A study of warp-knitted spacer fabrics as cushioning materials for human body protection

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A study of warp-knitted spacer fabrics as cushioning materials for human body protection

 

Author: Liu, Yanping
Title: A study of warp-knitted spacer fabrics as cushioning materials for human body protection
Degree: Ph.D.
Year: 2013
Subject: Sports injuries -- Prevention.
Athletics -- Equipment and supplies.
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xxv, 281 leaves : col. ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2652775
URI: http://theses.lib.polyu.edu.hk/handle/200/7247
Abstract: Sports related injuries from impact accidents have been identified as a major public health problem. To protect people from injuries, protective equipment has been developed by including cushioning materials, which are normally polymeric foams, to absorb the impact energy under compression actions at a relatively constant stress over a large range of strain, to keep the maximum load below some limit that tissue or bones can bear. Lower air permeability and moisture transmission capability of polymeric foams cannot well meet the comfort requirement of protective equipment. As a result, although a variety of impact protectors are available on the market, the use of them is often rejected by wearers. Warp-knitted spacer fabrics have a three-dimensional construction consisting of two separate outer fabric layers joined together but kept apart by spacer yarns. They have been recently proposed to be cushioning materials for replacing polymeric foams in developing impact protectors for human body protection to reduce the risk of sports injuries due to their good compressibility, high moisture conductivity, and excellent thermoregulation capability. It is crucial to develop impact protectors by using warp-knitted spacer fabrics with required properties. This requires a deep understanding of the deformation mechanism and quantitative structureproperty relationships of warp-knitted spacer fabrics under compression. Some effort has already been made to investigate the static compression behaviour of warp-knitted spacer fabrics. It has been shown that spacer fabrics can be produced to have the key feature of behaving as cushioning materials, providing three distinct stages under compression, described as linear elasticity, plateau and densification. However, all the warp-knitted spacer fabrics used in previous studies were designed empirically. There is a lack of systematic experimental investigations on the compression properties of warp-knitted spacer fabrics in terms of their structural parameters. In addition, no well accepted interpretation and theoretical model have been given to provide a reasonable understanding of the compression deformation mechanism. Clearly quantitative structureproperty relationships have also not been well established. In order to establish a clear picture for engineering the cushioning properties of warp-knitted spacer fabrics for human body protection, the purpose of this study was to identify the compression mechanism, to establish the quantitative structureproperty relationships under static compression, and to examine the dynamic and curved shape effects on the protective properties.
The study started with a preliminary experimental analysis on the compression properties of a warp-knitted spacer fabric with a typical structure under various test conditions. A potential deformation mechanism of the typical fabric was interpreted based on the analyses of the compression loaddisplacement curve obtained under a selected proper test condition and the cross-sectional pictures taken at different compression stages. Based on the compression behaviour of the typical spacer fabric and its structure analysis, an analytical model, without considering spacer monofilament radius, outer fabric layer thickness, contacts among spacer monofilaments and yarn material’s nonlinearity, was developed to predict the compression properties of the spacer fabric. With this analytical model, the structural parameters affecting the compression behaviour of warp-knitted spacer fabrics were identified. To fully take into account the structural details and the interactions among elements of the fabric, a precise geometric model from Micro X-ray CT scanning was used to build eight finite element (FE) models with different constraints on spacer monofilaments, outer layer thicknesses and compression test boundary conditions. With the FE models, the precise deformation mechanism was identified for the typical spacer fabric. The effects of spacer yarn inclination angle and fineness as well as fabric thickness on compression properties of warp-knitted spacer fabrics were also parametrically studied with six extra FE models that were built by extending one of the eight FE models whose result fits well the experimental result. The identified structural parameters were used to develop twelve warp-knitted spacer fabrics for human body protection. Their static compression and cushioning properties were characterised with the selected compression test method, while their impact compression properties were evaluated with a drop-weight impact tester and analysed in terms of impact contact forcedisplacement curve, energy absorbedcontact force curve, and transmitted forcetime curve. Finally, the twelve warp-knitted spacer fabrics with different laminated layers in hemispherical shape were tested by using the drop-weight impact tester according to the Europe Standard BS EN 1621-1:1998 to assess their protective properties. The experimental investigations, analytical modelling and finite element modelling clearly revealed the compression mechanism of the typical warp-knitted spacer fabric with typical cushioning properties. Furthermore, the developed FE models successfully bridged the fabric structural parameters, i.e., spacer yarn inclination angle, spacer yarn fineness and fabric thickness, with the compression loaddisplacement relationships, providing an effective approach to predict the compression behaviour of a spacer fabric with a specified structure quantitatively. The theoretical and experimental results suggested that spacer fabrics with larger spacer yarn inclination angles, higher fabric thicknesses, finer spacer yarns and larger size mesh of the outer layers have lower resistance under static flatwise compression. Under impact flatwise compression, spacer fabrics with coarser spacer yarns, small-size mesh or close structure outer fabric layers have lower peak contact forces and peak transmitted forces. In addition, an optimized fabric thickness and spacer yarn inclination exist for getting better protective performance. The impact tests in hemispherical shape showed that spacer fabrics with coarser spacer yarns, higher thicknesses, and more stable outer layer structures will have a better force attenuation capacity. Increasing the inclination of spacer monofilaments to around 35° will enhance their shear resistance and therefore increases the force attenuation of the spacer fabric under impact in hemispherical shape. Three layers of the spacer fabric knitted with chain plus inlay structure for both outer layers with a thickness about 2.5 cm in total can comply with the European Standard BS EN 1621-1:1998. The systematic study laid down a principle for engineering the cushioning properties of warp-knitted spacer fabrics for human body protection.

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