Study On Structure Formation And Modeling Of Negative Poisson’s Ratio Yarn

Negative Poisson’s ratio yarn is a new kind of yarn which will expand in radial direction under axial stretch, and it extends applied products for different properties with traditional yarn becoming thinner under axial stretch, especially for antimagnetic shield, water-repellency, warmth retention materials and other functional products materials. The properties of its products (woven, knitted and so on) are much more superior to traditional yarn. At present, researchers have been focusing on yarn’s fiber transfer and distribution, evenness, hairiness, strength, and production efficiency. There are less researching on yarn’s structure expansion under tension. It is a challenge to traditional yarn and also the paper’s innovation.This paper is to clearly interpret the mechanism of auxetic yarn’s negative Poisson’s ratio effect by the method of yarn’s design, manufacture and theoretical analysis, exploration for structural parameters for yarn with negative Poisson’s ratio effect based on the finite element analysis method, guidance to use the ring spinning system to manufacture yarns with negative Poisson’s ratio, establishment of geometry model of auxetic yarn at tensile conditions. So, the thesis mainly carried out the following specific researching work:(1)A negative Poisson’s ratio yarn model is designed by the method of finite element analysis software ABAQUS. The negative Poisson’s ratio yarn has two components. One component is wrapped yarn which is thinner, harder and has lower elongation, and the other component is core yarn which is thicker, softer and has higher elastic elongation. The wrap yarn winds around the core yarn. The paper also designed drawing simulation experiments by ABAQUS to obtain the deformation and change of the stress and strain under tensile load of negative Poisson’s ratio yarn. The change rule of the negative Poisson’s ratio yarn’s diameter was obtained from analysis of the database, then the change of Poisson’s ratio with the gradual increase of axial strain is derived.(2)To spin negative Poisson’s ratio yarn, this paper designed a new complex spinning technology in which there is an extra roller for feeding wrap yarn and controlling the feeding speed, where there is a rectangular groove on the right side of front upper roller. The wrap yarn was held by the outer roller and fed into the front upper roller’s rectangular groove, and then gathered at a certain angle with the core yarn vertically outputted from the center of front rollers, and finally the complex yarn was twisted to be yarn. Moreover, the paper studied the effects of ring spinning process parameters on the negative Poisson’s ratio yarn, and the influence rules of the speed of front rollers, the speed ratio of front roller to the outer rollers, spindle speed and the quality of traveler on the negative Poisson’s ratio yarn properties was obtained.(3)Finally, the curve of yarn’s Poisson’s ratio changes with the axial strain increasing was theoretically derived, and it was compared with the measured value to be modified. Conclusions can be drawn from the above researching work as following:(1) In Abaqus finite element analysis, the mesh size is smaller, the precision of the model is higher. But when the size decreases to a smaller size, the results of the analysis become stable such as stress and strain, etc. It shows that the density of the mesh has reached the accuracy requirement. In this paper the core yarn mesh size is0.06and the wrapped yarn mesh size is0.03(SI:mm);(2)Based on the mesh density meeting the demand of the data accuracy, this thesis has discussed the influence of the model’s parameters and draws following conclusions:①In ABAQUS finite element analysis, the smaller the load loading rate is, the more obvious the negative Poisson’s ratio effect of auxetic yarn is. In this paper, the tensile speed is set as follows:V1,=V2=0, V33=225(SI:mm);②On the basis of other structural parameters being constant, when the diameter ratio of core yarn to the wrapped yarn becomes larger, the negative Poisson’s ratio effect of auxetic composite yarn becomes more obvious;③The difference of tensile modulus between core yarn and the wrapped yam is bigger, i.e., the wrap yarn’s tensile modulus is higher than the core yarn’s tensile modulus, negative Poisson’s ratio effects of negative Poisson’s ratio yarn will be more significant;④The smaller the wrap yarn’s wrapping angle is, the more significant the negative Poisson’s ratio effect of negative Poisson’s ratio yarn is;(3)Based on the modified ring spun system, it added an additional set of rollers to control the wrap yarn and a small groove was opened on the front up roller, then, the auxetic yarn designed can be manufactured successfully. The influence of the spinning parameters on auxetic yarn’s structure and properties was also studied. When the spinning parameters are set as follows: front rollers’ speed as68.5r/min, additional rollers’ speed as76.5r/min, spindle speed as2575r/min and quality of traveler as0.15g, Poisson’s ratio value of the practical yarn is as low as-2.41.(4)Theoretical calculation of auxetic yarn’s Poisson’s ratio value against axial strain is conducted. Particular example is analyzed in this thesis in which the diameters of the core spandex filament and the wrap nylon filament yarn are0.52mm and0.13mm respectively, the Poisson’s ratio values of the core and wrap are0.475and0.3respectively, the initial wrap angle is30°. It is shown that the minimum value of Poisson’s ratio is obtained at the axial strain1/cos θ0-1, there exist good accordance between theoretical results and experimental results of the auxetic yarn’s Poisson’s ratio. It is helpful to analyze the structure and Poisson’s ratio of soft-hard ply-yarn under tension.