Tension and twist of chiral nanotubes: torsional buckling, mechanical response and indicators of failure

We report on molecular dynamic calculations of combined tension-torsion of chiral single-wall nanotubes. We work within the framework of objective structures that exploits symmetry groups to enable torsion of chiral nanotubes, in addition to non-equilibrium extension. We apply the method to study the mechanical response and failure of nanotubes of various chiralities. We find that three distinct regimes exist for nanotubes under twist: distorted but unbuckled, reversible torsional buckling and irreversible torsional buckling. When twisted but unbuckled nanotubes are subject to tension, there is minimal change in failure strain, whereas reversibly buckled nanotubes have substantially reduced failure strain and load. We also observe the evolution of the twisting moment during the elongation process while keeping the twist angle fixed. This evolution has two interesting and potentially useful features: first, some nanotubes 'unbuckle' in the process of extension, and second, there is a clear correlation between extrema in the evolution of the twisting moment and impending nanotube failure. Given the sensitivity of electrical properties in carbon nanotubes to torsion, and the recent demonstrations of measuring torsion-induced changes, the latter feature suggests the possibility of real-time diagnostics to detect critical mechanical events.