when it concerns stiffness than strength. For this chiral CNT under low twist-to-tension ratios, direct twisting has no impact on the failure strain (it equals the pure tensile failure strain) while inverse twisting has great influence on it (it is half the pure tensile strain). The magnitude of the twist-to-tension ratio also affects the post-buckling torsional stiffness of the chiral CNT: under combined tension-twisting, the post-failure structure of the CNT is more ductile for inverse twisting and more brittle for direct twisting. To the authors’ best knowledge, this is the first time the anisotropic behavior of chiral CNT under combined tensile-twisting loads is studied.