Plasticity of Carbon Nanotubes under a Combined Axial and Torsional Stress

Plasticity of carbon nanotubes (CNT) under a combined axial and torsional stress is studied. We apply a dislocation theory to CNT under a combined axial and torsional stress. It has been known that a pure axial tension can change chirality from (n,m) to (n,m-1). It is now revealed that any of six dislocation directions that change chiral indices from (n,m) to (n+1,m), (n-1,m), (n,m+1), (n,m-1), (n+1,m-1) or (n-1,m+1) can become the most energetically favorable direction depending on applied combined stress. In the hexagonal carbon network, a 5/7/7/5 dislocation dipole can be nucleated by a π/2 rotation of a single C-C bond (Stone-Wales transformation) and the motion of a 5/7 defect which is equivalent to a dislocation core allows above transformations to occur. This work is a general and important extension of previous study of CNT under tension and can explain molecular dynamics studies reported so far. From our results, one can estimate a combined stress optimized for any of six transformations to occur. The present finding therefore opens the way to manipulate the chirality of CNT and consequently their electronic transport properties.