Stick–slip behavior of para-aramid (Twaron®) fabric in yarn pull-out

Abstract

The aim of this study was to understand the stick–slip properties of para-aramid woven fabrics. Para-aramid Twaron CT®716 (CT716) and Twaron CT®714 (CT714) woven fabrics were used to conduct the pull-out tests. CT716 and CT714 woven fabrics have low and high fabric densities, respectively. Data generated from the single and multiple yarn pull-out tests using various lengths of CT716 and CT714 woven fabrics included fabric stick–slip force and accumulative retraction force. Stick–slip force and accumulative retraction force depend on fabric density and the number of pulled ends in the fabric. Stick–slip force and accumulative retraction force in the multiple-yarn pull-out test were higher than those of the single-yarn pull-out test. Stick–slip force and accumulative retraction force in single- and multiple-yarn pull-out tests in the dense CT716 fabric were higher than those of the loose CT714 fabric. In addition, long fabric samples showed high stick–slip force compared with that of the short fabric samples.

Keywords

Ballistic fabrics single and multiple yarn pull-outs stick–slip force accumulative retraction force pull-out fixture

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 by Kirkwood et al. as one end of the yarn being pulled out from the fabric structure by the motion of the penetrator. They reported that 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 by Erlich et al. 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 cross-over 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

A review of the factors that influence ballistic performance can be outlined as the material properties of the yarn, fabric structure and multiple plies, projectile geometry and velocity, friction between fabric and projectile and yarn-to-yarn in the fabric and far-field boundary conditions.^14, ¹⁵ The fabric maximum pull-out forces in para-aramid fabric structures have been investigated with regards to their ballistic performance. It was found that stitched ballistic layered structures showed high pull-out force which eventually enhanced the ballistic resistance of structures.¹⁶ The fabric displacement stage and crimp extension stage 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^17,18 and the crimp extension stage could be used to explain fabric failure under tensile loads.¹⁹ The stick–slip phenomenon has been identified in nature and has been used to explain seismic movement, the flow of glaciers²⁰ and textile materials,²¹ and even everyday life. The stick–slip phenomenon was considered during single- and multiple-yarn-ends pull-out in fabric.¹⁹ As seen in the literature, the friction in the stick–slip stage of pull-out in fabric structure was an important energy absorption mechanism for soft ballistics. 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

The woven fabric was constructed with the para-aramid type of fibers (Twaron® of Teijin, Japan). The fiber and fabric properties are presented in Table 1. Two types of fabrics were used. These were Twaron CT® 716 (CT716) and Twaron CT® 714 (CT714). They were both plain weave and the warp and filling yarn linear densities were 110 tex. The warp and filling densities of the CT716 and CT714 fabrics were 12.2 and 8.5 ends/cm, respectively. The weights of the fabric unit areas were 280 and 190 g/m², respectively. Water repellent treatment was also applied to both fabrics. Crimp measurement was performed using a Tautex digital instrument (James H. Heal Co., UK) according to ISO 7211-3. Fabric thickness measurement was performed using an R&B cloth thickness tester (James H. Heal Co., UK) according to ISO 5084. Fabric weight measurement was performed based on ISO 6348. The fabric’s initial angle between warp and weft was measured by an optical microscope (Olympus SZ61-TR).

Table 1.

Properties of high-modulus para-aramid Twaron CT® fibers²² and fabrics

Fiber type		Fiber diameter (µm)		Fiber density (g/cm³)		Tensile strength (GPa)	Tensile modulus (GPa)		Elongation at break (%)	Decomposition point (C°)
Twaron CT®, Teijin		12		1.45		3.2	115		2.9	550

Fabric type	Weave type	Yarn linear density (tex)		Density (per cm)		Weight (g/m²)	Crimp (%)		Thickness (mm)	Treatment
		warp	filling	warp	filling		warp	filling

Twaron CT® 714	Plain	110	110	8.5	8.5	190	1.38	1.98	0.30	Water repellent (wrt)
Twaron CT® 716	Plain	110	110	12.2	12.2	280	6.36	1.96	0.40	Water repellent (wrt)

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.¹⁸ 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 each of the consecutive intersecting points in the fabric during single or multiple yarn pull-out 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 as the accumulative retraction force’. Fabric crimp interchange during the pull-out test was ignored. The residual tension on the fabric due to 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.

Figure 1.

Pull-out fixture with fabric on the tensile testing instruments.¹⁸

Figure 2.

Schematic views of the fabric and yarn positions measured during pull-out test: (a) fabric position before pull-out test; (b) stick–slip stage of fabric position during pull-out test.

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, 2 and 3 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 CT716 and CT714 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, in CT716 and CT714 fabrics were considered. The curve in the kinetic region has one maxima and one minima for each of the two crossing points where from maximum to minimum (one minima) is called stick–slip and from minimum to maximum (one maxima) is called the accumulative retraction force due to the fabric structure. Figures 3(a) and (b) show the stick–slip stages of the single-yarn pull-out force–displacement curves for CT716 and CT714 fabrics, respectively. The 11 meso-cells of CT716 fabric were considered to investigate the stick–slip stage of the single-yarn pull-out force–displacement curve as seen in Figure 3(a) whereas the eight meso-cells of CT714 fabric were considered to investigate the stick–slip stage of the single yarn pull-out force–displacement curve as seen in Figure 3(b). On the other hand, Figures 4(a) and (b) show the stick–slip stages of the multiple yarn pull-out force–displacement curves for CT716 and CT714 fabrics, respectively. The 11 meso-cells of CT716 fabric were considered to investigate the stick–slip stage of the multiple yarn pull-out force–displacement curve as seen in Figure 4(a) whereas the eight meso-cells of CT714 fabric were considered to investigate the stick–slip stage of the multiple yarn pull-out force–displacement curve as seen in Figure 4(b).

Figure 3.

Stick–slip stages of single-yarn pull-out force–displacement curves with chosen number of meso-cells: (a) CT716 fabric (chosen number of meso-cells: 11); (b) CT714 fabric (chosen number of meso-cells: 8). (Fabric width: 300 mm; fabric length: 50 mm.).

Figure 4.

Stick–slip stages of multiple-yarn pull-out force–displacement curves with chosen number of meso-cells: (a) CT716 fabric (chosen number of meso-cells: 11); (b) CT714 fabric (chosen number of meso-cells: 8). (Fabric width: 300 mm; fabric length: 50 mm; pulled yarn ends: 3.).

Therefore, we considered only the first 11 meso-cells, where one meso-cell is composed of one stick and one slip region, for the CT716 fabric and the first 8 meso-cells for the CT714 fabric from single- and multiple-yarn pull-out force–displacement curves which were from the beginning of the maximum pull-out force to the corresponding number of meso-cells, respectively.

Figure 5 shows the schematic views of meso-cells which include one stick region and one slip region. In the stick region, there is pressure between the warp and weft yarns either on the front face or back face of the fabric during the pulling of the warp yarn as shown schematically in Figure 6. 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 6. The amount of pressure is proportional, as given in the following relationships. We also assume that dry fabric has no initial tension in the yarns and the only force in the yarn was pull-out force, F, during pull-out test and the pull-out force F is momentarily the same in all of the crossing points in the fabric:

F_{1} = \cos θ \cdot F

(1)

F_{2} = \sin θ \cdot F

(2)

where F is the pull-out force, θ is the initial crossing angle, F₁ is the in-plane direction pull-out force component and F₂ is the out-of-plane direction pull-out force component.

Figure 5.

The schematic views of stick–slip stage in the meso-cells of para-aramid fabric structures after pull-out force is applied.

Figure 6.

The schematic views of pull-out force components in the stick–slip stage of the para-aramid fabric after pull-out force is applied.

The initial crossing angle (θ) depends on the directional fabric density and directional crimp ratio. Under the pull-out force on warp yarn, fabric displacement and crimp extension stages occurred first.¹⁷ This causes straightening of the pulled warp yarn and θ is decreased from its initial value. The measured average initial θ values for CT716 and CT714 fabrics were 10.37° and 4.41°, respectively. Figure 7 shows the measured initial crossing angles of CT716 and CT714 fabric structures. If we use Equations (1) and (2), we get F₁ = 0.984 F and F₂ = 0.175 F for CT716, and F₁ = 0.996 F and F₂ = 0.087 F for CT714. As seen in the relations, the out-of-plane direction pull-out force, F₂, was very small and the in-plane direction pull-out force, F₁, was very high for both fabric structures. In the stick regions, the in-plane direction pull-out force component (F₁) is the most likely main force to generate pressure on the yarn in the fabric structure. In the slip regions, the out-of-plane direction pull-out force component (F₂) is the most likely force to generate pressure on the crossing part of the yarn in the fabric structure as shown in Figures 5 and 6. However, more research is required to define the yarn pressure in the slip region of the fabric during pull-out.

Figure 7.

The cross-sectional views of the measured initial crossing angle (θ) between warp and weft in dry form para-aramid fabric: (a) CT716 fabric structure; (b) CT714 fabric structure. (Optical microscope view,×20 magnification.).

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 and 4, 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 (S_k–S_p) whereas the increasing line corresponds to each accumulative retraction force by fabric structure (A_f) as shown in Figure 8. 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 the fabric structure coming from the remaining eight yarns in the end of the pulled yarn (warp) as shown in Figures 6 and 8. When the pull-out phenomena was repeated, the second decreasing line occurred due to the second yarn stick–slip region. Immediately afterwards, 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 ninth yarn was released from the pulled yarn.

Figure 8.

The schematic 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

The stick–slip force and accumulative retraction force obtained from the single-yarn pull-out force–displacement curve of CT716 fabric for 11 meso-cells and CT714 fabric for 8 meso-cells are presented in Tables 2 and 3, respectively. An example of the calculation of stick–slip force from the pull-out force–displacement curve of CT716 fabric in MC-1 (fabric length: 50 mm, number of pull-out ends:1) was given as stick force (S_k) − slip force (S_p) = 16.10575 −12.08432 =4.021 N and accumulative retraction force (A_f) =|12.08432 − 14.36516 | = 2.281 N. Figure 9 shows the relationship between stick–slip force and various fabric lengths in the single-yarn pull-out test of CT716 fabric. Figure 10 shows the relationship between stick–slip force and the number of meso-cells in the single-yarn pull-out test of CT716 fabric. Figure 11 shows the relationship between stick–slip force and various fabric lengths in the single-yarn pull-out test of CT714 fabric. Figure 12 shows the relationship between stick–slip force and the number of meso-cells in the single yarn pull-out test of CT714 fabric.

Figure 9.

Relationship between the stick–slip force and various fabric lengths in the single-yarn pull-out test of CT716 fabric.

Figure 10.

Relationship between the stick–slip force and the number of meso-cells in the single-yarn pull-out test of CT716 fabric.

Figure 11.

Relationship between the stick–slip force and various fabric lengths in the single-yarn pull-out test of CT714 fabric.

Figure 12.

Relationship between the stick–slip force and the number of meso-cells in the single-yarn pull-out test of CT714 fabric.

Table 2.

Stick–slip force and accumulative retraction force obtained from the single-yarn pull-out force–displacement curve of CT716 fabric for 11 meso-cells

Fabric length	Ends of yarn	MC-1		MC-2		MC-3		MC-4		MC-5		MC-6
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)
50 mm	1 yarn	4.021	2.281	2.421	1.080	2.691	1.210	2.421	1.080	1.881	1.070	1.070	0.670
100 mm	1 yarn	5.912	3.631	3.761	2.821	3.361	1.611	2.821	1.611	2.551	1.881	2.681	1.611
150 mm	1 yarn	7.393	2.551	4.832	2.151	4.432	2.281	3.891	2.151	3.491	2.011	3.221	2.011
200 mm	1 yarn	7.783	2.151	5.102	2.141	4.292	1.741	3.491	1.210	2.951	1.471	2.951	1.481
250 mm	1 yarn	6.312	2.151	4.161	2.011	3.621	1.881	3.361	1.751	3.091	1.751	2.961	2.021
300 mm	1 yarn	6.442	2.151	3.091	1.611	2.951	2.151	2.961	2.151	3.221	2.421	2.961	2.281
350 mm	1 yarn	6.172	1.741	2.551	1.481	2.421	1.611	2.411	1.871	2.551	1.881	2.281	1.751

		MC-7		MC-8		MC-9		MC-10		MC-11
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)

50 mm	1 yarn	2.021	1.210	1.341	0.810	1.080	0.270	1.210	0.810	0.940	0.540
100 mm	1 yarn	2.421	1.611	2.551	1.611	2.281	1.481	0.940	0.400	1.611	0.940
150 mm	1 yarn	2.821	1.751	2.691	1.881	2.681	1.741	2.551	1.751	2.281	1.611
200 mm	1 yarn	2.551	1.341	2.421	1.481	2.421	1.481	2.281	1.481	2.021	1.340
250 mm	1 yarn	2.821	2.011	2.951	2.151	2.821	2.151	2.961	2.151	2.821	2.151
300 mm	1 yarn	2.951	2.421	3.221	3.351	3.761	2.551	2.951	3.891	4.432	3.901
350 mm	1 yarn	2.551	2.011	1.881	1.340	2.281	1.881	2.281	1.741	2.141	1.741

MC, meso-cell; S_k − S_p, stick − slip; A_f, accumulative retraction force due to fabric structure.

Table 3.

Stick–slip force and accumulative retraction force obtained from the single-yarn pull-out force–displacement curve of CT714 fabric for 8 meso-cells

Fabric length	Ends of yarn	MC-1		MC-2		MC-3		MC-4
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)
50 mm	1 yarn	1.141	0.709	0.787	0.573	0.662	0.560	0.595	0.489
100 mm	1 yarn	2.411	1.481	1.881	1.470	1.881	1.351	1.621	1.350
150 mm	1 yarn	2.551	1.481	1.751	1.210	1.470	1.200	1.471	1.210
200 mm	1 yarn	2.551	1.070	1.611	1.080	1.481	1.210	1.340	1.070
250 mm	1 yarn	2.421	0.810	1.480	0.940	1.481	0.940	1.200	0.940
300 mm	1 yarn	3.901	1.481	2.551	1.481	2.021	1.340	1.871	1.340
350 mm	1 yarn	4.292	1.340	2.691	1.481	2.281	1.480	2.151	1.611

		MC-5		MC-6		MC-7		MC-8
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)

50 mm	1 yarn	0.675	0.483	0.580	0.404	0.501	0.404	0.498	0.333
100 mm	1 yarn	1.611	1.341	1.481	1.341	1.471	0.940	0.940	0.800
150 mm	1 yarn	1.621	1.350	1.481	1.210	1.481	1.341	1.341	1.210
200 mm	1 yarn	1.340	1.070	1.340	1.070	1.210	1.080	1.210	1.070
250 mm	1 yarn	1.210	1.070	1.341	1.070	1.341	1.070	1.200	1.070
300 mm	1 yarn	1.751	1.340	1.611	1.210	1.471	1.200	1.611	1.350
350 mm	1 yarn	2.151	1.611	2.011	1.611	2.011	1.611	1.881	1.611

MC, meso-cell; S_k − S_p, stick–slip; A_f, accumulative retraction force due to fabric structure.

As seen in Figures 9 and 11, and Tables 2 and 3, the warp directional single-yarn stick–slip force in the first meso-cell (MC-1) and eleventh meso-cell (MC-11) of CT716 and, in the first meso-cell (MC-1) and eighth meso-cell (MC-8) of CT714 fabric generally 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 CT716 and CT714 fabrics were higher than those in the MC-11 of CT716 and the MC-8 of CT714 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 CT716 fabric were higher than those in CT714 fabric due to fabric density. Fabric length considerably affected the stick–slip forces of dense CT716 fabric and loose CT714 fabric due to the increasing number of crossing points.

As seen in Figures 10 and 12, and Tables 2 and 3, the warp directional single-yarn stick–slip forces from MC-1 to MC-11 of the short and long CT716 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 short and long CT714 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 long CT716 fabric sample were higher than those of the short CT716 fabric sample due to the number of crossing points in the fabric during pull-out. In addition, the warp directional single-yarn stick–slip forces from MC-1 to MC-8 of the long CT714 fabric sample were higher than those of the short CT714 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 CT716 fabric were higher than those of the CT714 fabric due to fabric density. Fabric length considerably affected the stick–slip forces of the meso-cells of the CT716 and CT714 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 CT716 fabric for 11 meso-cells and those of CT714 fabric for 8 meso-cells are presented in Tables 2 and 3, respectively.

Figure 13 shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the single-yarn pull-out test of CT716 fabric. Figure 14 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 CT716 fabric. Figure 15 shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the single-yarn pull-out test of CT714 fabric. Figure 16 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 CT714 fabric.

Figure 13.

Relationship between the accumulative retraction force due to the fabric structure and various fabric lengths in the single-yarn pull-out test of CT716 fabric.

Figure 14.

Relationship between the accumulative retraction force due to the fabric structure and the number of meso-cells in the single-yarn pull-out test of CT716 fabric.

Figure 15.

Relationship between the accumulative retraction force due to the fabric structure and various fabric lengths in the single-yarn pull-out test of CT714 fabric.

Figure 16.

Relationship between accumulative retraction force due to fabric structure and the number of meso-cells in the single-yarn pull-out test of CT714 fabric.

As seen in Figures 13 and 15, and Tables 2 and 3, the warp directional single-yarn accumulative retraction forces in various fabric lengths of CT716 fabric varied from 1.741 to 3.631 N in MC-1 and from 0.540 to 3.901 N in MC-11. The warp directional single-yarn accumulative retraction forces in various fabric lengths of CT714 fabric varied from 1.481 to 0.709 N in MC-1 and from 1.611 to 0.333 N in MC-8. We did not find any significant differences in the MC-1 and MC-11 of various fabric lengths of CT716 fabric. However, the warp directional single-yarn accumulative retraction forces in MC-1 of CT716 and CT714 fabrics were higher than those in the MC-11 of the CT716 and MC-8 of CT714 fabrics due to the remaining crossing points in the fabric during pull-out. On the other hand, the warp directional single-yarn accumulative retraction forces in CT716 fabric were higher than those of CT714 fabric due to fabric density.

As seen in Figures 14 and 16, and Tables 2 and 3, the warp directional single-yarn accumulative retraction forces from MC-1 to MC-11 of the short CT716 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 long CT716 fabric samples were almost equal. In addition, the warp directional single-yarn accumulative retraction forces from MC-1 to MC-8 of the short CT714 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 long CT714 fabric samples were almost equal. The warp directional single-yarn accumulative retraction forces from MC-1 to MC-11 of the long CT716 fabric sample were higher than those of the short CT716 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 long CT714 fabric sample were higher than those of the short CT714 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 the meso-cells of the CT716 fabric were higher than those of the CT714 fabric due to fabric density.

Stick–slip force in multiple-yarn pull-out

The stick–slip force and accumulative retraction force obtained from the multiple-yarn pull-out force–displacement curves of CT716 fabric for 11 meso-cells and CT714 fabric for 8 meso-cells are presented in Tables 4 and 5, respectively. Figure 17 shows the relationship between stick–slip force and various fabric lengths in the multiple-yarn pull-out test of CT716 fabric. Figure 18 shows the relationship between stick–slip force and the number of meso-cells in the multiple-yarn pull-out test of CT716 fabric. Figure 19 shows the relationship between stick–slip force and various fabric lengths in the multiple-yarn pull-out test of CT714 fabric. Figure 20 shows the relationship between stick–slip force and the number of meso-cells in the multiple yarn pull-out test of CT714 fabric.

Table 4.

Stick–slip force and accumulative retraction force obtained from the multiple-yarn pull-out force–displacement curve of CT716 fabric for 11 meso-cells

Fabric length	Ends of yarn	MC-1		MC-2		MC-3		MC-4		MC-5		MC-6
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)
50 mm	2 yarns	10.074	3.361	3.091	1.751	2.551	2.821	2.821	2.151	1.881	1.341	0.670	1.210
50 mm	3 yarns	15.316	3.631	8.193	6.852	8.863	6.712	6.312	5.102	3.761	2.951	5.362	3.621
100 mm	2 yarns	10.074	2.281	3.491	2.551	2.821	2.021	3.361	2.151	2.551	1.881	2.551	1.741
100 mm	3 yarns	33.842	6.042	11.684	5.512	8.063	6.312	4.562	1.741	3.091	0.140	3.891	2.551
150 mm	2 yarns	18.927	4.021	9.663	4.702	6.312	0.940	3.091	1.341	3.891	2.551	3.361	4.162
150 mm	3 yarns	74.527	4.572	6.452	6.582	7.653	5.642	5.102	3.891	2.681	4.021	2.821	1.611
200 mm	2 yarns	19.067	0.540	7.113	3.751	6.982	4.302	5.912	4.702	5.642	4.572	3.891	1.871
200 mm	3 yarns	288.283	6.302	7.513	2.681	4.562	2.011	3.491	4.432	4.562	4.432	3.491	5.772
250 mm	2 yarns	22.288	1.210	5.912	0.540	0.810	1.881	5.102	1.070	1.611	1.881	8.183	1.470
250 mm	3 yarns	271.647	33.702	22.018	0.400	11.544	5.772	9.663	6.042	7.793	4.972	6.983	5.502
300 mm	2 yarns	6.722	1.080	15.716	1.481	10.474	0.940	8.993	0.800	7.923	1.751	6.983	1.881
300 mm	3 yarns	313.943	3.091	16.516	7.523	17.186	9.934	6.172	9.533	5.642	1.341	13.565	7.253
350 mm	2 yarns	20.687	11.824	16.926	12.354	16.786	11.014	18.257	11.674	16.376	11.544	15.706	13.155
350 mm	3 yarns	293.795	33.032	20.677	7.253	16.116	15.045	20.417	16.656	22.158	13.695	20.677	16.786

		MC-7		MC-8		MC-9		MC-10		MC-11
Fabric length	Ends of yarn	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)	S_k − S_p (N)	A_f (N)

50 mm	2 yarns	4.172	1.210	2.281	0.670	3.221	1.210	1.881	0.800	1.611	0.810
50 mm	3 yarns	4.302	3.491	4.161	2.961	6.722	3.631	6.582	3.361	6.712	2.951
100 mm	2 yarns	1.881	1.751	2.011	1.881	1.751	0.810	0.810	2.821	2.151	0.140
100 mm	3 yarns	4.161	2.951	2.151	2.709	3.491	1.471	7.001	0.140	1.350	0.410
150 mm	2 yarns	29.541	0.410	2.421	0.400	2.411	0.800	2.681	0.670	8.993	0.400
150 mm	3 yarns	2.141	0.800	20.947	0.140	11.014	2.151	13.295	2.421	13.165	2.691
200 mm	2 yarns	1.341	2.551	20.407	0.400	2.551	0.410	4.972	0.400	13.825	0.130
200 mm	3 yarns	3.221	2.151	10.214	0.940	6.042	0.940	4.832	1.080	4.432	0.800
250 mm	2 yarns	0.270	0.140	7.523	1.751	6.722	1.210	5.772	1.210	5.102	1.210
250 mm	3 yarns	8.863	3.891	9.123	3.891	8.863	4.031	8.593	4.692	8.593	4.842
300 mm	2 yarns	6.312	1.741	5.632	1.741	5.102	2.011	5.372	2.291	5.372	2.141
300 mm	3 yarns	2.951	8.863	16.786	7.523	3.901	7.793	15.846	8.323	4.562	7.783
350 mm	2 yarns	16.786	11.684	15.035	12.354	15.446	10.474	12.755	10.064	13.025	10.614
350 mm	3 yarns	21.488	16.656	20.947	15.175	18.397	15.976	18.267	15.446	20.007	16.386

MC, meso-cell; S_k − S_p, stick–slip; A_f, accumulative retraction force due to fabric structure.

Table 5.

Stick–slip force and accumulative retraction force obtained from the multiple-yarn pull-out force–displacement curve of CT714 fabric for 8 meso-cells

Fabric length	Ends of yarn	MC-1		MC-2		MC-3		MC-4
Fabric length	Ends of yarn	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)
50 mm	2 yarns	2.681	1.471	1.881	1.481	1.611	1.481	1.611	0.930
	3 yarns	3.221	1.351	1.881	1.470	1.611	1.481	2.021	1.210
	4 yarns	3.621	0.940	2.151	1.481	1.611	1.200	2.011	1.340
	5 yarns	4.432	1.751	2.551	1.471	2.811	1.741	2.011	1.340
	6 yarns	4.292	1.341	2.281	1.070	2.951	2.011	3.091	1.881
100 mm	2 yarns	4.031	2.291	3.091	2.411	2.551	1.881	2.281	1.611
	3 yarns	6.312	3.491	4.972	3.761	4.562	3.221	2.951	2.281
	4 yarns	10.344	2.951	6.302	3.891	4.162	2.821	4.972	3.891
	5 yarns	13.835	0.810	6.852	3.091	8.733	4.842	7.523	6.582
	6 yarns	24.839	1.470	5.502	1.340	4.021	1.741	4.702	1.210
150 mm	2 yarns	6.572	2.281	3.761	2.421	3.231	2.421	3.091	2.691
	3 yarns	10.614	2.151	6.042	3.361	4.702	3.351	4.962	3.761
	4 yarns	21.348	1.751	10.614	4.031	8.193	4.832	7.783	5.502
	5 yarns	30.751	1.480	19.067	5.232	14.505	7.253	12.354	8.063
	6 yarns	65.393	1.350	19.067	4.832	13.835	5.372	11.814	6.852
200 mm	2 yarns	7.253	2.411	4.432	2.961	4.162	2.811	3.491	2.561
	3 yarns	13.565	2.821	7.793	3.761	6.712	4.031	5.512	3.491
	4 yarns	27.530	1.881	16.646	5.632	13.425	7.253	11.814	7.243
	5 yarns	82.440	2.551	19.877	8.323	19.197	10.074	17.326	9.803
	6 yarns	165.019	4.562	18.397	5.642	17.456	6.712	14.765	3.221
250 mm	2 yarns	8.463	3.221	5.642	3.631	4.972	3.491	4.562	3.891
	3 yarns	17.456	3.491	11.004	5.772	9.403	5.912	8.733	6.312
	4 yarns	36.923	2.011	21.478	7.923	17.596	10.344	16.656	10.614
	5 yarns	125.955	0.400	13.555	0.130	10.334	2.811	10.204	1.341
	6 yarns	248.809	0.130	16.516	0.140	15.315	2.021	15.315	2.821
300 mm	2 yarns	6.452	2.281	3.891	2.151	3.221	2.551	3.761	2.821
	3 yarns	9.944	0.680	5.912	2.011	4.962	2.681	4.702	3.091
	4 yarns	39.874	0.800	8.723	1.210	7.793	2.421	5.912	2.021
	5 yarns	121.253	0.270	14.775	1.881	12.214	1.611	9.944	3.491
	6 yarns	271.777	0.530	15.706	2.281	13.425	3.221	12.354	0.540
350 mm	2 yarns	9.403	0.270	4.702	1.070	4.162	1.210	3.491	1.340
	3 yarns	22.828	0.130	13.695	1.881	10.614	4.171	9.673	4.562
	4 yarns	91.173	2.281	13.295	3.891	12.214	6.172	11.814	6.042
	5 yarns	205.304	2.281	20.948	6.182	16.656	4.702	14.775	7.253
	6 yarns	305.079	1.340	30.071	6.982	25.519	6.982	22.558	9.403

		MC-5		MC-6		MC-7		MC-8
Fabric length	Ends of yarn	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)	S_k − S_p(N)	A_f(N)

50 mm	2 yarns	1.341	1.080	1.080	0.940	1.340	0.670	0.940	0.810
	3 yarns	1.611	1.210	1.481	1.351	1.611	1.200	1.741	1.210
	4 yarns	1.741	1.210	1.751	1.210	2.021	1.210	1.340	0.810
	5 yarns	2.421	1.751	2.281	1.471	1.471	0.800	1.470	0.940
	6 yarns	2.281	0.940	2.151	0.940	1.341	0.540	1.210	0.940
100 mm	2 yarns	2.011	1.611	1.881	2.011	1.881	1.210	1.611	1.210
	3 yarns	3.621	3.621	3.221	2.011	2.821	2.151	3.361	3.231
	4 yarns	5.772	4.702	5.102	3.621	5.902	4.292	5.372	4.162
	5 yarns	8.323	6.842	8.053	4.962	7.113	4.432	5.242	3.091
	6 yarns	4.702	1.481	5.642	1.751	5.912	1.481	4.702	0.540
150 mm	2 yarns	2.961	2.551	3.081	2.551	2.821	2.281	2.821	2.281
	3 yarns	4.572	3.491	4.292	3.891	4.432	3.491	4.432	3.621
	4 yarns	7.113	4.832	6.842	5.632	6.042	4.832	6.842	4.292
	5 yarns	12.084	8.593	11.284	8.463	10.744	6.172	7.923	5.912
	6 yarns	11.284	6.312	8.993	3.621	8.463	4.161	7.513	5.642
200 mm	2 yarns	3.231	2.421	2.951	2.281	2.821	2.281	2.821	2.281
	3 yarns	5.362	3.891	5.102	3.351	5.102	4.031	5.242	4.302
	4 yarns	10.064	6.572	9.934	6.712	9.663	7.253	9.944	6.312
	5 yarns	15.576	8.993	11.944	7.243	10.334	8.193	11.414	8.053
	6 yarns	11.144	6.172	10.474	6.982	10.744	5.102	9.934	3.891
250 mm	2 yarns	4.562	3.491	4.302	3.361	3.901	3.091	3.621	2.951
	3 yarns	8.193	6.582	8.723	6.442	8.593	7.653	8.863	6.982
	4 yarns	14.505	9.934	14.495	11.944	14.895	12.755	15.716	12.084
	5 yarns	9.263	4.161	9.794	4.562	9.673	4.702	9.393	3.491
	6 yarns	12.755	2.151	9.934	2.411	11.684	3.631	10.874	3.221
300 mm	2 yarns	3.221	2.681	3.221	2.551	3.091	2.421	2.951	2.411
	3 yarns	4.832	2.681	4.021	2.951	4.432	3.491	4.432	3.361
	4 yarns	5.642	2.551	6.042	4.161	6.982	3.221	5.232	2.811
	5 yarns	9.393	1.741	8.053	2.951	8.463	2.561	6.983	2.141
	6 yarns	9.263	1.881	8.463	2.421	7.923	2.811	9.663	2.151
350 mm	2 yarns	3.221	1.340	2.811	1.471	2.821	1.480	2.681	1.471
	3 yarns	8.723	4.562	7.653	4.432	7.383	4.432	6.852	4.432
	4 yarns	11.684	6.712	11.414	6.983	11.814	7.523	11.554	7.253
	5 yarns	14.365	5.912	14.775	7.123	15.576	6.302	14.765	5.372
	6 yarns	21.218	9.933	19.337	8.863	17.456	10.474	18.256	8.593

MC, meso-cell; S_k − S_p, stick–slip; A_f, accumulative retraction force due to fabric structure.

Figure 17.

Relationship between the stick–slip force and various fabric lengths in the multiple-yarn pull-out test of CT716 fabric. (Pulled yarn ends: 3.).

Figure 18.

Relationship between the stick–slip force and the number of meso-cells in the multiple-yarn pull-out test of CT716 fabric. (Pulled yarn ends: 3.).

Figure 19.

Relationship between the stick–slip force and various fabric lengths in the multiple-yarn pull-out test of CT714 fabric. (Pulled yarn ends: 3.).

Figure 20.

Relationship between the stick–slip force and the number of meso-cells in the multiple-yarn pull-out test of CT714 fabric. (Pulled yarn ends: 3.).

As seen in Figures 17 and 19, and Tables 4 and 5, the warp directional multiple-yarn stick–slip forces in the MC-1 and MC-11 of CT716, and in the MC-1 and MC-8 of CT714 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 CT716 and CT714 fabrics were higher than those in the MC-11 of CT716 and in the MC-8 of CT714 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 CT716 fabric were higher than those of CT714 fabric due to fabric density. Fabric length and the number of pull-out ends considerably affected the stick–slip forces of the dense CT716 fabric and loose CT714 fabric.

As seen in Figures 18 and 20, and Tables 4 and 5, the warp directional multiple-yarn stick–slip forces from MC-1 to MC-11 of the short and long CT716 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 short and long CT714 fabric samples decreased due to the decreasing number of crossing points.

The warp directional multiple-yarn stick–slip forces from MC-1 to MC-11 of the long CT716 fabric sample were higher than those of the short CT716 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 long CT714 fabric sample were higher than those of the short CT714 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 CT716 fabric were higher than those of CT714 fabric due to fabric density. Fabric length and the number of pull-out ends affected the stick–slip forces in the meso-cells of CT716 and CT714 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 CT716 fabric for 11 meso-cells and CT714 fabric for 8 meso-cells are presented in Tables 4 and 5, respectively.

Figure 21 shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the multiple-yarn pull-out test of CT716 fabric. Figure 22 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 CT716 fabric. Figure 23 shows the relationship between accumulative retraction force due to fabric structure and various fabric lengths in the multiple yarn pull-out test of CT714 fabric. Figure 24 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 CT714 fabric.

Figure 21.

Relationship between the accumulative retraction force due to the fabric structure and various fabric lengths in the multiple-yarn pull-out test of CT716 fabric. (Pulled yarn ends: 3.).

Figure 22.

Relationship between the accumulative retraction force due to the fabric structure and the number of meso-cells in the multiple-yarn pull-out test of CT716 fabric. (Pulled yarn ends: 3.).

Figure 23.

Relationship between the accumulative retraction force due to the fabric structure and various fabric lengths in the multiple-yarn pull-out test of CT714 fabric. (Pulled yarn ends: 3.).

Figure 24.

Relationship between the accumulative retraction force due to the fabric structure and the number of meso-cells in the multiple-yarn pull-out test of CT714 fabric. (Pulled yarn ends: 3.).

As seen in Figures 21 and 23, and Tables 4 and 5, the warp directional multiple-yarn accumulative retraction force in various fabric lengths of CT716 fabric varied from 3.091 to 33.702 N in MC-1 and from 0.410 to 16.383 N in MC-11. The warp directional multiple-yarn accumulative retraction force in various fabric lengths of CT714 fabric varied from 0.130 to 3.491 N in MC-1 and from 1.210 to 6.982 N in MC-8. We did not find any significant differences in the MC-1 and MC-11 of various fabric lengths of CT716 fabric. However, the warp directional multiple-yarn accumulative retraction forces in the MC-1 of the CT716 fabric were higher than those in the MC-11 of CT716 but the warp directional multiple-yarn accumulative retraction forces in the MC-1 of the CT714 fabric were higher than those in the MC-8 of CT714. On the other hand, the warp directional multiple-yarn accumulative retraction forces in CT716 fabric were higher than those of CT714 fabric due to fabric density.

As seen in Figures 22 and 24, and Tables 4 and 5, the warp directional multiple-yarn accumulative retraction forces from MC-1 to MC-11 of the short CT716 fabric samples decreased due to the increasing number of released yarns (weft). However, the warp directional multiple-yarn accumulative retraction forces from MC-1 to MC-11 of the long CT716 fabric samples were almost equal except for MC-1. In addition, the warp directional multiple-yarn accumulative retraction forces from MC-1 to MC-8 of the short CT714 fabric samples slightly decreased due to the increasing number of released yarns (weft). However, the warp directional multiple-yarn accumulative retraction forces from MC-1 to MC-8 of the long CT714 fabric samples were almost equal except for MC-1 and MC-2. The warp directional multiple-yarn accumulative retraction forces from MC-1 to MC-11 of the long CT716 fabric sample were higher than those of the short CT716 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-1 to MC-8 of long CT714 fabric sample were higher than those of the short CT714 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 forces in the meso-cells of the CT716 fabric were higher than those of the CT714 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 high- and low-density para-aramid fabrics in soft ballistic applications. Data were generated from single- and multiple-yarn-ends pull-out tests for high-density CT716 and low-density CT714 para-aramid fabrics.

It was found that the decreasing line in the force–displacement curve corresponds to each stick–slip region (S_k − S_p) whereas the increasing line in the force–displacement curve corresponds to each accumulative retraction force by fabric structure (A_f). 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. The MC-1 was found to be the most critical cell due to the starting point of the yarn pulling region and it was related to fabric boundary. The amount of stick–slip force and accumulative retraction force in multiple-yarn pull-out were extremely nonlinear compared with those of the single-yarn pull-out. On the other hand, the amount of stick–slip force was related to the number of interlacement points in the fabric whereas the amount of accumulative retraction force was related to fabric structural response. These were probably significant results for the energy absorption of soft ballistic structures.

Stick–slip force and accumulative retraction force depended on fabric density and the number of pull-out yarn ends. In general, the stick–slip force and accumulative retraction force of high- and low-density fabrics obtained from the multiple-yarn pull-out test were higher than those of the single-yarn pull-out test. On the other hand, the stick–slip force and accumulative retraction force of dense fabrics were higher than those of loose fabrics. It was also found that the stick–slip force and accumulative retraction force of long fabrics were higher than those of short fabrics.

Future research should be conducted to find the analytical relation among stick–slip force, accumulative retraction force and yarn–fabric structural parameters for various fabric weaves. This could result in a multiaxially interlaced fabric with improved frictional properties which could be used in soft ballistic applications.

Footnotes

Acknowledgements

The author would like to thank Research Associate Mr Mahmut Korkmaz and Research Assistant Miss Gaye Yolacan for helping during preparation of the manuscript and some of the artwork.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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JWS

. High Performance Fibers, Cambridge: Woodhead Publishing/Boca Raton, FL: CRC Press, 2001.