A study of novel tubular braided structure with negative poisson's ratio

Abstract

Auxetics are the materials and structures that display a negative Poisson's ratio. Differing from the common materials which tend to be thinner when stretched, auxetics will actually expand in the transverse direction upon stretching. This novel and counterintuitive behaviour allows auxetics to exhibit a series of improved material properties, such as increased shear stiffness, higher energy absorption, enhanced acoustic behaviour and superior indentation resistance, compared with conventional positive Poisson's ratio materials. Thus, in the past three decades, auxetics have gained great popularity in research and numerous studies have been conducted to explore the nature of them. However, none of auxetic braided structures has been reported among the considerable number of auxetic materials and structures proposed, fabricated, or synthesized in the previous studies. The study on structural characterization, manufacturing process and structure-properties relationships of auxetic braided structures are still lacking. This study then was set to introduce auxetic effect into the field of braid, notably the field of tubular braid. The aims of this study were to propose novel tubular braided structures with negative Poisson's ratio, to explore the nature of their deformation mechanism and to investigate the effect of the structural parameters on the auxetic behaviour of developed braid. In order to reach the above goals, several types of auxetic braids were designed and fabricated using a standard or modified circular braiding technology. In the meantime, preliminary experimental investigations were conducted on those fabricated auxetic braids in order to evaluate and confirm the negative Poisson's ratio effect of them under tensile conditions. To examine the influence of the structural parameters on the auxetic behaviour of developed braid, a theoretical analysis on structure-property relationships was also adopted besides the experimental investigation. Based on the established analytical model, the Poisson's ratio of the structure at any given longitudinal strain can be calculated, and the effects of three structural parameters, i.e., the initial wrap angle, component yarns diameter and the Poisson's ratio of component yarns, on auxetic behaviour are studied. With conducted experimental investigation and theoretical analysis, it is possible to optimize the developed structure for desired performance criteria. It is expected that such a study could shed light on the development of the auxetic braided structures and provide an alternative way to fabricate auxetics from positive Poisson's ratio materials. Meanwhile, the developed new kinds of auxetic braids may solve the problems of unsolid knots that exist in currently commercialized medical sutures, shoelaces or secure threads.