An experimental study of the effect of ply orientation on ballistic impact performance of multi-ply fabric panels

Abstract

This paper presents a comprehensive experimental study of the effect of ply orientation on the ballistic performance of the multi-ply fabric panels. The fabric panels used in the experiments were constructed by laying plies of plain-woven fabrics in a selection of orientations. Particular attention was paid to the stacking sequence of fabrics with different laying angles. Ballistic impact tests were carried out on such angled fabric panels. The results showed significant improvement in energy-absorbing capacity of the angled multi-ply panel over the conventional aligned panel where all fabric plies are laid in the same orientation. The impact energy absorption by these angled fabric panels showed a 14% increase over the aligned counterpart, depending on the number of the plies. Optimizing the stacking sequence of the angled plies caused a 15% increase in impact energy absorption. For the panels with a large number of plies, such as the eight-ply panels, the effective panel construction can be determined according to the performance of the sub-level ply group.

Keywords

ballistic impact energy absorbing ply orientation body armors

High-strength fabrics that offer resistance to high-velocity impact are a suitable selection for applications such as ballistic body armors.¹ The protection against impact projectiles provided by the modern body armors has been greatly enhanced by innovations in polymeric fiber engineering. However, it seems that fibers available for ballistic protection are limited to only a few types, such as the aramid and high-density polyethylene, and superior fibers are yet to be announced for the improvement of body armor. On the other hand, the requirements of body armor have been made more challenging in that the body armor should provide better protection with reduced weight. Confronted by such a challenge, engineering design of ballistic fabrics and panels becomes a reasonable approach to provide better body armor protection using fibers that are currently available for this application. For most body armors, the same ballistic fabric is used for all layers of the panel, despite the observed variation in strain distribution and failure mode of different layers of fabric in the body armor.² Woven and unidirectional (UD) fabrics are widely used as panels for soft ballistic body armor. The complexity of the response of ballistic fabric to impact has precluded exhaustive experimental programs to explore the permutation of the many independent variables and observe changes in ballistic performance. Typical phenomenological studies have instead been forced to draw preliminary conclusions based on experimentation and these studies have significantly advanced the level of understanding of the impact event.³

In a plain-woven fabric, two sets of parallel threads intersect at 90°. This biaxial feature of woven fabric leads to the formation of a pyramid impact deformation zone with a square base as the projectile impacts on the fabric. The two diagonals of the square base always coincide with the warp and weft directions, which is clearly shown in Figure 1. The warp and weft yarns that are in direct contact with the projectile are known as the primary yarns, with the other yarns known as secondary yarns. When the projectile impacts on a fabric, a transverse deflection in the primary yarns is produced and longitudinal strain waves are generated that propagate at the sound speed of the material down the axis of the yarns.^4,5 The primary yarns contribute significantly to absorbing the kinetic energy of the projectile, which is transferred into strain and kinetic energy of the fabric, whereas the contribution of the secondary yarns to energy absorption is small.^6,7 Over the past few decades numerous studies have been carried out on understanding the ballistic impact behavior of single yarns and single-ply fabrics using various analytical,^8–14 numerical^15–26 and experimental techniques.^{1,3,16,27–30} Previous research work on ballistic fabrics has been reviewed by a number of investigators.^31–33
Figure 1.
Typical images of a projectile penetrating a plain-woven fabric.

To enhance the ballistic performance of the fabric panel for body armor, various construction methods have been developed, such as the use of combinations of materials with different properties in the panel. The interaction between the fabric layers becomes a crucial factor in determining the ballistic impact performance of the multi-ply fabric panel and the stacking sequence of the plies in the fabric panels shows a significant effect on energy absorption. Cunniff³ demonstrated this effect using a double-ply system. The first system, composed of one ply of 1000 denier Kevlar 29 fabric backed by one ply of 375 denier Spectra 1000 fabric, had a ballistic limit of 269 m/s. The second system reversed the order of the fabrics and the ballistic limit of the second system was only 114 m/s. When the same type materials were used in both first and second ply the order of the plies showed no effect on the energy absorption. A mismatch in material properties, producing a mismatch in transverse deflection, was considered as the main reason for the large difference in energy absorption.³ The narrow transverse deflection of the second ply was suggested as constraining the wider transverse deflection of the first ply. Reversing the order of the plies would have eliminated the contact between layers. These results seemed promising for the design of double-ply panels. However, there is a question as to whether the effective stacking order from double-ply systems is suitable for fabric panels with a large number of plies. Hence, further investigation of these effects may be needed, especially the effect of the sequence of the ply group on energy absorption of the multi-ply panels.

The orientation of plies within the panel influences the transverse deflection of the plies and a different ballistic resistance may therefore occur due to the mismatch of transverse deflection. Hearle and colleagues¹⁶ experimentally observed the shape of the transverse wave front of a two-layer panel and noted that the shape of the transverse deflection was octagonal if the yarn directions of each layer were set at 45° to each other, but they did not determine the ballistic impact performance. The octagonal form of the transverse wave in the two-layer case indicated some degree of interlayer interaction.

In a more recent study, the impact performance of multi-ply fabric panels was tested, using the two-ply and three-ply fabric panels with selected ply orientations.^22,23 The test results revealed that the energy absorption of the panels was significantly increased when each ply of the fabric in the panel was placed with a different orientation. Therefore, it is important to fundamentally understand the effect and mechanism of ply orientation on the energy absorption of each ply and the panel as a whole. This need is particularly valid for the multi-ply panels with a large number of fabric plies.

The main purpose of the study described in this paper is to explore experimentally the effect and mechanism of ply orientation, layering sequence of the plies and ply-group combinations within the panel. The comprehensive experimental programs were based on the use of aramid multi-ply panels with various panel constructions.

Experimental methods

Fabric specimens

The fabric used in the present study is plain woven. The ends/picks of the fabric are 7.5 per centimeter. The fabric is woven of aramid fiber-based yarn, which has a linear density of 158 tex. The volumetric density of the yarn is 1440 kg/m³. The measured areal density of the fabric is 260 g/m². A tensile test of the yarn was carried out to measure the mechanical properties and the elongation and Young's modulus are 4% and 93.5 GPa, respectively. The single fabric panel is a square of 240 mm by 240 mm with the thickness of 0.345 mm.

Construction code of multi-ply fabric panels

An x, y, z orthogonal coordinate system is used in describing the orientation of each fabric ply in the multi-ply panel shown in Figure 2. The orientations of the plies are specified by the angle θ with respect to the x-axis with θ the angle between the warp yarn and x-axis. The angle θ is positive in the counter clockwise direction. For example, the fabric panel consisting of four plies shown in Figure 2 is designated as [45/45/0/0]. This fabric panel contains two ply groups, the first containing two plies in the 45° direction, and the second containing two plies in the 0° direction. In contrast to UD composite stacking sequence convention, since each ply is a 0/90 woven fabric, an additional θ+90° layer is effectively present at each orientation.
Figure 2.
A schematic showing the x, y, z multi-ply panel coordinate system, the x₁, x₂, x₃ ply coordinate system and the layup of the panel.

When all the plies (all warp yarns in the fabric panel) are in the same direction, the panel is defined as an aligned fabric panel, for example, [0/0/0/0]. When all the warp yarns are in different directions the panel is defined as an angled fabric panel, for example, [0/45], [0/22.5/45/67.5], etc. The aligned fabric panel is normally applied in multi-ply body armor systems due to its simplicity.

Experimental plan

The fabric constructions used in the experimental tests are listed in Table 1, which is designed for investigating the following: (a) the effect of the ply orientations; (b) the effect of the layering sequence of the plies; and (c) the effect of ply-group combination within the panel.
Table 1.
Specifications of fabric panels with different numbers of plies

The two-ply panel system was selected to study the effect of ply orientation on the energy absorption. The first ply is fixed at the angle of 0° and the second ply is orientated in the angles of 15°, 30° and 45°.

The three-ply panel systems were designed to study the effect of the layering sequence of the rotated ply. Only one ply was oriented with an angle of 45° and positioned at the top, in the middle and at the bottom. Four combinations of the fabric panels were selected to perform the experimental tests.

When the number of plies is more than four, more ply combinations are possible. It is not economic to test all the possible combinations experimentally. According to the numerical study of the influence of ply combinations on ballistic impact resistance of multi-ply fabrics,²³ two cases from the four-ply panels and four cases from the eight-ply panels were selected to experimentally test their energy-absorbing capacity.

Clamp design

During the ballistic impact tests, the fabric panel experienced high levels of tension and this tensile force causes the sample edges to slip when an ordinary clamp is used. This is especially true when the sample contains a large number of plies. It has been found that slippage has a significant effect on energy dissipation. The ordinary clamp is unable to hold the fabric panels consistently and therefore a large scatter in energy absorption occurs, which has been seen in the experimental tests. In order to have a consistent experimental result, a novel clamp system was designed to prevent slippage, as shown in Figure 3. This clamp consists of a base plate, a top plate, four bars and eight sliders. The sliders contact the bars and when clamped these bars are pushed vertically and laterally, holding the fabric panel very tightly. Also, the top plate produces a frictional resistance to prevent fabrics from slippage. Experimental observations have confirmed that there is very little slippage using this new clamping system.
Figure 3.
Schematic showing the fabric clamping system: (a) three-dimensional drawing of the clamp; (b) details of the drawing; (c) overview of the clamp; and (d) section view showing how the fabric is tightly clamped.

Experimental apparatus

Figure 4 illustrates the ballistic impact test system used in this study. It comprises the gun, fabric target, timer and high-speed video camera. The impact tests were performed with multi-ply fabric panels held in the novel clamp. The specimens were impacted by a cylindrical bearing steel projectile fired at velocities ranging from 400 to 540 m/s. The projectile has a mass of one gram and its length and diameter are both 5.5 mm. No deformation was seen in the projectiles after the impact tests. The ballistic performance of the fabric panels was assessed by measuring the energy absorbed by the fabric panel on penetration. It is assumed that the loss of kinetic energy of the projectile equals the energy absorption by the fabric panel. The tests involved measuring the projectile velocity before and after the impact, called the strike velocity (V_s) and the residual velocity (V_r), respectively. The loss of kinetic energy ( $Δ E$ ) of the projectile can be determined using the following expression:
$Δ E = \frac{1}{2} m (V_{s}^{2} - V_{r}^{2}),$
where m is the mass of the projectile.
Figure 4.
Schematic showing the ballistic impact test system.

Results and discussion

V_s−V_r test results

The strike velocities of the projectile (V_s) are plotted against the projectile residual velocities (V_r). These V_s−V_r data are shown in Figures 5 –9. For simplicity, the effect of air friction is not taken into account. The dashed line represents the results that would be obtained if the fabric panel did not exist and therefore no reduction in velocity occurred. The slope of the dashed line would be one. The projectile velocity loss at the low impact velocity is larger compared with the high-velocity impact, especially when the impact velocity approaches the ballistic limit. The V_s−V_r data is nonlinear at the low-velocity impact and it becomes linear at the high-impact velocity.²³ A linear regression is performed for all data sets and the lines of best fit are represented by the solid line in the figures.
Figure 5.
Strike-residual velocity data of the single-ply fabric panel.

Figure 6.
Strike-residual velocity curve for two-ply panels.

Figure 7.
Strike-residual velocity data for three-ply panels.

Figure 8.
Strike-residual velocity data for four-ply panels.

Figure 9.
Strike-residual velocity data for all aligned fabric panels.

Figure 5 presents the V_s−V_r data from one-ply fabric. The measured strike velocities ranged from 450 to 535 m/s, which were of high-velocity impact and therefore a linear V_s−V_r data was seen. The solid line in Figure 5 represents its trend line with the slope of 1.021and this value is almost the same as the slope of the dashed line where the fabric panel is not present,

Figure 6 shows the V_s−V_r data of the two-ply panel system. Four predetermined ply orientations, [0/0], [0/15], [0/30] and [0/45], were tested to determine the effect of the ply orientations on the energy-absorbing capacity. As can be seen from the V_s−V_r data, the panel with a [0/45] structure shows lower residual velocities compared with the other three panel constructions. For example, when the strike velocity is 500 m/s, the corresponding residual velocities are approximately 460 m/s for [0/0], 458 m/s for [0/15], 455 m/s for [0/30] and 450 m/s for [0/45]. The test results show some effect of the ply orientation on the ballistic impact resistance.

Figure 7 shows the results from the three-ply fabric panel. The purpose of the tests is to investigate the effect of the sequence of the orientated ply. Among the three plies, only one ply was oriented with an angle of 45° and positioned at the top, in the middle and at the bottom to form a different layering sequence. Four combinations of the fabric panels were selected and they are [0/0/0], [0/0/45], [0/45/0] and [45/0/0]. The V_s−V_r data of the three-ply system confirmed that the position of the oriented ply has a significant effect on the resistance to ballistic impact. For the three-ply panel, the best ballistic resistance was seen when the oriented ply was positioned in the middle.

Figure 8 demonstrates the V_s−V_r data of the four-ply systems. Two panel structures, [0/0/0/0] and [0/22.5/45/67.5], were selected to study the resistance to ballistic impact. In the fabric panel [0/0/0/0], all plies are in the same orientation. In the fabric panel [0/22.5/45/67.5], a 22.5° angle exists between two adjacent plies. Comparison of these two fabric panels shows that the resistance to the ballistic impact in the multiple angled panel is superior to that of the single angled panel. For example, the residual velocities are approximately 420 and 410 m/s, respectively, at the strike velocity of 500 m/s.

All the experimental test results confirmed that the panels show an improved resistance to the ballistic impact when the panel contains angled plies. For the two-ply systems, the resistance to ballistic impact increases as the rotation angle increases, reaching a maximum at the angle of 45°. Similar to the two-ply panels, the three-ply system also shows the best ballistic impact resistance in the presence of the 45° ply. The sequence of the angled ply also plays an important role in the resistance to the ballistic impact.

The effect of ply numbers on the characteristic of the V_s−V_r data is shown in Figure 9 for the aligned fabric panels (one, two, three, four and eight plies). Obviously, for a given strike velocity, the residual velocities of the projectile decrease when the number of plies increases due to more material being involved in impact energy absorption. The slopes of the V_s−V_r data (trend line) gradually increase as the number of plies increases, which implies that the resistance to the ballistic impact increases with the increase of the number of plies. Also, importantly the ballistic limit of the panel would be expected to increase as the number of plies increases.

Effect of ply orientation on energy absorption

In the previous discussions, the strike velocities were plotted against the residual velocities and a linear relationship was shown over strike velocities in the range 400–540 m/s. Using linear regression of all the data, the average residual velocity for any given strike velocity can be obtained. For simplicity, in the following discussions one strike velocity has been selected (500 m/s) to compare the energy absorption of different panels. Figure 10 shows the results of the two-ply panel systems in which the energy absorption of the fabric panel increases as the orientation angles increase from 0° to 45°. Compared with the aligned panel [0/0], it is increased by 4.5% for [0/15], 9% for [0/30] and 11.3% for [0/45]. It is concluded that the ply orientations significantly influence the energy absorption of the fabric panel and the panel containing the even angular distribution shows the maximum energy-absorbing capacity. The reason for the increase in energy absorption is because the strain energy in the panel [0/45] is larger than that in the panel [0/0].²³
Figure 10.
Effect of ply orientation on energy absorption at 500 m/s strike velocity.

Effect of layering sequence of the plies on energy absorption

Figure 11 presents the results from the three-ply fabric panels. As can be seen, the locations of the angled ply also show a significant effect on the energy-absorbing capacity. The energy-absorbing capacity is greatest when the angled ply is positioned in the middle ([0/45/0]). Similar to the case of the two-ply panel, the energy absorption increases in the presence of the ply orientations. For example, the panel structure [0/45/0] increases the energy absorption by 15% in comparison with the aligned panel structure [0/0/0].
Figure 11.
Effect of ply orientation on energy absorption for three ply panels at 500 m/s strike velocity.

Effect of the ply-group combination within the panel

The choices of the ply-group combination are increased when the number of plies is more than four and, accordingly, many combinations can be created. Based on the numerical study,²³ the predicted lowest performing panel construction [0/0/0/0] and the predicted highest performing panel construction [0/22.5/45/67.5] were tested and the result is shown in Figure 12. The test result indicates that energy absorption is increased by 11.1% for the panel [0/22.5/45/67.5].
Figure 12.
Effect of ply orientation on energy absorption for four-ply panels at 500 m/s strike velocity.

The results from the eight-ply panels are shown in Figure 13 and indicate the importance of the ply-group combinations within the panels. The four-ply panels, for example, [0/0/0/0] or [0/22.5/45/67.5], can be considered as one ply group. Therefore, the eight-ply panel [0/45/0/45/0/45/0/45] can be rewritten as [0/45/0/45]₂ and the panel [0/22.5/45/67.5/0/22.5/67.5] as [0/22.5/45/67.5]₂, etc. The panel [0/22.5/45/67.5]₂ increases the energy absorption by 12.6%, compared with the panel structure [(0/0/0/0)₂]. It should be noted that the total increase in energy absorption is not the sum of the increase from the sub-level ply group. The amount of the increase of the energy absorption is no more than 15%.
Figure 13.
Effect of ply orientation on energy absorption for eight-ply panels at 500 m/s strike velocity.

The energy loss of the projectile is plotted against the numbers of the plies, shown in Figure 14. The graph clearly shows the advantage of the angled fabric panels (i.e. [0/45], [0/22.5/45/67.5], etc.) in energy absorption over aligned fabric panels (i.e. [0/0/0/0]). For the angled fabric panels, the energy absorption values from the best fabric assemblies are selected to compare with the aligned panel. These, as shown in Figures 10 –13, happen to be the ones where the yarn directions are most evenly distributed within 90°. As can be seen, the angled fabric panels always show a higher energy-absorbing capacity. The energy absorption increases almost linearly with the increase of the number of the plies.
Figure 14.
Plot of ply numbers against energy absorption at 500 m/s strike velocity.

Conclusions

A comprehensive experimental study has been carried out to investigate the ballistic impact resistance of multi-ply fabric panels. The present study investigated the following: (i) the effect of ply orientations; (ii) the effect of the layering sequence of plies; and (iii) the effect of the ply-group combination within the panels. The ballistic impact tests were conducted on the multi-ply panels, which consisted of various panel constructions. A novel clamp was designed in order to keep the experimental tests consistent. The main conclusions that can be drawn from the present experimental studies are as follows.

The experimental test results show that the ply orientations significantly affect the energy-absorbing capacity of the multi-ply fabrics. The angled fabric panels had a consistently higher energy-absorbing capacity compared to aligned fabric panels. The energy absorption in the angled fabric panels was 15% greater, depending on the number of the plies.

The layering sequence of the orientated plies also plays an important role in energy absorption. For the three-ply panels, the best energy absorption appears when the middle ply is orientated at an angle of 45°, in which case the energy absorption is increased by 15% over the case with the 45° ply on the outside.

For the panels with a large number of plies, such as the eight-ply panels, panel performance appears to be determined by the performance of the sub-level ply group. For example, the panel [0/22.5/45/67.5] shows the best performance among the four-ply panels and repeating this layup to produce the eight-ply panel gives the best performance among all the eight-ply panels tested.

A novel clamp system was designed in order to prevent the clamped edges from slipping. The experimental tests confirmed the effectiveness of the novel clamp, which gave more consistent results than the ordinary clamp system, especially for the multi-ply panels containing large numbers of plies.

Footnotes

Funding

This work was supported by a research grant from EPSRC/Dstl (EP/G042829/1).

Acknowledgment

© Crown copyright 2013. Published with the permission of the Defence Science and Technology Laboratory on behalf of the Controller of HMSO. The support from Teijin Aramid is also gratefully acknowledged.

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An experimental study of the effect of ply orientation on ballistic impact performance of multi-ply fabric panels

Abstract

Keywords

Experimental methods

Fabric specimens

Construction code of multi-ply fabric panels

Experimental plan

Clamp design

Experimental apparatus

Results and discussion

Vs−Vr test results

Effect of ply orientation on energy absorption

Effect of layering sequence of the plies on energy absorption

Effect of the ply-group combination within the panel

Conclusions

Footnotes

Funding

Acknowledgment

References

V_s−V_r test results