On the design and characterisation of low-stiffness auxetic yarns and fabrics

An auxetic material is one which exhibits a negative Poisson's ratio; it expands laterally when stretched longitudinally and contracts laterally when compressed longitudinally. The helical auxetic yarn is a novel fibre structure with a diverse range of potential applications. The unusual mechanical properties of the yarn can be determined by particular combinations of geometry and component material properties. This paper reports on the development of low-stiffness auxetic yarns and fabrics which offer a range of applications such as medical devices, particularly bandages, compression hosiery and support garments and fashion apparel. The mechanical performance of the yarns and fabrics is elucidated, with emphasis on the ability to exploit significant changes through a prescribed strain range. A yarn Poisson's ratio as low as -1.5 is demonstrated, and fabrics with in-plane and out-of-plane negative Poisson's ratios are illustrated. Stiffness is shown to be highly dependent upon yarn geometry.