Loading rate effects on dynamic out-of-plane yarn pull-out

Abstract

In this study, the mechanical response of a single yarn pull-out from single layers of Kevlar® and Twaron® fabric under out-of-plane loading at both quasi-static and dynamic rates was experimentally investigated. In order to perform the dynamic experiments, a pendulum impact setup was designed and constructed to pull out a single yarn dynamically. The pull-out load was measured directly by a load cell and the movement of the fabric was measured to portray the load–displacement history. The effects of transverse pressure, different weave direction, and loading rates were also investigated.

Keywords

yarn pull-out dynamic out-of-plane pendulum pressure

Woven fabrics with aramid fibers have gained great attention as protective systems from damage under ballistic impact for several decades. Aramid fibers are composed of long molecular chains produced from p-phenylene terephthalamides (PPTA), and popular examples of these materials include Kevlar® developed by DuPont, and Twaron® and Technora® produced by Teijin. These PPTA fibers provide unique characteristics of high strength, low density, and high flexibility in the woven structure, and are therefore in popular demand for body armor and aircraft turbine engine fragment barriers. The impact resistance of the fabrics under ballistic loading is known to be determined by various factors, such as single fiber properties, the fabric structure or weave type, geometry of the impacting projectile, projectile impact velocity, multiple ply interaction between the fabric layers, far-field boundary conditions away from the zone of impact, and inter-yarn friction.¹

It has been shown that this yarn–yarn friction increases the energy dissipated via frictional sliding, as well as yarn strain energy and yarn kinetic energy.² The ballistic limit (defined as the projectile velocity required to penetrate the fabric at least 50% of the time) of these fabrics was also shown to increase by approximately 10%, as demonstrated by computational results from Rao et al.³ comparing baseline plain-woven Kevlar® KM2 fabrics with and without inter-yarn friction. The effects of inter-yarn friction and yarn pull-out can be correlated to pull-out force and total pull-out energy. The results obtained using the force–displacement history can be used to develop friction models for yarn pull-out at different rates such that the models can be implemented in numerical simulations of fabrics under impact. Kirkwood et al.⁴ have demonstrated the usefulness of yarn pull-out analysis in predicting the ballistic performance of these bulletproof fabrics.

The relation between yarn pull-out and friction suggests that yarn pull-out is an important behavior to study the effect of inter-yarn friction in the woven structure. According to Dong et al.⁵ and Kirkwood et al.,⁴ yarn pull-out force in fabric has a correlation with the fabric's performance of ballistic resistance, as fabrics with higher yarn pull-out force displayed better capabilities in resisting ballistic impact. The main factors influencing the yarn pull-out force have been identified as fabric weave style, material properties, such as the yarn elastic modulus and ultimate strength and its constitutive fibers, fabric length, multiple-ply interaction, surface treatment, and pull-out loading rate.⁶

The behavior of yarn pull-out is usually illustrated in the curve of pull-out load versus displacement. Figure 1 shows a typical in-plane yarn pull-out response under quasi-static loading for a Kevlar® plain weave fabric. At the initial stage of pull-out of the yarn, the originally crimped yarn undergoes the process of uncrimping and the fabric holding the yarn is simultaneously deformed toward the direction of pull-out and loading. After the fabric is fully deformed, the yarn is further uncrimped until it reaches the point where the pull-out load is maximum, as shown in Figure 1(a). Once maximum uncrimping is achieved and the yarn is fully straightened, the yarn being pulled out starts translating. In the translation stage, stick-slip response is clearly observed via an oscillatory behavior (Figure 1b), as the yarn being pulled out is sticking when it passes under the yarn and slipping when it passes over the crossover points. The local maxima and minima are representative of the sticking and slipping motion in the yarn translation region, respectively. The mean value of the oscillation in the kinetic friction region also decreases due to the reduction in the number of crossover points as the yarn is being pulled out.

Figure 1.

In-plane yarn pull-out force versus displacement curve of a Kevlar® fabric depicting (a) yarn pull-out schematic and (b) force history showing yarn uncrimping and yarn translation regions.

Most of the available yarn pull-out experiments and computational models have been performed and simulated at quasi-static states and under loading along the in-plane directions. Some studies revealed that yarn pull-out load increased with transverse tension applied on the edges of the fabrics of Kevlar® KM2, Kevlar® 49, Kevlar® 29, Spectra^®, and Zylon®. Results revealed that increasing transverse in-plane tension not only increased the pull-out force, but also increased the total pull-out energy, which was defined as an area under the curve of pull-out force versus displacement.^4,7,8 Bazhenov et al.⁹ pulled out a single yarn in both warp and weft directions and found that the maximum pull-out force for warp yarns having more waviness was higher than that for weft yarns. However, the slope of the initial part of the force–displacement curve where the linear region was observed was higher when pulling out the weft yarn. Gawandi et al.⁸ investigated the effects of loading rate on Kevlar® under the transverse tension. Kevlar® KM2 fabrics coated with polymer Vamac® at different temperatures of heat-compression were tested for three different loading rates: 5.08, 50.8, and 508 mm/min. The results of their study revealed that these Vamac-coated fabrics displayed rate-sensitivity as the pull-out load was shown to increase with a higher in-plane pull-out velocity.

According to the frictional sliding law previously proposed by several authors,^10,11 one of the governing factors in the friction coefficient on single fiber pull-out was the sliding velocity. Applying this rate-dependent theory with experimental data enabled access not only to the qualitative aspects, but quantitative analyses of stick-slip behavior¹² as well. This study of rate dependency on single fiber pull-out suggests the necessity to investigate the frictional response through yarn pull-out at different velocities. Specifically, the study on frictional behavior at high rates is essential due to the fact that these woven fabric structures are mainly used in applications that are subjected to dynamic impact conditions.

Experimental, analytical, and numerical studies on yarn-pull-out at quasi-static rates on these PPTA materials have been already performed by numerous researchers; however, no experimental studies on the pull-out at higher dynamic rates were found, as the maximum loading rate of current single yarn pull-out experiments was found to be at approximately several hundred millimeters per minute, which was still in the quasi-static range. Even though the pull-out rates in this study (approximately 1 m/s) are increased by several magnitudes compared to previous literature, the achieved pull-out velocities in this study are still way below the ballistic limit of such bulletproof fabrics (which are typically hundreds of meters per second); the effects of yarn failure and breakage that occur during projectile penetration are relatively insignificant and therefore not taken into consideration. The main objective of this paper therefore is to investigate the inter-yarn frictional behavior of aramid plain-woven fabrics when subject to higher dynamic pull-out rates through yarn pull-out experiments. To achieve the objective of experimentally investigating the pull-out response of yarn at dynamic rates, a pendulum impact setup was introduced. In order to study the rate dependency, the results obtained from pull-out experiments at quasi-static rate will be presented. Additional studies on the effects of out-of-plane pressure and weave direction will be investigated as well.

Experimental procedure

In this study, the fabric samples were prepared using a Point Blank Pathfinder Special® bulletproof vest manufactured in 2006 and 2008 by Point Blank Body Armor, and consisted of 600d Kevlar® and 500d Twaron® samples. No further specification on the surface treatment was provided; therefore, the fabric samples were tested as-is. In these fabric layers, the warp-direction yarns have a thicker width than the weft-direction yarns, as informed by the manufacturer. As warp yarns are typically more crimped than weft yarns, the weave direction was also further verified by measuring the crimp ratios using the equation¹³

Crimp = \frac{L_{y} - L_{f}}{L_{f}} \times 100

(1)

where L_y and L_f are the yarn length and fabric length, respectively. Specifications of the fabrics are given in table 1.

All cut samples have 1-cm wide tails on each side to reduce the effects of irregular fabric widths after cutting, as can be observed in Figure 2(a). The fabric sample is clamped between aluminum fixtures, with a 1-mm thick sheet of Teflon on either side to ensure full contact of the fabric with the fixture, and to reduce possible friction with the aluminum fixture.

Figure 2.

(a) Yarn pull-out fixture on the MTS 810 for quasi-static experiments. The center hook is connected to the load cell, which records the corresponding pull-out load histories as the crosshead moves. (b) Setup for applying transverse out-of-plane pressure on the fabric. The applied transverse load is adjusted by changing the compressed length of the springs.

Quasi-static experiments were performed on the MTS 810 system servo-hydraulic system shown in Figure 2(a), with a crosshead speed of 1 mm/s. The pull-out load was measured using a load cell (Interface Inc.) with a maximum load capacity of 220.24 N.

Dynamic experiments were performed using a pendulum setup inspired by the Charpy impact test (ASTM E23, 6110, ISO 148-1 standard) in order to achieve much higher pull-out velocities compared to previous literature. A schematic of this pendulum setup is shown in Figure 3. The fabric fixture is mounted on a linearly sliding cart (Edmund Optics EDM-37365A). The center yarn to be pulled out of the fabric is attached to a quartz force transducer (Kistler 9712B50) with a hook. The quartz force transducer is then mounted onto a stationary post that is fixed on a surface, as shown in Figure 2(b). This design prevents inertia effects from getting into the measured pull-out force histories.

Figure 3.

Pendulum setup with fabric fixture mounted on linear sliding cart (a) before impact and (b) after impact, moving at velocity v. The hook is connected to the force transducer to measure the pull-out load.

The pendulum of mass 5.53 kg was raised to an initial vertical height of approximately 7 cm in order to achieve a theoretical post-impact cart velocity of 1.3 m/s. While higher velocities could be achieved theoretically, a higher pendulum impact velocity resulted in significant noise in the load signals and irregularity in the achieved cart velocities. Upon impact from the pendulum mass, the cart moves forward with a calculated velocity and the hook pulls out a single yarn. The cart displacement was measured using a magnetic linear band and encoder system (SIKO® MSK 230 and MB 320) mounted on the side of the linear cart system. Using the displacement–time history of the cart, the actual pull-out velocities achieved ranged between 0.9 and 1.2 m/s due to energy loss during impact and cart friction. A piece of rubber at the impact point between the linear slide and pendulum mass was used to reduce mechanical noise from shock.

The aluminum fabric fixtures for both quasi-static and dynamic experiments were also designed to allow a transverse out-of-plane pressure to be applied on the yarn being pulled out using a compression spring system. The amount of force applied on the fabric was varied at 0, 20, and 80 N. This was achieved by using eight compression springs with a known spring constant (4.72 N/mm) and adjusting their compression lengths, as shown in Figure 4.

Figure 4.

Setup for applying transverse out-of-plane pressure on the fabric. The applied transverse load is adjusted by changing the compressed length of the springs.

By assuming the transverse pressure applied is the sum of all the compressive forces from the springs divided by the area of the fabric, the calculated transverse pressures applied on the fabric samples were 0, 21.4, and 85.4 kPa, respectively. The opening width between the two aluminum plates was kept at a constant of 2.5 cm, as shown in Figure 5.

Figure 5.

Experimental setup with aluminum fixtures for (a) no transverse load on fabric and (b) transverse load on fabric. The principal yarn being pulled out is highlighted in bold.

While a larger number of experiments is desired, the availability of fabric samples was limited, and therefore each experiment for all parameters was performed three times to ensure repeatability.

Results and discussion

The yarn pull-out tests for these different parameters were performed, and the results for the yarn pull-out forces are presented in Table 2 and in the following sections.

Table 1.

Specifications of extracted yarns and tested fabrics¹⁴

Fiber	Kevlar®		Twaron®
Diameter (µm)	11.22 ± 0.16		9.37 ± 0.12
Density (kg/m³)	1440		1450
Fiber modulus (GPa)	109.39 ± 5.45		152.49 ± 9.22
Ultimate yield strength (GPa)	4.83 ± 0.29		4.74 ± 0.32
Failure strain (%)	4.42 ± 0.26		3.12 ± 0.22
Weave type	Plain		Plain
Yarn linear density (denier)	600		500
Yarn linear density (tex)	66.7		55.6
Ends per cm (warp)	12.00		11.49
Picks per cm (weft)	13.50		11.49
Crimp (%)	2.94 (warp)	1.01 (weft)	1.47 (warp)	1.07 (weft)

Table 2.

Yarn pull-out results for Kevlar® and Twaron® in the parameter space

Fabric	Direction	Yarn pull-out rate
		Quasi-static (1 mm/s)			Dynamic (~1 m/s)
		Out-of-plane pressure			Out-of-plane pressure
		0 kPa	21.4 kPa	85.4 kPa	0 kPa	21.4 kPa	85.4 kPa
Kevlar®	Warp	7	10	18	13.4	–	13.5
Kevlar®	Weft	5	6	8	10.0	–	10.5
Twaron®	Warp	1.9	2.7	3.4	5.00	–	5.10
Twaron®	Weft	1.8	1.9	3.3	1.65	–	2.20

For the purposes of discussion regarding the frictional behavior during the yarn pull-out process, we qualitatively using classical friction law relations in Equation (2) below:

F_{friction} = f (μ, N, L_{f}, A_{contact})

(2{\rm a})

A_{contact} = f (F_{normal}) = f (F_{int} + F_{ext})

(2{\rm b})

In Equation (2a), the frictional force is a function of several parameters as shown above, where μ is the coefficient of friction, N is the yarn density (ends or picks per unit length in the principal direction), L_f is the fabric length in the principal direction, and A_contact is the real contact area between the yarns at each crossing point. Newer theories of friction attempt to explain the dependence of friction on the real contact area between the two surfaces by taking into account the microscopic asperities, which is in turn a function of the total normal force. As we have an external out-of-plane force being applied on the fabric in this study, the total normal force is expressed as a function of F_int, the amount of contact force at each crossing point (also known as the internal pressure¹⁵), and F_ext, the amount of out-of-plane force applied outside the fabric at each crossing point. While friction is a complex phenomenon, the combined classical friction laws are sufficient to explain some of the macroscopic results obtained in this study.

As previously discussed, yarn pull-out load–displacement curves typically have two regions, namely the yarn uncrimping and yarn translation region. Although Figure 1 depicts the pull-out load history for in-plane yarn pull-out, these trends are similarly reflected in the out-of-plane pull-out curves, as shown in Figure 6 for both Kevlar® and Twaron® fabrics.

Figure 6.

Typical quasi-static out-of-plane yarn pull-out curves for (a) Kevlar® and (b) Twaron® in both weave directions.

Due to the existence of high-frequency background noise produced by mechanical shock from the impact, the Savitzsky–Golay smoothing filter was implemented to preserve local maxima and minima, thereby showing the oscillatory stick-slip behavior more clearly. During the yarn pull-out experiments, an out-of-plane fabric deformation was observed for both quasi-static and dynamic experiments, as shown in Figure 7. For in-plane experiments that were previously performed by several authors, the fabric deformation is in the form of a triangular extension about the principal yarn.¹⁵ For out-of-plane yarn pull-out, this fabric deformation results in a pyramidal fabric deformation, which also plays a role in the force–displacement history of the yarn pull-out process, as the fabric deforms during the yarn uncrimping phase and contributes to the fabric displacement.

Figure 7.

Out-of-plane fabric deformation when the center yarn is being pulled out.

The inter-yarn friction force decreases with the number of crossover points in the yarn translation region; therefore, the first analysis performed was to investigate the correlation between the yarn translation force and the number of remaining crossover points. Since each local maxima corresponds to the yarn passing under two crossover points on either end of the fabric, as illustrated in Figure 8, the local maxima was then normalized with respect to the number of crossover points remaining, as shown in Figure 9. The number of crossover points is easily calculated using the corresponding ends and picks per cm of the fabric.

Figure 8.

Schematic of crossover points corresponding to local maxima during the yarn translation phase.

Figure 9.

Normalized force (pull-out force per end/pick) against remaining crossover points.

Based on the force history curves shown in Figure 6, it was initially thought that the total yarn pull-out force in the translation phase decreases linearly with the number of remaining crossover points, that is, the pull-out force per crossover point remains constant, implying that the amount of kinetic friction is a simple linear relationship with the total number of contact points between the weave directions. From Figure 9, it was observed that the amount of kinetic friction per remaining crossover point actually decreases as the yarn starts to translate and pass under these crossover points. The behavior of this normalized load shows the non-linearity of this relationship, implying that the frictional behavior during the yarn translation phase is much more complicated than a direct correlation between the number of crossover yarns and level of frictional force.

It can also be observed from Figures 6(a) and (b) that the peak loads of the warp yarn pull-out from both fabric types have higher values than the weft-direction yarns. This is due to the warp-direction yarns having a higher crimp ratio (or ‘waviness’), resulting in a higher peak load in the yarn uncrimping region before yarn translation occurs. These results agree with the observations of Bazhenov⁹ that the weave direction with a higher crimp ratio has a higher peak load. A higher yarn density also creates higher internal contact forces (or internal pressure) at the crossing points,¹⁵ and using Equation (2), the total amount of friction is increased when the internal forces increase. The peak pull-out loads for the Kevlar® samples are also observed to be higher than that of the Twaron® samples for both warp and weft directions. This can be attributed to the higher number of ends and pick per centimeter of the Kevlar® samples, resulting in a tighter weave compared to the Twaron® fabric. The same mechanism as detailed above therefore explains how this higher peak uncrimping load is due to the higher yarn density (ends and picks/cm) in both weave directions for Kevlar®, resulting in a higher number of crossover points and therefore the total inter-yarn friction, effectively increasing the peak force value of the yarn uncrimping region.

Loading rate effect

The obtained load–displacement curves for both quasi-static and dynamic experiments were then compared to delineate the rate effects for the yarn pull-out mechanism. Besides comparing the peak loads for both pull-out rates, the fabric displacement at which the peak load occurs (i.e. the displacement of the fabric at the end of the yarn uncrimping region) and the yarn uncrimping energy were also compared. The total uncrimping energy is obtained by integrating the area enclosed by the force–displacement curve up to the point of peak load. This was computed for different loading rates under out-of-plane loading. Figures 10 and 11 summarize the results of peak pull-out force against pull-out velocity. Gawandi et al.⁸ presented results that depict a loading rate-independence of the peak loads in the case of baseline neat fabrics (i.e. non-coated KM2 fabrics), and an overall increase in peak load as a function of loading rate at 5.08, 50.8, and 508 mm/min for Vamac®-coated (pre-weaving) and modified polymer-coated (post-weaving) Kevlar KM2 fabric. Trends in a study performed by Kirkwood et al.⁷ on plain-woven Kevlar KM2 are in agreement as well, showing an increase in peak pull-out load from 5 to 500 mm/min loading rate, although any polymer coatings on the tested fabrics were not specified. Similarly, the experimental data comparison between dynamic and quasi-static loading rates in this study shows that a higher pull-out rate results in an increase in the peak load. Although the information was not provided by the manufacturer, it is highly likely that the fabrics used in this study were coated or post-treated before weaving.

Figure 10.

Yarn pull-out curves for Kevlar (a) warp and (b) weft yarns at different loading rates.

Figure 11.

Yarn pull-out curves for Twaron (a) warp and (b) weft yarns at different loading rates.

Figure 12(a) provides a clearer comparison between the peak pull-out loads at different loading rates. Figure 12(b) depicts the fabric displacement at which the peak pull-out load occurs (i.e. the end of the yarn uncrimping phase and the beginning of the yarn translation phase) with respect to the pull-out velocity. Comparison between quasi-static and dynamic results reveals that as the loading rate increases, the fabric displacement at the peak load increases. The fabric deformation (the pyramidal deformation observed in Figure 7) also contributes to the increase in fabric displacement at the peak load for high rates compared to quasi-static rates. Determining this displacement at the peak pull-out load facilitates the calculation of the yarn uncrimping energy (defined by the area under the force–displacement curve up to the peak force), which is plotted against the yarn pull-out velocity, as shown in Figure 12(c). In general, the yarn uncrimping energy increases with a higher pull-out velocity due to both an increased peak pull-out force as well as an increased fabric displacement. The change in both yarn uncrimping and yarn translation regions at different loading rates indicates that the total yarn pull-out energy at a higher pull-out rate is greater than that the total energy at a quasi-static pull-out rate, as shown by the increased area in the pull-out load against displacement curve.

Figure 12.

(a) Peak pull-out loads at dynamic and quasi-static pull-out rates. (b) Fabric displacement at peak load for dynamic and quasi-static experiments. (c) Total uncrimping energy for dynamic and quasi-static experiments, obtained by integrating the load–displacement curve in the static friction region.

Out-of-plane pressure effect

In practical applications, the fabrics are typically used in a multi-layer form. This configuration restricts the motion of the yarns in the out-of-plane direction. Such lateral restrictions in yarn motion may change the pull-out behavior due to a change in external normal force applied at each contact point, as per Equation (2). The out-of-plane pressure applied on the fabric increases the external normal force F_ext at each contact point, thereby increasing the total normal force during the yarn pull-out test, resulting in a higher frictional force being applied on the principal yarn. King¹⁶ describes a crimp interchange mechanism where, as one weave direction is straightened and becomes uncrimped, the other weave direction becomes more crimped, that is, increases in waviness in the out-of-plane direction. Therefore, when transverse pressure is applied, the single yarn takes a more tortuous path during uncrimping, as any out-of-plane movement in the opposing weave direction is impeded by this out-of-plane pressure, thereby increasing the peak load. This mechanism would apply for both in- and out-of-plane directions, as the pull-out phase requires straightening and crimp interchange regardless of pull-out direction.

It was observed from experimental results that at quasi-static rates, the pull-out load increases with increased out-of-plane pressure, which is in agreement with previous literature. It is noted in Figure 13(a) that for the Kevlar® fabric, the warp yarns experienced a much larger increase in peak load under increased pressure compared to the weft yarns. The peak pull-out load of the Twaron® fabric did not change significantly in both warp and weft directions. In fact, the peak pull-out loads increased by approximately the same amount as the rate was increased. However, at high rates, the effects from transverse pressure become negligible on the peak pull-out force for both fabrics in both directions, as indicated by Figure 13(b). It is possible that some inertial effects occur at dynamic pull-out rates especially at the far field, but further experimentation needs to be performed to investigate this phenomenon before satisfactory conclusions can be made.

Figure 13.

Out-of-plane yarn pull-out peak force for both fabrics under transverse pressure at (a) quasi-static rates and (b) high rates.

In order to further investigate the effects of out-of-plane pressure on the yarn pull-out dynamics, the typical force–displacements curves for Kevlar® and Twaron® in both warp and weft directions were compared for both transverse pressure levels of 0 and 85.4 kPa, as shown in Figures 14 and 15. These plots were obtained only for quasi-static rates, as the dynamic experiments did not show significant change with an increase in transverse pressure, as previously elaborated and depicted in Figure 13(b). It was observed from the quasi-static results that an increase in transverse pressure from 0 to 85.4 kPa has three distinct effects on the pull-out load history. Firstly, a comparatively non-linear decay in the average pull-out load in the yarn translation region can be observed when the transverse pressure is applied. This suggests that a transverse pressure not only impedes the transverse motion of the opposing weave direction, but also the motion of the yarn being pulled out as it goes through the crossover points. Secondly, from Figures 14 and 15, the peak pull-out load as well as the fabric displacement at the peak load both increase with a higher transverse pressure, which is more clearly visible in the Kevlar fabric. This results in an increase in uncrimping stiffness, as shown by the increase in the slope of the pull-out curve in the uncrimping region.

Figure 14.

Quasi-static load–displacement curves at zero and 85.4 kPa transverse pressure in (a) Kevlar® warp direction; (b) Kevlar® weft direction.

Figure 15.

Quasi-static load–displacement curves at zero and 85.4 kPa transverse pressure in (a) Twaron® warp direction; (b) Twaron® weft direction.

Thirdly, there was an observable non-linear ‘distribution’ in the maximum pull-out peaks across a wider displacement range, most visibly in the case of Twaron® fabric. Observations made during the quasi-static yarn pull-out process suggest that this anomalous distribution in the peak loads is a result of unsymmetrical yarn uncrimping and translation. With increased pressure in the out-of-plane direction, the initiation of yarn slippage and translation occurs at one end of the principal yarn, while the other end of the principal yarn is still being uncrimped, as schematically illustrated in Figure 16. These unsymmetrical uncrimping zones for each end are slightly shifted and overlapped, resulting in an overall force–displacement history that appears distributed across a broader displacement range.

Figure 16.

Unsymmetrical uncrimping and translation of the yarn under pressure.

Relevance of yarn pull-out results to ballistic performance of woven fabrics

As a projectile impacts a woven fabric structure, some of the projectile kinetic energy is absorbed and dissipated by the inter-yarn friction at these crossing points through mechanisms such as yarn uncrimping, yarn extension, out-of-plane fabric displacement, and yarn translation. While the ballistic limit of bullet-resistant fabrics far exceeds the pull-out velocities examined in this study, the results of the dynamic pull-out for both Kevlar and Twaron show a marked increase in the uncrimping load and uncrimping energy, which suggests that a larger portion of the projectile kinetic energy is being absorbed through the yarn pull-out mechanism, at least for velocities below the ballistic limit. Further experimentation is required to determine the significance of yarn pull-out as an effective mechanism for energy absorption for velocities near or higher than the ballistic limit, although yarn breakage at these velocities would potentially present difficulties when isolating the effects of out-of-plane yarn pull-out.

Conclusions

In this study, the dynamic mechanical response of the single yarn pull-out from both Kevlar® and Twaron® fabrics was experimentally investigated. A pendulum impact setup was developed for dynamic yarn pull-out (approximately 1 m/s). To reveal the rate effects, quasi-static (1 mm/s) experiments were performed using an MTS 810 system. In general, it was found that the Kevlar® fabric has a much higher pull-out peak load than the Twaron® fabric, and the warp-direction yarns for both fabrics took higher loads to pull them out than the weft (or fill) direction yarns.

Experimental results reveal that the behavior of yarn pull-out is sensitive to loading rate, as the peak pull-out load, displacement at the peak load, and the pull-out energy all increased with increasing loading rate. Transverse pressure effects showed more interesting behavior in the yarn pull-out mechanism. At quasi-static pull-out rates, the peak load was observed to increase, but the displacement at which the peak load occurs decreased with increasing pressure. However, the peak load did not change with increasing pressure at higher rates.

Footnotes

Funding

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

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