Fabrication and characterization of braided auxetic yarns based on a high-speed braiding machine

In recent years, three kinds of auxetic yarns have mainly been developed, namely helical auxetic yarns based on the ring spinning system and the hollow spindle system and braided auxetic yarns based on the braiding system. However, these auxetic yarns have some drawbacks, such as an unstable structure or no obvious auxetic effect. In this paper, auxetic yarns were successfully braided by wrap filaments and a core filament based on a braiding system, which overcame the slippage problem in the traditional helical yarn structure. Yarns were spun by different structural parameters of the core filament, the wrap filaments and braiding filament, including the diameter ratio, braiding speed and different numbers of wrap filaments. The results showed that all parameters have an impact on the Poisson’s ratio of auxetic yarns and, in addition, the braided auxetic yarns can cause the yarns to be in the pre-stretching state; once the stretching effect occurs, the Poisson's ratio will directly change from a negative value. Since the new auxetic yarns were produced using a high-speed braiding machine and filaments, this would pave the way for their mass production. This provides a certain research basis for mass production of the stable auxetic yarn structure, and based on the realization of the pre-stretching state, it also provides more possibilities for future research.