The influence of fiber migration on the mechanical properties of yarns with hierarchical helical structures

An analytical method is presented for studying the influence of fiber migration on the mechanical properties of a yarn with two-level helical structures. The theory of ideal migration is applied to the seven-ply yarn, which results in the exchange of the interior structure of the yarn. A bottom-up method for analyzing the internal forces and stresses of the yarn under axial tension and torsion is developed. The influence of fiber migration is demonstrated by making contrast between the mechanical responses of carbon nanotube yarns with and without fiber migration. The numerical results show that there is a periodical non-monotonic variation in both the internal forces and the stresses with the length of yarn. A stress concentration is revealed around the half-cycle migration point and the one-cycle migration point. It is shown that the chirality, initial helix angle, and the fiber migration pattern can be used to control the mechanical performance of yarns.