Molecular mechanics calculations together with coupling methods bridging molecular mechanics and finite crystal elasticity were used to simulate the fracture of defected C nanotubes and to compare with the available experimental results. The modified second generation Brenner potential (MTB-G2) was used for the calculations. These molecular mechanics calculations gave a fair agreement with quantum mechanical benchmarks, and indicated that one- and two-atom vacancies reduced the fracture strength of C nanotubes by 20 to 33% (whereas quantum mechanical calculations predicted 14 to 27%), but these fracture strengths were still much higher than experimental ones. It was then demonstrated that this experimental and theoretical discrepancy could be attributed to the presence of large-scale defects, such as those that could arise from oxidative purification processes. Simulations of multi-walled C nanotubes, and tubes twisted prior to tensile loading, revealed negligible effects upon fracture strength, and indicated that these were not the causes of low experimental values. The effects of chirality and tube diameter upon fracture strengths were also investigated.

Mechanics of Defects in Carbon Nanotubes - Atomistic and Multiscale Simulations. S.Zhang, S.L.Mielke, R.Khare, D.Troya, R.S.Ruoff, G.C.Schatz, T.Belytschko: Physical Review B, 2005, 71[11], 115403 (12pp)