Yarn Damage Evaluation in the Flat Knitting Process


 Textile yarns are subjected to numerous types of forces during knitting, usually leading to yarn damages, such as decrease in tensile, bending, shearing, and surface properties, which are closely related to different yarn properties, knitted structures/actions, and machine settings. This article comprehensively evaluated yarn damages in the computerized flat knitting process. Five different commercially available and commonly used yarns including cotton, wool, polyester, acrylic, and viscose were selected as raw materials, and the tensile, bending, shear, and frictional properties were investigated and compared before knitting and after being unraveled from plain- and rib-knitted fabrics, respectively. The results show that knitting actions/structures exhibit different damage extents for all different raw materials. It has been observed that the modulus is declined by 3–30% for bending, 2–10% for tensile, and 8–80% for shearing due to flat knitting action, respectively. The frictional coefficient of yarns also increased from 6 to 23%. As compared to yarn before knitting, the yarns unroved from plain and rib structures have been damaged to a great extent as a result of the loss of mechanical properties. The results are completely in agreement with the statistical analysis that clearly represents the significant loss in yarn properties during the knitting process. The microscopic analysis of the yarns clearly illustrates the effect of knitting action on yarn surface and mechanical properties. For yarn’s cross-sectional shearing properties testing, this article self-designed an innovative “Yarn Shear Testing Device.” The methodology and results are of great importance for improving the quality of knitted products, evaluating knitting yarns’ knittability, and in the development of high-performance technical textiles.


Introduction
Knittability and yarn damage evaluation in the knitting process receive increasingly more attention, which plays an important role in improving the quality of knitted articles, machine modifi cations, and even in the development of high-performance technical textiles. Researchers employed different methods to investigate yarn damage during knitting and evaluated the properties of knitted fabrics using different structures and machine settings [1][2][3][4]. Bhatt investigated that during loop formation, the yarn is subjected to tensile, bending, and torsional deformation. Bhatt also stated that the tensile, bending, and torsional rigidities of yarn are responsible for causing resistance to the deformation of yarn into a loop, and those properties also affect the magnitude and position of peak tension inside the knitting zone [5].
The effect of diverse integrated mechanism and knitting parameters on tensile strength failure of the reinforced unraveled yarns has been found in a good agreement of yarn damage and mechanical losses in a knitting process [6]. A quantitative approach has been developed to examine the degree of glass fi laments' breakage after the knitting process by unraveling the yarn from the knitted fabrics with the optimization of yarn tension, cam-setting, and some other machine parameters. The microscopic observations clearly indicate the yarn damage during the knitting process with the ultimate result that the tensile, bending, and frictional properties of the yarns play a vital role to determine the knittability of the yarns [7].
The installation of conductive yarns in fabrication (weaving, knitting, or embroidery) is getting more attention in the fashion industry and daily life usage products. In this scenario, the smart textiles worn by the people are under dynamic stresses. The study confi rms the damaging effects on the silvercoated nylon yarns in stitching or soutache embroidery. The mechanical behavior of bending, tensile, and shear stresses causes deterioration of yarns, which become worst with increasing mechanical forces and eventually result in the loss of electrical conductivity [8]. For textile materials, cut behavior can be proven to be a complicated process to study because a textile fabric is an array of fi ber bundles or yarns, put together in such a way that the fi nal product has stability and is locked together to give a one-piece structure [9].
During the knitting process, the proportion of fi ber damage for basalt fi lament yarn is lower than that of glass fi ber yarn as it has a higher value of the frictional coeffi cient. He further suggests that for suitable cam-setting, the yarn damage can be minimized for both basalt and glass fi lament yarns. For the similar cam setting, with the higher value of the frictional coeffi cient, the yarn has lower loop length, whereas the 1 x 1 rib produced with higher yarn frictional coeffi cient has longer loop length [10]. The effect of knitting action on the yarn surface results in protruding fi bers from the yarn surface because the shearing force of the knitting needles acts along the yarn's cross-section. The coarser yarns remain attached to the yarn surface, but the fi bers from fi ner yarn come out of the surface and result in transitory twisting change along the yarn, and the fi bers are sheared off from the yarn structure [11]. Many factors affect the yarn friction. Yarn stretching force is one of the leading factors, which infl uences all dynamic frictional features. The friction is also related to bending angle, yarn delivery speed, yarn surface properties, and so on. [12][13][14][15][16].
The literature review indicates that the factors infl uencing the yarn damages and knittability are complex. This article conducted a comprehensive evaluation of yarn damages before knitting and after being unraveled from plain, and 1 x 1 rib fabric knitted on the computerized fl at knitting machine. The mechanical properties of yarn including tensile, bending, shearing, and surface frictional properties were tested, and the microscopic analysis of unraveled yarns from plain and 1 x 1 rib fabric, as compared to the original yarns, has also been carried out. For yarn's cross-sectional shearing properties testing, this article self-designed an innovative "Yarn Shear Testing Device," which proved to be feasible and workable. The methodology and results could guide knitters to effectively improve the quality of knitted products, scientifi c evaluation of knittability of yarns, and even in the development of highperformance technical textiles.

Raw materials, knitted structures, and machines
In this study, fi ve different commercially available and commonly used yarns including cotton, acrylic, viscose, polyester, and wool are selected as raw materials, which are plied with the same count of 18/2 tex, then knitted into plain and 1 x 1 rib structures on the computerized Gauge 12 fl at knitting machine with the model of Long Xing (LXC-252SCV) from China. Yarn and fabric specifi cations are given in Table 1.

Tensile testing
The tensile testing is conducted by Instron tensile tester with 250 mm gauge length, testing speed 300 mm/min, and 0.5 cN/ tex preload by using standard ASTM D2256-02 in the standard atmospheric conditions (Temperature = 22°C and RH = 67%).

Bending testing
To conduct the yarn bending test, 100 pieces of sample yarns are arranged in parallel, which are in close contact with each other with the two ends fi xed by the adhesive tapes (as illustrated in Figure 1). Then, the bending test has been carried out using Kawabata Pure bending tester KES-FB2 with a bending width of 1 cm.

Shear testing
The yarn shearing test is performed by using Instron tensile tester with the speed of 300 mm/min, which is equipped with the self-designed "Yarn Shear Testing Device" as shown in Figure 2. The testing sample yarn is traversed under the mounted cutting edge, the upper clamp grips the two ends  of yarn and then moves upward to stretch, and the yarn gets sheared across the cutting edge.

Yarn surface properties testing
Two different methods have been adopted to evaluate changes in surface properties before and after being unraveled. These are the Kawabata FB-4 surface testing method and slope board methods, respectively.
KES FB-4 surface tester has been used to measure the surface roughness (SMD) and dynamic frictional coeffi cient (MIU) of the yarn samples in such a way that the yarns are closely arranged in a parallel way with an equal distribution of 14 yarns/cm on a 20 x 20 cm paper in the range of 12 cm by leaving 4 x 4 cm from both sides of the paper (as shown in Figure 3).
The slope board method is conducted to measure the yarn's static frictional coeffi cient. Yarns arranged in parallel on the glass-covered slope board. A 99.47 g slider block is prepared to slide over the attached yarns on the glass. Parallel arranged yarns are aligned under the slider surface, as demonstrated in Figure 4. The angle (θ) of slope board inclination can be adjusted. The angle q is marked when the slider starts to move.
The static frictional coeffi cient can be calculated by equation μ = tanθ.

Microscopic analysis of the yarns
To analyze the effect of knitting actions on the yarn surface, an optical microscope with the magnifi cation of 500 times has been used with the model (Panasonic WV-CP504DCH). Moreover, the effect of the knitting actions on the surface morphology of the yarns is also compared before knitting and after being unraveled from the knitted fabrics.

Tensile properties change
The stress-strain curves for the original yarns and the unraveled yarns from different knitted structures (i.e., plain and 1 x 1 rib) are plotted in Figure 5, whereas Figure 6 shows the tested yarn tensile modulus, which indicates that values of tensile modulus decrease obviously for all unraveled yarns in such a way, that is, the modulus of the yarn unraveled from 1 x 1 rib fabrics is lower than that of unraveled yarn from plain fabrics. However, the tensile strength of the unraveled yarns from 1 x 1 rib knit is lower than those of unraveled yarns from plainknitted fabrics. In addition, the elongation of all unraveled yarns increases, but the elongation of unraveled yarn from 1 x 1 rib is higher than that of the plain. The reason behind this fact is the geometrical deformation in the yarn caused by the knitting actions/structures. Moreover, it confi rms that the degree of geometrical deformation in the result of knitting process varies with respect to knitting structures, which affects the rate of loss of tensile modulus; which confi rms the fi ndings of Tian Hui's study [17], that is, reknitting after deknitting can increase the elongation of the fabric because of the much-crimped state that leads to bigger geometrical deformation, but the tensile strength reduces. Figure 5 shows that as compared to the original yarn, the maximum decrease in elastic modulus is 4.67% for unraveled yarns from plain-knitted fabrics and 9.12% for unraveled yarns from 1 x 1 rib-knitted fabrics, which also suggests that the knitting actions affect the different yarns to different degrees. The statistical test (t-test for means) in Table  2 points toward that the signifi cantly changed values in the tensile loss have been observed for all the yarns after knitting actions (plain knit and rib 1 x 1). The maximum signifi cant values of tensile properties were observed for the cotton unraveled yarns for both the knits (i.e., plain and rib 1 x 1). However, the least signifi cant results of the tensile values were observed for polyester unraveled yarns. It confi rms that knitting is a dynamic process, and both the knitting actions signifi cantly infl uenced the tensile properties of all different types of yarns opted in the experiment. Figure 7 shows the curves of bending moment against curvature for all the tested yarns. The values of bending modulus are given in Figure 8, which shows the loss in the bending modulus of the yarns unraveled from plain and 1 x 1 rib-knitted fabrics; however, the bending modulus of yarns unraveled from plain fabrics is higher than that of unraveled from 1 x 1 rib knit. The results can be attributed to the geometrical deformation in the unraveled yarns, which is related to the loop characteristics, such as loop length, loop shape, and the elastic properties of the yarn. The original yarn, which does not undergo the action of any external damaging forces, and the cohesive forces is 28.99% for unraveled yarns from plain-knitted fabrics and 30.44% for unraveled yarns from rib-knitted fabrics, which indicates that the yarn damage impact on bending properties is related to different knitting actions. Table 3 shows the effect of different knitting actions on yarn bending properties. The statistical test (t-test for means) in Table 3 confi rms that the knitting action has a signifi cant effect on different yarns. Signifi cant changes in the bending values have been observed for all the yarns before and after knitting actions (plain knit and rib 1 x 1). Highly signifi cant results of bending properties were observed for the polyester unraveled yarns for both the knits (i.e., plain and rib 1 x 1). However, the least signifi cant results of the bending values were observed for acrylic unraveled yarns. It confi rms that both the knitting structures/actions have a signifi cant effect on bending properties of different yarns selected for the experiment.

Shearing properties change
To assess the yarn damage in terms of shearing modulus, the shear stress-strain curves are drawn in Figure 9 for all the yarns under consideration, which shows the remarkable decrease of shear modulus for unraveled yarns. Figure 10 shows that the shearing modulus values sharply decrease for the unraveled yarns in such an order that the values of shearing modulus of the unraveled yarns from 1 x 1 rib are lower than those of the yarns unraveled from plain. The yarn's geometrical deformation (in terms of loop characteristics, i.e., loop shape, loop length, and elastic properties of yarns) due to knitting actions in the unraveled yarns increases the impact of the shear forces, and the highest impact is recorded for the yarns unraveled from among the fi bers play an important role in the behavior of yarn to act just like a straight rod and, therefore, give higher values of bending modulus. After being subjected to external forces during knitting (in the case of yarns unraveled from plain and 1 x 1 rib), the cohesive forces among the fi bers decrease, resulting in a decline in the bending modulus.
The cross-section of yarn unraveled from 1 x 1 rib becomes fl attened, and the cohesive forces among the fi bers are reduced that facilitate the yarn's bending. It can be found from the results in Figure 8 that the maximum loss in bending modulus   Table 4, that knitted actions (plain knit and rib 1 x 1) signifi cantly (p < 0.05) affect the yarns in terms of shearing properties loss for different yarn types.
The ultimate contraction in shear modulus is 60.66% for unraveled yarn from plain-knitted fabrics and 81.01% for unraveled yarn from rib-knitted fabrics, which implies that the yarn damage in terms of loss in shearing properties is very sensitive to different materials and different knitted structures.

Surface properties change
Figures 11 and 12 confi rm that the values of static frictional coeffi cient and dynamic frictional coeffi cient of yarns unraveled from 1 x 1 rib-knitted fabrics are greater than those of the values of yarns unraveled from plain-knitted fabrics. The results correlate with the fi ndings of Fouda et. al. [18] that the frictional coeffi cient for 1 x 1 rib knit was higher than that of the plain knits because the tension increased for 1 x 1 rib knit as compared to the plain knit. Furthermore, the frictional area between the yarn and the needle hook during the loop formation also increased. Static frictional values are higher than the dynamic frictional values as indicated in Figures 11 and 12. Figure 11 reveals that the highest increase in the static frictional coeffi cient is 10.54% for the unraveled yarn from plain and 23.97% for yarn unraveled from 1 x 1 rib-knitted fabrics. Figure  12 indicates the utmost increase in the dynamic frictional value (MIU) is 6.99% for unraveled yarn from plain and 24.72% for yarn unraveled from 1 x 1 rib-knitted fabrics, which suggests that the yarn surface damage in different knitting actions is perceptible for different types of yarn. Tables 5 and 6 illustrate    Figure 13 depicts that the values of surface roughness (SMD) are increased for all the yarns unraveled from 1 x 1 rib followed by the yarns unraveled from plain-knitted fabrics, as compared to original yarns. The reason behind this fact is that the yarn remains in contact with various yarn tensioner devices, contact the mean values and statistical data which indicate that the signifi cant change (p < 0.05) exists before and after the knitting actions/structures (plain knit and rib 1 x 1) for frictional and dynamic frictional coeffi cient values for all different types of yarn used in this study.   points, and knitting elements, and thereafter, in the knitting action, yarn-to-needle and yarn-to-yarn interactions contribute to the increased values of surface roughness. The utmost increase in surface roughness (SMD) for unraveled yarn from the plain is 12.64% and 39.49% for yarn unraveled from 1 x 1 rib-knitted fabrics, as shown in Figure 13. The statistical data are given in Table 7 that implies the signifi cant increase in surface roughness values after different knitting actions for all different types of yarn.

Microscopic images of yarns
To clarify the yarn damage more distinctly, optical microscopic analysis has been carried out as well. The knitting action damages the yarn structure, resulting in rupture of fi bers, which causes short fi bers to appear on the yarn surface. Figure  14 demonstrates the surface analysis of the original cotton, polyester, acrylic, wool, and viscose yarns, respectively. After the yarns being unraveled from the plain-and 1 x 1 rib-knitted fabrics, it can be observed that the number of protruded fi bers from the yarns unraveled from the 1 x 1 rib is greater than those of unraveled from plain-knitted fabrics. Moreover, the yarns unraveled from the plain and 1 x 1 rib are untwisted to some extent. The degree of untwisting is higher for the yarns unraveled from 1 x 1 rib than that of yarn unraveled from the plain, as shown in Figures 14(k-o) and 14(f-j).
The shearing force by needle hook along the cross-section of the yarn acts as a cutting edge on the yarn surface, which shears off fi bers from the yarn surface as confi rmed by previous

Conclusions and future works
According to the analytical results, conclusions can be drawn as follows: studies that yarn drawing-out and deformation along the yarn cross-section result in yarn untwisting [19,20]. Furthermore, from Figure 14(n), it can also be noticed that there is a pilling formation for the wool yarn unraveled from 1 x 1 rib-knitted fabrics due to its felting property.  The impact of knitting actions for different knitting structures is significant, which leads to the obvious contraction in the mechanical properties of the unraveled yarn including tensile, bending and shearing in such an order that the degree of contraction for shearing modulus loss is the highest, followed by the bending modulus loss is in the middle and the tensile loss is the lowest. However, the knitting process can improve the elongation of the unraveled yarns, which endows more geometrical deformation to the knitted structure.
The knitting yarn damages are closely related to different knitted actions/structures and different raw materials. The yarns sustain diverse extents of tensile, bending, and shearing forces in different knitting actions/structures. Contrarily, different raw materials exhibit different mechanical and surface properties, resulting in varying degrees of loss based on the aforementioned properties.
Both the slope board methods and KES-FB4 are feasible and workable for evaluating the yarn surface properties in terms of static frictional coefficient testing and surface roughness (SMD) and dynamic frictional coefficient (MIU) testing, respectively.
The experimental methodology and analytical methods are effective and comprehensive for evaluating the yarn damages in the knitting process. The results in this article are of great importance for improving the quality of knitted articles, evaluating knitting yarns' knittability, and in the development of high-performance technical textiles.
Future work should be carried on the high-speed circular knitting process, weaving process, and even braiding process, and so on, especially where the high-performance yarns, such as carbon, Kevlar, nano-fiber yarn, graphite yarn, are employed for technical application purposes because, in this case, the yarn damage should be particularly considered.