| Auxetic materials presented negative Poisson's ratio,which were different from traditional materials of positive Poisson's ratio.Due to the potential application in various fields,more and more scholars have been engaging in researching the auxetic materials and performances,including auxetic textile materials.In the past years,many scientists have made great efforts in the research of structure formation and property characterization of auxetic yarns and fabrics.The initially reported auxetic yarn was of two components with a core filament helically wrapped by a wrap filament.It was the difference in property and interact force between the two components that led to the increasing of contour diameter and auxetic behavior.According to the reported results,the complex yarn with a smaller initial helical angle presented a greater auxetic property.However,the free end part of complex yarn may present an unwinding behavior,which resulted in an unstable and incomplete structure.In addition,the residual torque from the spinning process could also lead to a kinking behavior.Both two problems above limited the helical auxetic yarn from theoretical researches to practical applications.At the same time,the interaction force between two components was not considered in the reported researches on both the geometrical model and finite element model of the helical auxetic yarns.It led to a difference between the theoretical values and the experimental results of Poisson's ratio.Thus,it is necessary to further analyze the compression force between two components of auxetic yarn to provide valuable information for material selection and structure design.This paper mainly focused on the structure formation and stability of the auxetic yarn.Three aspects in post-treatment process,structural design and preparation method were improved to obtain advanced auxetic yarns.The effect of structural parameters,including initial helical angle,diameter and modulus of yarn components,on the Poisson's ratio of the auxetic yarn were compared and analyzed.Finite element model and mechanical model were used to present the stretching mechanism under axial load including the variation of center line and radial strain of the auxetic yarn.Then,the predicting models could be used as a criterion for material selection and structural parameter design of auxetic yarn.Finally,the auxetic yarns were used as weft yarns to prepare woven fabrics.Testing and characterization of the basic properties of fabric samples were carried out to analyze the applicability of the auxetic yarn.Then,the potential advantages and expected effects of the auxetic yarns were put forward in the fields of wearable materials,coating materials and filtering materials.The results and conclusions of each part in details were as follows:(1)The first method was carried out based on the heat setting method of the post-processing technology of textiles.Third component of nylon filament with melting point of 80 ?C was added and the complex yarn with straight state was dealt with oven at the temperature that higher than the melting temperature.Then the Poisson's ratio under axial tension of the yarn after cooling was measured and calculated.Compared with the complex yarn without heat setting process,the results showed that both the structure and auxetic behavior of helical complex yarns were improved after heat setting process at 100 ?C as a result of the improved contacting effect.In the second method,a novel interlaced-helical auxetic structure was presented,where the core filament was helical wrapped symmetrically by two filaments.The novel composite yarn was prepared by the knitting machine.The axial tensile testing results showed that the interlaced-helical structure was more stable and auxetic than that of the helical wrapping yarn with the same materials and helical angle.Thirdly,the hollow spinning machine was selected to manufacture more stable helical auxetic yarn as a result of lower twisting and the ratio of the feeding speed and spinning tension of the two components.(2)Helical auxetic yarns with stable structure and negative Poisson's ratio were fabricated by three methods.The axial stretching and deformation property of helical auxetic yarn was analyzed with finite element software.The model included a cylindrical geometry structure of the core component and a helical geometry structure of the wrap component.The related parameters in the model such as the number of elements and the meshing size were verified.Effect of initial helical angle and Poisson's ratio of yarn components on the radial strain were compared and the results showed that the complex yarn presented better auxetic behavior with lower initial helix angle and Poisson's ratio of yarn components.During the axial stretching process,there were interaction forces between the yarn components,such as the compressive force between the contact surfaces during the stretching process.The effect on the radial strain,helical radius and helical angles of core component were analyzed by extracting a normal pressure on the contact surface and contour diameters of the model.In addition,a pair of helical auxetic yarns showed better expansion performance under axial tension and the porosity was being dynamic changed during the stretching process.(3)The contour diameter of the helical complex yarn increased and presented auxetic behavior due to the different property of the two components during the axial tensile process.In order to study the mechanism of this auxetic behavior,mechanical modeling of helical complex yarn was carried out.The relationship between the interaction force and structural parameters and the effect to the radial strain of the core filament were analyzed.Thus,the contour diameter and Poisson's ratio of complex yarns were calculated by considering the interaction force between the two components.The theoretical value and experimental results was verified to be in good consistency and it was expected to be a predicting model of Poisson's ratio of helical auxetic yarn.(4)The study of the structure formation and deformation behavior of auxetic yarn laid a foundation for its applicability.Therefore,woven fabrics with basic weaves were prepared with auxetic yarns.According to the testing and characterization results of fabric properties,the fabrics with unstable auxetic yarns presented the self-curling,self-folding and auxetic behaviors.Besides,the maximum value of negative Poisson's ratio reached to-0.5.However,the fabric composed of stable auxetic yarns by hollow spindle machine showed better properties in impact resistance and energy absorption.The breaking strain along the warp and weft direction was higher than the traditional fabrics and the behavior of stress concentration under loads was also greatly improved.Thus,the applicability of auxetic yarn could be expected in filter materials,coating materials,protective materials,fashion design and so on.In sum,the structure,deformation mechanism and applicability of auxetic yarns were studied in this work.The auxetic yarn with stable structure and negative Poisson's ratio could be continuously produced and the finite element model and mechanical model were provided to analyze the auxetic behavior.In addition,the discussion on the formability,properties and applicability of woven fabrics based on auxetic yarns was also beneficial to the development and application of related products of auxetic textile materials.It is expected that to be a reference for structural design,material selection and preparation of auxetic textiles. |
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