Irreversible deformation of carbon nanotubes under bending

Abstract

Carbon nanotubes are expected to be one of future fiber materials with extremely high tensile rigidity. Many reports concerning their physical and chemical properties have been published, while there are not enough studies on mechanical properties yet. In the present paper, the structural stability and the mechanical deformation behavior of nanotubes are investigated using the molecular dynamics. From relaxation process of a nanotube made of one graphite sheet (graphine) at room temperature, a tube with a relatively large radius is thermodynamically unstable due to forming the local planar substructure. This is the reason why it is the global minimum energy configuration of the original graphine. The multi-walled nanotube with weak van-der-Waals type interaction between layers is, thus, necessary to keep its shape round for the large-sized tubes. Nano-scaled tensile tests of both the normal and the helical tubes show that they have an extremely high elastic modulus of about 0.5 TPa whose order has been observed in the previous experimental works. As bending tests, a vertical following force is applied to the free end of a single-walled nanotube cantilever. The tube responds linearly (linear elastic relation) and then buckles at a certain critical load whose behavior is similar to the well-known macroscopic thin pipe over length scales. Two transition mechanisms related to topological changes of the basic carbon hexagons are observed; one is the creation of two pairs of pentagons and heptagons, and the other is the motion of a pair of them. Since the transformed configuration has been found to be the local minimum energy configuration, it remains after unloading. The helical tubes provide almost the same buckling behaviors notwithstanding lack of the geometric symmetry.