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Title: Study of auxetic warp knitted fabrics based on re-entrant geometry
Authors: Zhao, Shuaiquan
Degree: Ph.D.
Issue Date: 2021
Abstract: Auxetic fabrics are those with negative Poisson's ratio, which will expand laterally when stretched or shrink laterally when compressed. Due to this counter-intuitive behaviour, they have received extensive research. Recently, various auxetic textiles including auxetic fibers, auxetic yarns, auxetic woven fabrics as well as auxetic knitted fabrics, have been studied. Warp knitting technology is more flexible in fabricating net fabrics which makes it very suitable for fabricating auxetic structures. However, only limited numbers of auxetic warp knitted fabrics have been reported so far. This thesis aimed to design, develop and study a novel class of auxetic warp knitted fabrics with obvious and stable auxetic effect using conventional warp knitted machinery. The commonly used reentrant structure was selected as the basic geometry to fabricate auxetic warp knitted fabrics. To provide the fabrics with reentrant frames, both elastic yarns and stiff yarns were used with a special designed lapping movement to form stiff underlaps and elastic underlaps. Finally, several types of warp knitted fabrics with reentrant structure were developed. After fabrication, all the fabrics were subjected to a heat setting process to keep their shape stable. Then, single tensile test was conducted on an Instron tensile machine to assess the auxetic behaviour of the fabrics. Auxetic durability of the fabrics was studied under repeating tensile test. The selected fabric samples were tested under repeating tension both in wale direction and course direction. All the samples were tested with a repeating cycle of 100 up to a given tensile strain of 25%. Then deformation behaviours of the auxetic warp knitted fabrics were studied based on geometrical analysis. Models of auxetic behaviours both in wale direction and course direction were established. Finally, to simulate the auxetic behaviour, much more precise models in two directions were built and simulated using UG 10.0 and ANSYS 16.0 respectively.
The following conclusions can be obtained from this study: (1) Reentrant geometry can be realized by using warp knitting technology. Special arrangement of elastic yarns, stiff yarns and binding yarns combining with design of chain stitches can be used to develop auxetic warp knitted fabrics. Heat setting process can offer the auxetic fabric with a stable structure. (2) All the fabrics exhibit an obvious auxetic behaviour within a wide range of tensile strain both in wale direction and course direction. Especially, when stretched in wale direction, the fabrics keep an auxetic behaviour till breaking and the best auxetic effect of the fabrics is achieved in wale direction. (3) Auxetic effect decreased with the increase of tensile cycles. All the samples can keep their auxetic effect after 100 times of repeating test within a repeating tensile strain of 25%. The auxetic effect at higher tensile strain is much less to be affected by tensile cycles compared to that at lower tensile strain. At the same time, auxetic behaviour in course direction shows better durability. (4) Deformation behaviours in both wale and course directions were analyzed based on the geometrical models and semi-empirical equations in two directions were established. The models could well predict auxetic behaviour of fabrics with similar structures. Influences of angles between underlaps and diagonal ribs on auxetic effect of the fabric were predicted. In wale direction, with the increase of the angles, auxetic effect of the fabrics decreased while in course direction, with the increase of the angles, auxetic effect of the fabric increased. (5) Finite element results show the same variation trends as experimental results. Although the results from simulation are different from that of experimental ones, the differences between them are very small, which means finite element models based on real fabric can be used to simulate auxetic behaviours of the fabrics.
Subjects: Machine knitting
Textile fabrics
Hong Kong Polytechnic University -- Dissertations
Pages: vi, xii, 117 pages : color illustrations
Appears in Collections:Thesis

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