Analysis of warp-knitted fabric structure part IV: modeling the initial Poisson’s ratio in elastic deformation region

The effect of Poisson’s ratio on the mechanical properties of fabrics is undeniable. In the present study, a geometrical model for the Poisson’s ratio of two-guide-bar warp-knitted fabrics is developed, based on the presented models in the previous parts of this series. For this purpose, warp-knitted fabrics are categorized into two groups, according to their structures. Thereafter, a geometrical model is presented to predict the Poisson’s ratio of each group. To verify the generated models, theoretical results are compared with the available experimental data. The results show that the generated models are a suitable guidance to consider the Poisson’s ratio of warp-knitted fabrics.     

Design and manufacture of knitted tubular fabric with auxetic effect

Auxetic materials refer to materials that exhibit a negative Poisson’s ratio. They behave directly opposite to conventional materials by becoming wider when stretched and narrower when compressed. The unique property of this group of materials has led to them gaining a wider range of applications. This paper presents a study on the use of a flatbed knitting machine to fabricate and experiment on an arrow-headed auxetic tubular fabric. The study used three kinds of yarns: nylon (PA), PES, and cotton as the materials for knitting the tubular samples. Three variations of an arrow-head design (4 × 4, 6 × 6, and 8 × 8) were also used as the three main structural patterns. The results reveal that the ‘6 × 6’ structure gives the best auxetic effect among the three and the nylon (PA) yarn had the best auxeticity.     

A study of carboxylic acids and nano-TiO2 effects on stress relaxation of cotton fabrics

This study presents an evaluation on stress relaxation of cotton fabrics before and after performing the finishing process. The process includes a series of treatments on fabric samples through the Pad-dry curing method. A group of organic acids along with a proportion of catalyst and co-catalyst mixture constituted the treatment agents. Stress relaxation behavior of untreated and treated samples was assessed along two different directions of weft and warp using stress relaxation tester. In addition, an Italian Mesdan tensile tester was applied to measure physical and mechanical properties such as elongation, strength, Poisson’s ratio, and shear modulus. In the mean time scanning electron microscopy was applied to consider surface of samples before and after treatments. The results prove that the stress relaxation of treated samples has shown reasonably acceptable values when compared with those obtained for untreated one. Furthermore, the curve fitting of Maxwell’s model over the experimental data also justified that an interlaced model is more appropriate for explaining the stress relaxation in cotton fabrics. Beside the result of stress relaxation, Poisson’s ratio, and shear modulus illustrated remarkable increments. On the other hand, a reverse trend was observed for tensile properties in both directions. Orthotropic feature evaluation of fabrics toward both the direction of warp and weft also infers different response of stress relaxation in each direction.     

An explanation of warp knitted fabric deformation under uni-axial tension by a novel index as shape ratio

Although the Poisson’s ratio (PR) can predict the deformation of engineering materials under uniaxial tension, but seems that it cannot predict well the deformation of structures such as fabrics. It can be attributed to the lack of continuity among constituent elements that leads to some distortion of them during tension. Therefore, the PR is not a suitable tool to explain precisely the deformation of structures such as fabrics. For this reason, a new geometrical index was defined to quantify the deformation of fabrics in order to use instead of PR. This index named shape ratio is obtained by division of the variation of squared perimeter to the area of a printed square on the fabric under uniaxial tension. To check the defined index, warp-knitted fabrics with various structures such as Tricot, Locknit, and Three- and Four-needle satin were produced and tested using tensile tester. The results show that shape ratio can predict the deformation of warp-knitted fabrics as an orthotropic structure rather than the PR.     

双组分水性聚氨酯织物涂层胶的合成及性能测试

制备3种硬段含量及亲水基团含量的水性聚氨酯(WPU)乳液,并与可水分散多异氰酸酯按4种不同的质量配比混合,制得双组分WPU织物涂层胶。研究硬段含量和亲水基团含量,以及WPU乳液与可水分散多异氰酸酯的质量配比,对双组分WPU织物涂层胶性能的影响。结果表明,双组分WPU织物涂层胶具有良好的防水透气性能;WPU乳液中硬段含量和亲水基团含量都会影响双组分WPU织物涂层胶的防水透气性能;在可水分散多异氰酸酯用量较少时,适当增加用量有利于提高涂层织物的耐静水压,但用量不宜太高。     

Deformation characteristics of flexible PU-coated multi-axial warp knitted fabrics under uniaxial tensile

Based on the characteristics of high tenacity, low density, flexible design, the coated fabric has emerged as a promising construct for tensile test. Here, it reports the deformation mechanism of a PU-coated multi-axial warp-knitted fabric under uniaxial tensile via a digital image correlation system. Specimens produced by us are chase in three fibre-reinforced directions namely 0°, 45° and 90°, respectively. The result shows that the crease does not have an effect on the deformations in the small deformation range (strain < 5%), and the deformation was evidently occurred near the fixed end lags because of the faster movement of specimen at the loading end, and the transverse shrinking at the edges is faster than that in the middle of the specimen. In the large deformation range, the strain fields are inconsistent at the same displacement revealing that the deformations of this kind of flexible coated fabric is in-homogeneous under tensile.     

Investigating the effect of weight reduction treatment on Poisson’s ratio of microfiber polyester woven fabric

Polyester is one of the important fibers among man-made fibers. One of the finishing processes for improving the performance of polyester fabrics is weight reduction treatment. This treatment has a considerable effect on the mechanical properties of such fabrics. In this research, the effect of weight reduction treatment on changes in the Poisson’s ratio of a microfiber polyester woven fabric treated with four different sodium hydroxide concentrations was studied. Poisson’s ratio of the fabric at 10 levels of extension was measured. Results show that from 1% to about 5%, the Poisson’s ratio increases non-linearly. After this point up to 10%, the Poisson ratio decreases by a linear trend. There is a significant variation in the results at lower extensions, and by increase in extension, this variation reduces. Statistical analysis showed that weight reduction at percentages lower than 25% does not affect the Poisson’s ratio of the fabric. In the range of 6–10%, there is a high non-linear correlation between the weft crimp and Poisson’s ratio. In addition, there is a high non-linear correlation between Poisson’s ratio and fabric cover factor.     

Theoretical and experimental investigation of tensile properties of net warp-knitted spacer fabrics

Abstract

This work aimed to investigate the tensile behavior of net warp-knitted spacer fabrics using the experimental and theoretical approaches. Tensile behavior of the warp-knitted spacer fabrics is one of the fundamental parameters which rarely considered in the literature. Therefore, the net spacer fabrics were made with three different variables (fabric thickness, size of meshes, and the position of meshes of the outer layer with respect to each other) in order to study the effects of structural parameters on the tensile behavior. A 2D net-fabric which used for the outer fabrics was made for in-depth considerations and also the comparison between the 2D fabrics and spacer fabrics. The initial linear elastic modulus of the 2D fabrics and the initial linear elastic modulus of the spacer fabrics were selected for the assessment of the tensile characteristics. First, a mechanical model was developed based on the geometrical structures and material properties of the 2D fabrics using energy method and Castigliano’s theorem. Then, the effects of spacer yarns on the tensile behavior of spacer fabrics were taken into account. The latter model was proposed using the bending theory of a curved bar. Experimental data were compared with the values obtained from the theoretical approaches and the end results showed that the developed models can predict the modulus reasonably. Moreover, the results indicated that the tensile behavior of the spacer fabrics are independent of the position of the holes in the outer layer whilst the fabric thickness and size of meshes affect the tensile modulus.     

The formation and performance of auxetic textiles. Part II: geometry and structural properties

Some exceptional materials become fatter when stretched and are described as auxetics or having negative Poisson’s ratio. Auxetic textiles belong to this class of extraordinary materials that are increasingly attaining some prominence in many applications of technical textiles. We have sustained the efforts to fabricate auxetic fabric structures based on non‐auxetic yarns. The focus is to combine our knowledge of geometry and fabric structural characteristics to engineer auxetic textiles and to determine the properties of such auxetic textile fabrics. To realize our objective, we designed and investigated hexagonal knit structures as auxetic textiles offering optimum performance. The factors that influence Poisson’s ratio are identified as yarn type, number of chain courses and strain level. Also, a method has been developed for quantifying the geometrical structural unit cell of the auxetic structure based on measured parameters, namely a ₁, a ₂, b ₁, b ₂, h and c, as detailed in this paper.