纬编针织物双向拉伸性能研究
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摘要
纬编针织物生产流程短、原料适应性广、工艺灵活多变,而且还可进行成型编织,产品具有较好的性价比,应用前景广阔。纬编针织物不仅广泛用于服装、装饰等领域,而且在产业用方面也占有一席之地,特别是用于各类柔性复合材料,纬编针织物更具有其它织物无可比拟的优势。纬编针织的拉伸性能是一项重要而又基础的性能,该性能的好坏直接影响到产品的生产、使用等多个方面,还与服装的耐久性、舒适性等密切相关。纬编针织物具有线圈结构,织物受外力拉伸时,因纱线的滑移和伸展产生较大的变形,并且在织物纵向与横向之间可以相互转移,构成拉伸过程中的不确定性、多样性和复杂性。纬编针织物双向拉伸是指同时在两个相互垂直方向上进行的拉伸；或者是织物只在一个方向进行拉伸,而在与拉伸方向垂直的方向上保持织物尺寸不变(SBE)。由于双向拉伸可以提供更多的力学信息,与实际受力情况较为接近,越来越引起人们的关注,特别是近年来,随着纬编复合材料的快速发展,针织物力学性能的研究重新引起人们浓厚的兴趣。本课题围绕纬编针织物双向拉伸性能展开研究。
通过对纬编针织物双向拉伸断裂性能和双向拉伸弹性等的研究,主要目的是：(1)对纬编针织物双向拉伸断裂和双向拉伸弹性的测试条件进行实验研究,为今后进行相关的性能研究以及建立双向拉伸性能测试标准提供参考。(2)对纬编针织物的双向拉伸断裂性能的破坏机理进行分析,确立曲线模型,并对纵、横向之间的相互影响进行研究,为进一步的理论研究提供依据,并为产品的设计提供参考；对应力松弛特性的研究和小应变下拉伸性能的预测,为今后继续进行更深入的研究做一些有益的尝试。(3)研究纬编针织物的双向拉伸性能与顶破性能之间的相关性,为实际中选用简单、实用的力学测试方法提供理论依据。
课题从双向拉伸断裂性能实验研究入手,首先分析测试条件中的试样形状、双向拉伸速度及速比、预加张力等对测试结果的影响；并选取典型试样,分析原料、组织以及织物结构参数对纬编针织物双向拉伸性能的影响。研究结果表明：双向拉伸断裂测试,采用方形试样测试较为方便和稳定；双向拉伸速度均为100mm/min的等速双向拉伸,较为稳定和实用；测试预加张力采用1N为宜；此外,由于纬编织物纵、横向之间相互影响,在不同的拉伸条件下,得到的应力-应变曲线不同,不具可比性。纬编针织物的双向拉伸性能与织物的多种因素相关,原料不同的平针组织织物,应力-应变曲线非常类似；相同原料不同织物组织结构的织物试验显示：双罗纹组织比平针组织的断裂应力和变形都大；罗纹组织表现出容易变形的特性,特别是横向,初始模量较小,最终变形较大；密度对织物的拉伸曲线有一定影响,特别是对横向影响显著,密度增加,织物初始模量增加,最终断裂应力增大。
在实验研究的基础上,对纬编针织物的拉伸断裂机理及典型曲线进行了分析；并对不同拉伸边界条件下的拉伸变形进行了实验研究和模型预测。纬编针织物由于每一个线圈一般是由一根纱线组成,拉伸时易产生转移和变形,使纬编针织物双向拉伸应力-应变曲线呈下凹形单调增加,曲线的初始模量较小,之后模量增加。对典型应力-应变曲线采用指数函数和幂函数进行非线性拟合,结果显示：幂函数σ=aεb-c拟合效果最好。由于纬编针织物中线圈排列的定向性,织物纵、横向拉伸性能不同,纬编针织物纵向表现出较横向高的应力值,从破坏形式上看,等速双向拉伸时,损坏一般首先出现在纬向,之后外力引起更多纱线断裂或脱散,而使织物解体。
双向拉伸时,纵、横两个主方向之间关系十分密切。当一个方向受拉伸时,与之垂直的另一方向即为拉伸方向的边界,这时即使固定拉伸方向的拉伸条件,但只要边界条件变化,就会引起拉伸方向应力-应变关系的变化。边界不受约束,拉伸方向产生的变形较大,当边界条件由固定到拉伸速度为50 mm/min→100 mm/min→200 mm/min变化时,拉伸方向的变形逐步减少,同应变下应力增加,初始模量增加,拉伸比功减少,两个主方向之间呈现密切的关联性。对此,借助POPPER理论对最终变形进行预测,理论与实际结果基本一致。
目前,在纬编针织产品实际测试中,常用顶破测试来替代拉伸性能测试,课题在对顶破受力和变形分析之后,确认纬编针织物顶破性能从本质上讲是一种多向受拉的力学作用形式,与双向拉伸性能关系密切。两者从最终的破坏形式、负荷-变形曲线形状都有类似之处,但与双向拉伸曲线相比,顶破负荷-变形曲线后期模量变化较快,顶破断裂功小于等速双向拉伸纵、横向拉伸功之和。对比双向拉伸影响因素,双向拉伸性能的初始模量受组织和密度的影响较大；拉伸变形受原料、组织的影响较大；拉伸强力则与原料、组织、密度均相关。而顶破性能受原料和组织的影响较大,密度的影响稍小。课题选用五十多种不同的纬编针织物,同时进行顶破测试与等速双向拉伸测试。经过对单项强力指标和变形指标的相关性分析,利用相关程度最高的力和变形指标建立对应关系,将顶破测试得到的顶破强力Fb、顶破高度h视为第一组变量,等速双向拉伸得到的横向强力Fct、伸长变形ε视为第二组变量,采用典型分析的方法,对两组变量进行相关分析,结果表明：通常用于针织产品考核的顶破测试与等速双向拉伸性能有着很好的相关性。
论文对双向拉伸弹性的研究,从测试方法和测试条件着手。研究认为：由于纬编针织物结构特点,在进行双向拉伸弹性测试时,纵向与横向间相互影响,无法实现真正意义上的双向同时拉伸。因此,在进行双向拉伸弹性测试时,可采用一向固定、另一向等速拉伸的定伸长或定负荷弹性测试方法。纬编针织物定伸长和定负荷双向弹性测试中,测试条件的变化对测试结果有较大影响,定伸长(或定负荷)值增大,无论是横向还是纵向,达到定伸长(或定负荷)值时的应力(或应变)都会增大,而弹性回复率减小；无论是横向还是纵向,拉伸定伸长或定负荷作用时间增加,弹性回复率随之减少,而随着回复停顿时间的增加,弹性回复率随之增加,定伸长与定负荷测试具有类似的结果；随着往复拉伸循环次数的增加,各次定伸长(或定负荷)达到的应力值(或应变值)不断增大,纵、横向的弹性回复率都随着变形次数的增加而增大,累积变形不断减少。因此,对于该类测试,不同的测试条件,测试得到的弹性回复率以及曲线等不具可比性。
论文比较了纬编针织物在各个方向的双向拉伸弹性,发现其拉伸弹性具有明显的各向异性。初始模量无论是在进行定伸长还是定负荷拉伸时,各方向的规律基本是一定的,即在纵向拉伸(0°)时,初始模量值最大,而横向拉伸(90°)时,初始模量的值最小；在各方向采取相同的定伸长值时,达到设定伸长时的应力值不同,纵向拉伸的应力值最大,横向拉伸的应力值最小,在由纵向拉伸向横向拉伸过渡的过程中,应力值基本在减小；在各方向采取相同的定负荷值时,达到设定负荷时的应变值不同,横向拉伸的应变值最大,纵向拉伸的应变值较小；无论在定伸长还是定负荷拉伸条件下,各个方向的弹性回复率都不相同,且纵向具有较大的弹性回复率。对于实际中采用较多的SBE定伸长拉伸弹性,可以利用测试指标进行预测,理论与试验曲线符合较好。
论文在对双向拉伸性能进行实验研究的基础上,证实纬编针织物具有非线性的粘弹特性,利用非线性三元件力学模型能较好地模拟纬编针织物的应力松弛特性,模型拟合曲线与实测曲线较为接近。当一向固定一向拉伸的双向拉伸时,不同原料、组织、密度的纬编针织物得到的应力松弛曲线规律不同,即使同种试样其拉伸方向不同,结果也有差异,其模型参数是不同的；纬编针织物的应力松弛时间减小,表明织物松弛的较快,松弛现象明显；反之,松弛时间增加,粘性流动成分降低,弹性增加。同种织物拉伸伸长增加,曲线松弛量大而且快；罗纹组织的松弛时间比平针的大；密度高的平针织物松弛时间大于密度低的平针织物,这一结果与弹性试验结果基本相符,织物应力松弛与织物弹性回复性能之间有着一定的相关性。
论文最后还对小应变下的拉伸测试进行了预测,结果表明：采用线性化理论,借助复合材料的板材理论,预测小应变下的应力-应变关系可行。
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/min→100 mm/min→ 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.
通过对纬编针织物双向拉伸断裂性能和双向拉伸弹性等的研究,主要目的是：(1)对纬编针织物双向拉伸断裂和双向拉伸弹性的测试条件进行实验研究,为今后进行相关的性能研究以及建立双向拉伸性能测试标准提供参考。(2)对纬编针织物的双向拉伸断裂性能的破坏机理进行分析,确立曲线模型,并对纵、横向之间的相互影响进行研究,为进一步的理论研究提供依据,并为产品的设计提供参考；对应力松弛特性的研究和小应变下拉伸性能的预测,为今后继续进行更深入的研究做一些有益的尝试。(3)研究纬编针织物的双向拉伸性能与顶破性能之间的相关性,为实际中选用简单、实用的力学测试方法提供理论依据。
课题从双向拉伸断裂性能实验研究入手,首先分析测试条件中的试样形状、双向拉伸速度及速比、预加张力等对测试结果的影响；并选取典型试样,分析原料、组织以及织物结构参数对纬编针织物双向拉伸性能的影响。研究结果表明：双向拉伸断裂测试,采用方形试样测试较为方便和稳定；双向拉伸速度均为100mm/min的等速双向拉伸,较为稳定和实用；测试预加张力采用1N为宜；此外,由于纬编织物纵、横向之间相互影响,在不同的拉伸条件下,得到的应力-应变曲线不同,不具可比性。纬编针织物的双向拉伸性能与织物的多种因素相关,原料不同的平针组织织物,应力-应变曲线非常类似；相同原料不同织物组织结构的织物试验显示：双罗纹组织比平针组织的断裂应力和变形都大；罗纹组织表现出容易变形的特性,特别是横向,初始模量较小,最终变形较大；密度对织物的拉伸曲线有一定影响,特别是对横向影响显著,密度增加,织物初始模量增加,最终断裂应力增大。
在实验研究的基础上,对纬编针织物的拉伸断裂机理及典型曲线进行了分析；并对不同拉伸边界条件下的拉伸变形进行了实验研究和模型预测。纬编针织物由于每一个线圈一般是由一根纱线组成,拉伸时易产生转移和变形,使纬编针织物双向拉伸应力-应变曲线呈下凹形单调增加,曲线的初始模量较小,之后模量增加。对典型应力-应变曲线采用指数函数和幂函数进行非线性拟合,结果显示：幂函数σ=aεb-c拟合效果最好。由于纬编针织物中线圈排列的定向性,织物纵、横向拉伸性能不同,纬编针织物纵向表现出较横向高的应力值,从破坏形式上看,等速双向拉伸时,损坏一般首先出现在纬向,之后外力引起更多纱线断裂或脱散,而使织物解体。
双向拉伸时,纵、横两个主方向之间关系十分密切。当一个方向受拉伸时,与之垂直的另一方向即为拉伸方向的边界,这时即使固定拉伸方向的拉伸条件,但只要边界条件变化,就会引起拉伸方向应力-应变关系的变化。边界不受约束,拉伸方向产生的变形较大,当边界条件由固定到拉伸速度为50 mm/min→100 mm/min→200 mm/min变化时,拉伸方向的变形逐步减少,同应变下应力增加,初始模量增加,拉伸比功减少,两个主方向之间呈现密切的关联性。对此,借助POPPER理论对最终变形进行预测,理论与实际结果基本一致。
目前,在纬编针织产品实际测试中,常用顶破测试来替代拉伸性能测试,课题在对顶破受力和变形分析之后,确认纬编针织物顶破性能从本质上讲是一种多向受拉的力学作用形式,与双向拉伸性能关系密切。两者从最终的破坏形式、负荷-变形曲线形状都有类似之处,但与双向拉伸曲线相比,顶破负荷-变形曲线后期模量变化较快,顶破断裂功小于等速双向拉伸纵、横向拉伸功之和。对比双向拉伸影响因素,双向拉伸性能的初始模量受组织和密度的影响较大；拉伸变形受原料、组织的影响较大；拉伸强力则与原料、组织、密度均相关。而顶破性能受原料和组织的影响较大,密度的影响稍小。课题选用五十多种不同的纬编针织物,同时进行顶破测试与等速双向拉伸测试。经过对单项强力指标和变形指标的相关性分析,利用相关程度最高的力和变形指标建立对应关系,将顶破测试得到的顶破强力Fb、顶破高度h视为第一组变量,等速双向拉伸得到的横向强力Fct、伸长变形ε视为第二组变量,采用典型分析的方法,对两组变量进行相关分析,结果表明：通常用于针织产品考核的顶破测试与等速双向拉伸性能有着很好的相关性。
论文对双向拉伸弹性的研究,从测试方法和测试条件着手。研究认为：由于纬编针织物结构特点,在进行双向拉伸弹性测试时,纵向与横向间相互影响,无法实现真正意义上的双向同时拉伸。因此,在进行双向拉伸弹性测试时,可采用一向固定、另一向等速拉伸的定伸长或定负荷弹性测试方法。纬编针织物定伸长和定负荷双向弹性测试中,测试条件的变化对测试结果有较大影响,定伸长(或定负荷)值增大,无论是横向还是纵向,达到定伸长(或定负荷)值时的应力(或应变)都会增大,而弹性回复率减小；无论是横向还是纵向,拉伸定伸长或定负荷作用时间增加,弹性回复率随之减少,而随着回复停顿时间的增加,弹性回复率随之增加,定伸长与定负荷测试具有类似的结果；随着往复拉伸循环次数的增加,各次定伸长(或定负荷)达到的应力值(或应变值)不断增大,纵、横向的弹性回复率都随着变形次数的增加而增大,累积变形不断减少。因此,对于该类测试,不同的测试条件,测试得到的弹性回复率以及曲线等不具可比性。
论文比较了纬编针织物在各个方向的双向拉伸弹性,发现其拉伸弹性具有明显的各向异性。初始模量无论是在进行定伸长还是定负荷拉伸时,各方向的规律基本是一定的,即在纵向拉伸(0°)时,初始模量值最大,而横向拉伸(90°)时,初始模量的值最小；在各方向采取相同的定伸长值时,达到设定伸长时的应力值不同,纵向拉伸的应力值最大,横向拉伸的应力值最小,在由纵向拉伸向横向拉伸过渡的过程中,应力值基本在减小；在各方向采取相同的定负荷值时,达到设定负荷时的应变值不同,横向拉伸的应变值最大,纵向拉伸的应变值较小；无论在定伸长还是定负荷拉伸条件下,各个方向的弹性回复率都不相同,且纵向具有较大的弹性回复率。对于实际中采用较多的SBE定伸长拉伸弹性,可以利用测试指标进行预测,理论与试验曲线符合较好。
论文在对双向拉伸性能进行实验研究的基础上,证实纬编针织物具有非线性的粘弹特性,利用非线性三元件力学模型能较好地模拟纬编针织物的应力松弛特性,模型拟合曲线与实测曲线较为接近。当一向固定一向拉伸的双向拉伸时,不同原料、组织、密度的纬编针织物得到的应力松弛曲线规律不同,即使同种试样其拉伸方向不同,结果也有差异,其模型参数是不同的；纬编针织物的应力松弛时间减小,表明织物松弛的较快,松弛现象明显；反之,松弛时间增加,粘性流动成分降低,弹性增加。同种织物拉伸伸长增加,曲线松弛量大而且快；罗纹组织的松弛时间比平针的大；密度高的平针织物松弛时间大于密度低的平针织物,这一结果与弹性试验结果基本相符,织物应力松弛与织物弹性回复性能之间有着一定的相关性。
论文最后还对小应变下的拉伸测试进行了预测,结果表明：采用线性化理论,借助复合材料的板材理论,预测小应变下的应力-应变关系可行。
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/min→100 mm/min→ 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.
引文
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