Strain characteristics of cotton yarns depending on the strain rate and methods of their manufacture

One of the main reasons for the nonlinearity of the tension diagrams of cotton yarns is the variability of their moduli of elasticity and plasticity under strain. The changes in strain moduli obtained from the tensile diagrams confirm this. The strain curve has ten parameters, the values of which depend on the method of yarn manufacture and the strain rate. Based on the results of processing the tension diagrams of cotton yarn, obtained by carded and combed systems by ring and rotor spinning (CD-carded ring-spun, OE-carded rotor-spun, CM-combed ring-spun), at strain rates from 0,0033 s-1 to 0,033 s-1, the values of these ten parameters were determined and analyzed. Their quantitative and qualitative dependence on the method of their manufacture and strain rate are shown.


Introduction
Experimental determination of the strength of cotton yarns, a decently time-consuming and time-consuming problem. Determining the strength of yarns at high speeds of their movement or at high speeds of their deformation is a complex experimental problem, so the strength of yarns is determined, evaluated and predicted by formulas developed based on the analysis of experimental results [1]. These formulas are called empirical. Currently, there are no empirical formulas for determining the strength of yarns, depending on their types or the method of production [2].
The solution of suitable problems of textile threads and yarns during their movement in the technological processes of textile production requires knowledge of the law of their deformation. As reported, the real mathematical model or the law of deformation is nonlinear [3]. The nonlinearity of the yarn deformation law is suspended by the replacement of it is structures during stretching. This is an experimentally proven fact even in classical works [4], it is necessary to load it into the laws of deformation of cotton yarn. This path led to the development of a physical nonlinear elastic-viscoplastic law of deformation of cotton yarn in [5].
The nonlinear strain of textile cotton yarns (hereinafter referred to as yarn) under stretching to breakage has attracted the attention of researchers for a long time [6]. The manifestation of the nonlinear properties of the yarn was explained by the change in strain modulus of the yarn under its stretching. This concept was developed in [7]. Indeed, when the yarn is stretched, its fibrous structure undergoes changes. There occurs a mutual relative mixing of fibers and changes in the structure of the yarn appear at the beginning of strain. Consequently, the mechanical properties of the yarn change as well. The regularities of changes in mechanical characteristics of carded rotor spun yarn are discussed in detail in [8]. The results of experiments on stretching yarns to breakage, for three types of yarns, at different rates of stretching are given in [9].

Materials and Methods
In [9], the yarns produced by carded and combed systems by ring and rotor methods with a linear density  T 29.0 tex were used in experiments. The speed of their motion varied within the limits of the capability of the "Statimat C" tensile installation from   100 mm/min to 1000 mm/min. The strain rate, at a base yarn length  0 L 500 mm, varied from    0,0033 s -1 to    0,033 s -1 . Since these values of the strain rate are close to the static stretching of the yarn, the stretching process is considered quasi-static. It is shown in [5][6][7][8] that when the yarn is stretched to breakage, in the process of yarn strain all stages of strain are manifested -elastic, visco-elastic, elastic-viscoplastic strain. Therefore, the general concept of strain modulus is introduced, which includes elastic, viscous, plastic moduli. This strain modulus is actually a quasi-static modulus E S .

Results and Discussion
In [5][6][7][8] it is shown that in the process of yarn stretching, the strain modulus, depending on the strain itself, changes nonlinearly. A nonlinear function    S E characterizing changes in strain modulus of the yarn is shown in Fig. 1, for a combed ring yarn with a linear density  T 29.0 tex, obtained by stretching it to breakage at a speed of 1000 mm/min, determined from the diagram    F according to the methods described in [4][5][6]. The curve in Fig. 1 is the secant modulus of strain when the yarn is stretched from the initial value 0   to breakage k    . Curve 1, as seen from Fig. 1, has ten characteristic values for five points  Fig. 1 can be analytically described [7,8] (Fig. 1), which are necessary to define the yarn strength according to the method described in [5].
on the strain rate for the above types of yarn was determined for ten values of strain rate.   The averaged values of all 14 strain parameters according to the methods of their production are shown in Table 1. Based on the data from Table 1, the differences in the values of the parameters are determined in percent and shown in Table 2.  Table 2 shows a comparative assessment of cotton yarns manufactured by three spinning systems used in experiments, where number 1 corresponds to carded rotor yarn, 2 -to carded ring yarn and 3 -to combed ring yarn. The values of the averaged strain parameters were compared for these types of yarns.
It can be clearly seen that the yarn produced by the combed system in the ring-spun method is the strongest yarn. The values of the strain parameters of carded ring yarn are 5-33% greater in terms of strain moduli E compared to carded rotor-spun yarns, and, with the exception of values  E  E  E  E  E          ,  ,  ,  ,  ,  ,  ,  , up to 20-30% depend on the yarn production methods.
This means that when predicting the strength of yarns produced by different methods, this circumstance must be taken into account.