Research On Biaxial Tensile Properties Of Weft-Knitted Fabric

Weft knitted fabric has unparalleled advantages, such as short and flexible process, wide raw materials adaptability, and good cost performance. Weft knitted fabric has prospective applications, which is not only widely used in clothing, decoration, but also in the industrial area, especially for various types of flexible composites. Tensile properties of weft knitted fabric is an important and basic performance, which impacts directly on process, use and other aspects, it also closely related to clothing durability, comfort and etc. With special loop structure, the fabric comes into deforming due to yarn slippage and stretching under external tension. The vertical and horizontal transformation of loop contributes to the uncertainty, diversity and complexity during stretching process. Biaxial extension means fabric is stretched at both perpendicular directions, or strip biaxial extension (SBE) means fabric is stretched at one direction only, and additional tension to maintain the same size at the other direction. The biaxial extension draws increasing attention since it is closer to the actual situation during wearing and its measurement results can provide more information. In recent years, with rapid development of weft knitted composite, the mechanical properties of knitted fabric attracted renewed interest. The paper works on biaxial tensile properties of weft knitted fabric.A tensile fracture behavior and elasticity properties under biaxial extension were studied, the main purposes are to (1) investigate the testing conditions of tensile fracture behavior and elasticity properties, which can provide reference to the related studies and measurement standards of biaxial extension. (2) analyze the failure mechanism of a tensile fracture property under biaxial extension, establish the curve model, and study the vertical and horizontal interaction, which can provide experiment data for further theoretical research and reference to product design; predict the stress relaxation and small-strain tensile properties, which will contribute to further research; (3) figure out the correlation between biaxial tensile properties and bursting properties, which provide theoretical basis for testing method.First, the study focuses on the tensile fracture behavior under biaxial extension. (1) Analyze the testing conditions including specimen shape, tensile speeds at vertical and horizontal directions, the speed ratio and initial tension; (2) Select a representative sample, analyze raw materials and fabric structural parameters, investigate the effect on biaxial tensile properties of weft knitted fabric. In the biaxial tensile test, square specimen should be used which contribute to more convenient and stable test; constant speed of 100 mm/min in both directions is good for stable and practical test; 1N force was used as the pre-tension; In addition, it was not comparable among the results under different stretching modes, because the vertical and horizontal loop interaction of knitted fabric lead to the different stress-strain curves. A variety of fabric factors have influence on its biaxial tensile properties, the stress-strain curves were very similar among fabrics with varied raw materials; the same raw material fabric with different structures, compared to plain knitted fabric, rib knitted fabric has larger fracture stress and deformation, especially in its horizontal direction, lower initial modulus, and larger in the ultimate deformation. The fabric density has a certain influence on tensile curve, especially on the horizontal direction, with increasing density, the fabric initial modulus was increasing, and the final fracture stress was increasing.Based on the experimental measurement, the typical curves and tensile fracture mechanisms were analyzed, and the boundary conditions of different tensile stretching deformation were studied. The weft knitted fabric was easy to deform under stretching due to the special loop structure, which leads to the monotone increasing in its stress-strain curve, the initial modulus was small and then the modulus rose. For the typical stress-strain curve, nonlinear fitting method was used, compared to exponential function, power functionσ= aεb-c fitted better. The longitudinal and transverse tensile properties of weft knitted fabric were different due to the oriented loop arrangement; the longitudinal stress values were higher than the horizontal one. The results of biaxial extension at same constant speed shown that the damage appeared first in the weft and then more yarns were broken under external force, eventually the fabric was disintegrated.For the biaxial extension, the test conditions at vertical and horizontal directions interacted each other. When one direction was fixed, changes in the other direction made the tensile stress—strain curve variation. When the boundary conditions from free, a fixed to a tensile speed 50 mm/minâ†'100 mm/minâ†' 200 mm/min, the deformation at the tensile direction gradually reduced, stress and initial modulus increased at the constant strain, the properties at vertical and horizontal directions of weft knitted fabric were closely correlation, the POPPER theory was used to predict the deformation under biaxial extension, the results shown that the theory and experiment were highly consistent.Currently, bursting test is used to replace tensile test for knitted fabric, this paper confirmed that weft knitted fabric was essentially to be multi-directions stretched, which closely related to its biaxial extension properties. Both of bursting test and tensile test, the specimen were generated similar destruction form and force-elongation curve, but compared to the biaxial extension at same constant speed, modulus in the late of bursting curve produced rapid changes, bursting work was smaller than others. Factors influenced on the biaxial extension properties, such as the fabric structure and density effect greatly on its initial modulus, raw materials and fabric structure impacted greatly on tensile deformation, and raw materials, fabric structure and density impacted greatly on tensile strength. But raw material and fabric structure impacted greatly on bursting test, density has a smaller effect on brusting test than biaxial extension test. More than 50 kinds of weft knitted fabrics were chosen to carry on bursting test and biaxial extension test. Through analyzing the correlation between individual strength indicator and deformation indicators, found that there was good correlation between them.This paper investigates measurement method and test conditions for biaxial extension of weft knitted fabric. The results shown that it hardly carry on the simultaneously stretching at both directions, because of special loop structure and Longitudinal & transverse interaction. Therefore, for biaxial tensile stretching elasticity test, the fabric at one direction should be fixed, and one at the other direction can be tested under the constant tension or load. The test conditions impacted greatly on results, the related stress(or strain) would increase if the constant elongation (or constant load) increased, however, the elastic recovery rate decreased. The elastic recovery rate would decease when the tensile load was applied on any longer time; however the elastic recovery rate would increase if the standing time was increasing. With the increasing stretching cycles, the cumulative deformation reduced. Therefore, the test results were not comparable under different test conditions, test indicators, and the response curves.Comparisons with the biaxial tensile deformation and elastic properties at different directions, it is found that stretching has a significant anisotropy. For the tensile test under constant elongation or constant load, the tested tendencies were basically similar; the tensile stress at longitudinal direction was largest and the one at transverse direction was smallest when the both of elongations were fixed. The elastic recovery rates were different under the constant elongation or load extensions.Based on the experimental study of biaxial tensile properties, the weft knitted fabric has non-linear viscoelastic properties, the nonlinear three-element mechanical modal can be used to simulate the fabric characteristics of stress relaxation, the theory results fitted with the measured ones better. The stress relaxation curves were different from the fabrics with varied parameters such as raw materials, structure and density. For a sample, the properties of fabric at longitudinal direction also were different from the other direction. The time for stress relaxation of weft knitted fabric was shorter, the stress relaxation was faster, vice versa. If increasing the elongation for the same fabric, the curve relaxation was large and fast; compared to plain fabric, the relaxation time for rib fabric was longer, plain fabric with high density was longer than the one with low density. The results consistent with the experiment data so there was a certain correlation between stress relaxation and elastic recovery rate of weft knitted fabric.At last, this paper predicts the tensile properties of weft knitted fabric under small strain. Results show that it is possible to predict it using linear theory and plate theory of composite materials.