Structure Forming And Characterization Of Braided Auxetic Yarns

Auxetic yarns are yarns whose radius expands when the yarn is stretched axially.The traditional helical auxetic yarns is easy to untwist and the structure is unstable,which needs to be processed by heat setting in the later stage.In order to solve the above problems,we choose to use high speed braiding machine to prepare the auxetic yarns,in order to get a stable structure and more lasting negative Poisson’s ratio effect of auxetic yarns.Braided auxetic yarns can not only be used in daily protective products such as wrist guards and maternity clothing,but also in functional materials such as breathable materials and electromagnetic shielding materials.Therefore,this study was carried out.This paper focuses on the structure design of braided auxetic yarns and the optimization of process parameters.Based on the high-speed braiding machine,the braided auxetic yarns is prepared.At the same time,through the construction of the geometric model of yarn,the change curve of Poisson’s ratio of braided auxetic yarns is deduced.To prepare the composite braiding auxetic yarn with a more durable negative Poisson’s ratio effect,the specific research results are as follows:(1)To adopting high-speed braiding machine as the basic manufacturing machine,and adopt the combination of the core yarn,stiff yarn and wrap yarns to get stable performance braided auxetic yarns,and change the number of braided yarns,core yarn thickness and spindle speed of three process parameters,the preparation of braided auxetic yarns,using high speed camera stretching process,late in the way to deal with Image analysis,The Poisson’s ratio of braided auxetic yarns was measured and the relative Poisson’s ratio was plotted.The test results show that the braided auxetic yarns has the“pre-auxetic state” which the traditional helical auxetic yarns does not have,and the yarn directly presents negative Poisson’s ratio effect under low axial strain.At the same time,reducing the diameter of core yarn and the number of braided yarns roots can effectively improve the negative Poisson’s ratio effect of braided auxetic yarns.By increasing the spindle speed to a limited extent,it can not only ensure the obvious negative Poisson’s ratio effect,but also prepare more braided auxetic yarns in the same time.(2)The theoretical model of braided auxetic yarns was established: The change track of the center line in the process of braided auxetic yarns deformation was analyzed,and the trend of the overall diameter changing with the increase of axial strain was analyzed.Then the Poisson’s ratio was combined with the definition of Poisson’s ratio,and the correlation curve of Poisson’s ratio changing with the change of axial strain was drawn.The corresponding theoretical curve was obtained by taking into account the actual core yarn diameter,wrap yarns diameter,stiff yarn diameter and other relevant parameters.The Poisson’s ratio curves of braided auxetic yarns with the same technological parameters as the actual preparation are compared and analyzed.The three images have a good unity and consistency,which provides a theoretical basis for predicting the change of Poisson’s ratio of braided auxetic yarns and lays a theoretical foundation for the preparation of braided auxetic yarns with excellent performance;(3)Preparation and mechanism analysis of braided auxetic yarns with persistent negative Poisson’s ratio effect: The braided auxetic yarns was prepared into composite braided auxetic yarns.In the preparation process,core yarns of different diameters were used as independent variables to maintain the consistency of other variables such as the number of braided yarns and spindle speed.The Poisson’s ratio of tensile yarns of composite braided auxetic yarns was measured by means of photo recording and image analysis,and the correlation curve was drawn.The data showed that the critical state of Poisson’s ratio of composite braided auxetic yarns was basically the same as that of braided auxetic yarns.The minimum Poisson’s ratio has no obvious change,but the duration of negative Poisson’s ratio effect increases significantly,which provides a certain experimental basis for the subsequent production of related materials with more durable Poisson’s ratio effect.