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Title: Study of auxetic warp-knitted spacer fabrics
Authors: Wang, Zhengyue
Degree: Ph.D.
Issue Date: 2015
Abstract: Conventional fabrics usually exhibit a positive Poisson’s ratio (PR), i.e. they laterally shrink when stretched and laterally expand when compressed. On the contrary, auxetic fabrics exhibit a negative PR, i.e. they laterally expand when stretched or laterally shrink when compressed. The auxetic behaviour equips fabrics with a lot of enhanced properties such as enhanced formability, enhanced air permeability and reduced garment pressure, etc. The enhanced properties make auxetic fabrics very attractive for many applications in many different fields such as functional garment, health care and sports, etc. Although some auxetic fabrics have been designed and developed in recent years, some limitations still exist. In addition to low auxetic effect, most of auxetic fabrics reported in literature are unstable. Some of them lost their auxetic effect even after one stretch. So far systematic investigations on the auxetic fabrics are still missing. This study aimed to develop a new kind of auxetic fabric with good and stable auxetic effect, and to systematically investigate the deformation behaviour of these new auxetic fabrics. Three warp-knitted spacer fabrics were selected to produce auxetic fabrics in this study due to their stable structure and wide usage. Through a study of the auxetic geometries, a new geometry was first specially designed for manufacturing warp-knitted spacer fabrics. Then the base warp-knitted spacer fabrics were fabricated on the double needle bar Rashcel machine. The base fabrics were transformed to have the designed geometry by compression and heat setting post-processing to obtain auxetic warp-knitted spacer fabrics. To investigate the properties of the developed auxetic fabrics, a series of experiments was carried out by using an Instron 5566 machine. The auxetic effect, tensile properties and formability of the fabrics were measured and analysed. The fabric deformation behaviour under a tensile condition was also investigated by observing and examining the unit change of outer layer. Two geometric models were then built to predict the deformation behaviour of the auxetic warp-knitted spacer fabrics in the course and wale directions. Finally, the precise outer layer geometry was obtained with a Micro X-ray CT machine and the models were built based on the precise geometry with the Finite Element (FE) Method using ANSYS 13.0 software for simulating the deformation behaviour of the auxetic fabrics.
After the experimental and theoretical investigations, the main conclusions of this study can be drawn as follows: (1) The auxetic warp-knitted spacer fabrics can be successfully developed based on the warp-knitted spacer fabrics with the compression and heat setting method to achieve the specially designed auxetic geometry. (2) The auxetic warp-knitted fabrics showed very good auxetic effect in all directions in the outer layer planes. The best auxetic effect was found in the course direction, in which a negative PR of -2.5 was obtained. The auxetic warp-knitted spacer fabrics also showed very good retentivity of its auxetic effect. The auxetic behaviour of the fabrics tended to be stabilized after the second tensile cycle and about 65% of auxetic effect was still kept after ten tensile cycles. (3) The auxetic effect of auxetic warp-knitted spacer fabrics was affected by both the outer layer and spacer layer structures. Better auxetic effect can be obtained with simpler spacer layer structure, smaller angle α0 and β0 and longer rib lengths. (4) The auxetic warp-knitted spacer fabrics showed much better formability compared with the conventional warp-knitted spacer fabrics. For the fabric with the same thickness and materials, better auxetic effect leads to better formability of the fabric. (5) The geometrical models of the auxetic fabrics showed that in specific rib lengths, when extended in the course direction, the PR is only affected by α0, and when extended in the wale direction, the PR is only affected by β0. The increase of α0 can lead to the reduced auxetic effect and the decrease of β0 can maintain auxetic effect for a larger range of tensile strain. (6) The FEM model showed that the simulated deformation was in close agreement with the experiment deformation, and the simulated negative PR value fits well to the experiment negative PR value.
Subjects: Textile fabrics -- Testing.
Textile fabrics -- Mechanical properties.
Hong Kong Polytechnic University -- Dissertations
Pages: xviii, 163 leaves : illustrations (some color) ; 30 cm
Appears in Collections:Thesis

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