Study of auxetic woven fabrics

Abstract

The conventional fabrics have positive Poisson's ratio and upon stretching in one direction, they undergo contraction in the transverse direction which is known as lateral shrinkage. Unlike conventional fabrics, auxetic fabrics have the unusual property of lateral expansion which means that they possess negative Poisson's ratio or auxetic behaviour. The properties linked with the auxetic behaviour of auxetic fabrics include improved comfort and shape fitting at joint parts, increased porosity under stress, synclastic behaviour for better formability, etc. These counterintuitive properties make auxetic fabrics attractive for many applications such as, highly stretchable sports garment which can acquire different body shapes during movement or exercise, stretchable textile carriers, clothing with enhanced longevity and a fabric for clothing products which provide comfort and ability to mould and move easily in accordance with body movements. Up till today, auxetic fabrics have been produced based on two approaches. Firstly, using special auxetic yarns either in warp direction or in the weft direction and weaving technology to fabricate auxetic fabrics. However, auxetic woven fabrics developed to date based on this approach have some drawbacks. The major drawback associated with such fabrics is that there are very few auxetic yarns available and are expensive. Another drawback of such fabrics is that the auxetic behaviour of auxetic yarns cannot be transferred completely into the woven fabric due to the woven structural limitations. Therefore, in such fabrics, the smaller auxetic behaviour is achieved only in one direction. The other approach adopted to produce auxetic fabrics is based on using conventional yarns and knitting technology but most of the developed auxetic knitted fabrics also have certain limitations. They have a higher thickness, lower structural stability, lower elastic recovery and due to complicated geometrical structures, most of the auxetic knitted fabrics could not be produced on a larger scale. Auxetic fabrics produced by using the second approach have gained extraordinary curiosity of researcher in recent past years. Though auxetic fabrics have successfully developed by using conventional yarns and knitting technology, auxetic fabric developed by using conventional yarns and weaving technology is a research area that is still unaddressed. The auxetic woven fabrics made of conventional yarns having high extensibility and NPR in both principal directions reduced thickness, and better formability that can easily be shaped into garments are still not developed. Such innovative fabrics may have a great potential for clothing application as mentioned above. This study aimed to design, fabricate and analyze geometrically a novel class of stretchable auxetic woven fabrics using conventional yarns and machinery. The fabric structure was first designed based on auxetic geometries including foldable geometries, rotating quadrilaterals geometry and an approximation of re-entrant hexagonal geometry by using a combination of loose weave and tight weave within the unit cell of interlacement pattern, and then fabricated on a conventional rapier weaving machine by using readily and inexpensively available conventional elastic and non-elastic yarns. To confirm the auxetic behaviour, the obtained fabrics were tested on an Instron 5566 tensile testing machine. The engineering strains of the fabric structure in both tensile direction and transversal direction were calculated, and the Poisson's ratio was calculated at every 1-2% of tensile strain. The developed auxetic woven fabrics exhibited negative Poisson's ratio over a wide range of tensile strain. One of the obtained fabrics was also subjected to a geometrical analysis. The geometrical unit cell of the fabric structure was identified and the change in the geometry of the fabric structural unit cell at different tensile strains was observed when the fabric was stretched in two principal directions. In view of the observations, a geometrical model for each stretch direction was then proposed. Based on proposed geometrical models, the relationship between tensile strain and Poisson's ratio was established by constituting semi empirical-equations for both stretch directions. The constituted equations were validated by calculating Poisson's ratio at different tensile strains which showed that they fit well with experimental results. The constituted equations could, therefore, be used in the design of bi-stretch auxetic woven fabrics based on an approximation of re-entrant hexagonal geometry and prediction of their auxetic behaviour at different tensile strains. The following major conclusions can be drawn from this study: (1) The auxetic woven fabrics can be developed using conventional elastic yarns, non-elastic yarns and weaving technology. (2) The auxetic behaviour can be induced into the woven fabric structure by realizing auxetic geometrical structures into woven fabric structure through the creation of the phenomenon of differential shrinkage/ non-uniform contraction profile within the unit cell of the fabric structure. Such geometrical structures include foldable geometries, rotating quadrilaterals geometry and an approximation of re-entrant hexagonal geometry. (3) The phenomenon of differential shrinkage/ non-uniform contraction profile can be created by employing the combinations of loose weave and tight weave within the unit cell of interlacement pattern and by using elastic and non-elastic yarns. This phenomenon can be created in one direction to produce uni-stretch auxetic woven fabrics or in two directions to produce bi-stretch auxetic woven fabrics. (4) In the case of uni-stretch auxetic woven fabrics, the auxetic behaviour is achieved in one direction only, while in the case of bi-stretch auxetic woven fabrics, the auxetic behaviour is achieved in two directions. (5) The auxetic behaviour in case of bi-stretch auxetic woven fabrics is influenced by stretch directions and weft yarn arrangement. It is usually larger when the fabric is stretched along warp direction than that of when stretched along weft direction. and all elastic weft yarn arrangement. The larger auxetic effect is achieved for all elastic weft yarn arrangement. (6) In case of bi-stretch auxetic woven fabrics based on foldable geometry, the auxetic effect is significantly influenced by the float length of loose weave and weft yarn arrangements. The larger auxetic effect is produced by loose weave's float length (3). (7) The geometrical analysis of bi-stretch auxetic woven fabrics based on re-entrant hexagonal geometry showed that the rib segments of the re-entrant hexagonal unit cells are not kept constant under extension due to easy deformation of the fabric structure. Because of different deformation behaviour of fabric in the warp and weft direction, different geometrical models are required to estimate the deformation behaviour in each direction. Further, the auxetic effect of the fabric structure is greatly affected by geometrical parameters a and b which have different variation trends when stretched in the warp and weft direction.