Auxetic Yarn Research Articles

Mechanical analysis of the auxetic behavior of novel braided tubular structures by the finite element method

Auxetic materials are characterized with counter intuitive mechanical behavior, that is, a negative Poisson's ratio, in which the material expands in the direction perpendicular to the elongation direction. This intriguing property can be used to engineer structures with various porosity for ‘smart filters’ where the permeability changes in response to exterior force. In this research, the mechanical behavior of braided tubular structures that comprise helical auxetic yarns (HAYs) has been investigated. The permeability of the structure can be altered with the application of different levels of tensile force. Both HAYs and biaxial structures are fabricated on a Maypole braiding machine. The novelty of this braided tubular structure is that its open area decreases when stretched due to the local auxetic effect. A multi-scale finite element model is established to analyze the auxetic effect and the relationship between tensile force and the open area. The open area of the biaxial structure is measured by a camera system that is installed next to an Instron machine while the sample is subjected to tension. The experimental results show that the open area in the biaxial structure is reduced due to the auxetic effect of each HAY. The model gives a reasonable prediction compared to the experimental data, even though deviation exists, which is probably due to the nonlinear behavior from contact and large deformation.

Performance of novel auxetic woven fabrics produced using Helical Auxetic Yarn

The auxetic materials and structures exhibiting negative Poisson’s ratio are essentially used in filtration, medical devices, energy absorbing applications, etc. The auxetic materials excel at traditional materials due to better mechanical performance. This paper presents the auxetic fabrics produced using Helical Auxetic Yarn (HAY). The HAY was developed by wrapping a stiffer yarn (multifilament Kevlar) on core yarn (multifilament Polypropylene), by varying wrap to core angle from 8° to 20°. The recent research works report the auxeticity of HAY woven fabric, but the scope is limited to a particular weave pattern with a certain warp to core angle. In this study, the developed HAYs were used to weave fabrics in four different weaving patterns. The woven fabrics were produced using HAY as warp and Kevlar as weft. Furthermore, auxetic effect exhibited by HAY and subsequent woven fabrics was studied. It was found that the lower wrap to core angle in HAY yielded higher auxetic effect. Moreover, the matt woven fabric developed using 8° HAY exhibited maximum auxetic effect. The developed structure can be effectively used for filtration or energy absorbing applications.

Numerical Analysis of Auxetic Yarn with Composite Matrix Interaction

A modelling method of auxetic yarn (Helical Auxetic Yarn type) cooperating with the composite matrix was presented. The two-component yarn surrounded by a flexible material is a fragment of composite, that consists of an auxetic fabric as a reinforcement and a flexible matrix. Such composite can be used in energy absorbing structures. The HAY as an auxetic material is characterized by a negative Poisson’s ratio. The stiff fiber spirally wrapped around the elastic core with a larger diameter gives this structure anomalous behaviour during tensile. Stretching of such yarn increases its dimensions in transverse directions. Helical Auxetic Yarns are sewn into fabric which is durable, flexible and light. The conducted analysis of the fragment of the composite reflecting one yarn allows for pre-adjustment of values of matrix material parameters. The purpose of the modelling is to determine the values of a composite matrix parameters that should allow the effective work of auxetic yarn. If the matrix were too stiff, it would block the capability of bending the core fiber during stretching and the auxetic phenomenon would be cancelled. The key problem is performing these analyses with appropriate value of the Young’s modulus of the composite matrix material to get the optimal composite structure. Finite Element Method analyses were conducted in LS-Dyna solver. Discrete model was build using hexagonal, solids elements. It was assumed that the matrix completely filled the spaces between the fibers, and at the outside it had the shape of the cylinder. The choice of such matrix shape allowed to carry out proper interpretation of the displacement values of nodes located at the boundary plane of the matrix model. The displacement of these nodes determines the proper work of the auxetic yarn. Relevant constitutive material models were used to reflect the work of the aramid wrap as well as the elastomeric core and matrix. Nodal displacement was applied to the one of the end of model to simulate tension under static conditions. FEM analyses were based on the implicit method, which is well suited for simulation of static processes, where final state is the most interesting. Based on the results of the conducted analyses, it was possible to choose the appropriate matrix material of the composite without preparation of many composite samples with various elastomer matrix and carrying out many experimental tests.

The manufacture and characterization of auxetic, self-curling, and self-folding woven fabrics by helical auxetic yarns

Auxetic fabrics with traditional filaments and auxetic structures have been provided by knitting method; however, the auxetic behavior and applicability of woven fabric with auxetic yarns remain to be studied. Thus, the paper aims to present the special characters of woven fabrics with heliacal auxetic yarns. Auxetic yarns with the maximum value of Poisson's ratio −0.88 were used as weft yarns to do the weaving by the semi-automatic loom. Then the properties of the fabrics have been tested and analyzed under tensions of different directions (warp, weft, and diagonal). The results indicated that the fabric presented auxetic effect with the maximum value of Poisson's ratio −0.3 under diagonal tension and also showed self-curling and self-folding behavior in natural state. Moreover, the relationship between properties and fabric weaves was also discussed and analyzed. It is expected that the study of fabrics with helical auxetic yarns could promote the practical applications of auxetic textiles such as the self-folding property for smart cladding materials.

Auxetic Yarn Made with Circular Braiding Technology

This paper reports a new type of auxetic yarn made with circular braiding technology to overcome the yarn slippage problem in the conventional helical auxetic yarn structure. Various auxetic yarn structures are firstly produced on a vertical circular braiding machine with 16 yarn carries by changing the number and arrangement of stiff yarn as well as diameter of elastic yarn and core yarn, and then tested under single and repeating tensile conditions. The results show that not only yarn slippage problem could be overcome in the newly developed auxetic yarn structure, but also an early activation and a higher magnitude of negative Poisson's ratio could be achieved. The results also show that all structural parameters including stiff yarn number and arrangement, core yarn, and elastic yarn diameter have an influence on the auxetic effect of the novel yarn structure. Since the new auxetic yarn is produced by using standard braiding manufacturing technique and conventional non‐auxetic yarns, there are few technological barriers for its large‐scale manufacture for practical application.

Finite element simulation of an auxetic plied yarn structure

Auxetic plied yarn is a novel type of yarn structure formed with two types of component yarns with different moduli. Although the effect of structural parameters on its negative Poisson's ratio behavior has been investigated by both experimental and geometrical analyses, the influence of material properties, including yarn tensile modulus and friction, has not been studied yet. In this work, these two factors are further investigated via finite element simulation. The finite element model is first created by using the commercial software ANSYS 15.0. Then, the simulation results are compared with the experimental and analytical data. Finally, the effects of the tilt angle of stiff yarn, yarn friction and tensile modulus are discussed. It is expected that the outcomes of this work would be useful to guide the design and fabrication of this type of auxetic yarn structure.

A Novel Interlaced-helical Wrapping Yarn with Negative Poisson’s Ratio

The two-component complex yarn with helical wrapping structure has been reported with auxetic behavior and unique performance, however, the residual torque could lead to the slippage of the wrap component from the core in the natural state and poor stability and applicability. Therefore, this paper aims to introduce a novel interlaced-helical wrapping yarn with negative Poisson’s ratio, stable structure, practicability and continuous preparation. The differences in geometry and auxetic behavior between helical structure and interlaced-helical structure were presented and analyzed. Both the structure stability and auxetic effect of the interlaced-helical complex yarn have advantages over the helical wrap yarn with same materials and initial wrap angle. It can be shown that the interlaced-helical wrap yarns with lower initial wrap angle and bigger tensile modulus of the stiff yarns will produce better auxetic effect. Besides, the deformation mechanism and the manufacturing process were described in detail. Several kinds of yarns have been fabricated to discuss the effect of initial wrap angle and tensile modulus on the Poisson’s ratio of the structure. The maximum value of Poisson’s ratio for the yarns manufactured was -2.8. It is also expected that auxetic yarns with novel braided structure can prompt the applications of auxetic textiles.

A theoretical analysis of deformation behavior of auxetic plied yarn structure

This paper presents a theoretical analysis of the auxetic plied yarn (APY) structure formed with two types of single yarns having different diameter and modulus. A model which can be used to predict its deformation behavior under axial extension is developed based on the theoretical analysis. The developed model is first compared with the experimental data obtained in the previous study, and then used to predict the effects of different structural and material parameters on the auxetic behavior of the APY. The calculation results show that the developed model can correctly predict the variation trend of the auxetic behavior of the APY, which first increases and then decrease with the increase of the axial strain. The calculation results also indicate that the auxetic behavior of the APY simultaneously depends on the diameter ratio of the soft yarn and stiff yarn as well as the ratio between the pitch length and stiff yarn diameter. The study provides a way to design and fabricate APYs with the same auxetic behavior by using different soft and stiff yarns as long as these two ratios are kept unchanged.

Study on the structure formation and heat treatment of helical auxetic complex yarn

Currently, auxetic yarns consisting of a core filament and a wrap filament are generally utilized as complex materials, which are attributed to residual torque. Thus, the main content dealt with in this paper was to analyze the structure formation and effect of heat treatment of helical auxetic complex yarn spun by a modified ring-spinning system. Firstly, an orthogonal experiment with three factors and three levels was carried out to study the effects of diameter ratio, yarn twists and initial helical angle on the structure and Poisson’s ratio of complex yarn. Secondly, a heat treatment process with polyamide filament with low melting point (80℃) was conducted to improve the structural stability of auxetic complex yarn. It was found that the heat treatment process can not only improve the structural stability of complex yarn, but also promote the expansion effect. By comparing experimental and theoretical results, it was found that the variation of Poisson’s ratio with axial strain had good consistency. Therefore, the heat treatment method can effectively solve the slippage problem of helical auxetic complex yarn.

Woven Fabrics Made of Auxetic Plied Yarns.

Auxetic plied yarns are specially constructed with two types of single yarns of different sizes and moduli. This paper investigates how to use these types of yarns to produce woven fabrics with auxetic effects. Four-ply auxetic yarns were first incorporated into a series of woven fabrics with different design parameters to study their auxetic behavior and percent open area during extension. Effects of auxetic plied yarn arrangement, single component yarn properties, weft yarn type, and weave structure were then evaluated. Additional double helical yarn (DHY) and 6-ply auxetic yarn woven fabrics were also made for comparison. The results show that the alternative arrangement of S- and Z-twisted 4-ply auxetic yarns in a woven fabric can generate a higher negative Poisson’s ratio (NPR) of the fabric. While the higher single stiff yarn modulus of auxetic yarn can result in greater NPR behavior, finer soft auxetic yarn does not necessarily generate such an effect. Weft yarns with low modulus and short float over the 4-ply auxetic yarns in fabric structure are favorable for producing high NPR behavior. The weft cover factor greatly affects the variation of the percent open area of the 4-ply auxetic yarn fabrics during extension. When different kinds of helical auxetic yarns (HAYs) are made into fabrics, the fabric made of DHY does not have the highest NPR effect but it has the highest percent open area, which increases with increasing tensile strain.

Large-scale manufacturing of helical auxetic yarns using a novel semi-coextrusion process

This paper introduces a novel extrusion process for manufacturing helical auxetic yarn (HAY). A range of semi-coextruded HAYs have been manufactured in a cost-effective, consistent and readily usable form. The semi-coextruded HAYs were compared to the conventional spun yarns in terms of tensile properties and auxetic behavior. The results show the presence of the auxetic effect in newly fabricated semi-coextruded HAYs. Similar to the traditional spun HAYs, the new HAYs are sensitive to parameters such as the initial wrap angle, the core/wrap diameter ratio and component moduli. Importantly, a few new manufacturing parameters have been identified for tailoring the auxetic behavior of the semi-coextruded HAYs. The semi-coextruded HAYs are auxetic when an instantaneous true Poisson’s ratio analysis method is applied. The semi-coextruded HAYs give a larger maximum negative Poisson’s ratio than the conventional HAYs due to the advantages of the pre-formed helical wrap structure.

Tensile and Deformation Behavior of Auxetic Plied Yarns

This paper presents an experimental study of tensile and deformation behavior of an auxetic plied yarn structure. The basic configuration of the plied structure formed with two soft yarns and two stiff yarns was firstly introduced. Based on this 4‐ply helix structure, six kinds of yarn samples were made by different combinations of soft yarns, stiff yarns and twist levels. These yarn samples were first tested and the effects of different design parameters were analyzed. Apart from the tensile test, changes in the internal structure of two samples upon stretching were also studied through microscopic examination. Finally, double helix yarn and 6‐ply auxetic yarn were made for a comparative purpose to investigate the effect of helical structure. The study shows that all of the 4‐ply auxetic yarn samples exhibit negative Poisson's ratio (NPR) throughout the entire tensile process, and the magnitude of NPR gets larger with a smaller soft yarn diameter, a higher tensile modulus of stiff yarn and a lower twist level. The study also shows that auxetic behavior inside the 4‐ply structure is generated by the interplay between the soft yarns and stiff yarns and 4‐ply auxetic yarns have the advantages to produce immediate auxetic effect upon stretching.

Superelastic Helical Auxetic Structures with High Values of Poisson's Ratio

This paper introduces a novel superelastic shape memory alloy used yarn structure derived from the helical auxetic yarn (HAY) to obtain more stable structures and high values of poisson's ratio. The geometry of structure is defined and producing process described. A range of structures have been manufactured that vary the geometric parametre of structure. An experimental test bench has been set up and a systematic study has been completed to evaluate the effect on the auxetic behaviour of the geometry. Superelastic wrap wires were spun around the rubber core filament in same helical angel but different diameter ratio. Experimental results showed that the structure expanded transversely under tension and existed high negative Poisson's ratio values. Combination of superelastic and auxetic behaviours in textile structure can be useful in many applications, especially in medical and technical textiles applications.

Parametric sensitivity analysis to maximise auxetic effect of polymeric fibre based helical yarn

Studies on designing polymeric fibres based helical auxetic yarn (HAY) to maximise their auxetic effect are yet to propose optimised design configurations for general impact mitigation applications. This study therefore presents optimal design parameters through analytical calculations and finite element (FE) method. Three main design parameters were considered which includes Poisson’s ratio, core/wrap diameter ratio, and starting wrap angle. The Poisson’s ratio of the HAY was calculated by measuring its total diameter at a given rate of strain. The investigation found here to be a starting wrap angle of a HAY (critical angle) that resulted in the highest possible exhibiting of the auxetic effect. The critical angle was determined to be 7°, and a maximum NPR of −12.04 was achieved with this design.

Dynamic thermo-mechanical and impact properties of helical auxetic yarns

Abstract This paper presents an experimental investigation of the dynamic thermo-mechanical and impact properties of helical auxetic yarns (HAYs). A series of thermoplastic polyurethane (TPU) core fibres fabricated using an extrusion process have been wrapped with either ultra-high-molecular-weight polyethylene (UHMWPE) wrap or stainless steel wire wrap to form helical auxetic yarns. Dynamic mechanical analysis (DMA) measurements indicated that the core/wrap diameter ratio and the initial wrap angle influenced significantly the dynamic thermo-mechanical behaviour of HAYs. The impact test results have shown that the fibre property, impact velocity and the initial wrap angle had great effect on the impact response of a HAY. Importantly, in this work it is shown that an optimal wrap angle can be found to give the best combination of stiffness, energy absorption and auxetic performance of HAYs.

Varying the performance of helical auxetic yarns by altering component properties and geometry

This paper presents a systematic study of the helical auxetic yarn (HAY) via careful in-house fabrication and characterisation of a wide range of polymeric fibres and yarns. It provides a better understanding of the auxetic behaviour of the HAY in order to tailor their properties for specific applications. The study focused on three parameters: component moduli, the core/wrap diameter ratio and the initial wrap angle. The results show that a larger difference in component moduli, a higher core/wrap diameter ratio and a lower initial wrap angle can produce a larger maximum negative Poisson’s ratio value and thereby a better auxetic performance for HAYs. All three parameters could be carefully utilised when in combination to achieve the required auxetic behaviour of HAYs. Moreover, the instantaneous true Poisson’s ratio analysis accurately presents the instantaneous behaviour of highly strain dependent HAYs.

Design strategy for auxetic dual helix yarn systems

Auxetic materials exhibit negative Poisson’s ratio and they expand in the lateral direction on stretching. In the present study, the auxetic behavior of dual helix yarn (DHY) system has been predicted through energy minimization approach. It has been proposed that the interplay between the bending and tensile energies is responsible for the generation of the auxetic behavior in a typical DHY system. The DHY system can be potentially tailored for a large negative Poisson’s ratio under a broader “window” of axial tensile strains by simply downsizing the diameter of carbon based wrap yarn from micron to nanoscale. A comparison has been made between the theoretical and experimental results obtained from the literature.

The fabrication and mechanical properties of a novel 3-component auxetic structure for composites

Abstract Functional auxetic composite materials can be fabricated from conventional or from auxetic components. The helical auxetic yarn (HAY) is a recently invented auxetic reinforcing component for composite materials. This paper investigates the Poisson's ratio behaviour of a further development of the HAY, needed for many practical applications. The 3-component auxetic yarn is based on a stiff wrap fibre (the first component) helically wound around an elastomeric core fibre (the second component) coated by a sheath (the third component). The resultant structure can overcome problems such as slippage of the wrap and changes in wrapping angles previously encountered during the manufacture and utilisation of the two-component HAY. The mechanical performance of conventional and novel systems is investigated; with emphasis on the differences between the engineering and true Poisson's ratio. The importance of the utilisation of a true tensile modulus and a true Poisson's ratio is demonstrated. This is the first time reported in the literature that an experimental auxetic effect analysis of HAYs was carried out by comparing true and engineering Poisson's ratio. We show that depending on the coating thickness of the third component, the 3-component auxetic system can demonstrate auxetic behaviour, and the coating thickness can be employed as a new design parameter to tailor both the Poisson's ratio and modulus of this novel composite reinforcement for a wide range of applications.

Study on negative Poisson’s ratio of auxetic yarn under tension: Part 2 – Experimental verification

Auxetic yarn was successfully produced by a wrap filament and a core filament based on a ring-spinning system, which showed an auxetic effect when stretched, that is, expanding transversely under tension. Yarns were spun by different structural parameters of the core filament and the wrap filament, including diameter ratio, helical angle and tensile modulus of the wrap filament. Corresponding tensile tests were utilized to analyze the effects of structural parameters and performance on outer contour deformation of auxetic yarn. Experimental results showed that the outer contour of auxetic yarn expanded transversely under tension and there existed significant negative Poisson's ratio values in the Poisson's ratio-strain curve, and it was found that the auxetic effect became more observable with a higher diameter ratio of the core filament to the wrap filament, and lower helical angle and larger tensile modulus of the wrap filament. Moreover, compared with theoretical results by theoretical modeling in Part I, good relations of Poisson's ratio-strain curves were obtained between experimental and theoretical results. This proves the effectiveness of spinning auxetic yarn by selecting reasonable components of yarn and it is feasible to design yarn with a negative Poisson's ratio effect.

Study on negative Poisson’s ratio of auxetic yarn under tension: Part 1 – Theoretical analysis

The main content dealt with in the paper is to make a theoretical analysis of the contour diameter of auxetic yarn under tension, which expands transversely under axial tension. The auxetic yarn is designed by two components, that is, a core filament and a wrap filament. Compared with the wrap filament, the core filament is thicker, softer and much more extensible. In the initial status, the core filament is straight in the center of the auxetic yarn and the wrap filament wraps the core filament helically in a certain helical angle. Based on theoretical analysis, the formula of negative Poisson’s ratio of stretched auxetic yarn is obtained. It is found that the value of Poisson’s ratio is related to axial strain, helical angle and the diameter ratio of the core filament to the wrap filament. An applied example is presented to verify the Poisson’s ratio value-strain curve of auxetic yarn. In order to better investigate effects on negative Poisson’s ratio, the effect of different parameters of yarn, including mesh size, diameter ratio, tensile modulus and helical angle of the wrap filament, is investigated using finite element analysis based on ABAQUS software to analyze the negative Poisson’s ratio of auxetic yarn when stretched. It verified that yarn spun by two different components produced a negative Poisson’s ratio effect under tension and structure parameters of auxetic yarn significantly affected negative Poisson's ratio values. It is expected that this study could help us understanding the deformation mechanism of auxetic yarn under axial tension load.