Mechanical properties of twisted CNT fibers: A molecular dynamic study

Carbon nanotubes (CNTs) are regarded as next-generation metamaterials with excellent mechanical properties. CNT fibers (CNFs), as one-dimensional materials assembled by large amount of CNTs, possess promising me-chanical properties and could potentially be used in many fields. However, due to the difference of fabrication methods, the twisting degree and length of CNTs in CNFs are distinct which is vital for mechanical design and optimization of super-strong CNFs. Thus, in this work, coarse-grained molecular dynamic simulation method is utilized to investigate the mechanical mechanisms of CNF with different twisting degree and CNT length. From the results of tensile simulations, it can be found that CNF possess successive bearing capacity via bond stretching and bond fracture bearing modes. Besides, mechanical properties of CNFs could be efficiently tuned by changing corresponding twisting degree. As for defective CNFs, it is shown that, due to the existence of short CNTs, the van der Waals interaction is improved. Furthermore, the transformation of CNTs from curve geometry to straight geometry could dramatically improve the stretchability of CNFs. And fracture stress and Young's modulus of CNFs decreases as twist lap increases, while the influence of twisting operation on toughness of CNFs is negli-gible. Besides, when CNF with different twist laps and delete percentage are compressed, a new phenomenon that a hole in the center of CNF would form when CNF are compressed is found which indicates that external loadings (tension and compression) could change mechanical behaviors of CNF. This work systematically investigates mechanical properties of CNFs with different degree of n and gamma and sheds lights into the construction and optimization of super-strong CNFs.