Effect of randomly occurring Stone-Wales defects on mechanical properties of carbon nanotubes using atomistic simulation

The remarkable mechanical properties of carbon nanotubes (CNTs) have generated a lot of interest in recent years. While CNTs are found to have ultra-high stiffness and strength, an enormous scatter is also observed in available laboratory results. This randomness is partly due to the presence of nanoscale defects, heterogeneities etc, and this paper studies the effects of randomly distributed Stone-Wales (SW or 5-7-7-5) defects on the mechanical properties of single-walled nanotubes (SWNTs) using the technique of atomistic simulation (AS). A Matern hard-core random field applied on a finite cylindrical surface is used to describe the spatial distribution of the Stone-Wales defects. We simulate a set of displacement controlled tensile loadings up to fracture of SWNTs with (6, 6) armchair and (10, 0) zigzag configurations and aspect ratio around six. A modified Morse potential is adopted to model the interatomic forces. We find that fracture invariably initiates from a defect if one is present; for a defect-free tube the crack initiates at quite random locations. The force-displacement curve typically behaves almost linearly up to about half way, although there is no obvious yield point. Three mechanical properties-stiffness, ultimate strength and ultimate strain-are calculated from the simulated force and displacement time histories. The randomness in mechanical behaviour resulting only from initial velocity distribution was found to be insignificant at room temperature. The mean values of stiffness, ultimate strength and ultimate strain of the tube decrease as the average number of defects increases, although the coefficients of variation do not show such a monotonic trend. The introduction of an additional defect has the most pronounced effect on the randomness in mechanical properties when the tube is originally defect free. We also find that, for a given mean number of defects in the tube, the zigzag configuration has less strength and less ultimate strain on average, but more uncertainty in its stiffness and ultimate strain, compared with the armchair tube.