Irreversible deformation of carbon nanotubes under bending

Carbon nanotubes are expected to be one of future fiber materials with extr emely high tensile rigidity. Many reports concerning their physical and che mical 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 thermodynami cally unstable due to forming the local planar substructure. This is the re ason why it is the global minimum energy configuration of the original grap hine. The multi-walled nanotube with weak van-der-Waals type interaction be tween layers is, thus, necessary to keep its shape round for the large-size d tubes. Nano-scaled tensile tests of both the normal and the helical tubes show tha t 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 ela stic relation) and then buckles at a certain critical load whose behavior i s similar to the well-known macroscopic thin pipe over length scales. Two t ransition mechanisms related to topological changes of the basic carbon hex agons are observed; one is the creation of two pairs of pentagons and hepta gons, 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 buck ling behaviors notwithstanding lack of the geometric symmetry.