Tight-binding molecular dynamics were applied to single-walled C nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be ~1.1TPa, and brittle rupture occurred at a strain of 17% under quasi-static loading. The predicted modulus was consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two C atoms, and corresponded to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurred at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by ~160GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by ~90GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus was lowered by only ~100GPa, but with a lower critical strain of 11%. In all cases, there was noticeable strain softening, for instance, leading to an ~250GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5GPa) even as the nanotube unravelled. Since the statistical likelihood of defects occurring near each other increased with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.

Tight-Binding Molecular Dynamics Study of the Role of Defects on Carbon Nanotube Moduli and Failure. R.W.Haskins, R.S.Maier, R.M.Ebeling, C.P.Marsh, D.L.Majure, A.J.Bednar, C.R.Welch, B.C.Barker, D.T.Wu: Journal of Chemical Physics, 2007, 127[7], 074708