Abstract
The aim of this study was to understand the stick-slip properties of para-aramid woven fabrics. For this reasons, pull-out test was conducted on para-aramid Kevlar® 29 and Kevlar® 129 woven fabrics. The force–displacement curve from each fabric sample was obtained. The stick-slip region and accumulative retraction force region were defined based on the force–displacement curve. It was found that stick-slip and accumulative retraction forces depend on fabric density and the number of pulled ends in the fabric. Stick-slip and accumulative retraction forces in the multiple yarn pull-out test were higher than those of the single yarn pull-out test. Stick-slip and accumulative retraction forces in single and multiple yarn pull-out tests in the dense K29 fabric were higher than those of the loose K129 fabric. In addition, long fabric samples showed high stick-slip force compared to that of the short fabric samples.
Keywords
Introduction
Ballistic fabrics with higher pull-out force have been shown to perform favorably in impact tests [1]. Some studies have stated that to understand the mechanism of yarn pull-out it is necessary to understand the role of yarn pull-out friction in fabrics and engineering frictional properties to enhance their ballistic performance. Yarn pull-out was defined as one end of the yarn pulled out from the fabric structure by the motion of the penetrator. The force required to pull the yarn from the fabric structure was the sum of the frictional forces between the yarn sets at all intersecting points [2, 3]. The three distinct modes of fabric failure observed in slow penetration tests were yarn pull-out, local yarn rupture, and remote yarn failure [4]. Ballistic performance depends upon friction and material properties such as elastic modulus and strength of the yarn [5]. Another study revealed that very high inter-yarn friction could lead to premature yarn rupture during impact load and eventually reduce the energy absorbing ability of the fabric. In addition, the crimp in the woven fabric could be considered as another factor [6, 7]. On the other hand, the tribological behavior of woven fabric made from Kevlar® yarns of different linear densities was compared with the friction properties of their constituent yarns using different surface treatments. Both yarn texture and surface treatment were seen to have an influence on the friction coefficient. Linear density and woven structure had the largest impact on friction [8]. The softening treatment of fabric was shown to reduce inter-yarn adhesion and inter-yarn sliding friction [9]. Frictional processes within a fabric are important for both normal indentation and ballistic deformations as they control the effective stiffness of the material. It was found that fabrics with high friction and the lowest effective moduli dissipated larger amounts of energy relative to fabrics with lower friction. Relatively small changes in friction produced much greater changes in the deformational behavior of an assembly of cross-over contacts [10].
Modeling studies have shown that friction contributed to delaying fabric failure and increasing impact load thus allowing the fabric to absorb more energy. Also, it was reported that fabric boundary condition was a factor that influenced friction [11]. Projectile-fabric friction delayed yarn breakage by distributing the maximum stress along the periphery of the projectile-fabric contact zone. The delay of yarn breakage substantially increased the fabric’s energy absorption during the later stages of impact. Yarn-to-yarn friction hindered the relative motion between yarns and thus resisted decrimping of fabric weave tightness. It induced the fabric to fail earlier during the impact process [12]. The effect of yarn slippage at the crossover point as well as within the clamp was modeled and yarn fracture during impact in single ply woven fabric was determined using a kinetic energy relation [13].
The fabric maximum pull-out forces in para-aramid fabric structures have been investigated with regard to their ballistic performance. It was found that stitched ballistic layered structures showed high pull-out force which eventually enhanced the ballistic resistance of the structures [14]. The fabric displacement and crimp extension stages in single and multiple yarn ends pull-out have been investigated. It was concluded that the fabric displacement stage could be utilized to determine fabric shear behavior [15, 16] and the crimp extension stage could be used to explain the fabric failure under tensile loads [17]. The stick-slip phenomenon has been identified in nature and has been used to explain the seismic movement, the flow of glaciers [18] and textile materials [19], and even everyday life. The stick-slip phenomenon was considered during single and multiple yarn ends pull-out in fabric [17]. Therefore, the aim of this study was to understand the behavior of the stick-slip stage of para-aramid single woven fabric under single and multiple yarn pull-outs.
Materials and methods
Para-aramid fiber and woven fabrics
Properties of high modulus para-aramid fiber [20] and fabrics.
Pull-out tests
Pull-out tests were conducted to determine the yarn-to-yarn friction on single or multiple yarn ends in the frayed edge of the plain fabric structure. For this reason, a pull-out fixture was developed. Figure 1 shows the fixture and the pull-out test carried out in the testing instrument [16]. Fabric from both edges was clamped. In this set-up, the fabric stick-slip stage was defined as ‘the end of one yarn set (either warp or weft) passes through from the each of consecutive intersecting points in the fabric during single of multiple yarn pull-outs after the maximum pull-out force stage is completed’. Figure 2 shows the schematic views of the fixture and pull-out test during the stick-slip stage. In addition, ‘the pulled yarn end in the fabric is released from the each yarn which is normal to the pulled yarn direction in where the response of the remaining part of the pulled yarn in the fabric is defined the accumulative retraction force’. Fabric crimp interchange during the pull-out test was ignored. The residual tension on the fabric due to the clamped fabric edges was also ignored. The yarn slippages and yarn flattening in warp and weft directions in the fabric interlacement regions were not considered for simplification purposes. It was also observed that some of the filaments in the pulled yarn structure were broken. These broken filaments were also ignored. The testing instrument used was the Instron 4411 and the testing speed was 100 mm/min.
Pull-out fixture with fabric on the tensile testing instruments [16]. Schematic views of the fabric and yarn positions measured during pull-out test: (a) fabric position before pull-out test and (b) stick-slip stage of fabric position during pull-out test. Stick-slip stages of single yarn pull-out force–displacement curves: (a) K29 fabric and (b) K129 fabric (fabric width: 300 mm, fabric length: 50 mm). Meso-cells in stick-slip stages of single yarn pull-out force–displacement curve: (a) K29 fabric and (b) K129 fabric (fabric width: 300 mm, fabric length: 50 mm). Stick-slip stages of multiple yarn pull-out force–displacement curves: (a) K29 fabric and (b) K129 fabric (fabric width: 300 mm, fabric length: 50 mm, and pulled yarn ends: 3). Meso-cells in stick-slip stages of multiple yarn pull-out force–displacement curve: (a) K29 fabric and (b) K129 fabric (fabric width: 300 mm, fabric length: 50 mm, and pulled yarn ends: 3).
Fabric dimensions for performing the pull-out test were prepared as a fabric width of 360 mm for the total sample dimension and 300 mm for the sample dimension in the fixture. Fabric lengths ranged from 50 to 350 mm at 50 mm increments. The pull-out direction was in the warp direction of the fabrics. The frayed yarn length of the sample was 150 mm and the total edge length holding the sample in the fixture edge was 60 mm. In the single yarn pull-out test, only one yarn was pulled from the middle of the fabric sample. In the multiple yarn pull-out test, two and three yarns were pulled from the middle of the each fabric sample. The Instron 4411 pull head draws individual yarn ends from the frayed edge of the single fabric.
Results and discussion
Stick-slip stage in the yarn pull-out
Single and multiple yarn pull-out tests on K29 (dense fabric) and K129 (loose fabric) samples were carried out. Single and multiple yarn pull-out force–displacement curves were obtained. In the yarn pull-out force–displacement curve, the stick-slip stages of the kinetic friction part, which was from the beginning of the maximum pull-out force to the end of the yarn pull-out test, were considered. The curve in the kinetic region has one maxima and one minima for each two crossing points where from maximum to minimum (one minima) is called stick-slip and from minimum to maximum (one maxima) is called accumulative retraction forces due to fabric structure. Figure 3(a) and (b) show the stick-slip stages of the single yarn pull-out force–displacement curves. In addition, we considered only the first 11 meso-cells for the K29 fabric and the first eight meso-cells for the K129 fabric which were from the beginning of the maximum pull-out force to the corresponding number of meso-cells depending on fabric density. Figure 4(a) and (b) show the stick-slip stages of the single yarn pull-out force–displacement curves for the corresponding meso-cells in K29 and K129 fabrics. On the other hand, Figure 5(a) and (b) show the stick-slip stages of the multiple yarn pull-out force–displacement curves. Figure 6(a) and (b) show the stick-slip stages of the multiple yarn pull-out force–displacement curves for the corresponding meso-cells in K29 and K129 fabrics.
One meso-cell is composed of one stick region and one slip region as shown in Figure 7. In the stick region, there is pressure between the warp and weft yarns either in the front face or back face of the fabric during the pulling of the warp yarn as shown schematically in Figure 8. In the slip region, there is pressure between the warp and weft yarns where the warp is crossed during the pulling of the warp yarn as shown in Figure 8. The amount of pressure is proportional, as given in the following relationships
The schematical views of slip-stick stage in the meso-cells of para-aramid fabric structure. The schematical views of pull-out force components in slip-stick stage of the para-aramid fabric.
The initial crossing angle (θ) depends on directional fabric density and directional crimp ratio. Under the pull-out force on warp yarn, fabric displacement and crimp extension stages occurred first [15]. This causes straightening of the pulled warp yarn and θ is decreased from its initial value. The measured average initial θ values for dense and loose fabrics were 10°.12′ and 5°.07′, respectively. Figure 9 shows the measured initial crossing angles of dense and loose fabric structures. If we use equations (1) and (2), we get F1 = 0.175 F and F2 = 0.984 F for K29, and F1 = 0.087 F and F2 = 0.996 F for K129. As seen in the relations, the out-of-plane direction pull-out force, F1, was very small and the in-plane direction pull-out force, F2, was very high for both fabric structures. In the stick regions, the in-plane direction pull-out force component (F2) is most likely to main force to generate pressure on the yarn in the fabric structure. In slip regions, out-of-plane direction pull-out force component (F1) is most likely to force to generate pressure on the crossing part of the yarn in the fabric structure as shown in Figures 7 and 8. However, more research is required to define the yarn pressure in the slip region of the fabric during pull-out.
The cross-sectional views of the measured initial crossing angle (θ) between warp and weft in dry form para-aramid fabric: (a) K29 fabric structure and (b) K129 fabric structure (optical microscope view, magnification: 20×).
When we look at the meso-cells in the stick-slip stages of the single and multiple yarn pull-out force–displacement curves in Figures 3 to 6, there is an exponential function which has periodic decrease and increase lines. It is most likely that the decreasing line corresponds to each stick-slip region whereas, the increasing line corresponds to each accumulative retraction force by fabric structure as shown in Figure 10. After the maximum pull-out force stage was completed, the first decreasing line occurred due to the first yarn stick-slip region. When the first yarn (weft) was released from the fabric structure, the first increasing line occurred due to accumulative retraction force by fabric structure coming from the remaining eight yarns in the end of the pulled yarn (warp) as shown in Figures 8 and 10. When the pull-out phenomena were repeated, the second decreasing line occurred due to the second yarn stick-slip region. Immediately afterward, the second yarn was released from the fabric structure and the second increasing line occurred due to accumulative retraction force by the fabric structure coming from the remaining seven yarns in the end of the pulled yarn. This phenomenon was repeated until the nineth yarn was released from the pulled yarn.
The schematical views of stick-slip stage in the representative pull-out force–displacement curve of para-aramid fabric during pull-out.
Stick-slip force in single yarn pull-out
Stick-slip and accumulative retraction forces obtained from the single yarn pull-out force–displacement curve of K29 fabric for 11 meso-cells.
MC: meso-cell; Sk-Sp: stick-slip; and Af: accumulative retraction force due to fabric structure.
As seen in Figures 11 and 12, and Tables 2 and 3, the warp directional single yarn stick-slip force in the MC-1 and the MC-11 of K29 and, in the MC-1 and the MC-8 of K129 fabric slightly increased when the fabric length increased due to the increasing number of crossing points. The warp directional single yarn stick-slip forces in the MC-1 of K29 and K129 fabrics were higher than those in the MC-11 of K29 and the MC-8 of K129 fabrics due to the remaining crossing points in the fabric during pull-out. On the other hand, the warp directional single yarn stick-slip forces in K29 fabric were higher than those in K129 fabric due to fabric density. Fabric length considerably affected the stick-slip forces of dense K29 fabric and loose K129 fabric due to the increasing number of crossing points.
(a) Relationship between stick-slip force and various fabric lengths in single yarn pull-out test of K29 fabric and (b) relationship between stick-slip force and the number of meso-cells in single yarn pull-out test of K29 fabric. (a) Relationship between stick-slip force and various fabric lengths in single yarn pull-out test of K129 fabric and (b) relationship between stick-slip force and the number of meso-cells in single yarn pull-out test of K129 fabric.
As seen in Figures 11 and 12, and Tables 2 and 3, the warp directional single yarn stick-slip forces from MC-1 to MC-11 of the short (fabric length:50 mm) and long (fabric length:350 mm) K29 fabric samples decreased due to the decreasing number of crossing points. In addition, the warp directional single yarn stick-slip forces from MC-1 to MC-8 of the 50 and 350 mm length of K129 fabric samples decreased due to the decreasing number of crossing points.
The warp directional single yarn stick-slip forces from MC-1 to MC-11 of the 350 mm length of dense fabric sample were higher than those of the 50 mm length of dense fabric sample due to the number of crossing points in the fabric during pull-out.
The warp directional single yarn stick-slip forces from MC-1 to MC-8 of the 350 mm length of loose fabric sample were higher than those of the 50 mm length of loose fabric sample due to the number of crossing points in the fabric during pull-out.
On the other hand, the warp directional single yarn stick-slip forces in the meso-cells of the K29 fabric were higher than those of the K129 fabric due to fabric density. Fabric length considerably affected the stick-slip forces of the meso-cells of the K29 and K129 fabrics due to the number of crossing points.
Accumulative retraction force due to fabric structure in single yarn pull-out
The accumulative retraction forces obtained from the single yarn pull-out force–displacement curves of K29 fabric for 11 meso-cells and those of K129 fabric for eight meso-cells are presented in Tables 2 and 3, respectively.
Figure 13(a) shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the single yarn pull-out test of K29 fabric. Figure 13(b) shows the relationship between accumulative retraction force due to fabric structure and the number of meso-cells in the single yarn pull-out test of K29 fabric. Figure 14(a) shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the single yarn pull-out test of K129 fabric. Figure 14(b) shows the relationship between accumulative retraction force due to fabric structure and the number of meso-cells in the single yarn pull-out test of K129 fabric.
As seen in Figures 13 and 14, and Tables 2 and 3, the warp directional single yarn accumulative retraction forces in various fabric lengths of dense fabric varied from 1.481–2.281 N in MC-1 and from 0.400–2.551 N in MC-11. The warp directional single yarn accumulative retraction forces in various fabric lengths of loose fabric varied from 1.070–2.421 N in MC-1 and from 0.400–1.481 N in MC-8. We did not find any significant differences in the MC-1 and MC-11 of various fabric lengths of dense fabric. On the other hand, the warp directional single yarn accumulative retraction forces in dense fabric were higher than those of loose fabric due to fabric density.
(a) Relationship between accumulative retraction force due to fabric structure and various fabric lengths in single yarn pull-out test of K29 fabric and (b) relationship between accumulative retraction force due to fabric structure and the number of meso-cells in single yarn pull-out test of K29 fabric. (a) Relationship between accumulative retraction force due to fabric structure and various fabric lengths in single yarn pull-out test of K129 fabric and (b) relationship between accumulative retraction force due to fabric structure and the number of meso-cells in single yarn pull-out test of K129 fabric.
As seen in Figures 13 and 14, and Tables 2 and 3, the warp directional single yarn accumulative retraction forces from MC-1 to MC-11 of the 50 mm length of dense fabric samples decreased due to the increasing number of released yarns (weft). However, the warp directional single yarn accumulative retraction forces from MC-1 to MC-11 of the 350 mm length of dense fabric samples were almost equal. In addition, the warp directional single yarn accumulative retraction forces from MC-1 to MC-8 of the 50 mm length of loose fabric samples decreased due to the increasing number of released yarns (weft). However, the warp directional single yarn accumulative retraction forces from MC-1 to MC-8 of the 350 mm length of loose fabric samples were almost equal. The warp directional single yarn accumulative retraction forces from MC-2 to MC-11 of the 350 length of dense fabric sample were higher than those of the 50 mm length of dense fabric sample due to the number of crossing points in the fabric during pull-out. The warp directional single yarn accumulative retraction forces from MC-1 to MC-8 of the 350 mm length of loose fabric sample were higher than those of the 50 mm length of loose fabric sample due to the number of crossing points in the fabric during pull-out. On the other hand, the warp directional single yarn accumulative retraction forces in meso-cells of K29 fabric were higher than those of K129 fabric due to fabric density.
Stick-slip force in multiple yarn pull-out
Stick-slip and accumulative retraction forces obtained from the single yarn pull-out force–displacement curve of K129 fabric for eight meso-cells.
MC: meso-cell; Sk-Sp: stick-slip; and Af: accumulative retraction force due to fabric structure.
Stick-slip and accumulative retraction forces obtained from the multiple yarn pull-out force–displacement curve of K29 fabric for 11 meso-cells.
MC: meso-cell; Sk-Sp: stick-slip; and Af: accumulative retraction force due to fabric structure.
Stick-slip and accumulative retraction forces obtained from the multiple yarn pull-out force–displacement curve of K129 fabric for eight meso-cells.
MC: meso-cell; Sk-Sp: stick-slip; and Af: accumulative retraction force due to fabric structure.
As seen in Figures 15 and 16, and Tables 4 and 5, the warp directional multiple yarn stick-slip forces in the MC-1 and MC-11 of dense and in the MC-1 and MC-8 of loose fabric generally increased when the fabric length increased due to the increasing number of crossing points. The warp directional multiple yarn stick-slip forces in the MC-1 of K29 and K129 fabrics were higher than those in the MC-11 of K29 and in the MC-8 of K129 fabrics due to the remaining crossing points in the fabric during pull-out. On the other hand, the warp directional multiple yarn stick-slip forces in K29 fabric were higher than those of K129 fabric due to fabric density. Fabric length and the number of pull-out ends considerably affected the stick-slip forces of the dense K29 fabric and loose K129 fabric.
(a) Relationship between stick-slip force and various fabric lengths in multiple yarn pull-out test of K29 fabric (pulled yarn ends: 2) and (b) relationship between stick-slip force and the number of meso-cells in multiple yarn pull-out test of K29 fabric (pulled yarn ends: 2). (a) Relationship between stick-slip force and various fabric lengths in multiple yarn pull-out test of K129 fabric (pulled yarn ends: 3) and (b) relationship between stick-slip force and the number of meso-cells in multiple yarn pull-out test of K129 fabric (pulled yarn ends: 3).
As seen in Figures 15 and 16, and Tables 4 and 5, the warp directional multiple yarn stick-slip forces from MC-1 to MC-11 of the 50 and 350 mm length of dense fabric samples decreased due to the decreasing number of crossing points. In addition, the warp directional multiple yarn stick-slip forces from MC-1 to MC-8 of the 50 and 350 mm length of loose fabric samples decreased due to the decreasing number of crossing points.
The warp directional single yarn stick-slip forces from MC-1 to MC-11 of the 350 mm length of dense K29 fabric sample were higher than those of the 50 mm length of dense K29 fabric sample due to the number of crossing points in the fabric during pull-out. The warp directional multiple yarn stick-slip forces from MC-1 to MC-8 of the 350 mm length of loose K129 fabric sample were higher than those of the 50 mm length of loose K129 fabric sample due to the number of crossing points in the fabric during pull-out. On the other hand, the warp directional multiple yarn stick-slip forces in the meso-cells of K29 fabric were higher than those of K129 fabric due to fabric density. Fabric length and the number of pull-out ends affected the stick-slip forces in the meso-cells of K29 and K129 fabrics due to the number of crossing points.
Accumulative retraction force due to fabric structure in multiple yarn pull-out
The accumulative retraction force obtained from the multiple yarn pull-out force–displacement curves of K29 fabric for 11 meso-cells and K129 fabric for eight meso-cells are presented in Tables 4 and 5, respectively.
Figure 17(a) shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the multiple yarn pull-out test of K29 fabric. Figure 17(b) shows the relationship between accumulative retraction force due to fabric structure and the number of meso-cells in the multiple yarn pull-out test of K29 fabric. Figure 18(a) shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the multiple yarn pull-out test of K129 fabric. Figure 18(b) shows the relationship between accumulative retraction force due to fabric structure and the number of meso-cells in the multiple yarn pull-out test of K129 fabric.
As seen in Figures 17 and 18, and Tables 4 and 5, the warp directional multiple yarn accumulative retraction force in various fabric lengths of K29 fabric varied from 2.011–4.972 N in MC-1 and from 0.130–7.383 N in MC-11. The warp directional multiple yarn accumulative retraction force in various fabric lengths of K129 fabric varied from 1.210–3.891 N in MC-1 and from 1.210–5.372 N in MC-8. We did not find any significant differences in the MC-1 and MC-11 of various fabric lengths of K29 fabric. On the other hand, the warp directional multiple yarn accumulative retraction forces in K29 fabric were higher than those of K129 fabric due to fabric density.
(a) Relationship between accumulative retraction force due to fabric structure and various fabric lengths in multiple yarn pull-out test of K29 fabric (pulled yarn ends: 2) and (b) relationship between accumulative retraction force due to fabric structure and the number of meso-cells in multiple yarn pull-out test of K29 fabric (pulled yarn ends: 2). (a) Relationship between accumulative retraction force due to fabric structure and various fabric lengths in multiple yarn pull-out test of K129 fabric (pulled yarn ends: 3) and (b) relationship between accumulative retraction force due to fabric structure and the number of meso-cells in multiple yarn pull-out test of K129 fabric (pulled yarn ends: 3).
As seen in Figures 17 and 18, and Tables 4 and 5, the warp directional multiple yarn accumulative retraction forces from MC-1 to MC-11 of the 50 mm length of dense fabric samples decreased due to the increasing number of released yarns (weft). However, the warp directional multiple yarn accumulative retraction forces from MC-2 to MC-11 of the 350 mm length of dense fabric samples were almost equal. In addition, the warp directional multiple yarn accumulative retraction forces from MC-1 to MC-8 of the 50 mm length of loose fabric samples decreased due to the increasing number of released yarns (weft). However, the warp directional multiple yarn accumulative retraction forces from MC-6 to MC-8 of the 350 mm length of loose fabric samples were almost equal. The warp directional multiple yarn accumulative retraction forces from MC-1 to MC-11 of the 350 mm length of dense K29 fabric sample were higher than those of the 50 mm length of dense K29 fabric sample due to the number of crossing points in the fabric during pull-out. The warp directional multiple yarn accumulative retraction forces from MC-3 to MC-8 of the 350 mm length of loose K129 fabric sample were higher than those of the 50 mm length of loose K129 fabric sample due to the number of crossing points in the fabric during pull-out. On the other hand, the warp directional multiple yarn accumulative retraction force in the meso-cells of K29 fabric were higher than those of K129 fabric due to fabric density.
Conclusions
Single and multiple yarn pull-out tests were conducted in order to understand the stick-slip stage properties of K29(dense) and K129(loose) para-aramid fabrics in soft ballistic applications. It was found that the decreasing line in the force–displacement curve corresponds to each stick-slip region whereas the increasing line in the force–displacement curve corresponds to each accumulative retraction force by fabric structure. The warp directional single and multiple yarn stick-slip and accumulative retraction forces in the MC-1 of dense and loose fabrics were generally higher than those in the MC-11 of dense and the MC-8 of loose fabrics due to the number of interlacement points in the fabric.
Stick-slip and accumulative retraction forces depended on fabric density and the number of pulled yarn ends. In general, the stick-slip and accumulative retraction forces of dense and loose fabrics obtained from the multiple yarn pull-out tests were higher than those of the single yarn pull-out test. On the other hand, the stick-slip and accumulative retraction forces of dense fabrics were higher than those of loose fabrics. It was also found that the stick-slip and accumulative retraction forces of long fabrics were higher than those of short fabrics due to the increasing number of crossing points.
Future research should be conducted to find the analytical relation among stick-slip force, accumulative retraction force, and yarn-fabric structural parameters. This could result in a multiaxially interlaced fabric with improved frictional properties which could be used in soft ballistic applications.
Footnotes
Acknowledgments
The author thank the Research Associate Mr Mahmut Korkmaz and Research Assistant Miss Gaye Yolacan for helping during the preparation of the manuscript and some artworks.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.