Mechanics of defects in carbon nanotubes: Atomistic and multiscale simulations

Molecular mechanics (MM) calculations together with coupling methods bridging MM and finite crystal elasticity are employed to simulate the fracture of defected carbon nanotubes (CNTs) and to compare with the available experimental results. The modified second generation Brenner potential (MTB-G2) is adopted in the calculations. Our MM calculations show fair agreement with quantum mechanical (QM) benchmarks, and indicate that one- and two-atom vacancies reduce the fracture strength of CNTs by 20%-33% (whereas the QM calculations predict 14%-27%), but these fracture strengths are still much higher than the experimental data. We then demonstrate that this experimental and theoretical discrepancy can be attributed to the presence of large-scale defects, such as those that may arise from oxidative purification processes. Simulations on multiwalled CNTs and tubes twisted prior to tensile loading show negligible effects on the fracture strength, which indicates that these are not the causes of low experimental values. The effects of chirality and tube diameter on fracture strengths are also investigated.