|Author:||Ng, Wing Sum|
|Title:||A study of auxetic plied yarns and their resultant auxetic fabrics|
|Advisors:||Hu, Hong (ITC)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Textile fabrics -- Mechanical properties
|Department:||Institute of Textiles and Clothing|
|Pages:||xiv, 144 pages : color illustrations|
|Abstract:||Poisson's ratio in tensile deformation is one of the fundamental properties of textile materials. Over the past few decades, science and technology have been crossed to alter the positive Poisson's ratio behavior in conventional materials. Innovative auxetic textiles have been developed utilizing the auxetic behavior to create unique mechanical properties for various kinds of specific applications. Recently, auxetic plied yarn structure has been proposed and fabricated. Arranging two groups of yarns with different thickness and stretching properties in a specific configuration, different types of auxetic plied yarn structure can be developed to exhibit negative Poisson's ratio (NPR) behavior. This is a very interesting field of research but remains largely unexplored. Therefore, this project is conducted with the aim to study this novel kind of auxetic plied yarn structure and their resultant auxetic fabrics. A range of 4-ply auxetic yarn samples that vary in material properties and geometry were fabricated and subjected to tensile test to evaluate the effects of different design parameters on the tensile properties and auxetic behavior of the 4-ply auxetic yarn structure. Additional double helix yarn and 6-ply auxetic yarn samples were made for a comparative purpose to investigate the effect of helical structure under monotonic and cyclic tensile loading. The study shows that under proper diameter ratio, 4-ply auxetic yarns exhibit a NPR during extension, and the magnitude is greater with a smaller soft yarn diameter, a larger stiff yarn diameter, a higher tensile modulus of stiff yarn and a lower twist level. By comparing the auxetic behavior of the three helical auxetic yarns (HAYs), the results reveal that 4-ply auxetic yarns have the advantages to produce immediate auxetic effect upon stretching. Under cyclic loading, three HAYs demonstrate a similar elastic recovery behavior regardless of their helical structures and extension levels.|
Apart from the tensile test, changes in the internal structure of auxetic plied yarns upon stretching were investigated through microscopic examination. The study shows that auxetic behaviors inside the 4-ply and 6-ply structures are generated by the interplay between the soft yarns and the stiff yarns. Stiff yarns with higher tensile modulus clearly demonstrate a higher migration intensity inside the 4-ply auxetic yarn structure. Finally, 4-ply auxetic yarns were further incorporated into a series of woven fabric to evaluate the effect of auxetic plied yarn arrangement, weft type, 4-ply auxetic yarn properties and weave structure on the auxetic behavior and percent open area of the resultant fabrics. Additional DHY and 6-ply auxetic yarn fabric samples were made to compare with the fabric sample made of 4-ply auxetic yarn. The results show that different kinds of auxetic plied yarn arrangement produce similar auxetic effect, while higher stiff yarn modulus results in a higher auxetic behavior but finer soft yarn does not necessarily generate a higher NPR. Weft yarns with low modulus and short float are favorable to produce high auxetic effect. On the other hand, weft cover factor has a great influence on the percent open area of the auxetic fabrics made of 4-ply auxetic yarns. Open pore properties of the fabric samples made of various HAYs are vastly different. However, such differences are not correlated to their auxetic performances.
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