Mechanical properties of carbon nanotube ropes with hierarchical helical structures

Hierarchical helical structures widely exist in both artificial and biological materials. Such nanomaterials as carbon nanotube ropes with hierarchical helical structures hold a promise for potential applications, for instance, in aerospace and medical engineering. In the present paper, a bottom-up theoretical model is established to investigate the mechanical properties of this kind of novel nanomaterials. The geometry of a rope with a hierarchy of chirality is first formulated. On the basis of the analysis of the internal forces and deformations of a single helical ply, a theoretical model is provided to predict the mechanical responses of multi-level helical materials. The effect of hierarchical helical structures is revealed by comparing the properties between a carbon nanotube rope with two-level helical structure and its counterpart bundle consisting of straight carbon nanotubes. The dependence of the mechanical properties of materials on the initial helical angles, fiber numbers, and handednesses at different structural levels are examined. Carbon nanotube ropes are found with higher deformation ability and elastic property which can be easily tuned via their microstructural parameters. This work helps understand the behavior of chiral materials and also provides inspirations for optimal design of advanced nanomaterials with hierarchical helical structures.