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Influence of material selection on hybrid design of soft body

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Citation for published version (APA): Yang, Y., & Chen, X. (2018). Influence of material selection on hybrid design of soft body armour.

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Download date:04. Oct. 2021 Proceedings of the 8th World Conference on 3D Fabrics and Their Applications Manchester, UK, 28-29 March 2018

Influence of material selection on hybrid design of soft body armour

Yanfei Yang1,2, Xiaogang Chen2 1 School of , Zhongyuan University of Technology, Zhengzhou, Henan 450007, China 2 School of Materials, University of Manchester, Sackville Street, Manchester M13 9PL, UK

Abstract. This study aims to optimise the construction of soft body armour panels by hybridization. Different ballistic responses of woven fabrics and Ultra High Molecular Weight (UHMWPE) Uni-directional (UD) laminates are investigated for material selection, which is conducted through ballistic test and fractorgaphic observation. According to ballistic test, with increasing of layers in a panel, SEA of woven panel shows a decrease trend, and Dyneema UD panel exhibits an increasing trend. According to fractorgraphic analysis, Twaron fabric has large transverse deformation under ballistic impact, particularly for back layers. This results in large BFS. For Dyneema UD, thermal damage can result in performance degradation, especially for front layers. Therefore, it is not appropriate for the striking face of a panel. Based on above findings, an optimized hybrid panel is designed by combing Twaron woven fabric before Dyneema UD. Ballistic results show that such hybridization is benefit for energy absorption of the panel and minimize BFS in comparison with other hybrid panel.

Key words: ballistic responses, aramid fabric, Ultra High Molecular Weight Polyethylene Uni-directional Laminate, Finite Element (FE) analysis

1. Introduction

Performance of ballistic body armour is always pushed by updating of operations, and ammunition. Ballistic body armour is required not only to resist perforation of bullet or , but also to minimize Back Face Signature (BFS) and reduce weight as much as possible. Since the 20th century, with the advent of high performance synthetic fibres, such as , aramid, and high modulus polyethylene, performance of ballistic body armour is greatly improved, and weight is significantly reduced. All these high performance fibres used for ballistic protection possess superior mechanical properties of high strength, high moduli and low mass density, in particularly, aramid fibres and Ultra High Molecular Weight Polyethylene (UHMWPE) fibres are two most popular high performance fibres for soft body armour.

Aramid fibre is aromatic , which was first introduced by DuPont in the early 1960s. Aramid fibres consist of long molecular chains with aromatic structures and amide groups. Any one structure of aromatic and amide is very strong[1]. What’s more, these molecular chains are highly oriented parallel with many inter-chain bonding (hydrogen-bonding) as shown in Fig. 2.10. These special molecular structures of aramid fibre contribute to superior material properties. The fibres possess the high tensile strength with 5 times than . The tensile modulus is up to 100GPa. Aramid fibres possess excellent heat resistance. The aromatic rings in molecular chains assure stable thermal properties. Such fibres do not melt or support combustion. The decomposing temperature is at about 427°C to 482ºC [2].

Fig. 1 Molecular structures of PPTA [3] Fig. 2 Polyethylene unit [4]

UHMWPE fibres are commercialized in the late 1970s by the Dutch chemical company DSM®. UHMWPE fibre is made up of extremely long chains of polyethylene with a large molecular mass numbering in the millions [4]. During the drawing process, UHMWPE fibres can attain a high degree of chain extension with parallel orientation greater than 95% and a high level of crystallinity up to 85%. This results in high tensile strength. Due to extreme high tensile properties and the lowest density among all fibres (0.97 g/cm3), it is reported that this kind of fibre has superior energy absorption capacity with four times of aramid fibre [5]. However, due to the weak bonding between olefin molecules, UHMWPE fibres have poorer heat resistance with relatively low melting point (130-145°C) [6]. This has been some concerns regarding the thermal damage effect on ballistic performance of soft body armour.

Soft body is usually manufactured by layering up numerous aramid woven fabrics (such as ® and Twaron®) or Uni-directional laminates (such as Dyneema® and Spectra®). Usually a ballistic proof vest varies from 10 to 50 layers with weight around 3-5kg [7]. However, combining same fabric layers is not the most efficient way in providing the best ballistic performance. In addition, there is no one material that can provide all required properties for ballistic protection. To make best use of materials properties, different ballistic materials can be combined together to make a hybrid armour panel.

Karahan [8] experimentally observed that hybrid panels combining para-aramid woven fabrics and K-Flex ® UD laminate can achieve around 4.5% reduction in BFS and 8.5% improvement in energy absorption per unit weight compared to 100% woven fabric panels. Cunniff [9] found when a high modulus material (Spectra®) was placed on the striking face and a low modulus material (Kevlar®) at back, the fabric panel had a reduction of energy absorption by over 80% than that of the fabric panel with a reverse layering sequence. Park [10] observed the hybrid panel with materials in the order of decreasing modulus enhanced the penetration resistance, and in the order of increasing modulus can obtain the enhanced trauma resistance. Rahman [11] numerically showed that the contact force of the woven panel was higher than that of the rigid panel. This results in more energy absorption in the woven panel. Many patents and some commercial hybrid products have been proved to be very efficient in providing superior ballistic performances with reduced weights [12-15]. However, most of these studies on hybrid armour panel just focus on the hybridisation effect. Mechanisms relating to the hybridisation have not been fully understood.

This research aims to optimise the construction of soft body armour panels by hybridization. Two most popular ballistic materials, aramid woven fabrics and UHMWPE uni-directional (UD) laminates are investigated on their advantages and weak points for ballistic material selection. This provides a guide for hybrid design of soft body armour.

2. Experiment

2.1 Ballistic test Two popular ballistic materials, aramid woven fabrics and UHMWPE UD laminates, are made into ballistic panels, as shown in Table 1. Woven fabrics are manufactured with two different weave densities by Twaron® fibres from ®. UHMWPE UD laminates are Dyneema® SB71 and Dyneema® SB51 from DSM®. In order to identify the best construction of armour panels, hybrid panels are manufactured with these two materials in different orders, as shown in Table 2. In hybrid panels, each component was listed in the layering order which is from the striking face to the exiting face and separated by the ‘/’. The subscript represents the number of layers of one component.

Table 1 Material properties

Yarn count Weave density Thickness Areal density Material Code (tex) (ends/cm) (mm) (g/m2)

Twaron fabric 8F 93 8.3 0.20 155.35

11F 93 11 0.26 196.85

Dyneema UD 7U / / 0.24 186.94 5U / . 0.30 252.98

Table 2 Hybrid panels Areal density Code Layer order Number of layers (g/m2)

7U /8F 2 342.13

8F/7U 2 342.13

7U /11F 6 1029.69

11F/7U 6 1029.69

8F8/7U16 24 4248.80

7U16/8F8 24 4248.80

7U8/8F8/7U8 24 4248.80

8F4/7U16/8F4 24 4248.80

2.2 Ballistic test Ballistic tests were conducted at the ballistic laboratory in the University of Manchester. A steel cylindrical projectile (5.5mm in diameter and height, 1.004 (+0.008) g) is used as Fragment Simulation (FSP) for impacting on panels in this study. It is fired by a machine simulating hand gun and propelled by gunpowder. The impact velocities are in the range of 460–500 m/s. The average striking velocity on fabric was calculated as 483 m/s, which is the velocity of the projectile contacting the panel.

In the perforation test, a panel was clamped in a square steel frame with dimensions of 24cm×24cm. Due to variability of exiting velocities of a projectile, energy absorption of a perforated panel is determined by ten shots. According to the striking velocity and residual velocity of the projectile, energy absorption in the panel can be calculated.

In the non-perforation test, a panel is placed before a clay box with a size of 24×24×10cm. The backing material is Roma Plastilina No.1 (RP#1) oil-based modelling clay. According to the NIJ standard[16], the clay should be put into an oven at 38oC above 3 hours before the non-perforation ballistic test. After the impact, an indentation is remained in clay. As the non-perforation test results are relative stable, BFS of a non-perforated panel is determined by five repeat ballistic tests.

2.3 Photographic examination Some ballistic responses of armour panel can be reflected by failure morphology of yarns and fibres in fabric. In this study, failure modes of fibres in different post-impact panels are examined in details. Global failure characteristics of post-impact panels are examined using an optical microscope (Projectina CCD-1300PQC) at magnifications from 5 to 50. The individual broken fibres and fractured yarns are closely observed on Scanning Electron Microscope (Hitachi S3000) with magnifications from 15 to 1000.

3. Results and discussions

3.1 Specific energy absorption To compare energy absorption capacity of different panels, the normalised energy absorption (SEA) is used to remove the effect of the areal density. According to ballistic test results of perforated panels (as shown in Fig.3), specific energy absorption (SEA) of Tawron woven fabrics shows a decreasing trend with increasing number of layers in the panel. This indicatesthat energy absorption efficiency of each fabric layer is decreased. Cunniff [9] explained that such deleterious system effects of the panel were due to the possible constraint of transverse deflection by the subsequent layers on the front layers. As a result, interference between layers may prevent stacked layers from achieving their individual energy absorption capacities.

However, under the same ballistic condition, SEA of Dyneema UD panels had an increasing trend with more layers added in the panel as shown in Fig. 1. Such trend is very different from that of woven panel. In particular, when the number of layers is above 9 layers, SEA seems to have a sharp increase. Such inverse results are speculated due to different failure mode of Dyneema UD during the impact process. Therefore, failure modes of these two materials are investigated in a following section. 500

400 Twaron 8F Twaron 11F 300 Dyneema UD SB71

Dyneema UD SB51 /g) 2 200

(J.cm 100

0

Specific energy absorption energy Specific 0 1 2 3 4 5 6 7 8 9 10 Number of layers Fig. 3 The specific energy absorption of Twaron and Dyneema UD panel

3.2 BFS

At non-perforation case, BFS of 24-layer Twaron woven panel 8F24 and 20-layer Dyneema UD panel 7U20 which has the same areal density were investigated. After ballistic impact on these two panels, indentation is left in clay. A conical indentation was found behind Twaron woven panels with round edge in the clay as shown in Fig.4(a). Behind Dyneema UD panels, the indentation exhibited a conical bottom and square edge in the clay as shown in Fig. 4 (b).

(a) Behind Twaron woven panel (b) Behind Dyneema UD panel Fig. 4 The indentations in the clay behind (a) Twaron woven fabric panel; (b) Dyneema UD panel

2 8F24 : 3782 g/m 2 7U20 : 3740 g/m

Fig. 5 Wax mould of the indentation behind Twaron woven panel 8F24 (left) and Dyneema 7U20 (right) To quantify different configurations of indentation behind woven panel and Dyneema UD panel, the exact mould of the indentation was taken by wax mould as shown in Fig.5. It can be found clearly that the indentation behind Twaron woven panel was deep and round like a bowl shape. While the indentation behind the Dyneema UD panel was much shallow and wider like a plate shape. The bottom of the indentation behind Twaron woven panel was larger than that of Dyneema UD panel. The average width of the indentation edge in clay behind the Dyneema UD panel (51.25 mm) was about one-third (36.19%) larger than that of the Twaron woven panels (37.63 mm).

In order to identify the influence on BFS, three panels are made with close areal density. BFS behind

Dyneema UD panel (11.81 mm) was less than a half as that of Twaron woven panels 8F24 (27.59 mm), and is only 60.28% as that of the woven panel 11F19(19.59 mm). Such findings indicate that impact stress wave on Dyneema UD can propagate more quickly than that of Twaron woven fabric. Therefore, Dyneema UD has advantage of minimize BFS for ballistic resistance.

35 2 8F24: 3728.40 g/m 2 30 11F19: 3740.15g/m 2 25 7U20: 3738.80g/m 20 15

BFS (mm) 10 5 0 8F24 11F19 7U20

Fig.6 BFS of woven panels and Dyneema UD panel

3.3 Failure modes According to photographic examination, it is found that Twaron woven fabric and Dyneema UD exhibits different failure modes under ballistic impact.

In the perforated Twaron woven panel 8F9, the woven panel produced transverse deflection and primary yarns were stretched and pulled out of fabric. As a result, some creases were left along primary yarns on post-impact fabric layers. According to Shim, the creases on the post-impact fabric give an indication of the extent of material deflection [17]. On the last layer ply-9 of Twaron woven panel 11F9, more severity of crease can be observed on the last layer ply-9 (as shown in Fig.7 (b) ) than that of the first layer ply-1 ( as shown in Fig.7 (a) ). This suggested the last back layer experienced more significant transverse deflection than that of front layers.

According to SEM observation, the axial splitting is the dominant failure morphology of fibres on every layer. This indicated that fibres experienced extreme high stress under ballistic impact. When the projectile impact on a fabric, the primary yarns under the projectile were compressed and sheared transversely. At the same time, the stress propagated quickly from the impact point along the primary yarns. The fibres on the primary yarns were stretched and experienced increasing tensile stress at high strain rate until fracture.

(a) (b)

(c) (d)

Fig. 7 Fractured fibres in the post-impact Twaron woven panel 8F9

(a) (b)

(c) (d)

Fig. 8 Fractured fibres in the post-impact Dyneema UD panel 7U9 On the post-impact UD layer, severe generator strip and ply splitting was found on the striking and exit face as shown in Fig. 8(a). The generator strip was typical ply delamination in the thin laminate when fracture occurred [19]. According to Shim [17], ply splitting in a thin laminate was induced by shear through the thickness and combined modes of tensile failure.

From SEM observation on the exit face of Dyneema UD panel, in the region at impact site, the resin exhibited melting and gross drawing. These different appearances indicated that thermal damage occurred in a local region around impact region. Around the perforated hole, the fractured Dyneema fibres were observed by SEM at higher magnification. Most broken fibre ends had a smooth, partly globular ends as shown in Fig. 8 (c) and (d). It is the typical thermal damage appearance of the material. Such failure characteristics confirmed that fractured Dyneema fibres experienced significant thermal damage under ballistic impact.

According to above fractographic analysis, under ballistic impact, failure mode of Twaron fabric can be identified to be axil splitting, due to tensile stress and shear stress. Back layers can produce more large transverse deformation than that of front layers in a panel. For Dyneema UD, due to lower melt point, thermal damage was the dominant failure mode of Dyneema fibres under ballistic impact. This can result in ballistic performance degradation. In particular, front layers in UD panel have been influenced more significantly. With more layers added in UD panel, back layers have more time to propagate stress wave before fracture, and be engaged in energy absorption. Therefore, in Dyneema UD panel energy absorption can increase with increasing number of UD layers.

3.4 Design of hybrid panel

According to above failure mode analysis, Twaron fabric and Dyneema UD exhibits different advantage and weak point on ballistic performance. Twaron fabric has large transverse deformation under ballistic impact, particularly for back layers. This results in large BFS. For Dyneema UD, thermal damage can result in performance degradation, especially for front layers. Therefore, it is not appropriate for the striking face of a panel. However, Dyneema UD has advantage of minimize BFS for ballistic resistance. In this study, a hybrid panel is designed by combing Twaron woven fabric on the striking face and Dyneema UD layer at the back of the panel. In order to identify positive effect of ballistic performance of such design, some other hybrid panels with different layering up sequences are also used for comparison.

For the perforation case, two-layer hybrid panels with a Twaron fabric layer and a Dyneema UD layer in reverse sequences were investigated. According to ballistic tests results (as shown in Fig.9), when Twaron fabric is placed before Dyneema UD, energy absorption efficiency of panels is obviously increased than that of the reverse combing sequence. The specific energy absorption of the panel 8F/7U is increased 18.71% in comparison with the panel 7U/8F. When the fabric 11F is combined before Dyneema UD, energy absorption of the hybrid panel 11F/7U is improved 8.99% than the panel 7U/11F. This result indicates that the designed hybrid panel with Twaron fabric before Dyneema UD is benefit for energy absorption of whole panel. 400 350 300 250 /g) /g) 2 200

(J.cm 150 100

Specific energy absorption energy Specific 50 0 8F/7U 7U/8F 11F/7U 7U/11F

Fig. 9 Specific energy absorption of hybrid material panels

For the non-perforated panel, the layering sequence of hybridisation also has influence on BFS of the hybrid panel. When Twaron fabric layers and Dyneema UD layers are combined in hybrid panels, there are four types of layering sequences. Fig. 10 shows BFS behind four types of different hybrid panels. These results indicate that when Dyneema UD layers are placed behind woven fabric layers, the hybrid panel has lower BFS than that of other layering sequences. This means that combining Twaron woven fabric layers on the striking face and Dyneema UD layers on the exiting face is beneficial to minimize BFS of the hybrid panels.

16 14 12 10 8

BFS (mm) 6 4 2 0

8F8/U16 8F8/7U16 U16/8F8 7U16/8F8 U8/8F8/U8 7U8/8F8/7U8 8F4/U16/8F4 8F4/7U16/8F4

Fig. 10 Effect of the layering up sequence on the BFS of panels

4. Conclusions

This study investigated different ballistic responses of Twaron woven fabric and Dyneema UD laminate. According to ballistic test, with increasing of layers in a panel, SEA of Twaron woven panel shows a decrease trend, and Dyneema UD panel exhibits an increasing trend. According to fractorgraphic analysis, thermal damage of Dyneema UD can be identified due to generated heat during ballistic impact. This results in ballistic performance degradation. Based on above findings, an optimized hybrid panel is designed by combing Twaron woven fabric before Dyneema UD. Ballistic results shows that such hybridization is benefit for energy absorption of the panel and minimize the BFS.

Acknowledgements

This work is partially financially supported by project supported by National Science Foundation for Young Scientists of China (Grant No.11702337). The authors are grateful to Henan Collaborative Innovation Center of Textile and Garment Industry for their assistance, and to the Weaving and Ballistic Protection Laboratories at the University of Manchester UK.

Reference:

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