Auxetic materials expand in the transverse direction under tensile loading in the longitudinal direction, or contract when compressed, in contrast to the conventional materials. [1] Negative Poisson's ratio (NPR) was reported in natural materials by Viogt in the late 19th century. [2] Evans et al. advised that NPR structures and materials could be renamed as "auxetics" for convenience from the Greek word "auxetos." [3] The auxetic characteristics have been reported on fundamental theory in 1989, [4] and different structures including re-entrant structure, [5] folded structure, [6] and honeycomb structure. [7,8] Also, many auxetic materials have been discovered up to date. Iron pyrites were found auxetic with the Poisson's ratio of À0.14, [9] polyurethane foams were reported for a NPR value of À0.7, [5] and warp-knitted and weft-knitted fabrics were found to demonstrate NPR effect too. [10,11] Alderson and Evans experimented on microporous ultrahigh molecular-weight polyethylene (UHMWPE) and found the Poisson's ratio ranged between 0 and À1.24. [12] There are two general approaches for making auxetic textiles. The first one is using the conventional yarns with specially designed fabric structures, such as the reentrant structures and rotating structures for knitted fabrics [13] and bistretch auxetic woven fabrics with re-entrant hexagonal geometry. [14] The other approach is to use auxetic yarns directly to make fabrics. Some researchers produced auxetic woven fabrics using double helix yarns, and their studies showed that such fabrics demonstrated the some level of auxeticity and increased porosity. [15,16] An out-of-register double helical yarns were used to produce double-pick woven fabrics by Miller et al. [17] which showed that the two-layer fabric assembly had the auxetic effect with the Poisson's ratio to be À0.1. In addition, Hu and coworkers [18,19] created an auxetic yarn with a fourply structure with the Poisson's ratio of about À2.0, and it was used to make woven auxetic fabrics which demonstrated much smaller auxeticity. It has to be said that despite the progress in making fabrics from auxetic yarns, the responses of the fabrics under tensile loading need further clarification. High modulus fibers together with an elastic core element are the most commonly used materials for manufacturing auxetic yarns. Helical auxetic yarns (HAYs) are one type of the most promising yarns for exploitation according to Hook. [20,21] HAYs have been made of two or more plies of component yarns with different diameters and with extremely different elastic moduli. These plies were twisted together, with the thinner and stiffer yarn wrapping over the thicker and more elastic core ply. When stretched, the two types of plies would swap their roles, leading to a situation where the thinner and stiffer ply became the straight core and the thicker and more elastic ply turned into the binder, as shown in Figure 1. However, it remains to be a challenge to produce stable helical yarns, with matched lengths of the two yarn plies, t...
Auxetic materials, including textiles, exhibit a negative Poisson’s ratio (NPR), which is of interest for many applications. This research aims to optimize the structural parameters of helical auxetic yarns (HAYs) and to evaluate the auxetic performance of these yarns. The research reports on the improvement of auxetic yarn quality and the yarn auxeticity through studying the effect of helical angles, diameter ratio and tensile moduli of the two plies, as well as the binder filament feeding. The maximum NPR of the optimized auxetic yarns was experimentally achieved as low as –9.6, with the helical angle of around 14.0° on average using the optimal machine setting. The optimized yarn parameters enabled the making of high-quality auxetic yarns with a wider range of machine settings than before. In parallel, theoretical and numerical studies were carried out for the engineering design of auxetic yarns, which enabled comparisons among the experimental results, calculated results and results from finite element analysis. The comparison showed that a lower initial helical angle, higher tensile modulus of the wrap ply and lower tensile modulus of the core ply led to a higher auxetic effect. A new finding is reported in that a concave relationship between the diameter ratio and the NPR was discovered. The results of this study could assist researchers in producing HAYs, and this type of HAY could be used for many potential applications, such as filtration and impact protection.
The paper presents a study on woven fabrics made of helical auxetic yarns (HAYs) and their key factors on Poisson’s ratio under tension. The work aims to create and evaluate auxetic woven fabrics with optimal parameters for achieving better auxeticity including weave structure, wrapping angle of the auxetic yarn, thickness of the auxetic yarn and properties of the warp yarn. The maximum negative Poisson’s ratio (NPR) of the woven fabric can be achieved as low as -2.92 for experiments. Then, a numerical study has been carried out as well to assist the development of auxetic woven fabrics. The findings of this paper showed longer float length, lower wrapping angle of the auxetic yarn, a thinner diameter of the auxetic yarn as well as lower tensile modulus of the warp yarn led to higher auxetic behaviour. This can also provide a reference for researchers to select the best parameters for producing the auxetic woven fabrics.
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