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.

The main content of this article theoretically analyzes the yarn diameter and Poisson's ratio variation of braided auxetic yarns under axial strain. Braided auxetic yarns are composed of three components: wrap yarns, the core yarn, and the stiff yarn. The core yarn has a larger diameter and smaller elastic modulus than the other two kinds of yarns, but its rigidity is lower. There is a lack of theoretical basis for predicting the Poisson's ratio of braided auxetic yarn. Based on the geometric model, the curve of the Poisson's ratio has been determined and the Poisson's ratio trend of braided auxetic yarns is deduced, aiming to predict the Poisson's ratio of braided auxetic yarns under axial strain with accurate yarn parameters. The experimental results show that the minimum Poisson's ratio of theoretical value and actual value appears under the same axial strain, and the variation trend of Poisson's ratio of both values is consistent. The Poisson's ratio curve of braided auxetic yarns is predicted, which lays a theoretical foundation for the development of auxetic materials and also provides more possibilities for subsequent research.