Abstract
Traditionally, the helmet shell has been used to provide protection against head injuries and fatalities caused by ballistic threats. In this study, because of the high cost of aramid fibres and the necessity for environmentally friendly alternatives, a portion of aramid was replaced with plain woven kenaf fibre, with different arrangements and thicknesses, without jeopardising the requirements demanded by U.S. Army helmet specifications. Furthermore, novel helmets were produced and tested to reduce the dependency on the ballistic resistance components. Their use could lead to helmets that are less costly and more easily available than conventional helmet armour. The hybrid materials subjected to ballistic tests were composed of 19 layers and were fabricated by the hot press technique using different numbers and configurations of plain woven kenaf and aramid layers. In the case of ballistic performance tests, a positive effect was found for the hybridisation of kenaf and aramid laminated composites.
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Ballistic Impact Resistance of Plain Woven Kenaf/Aramid Reinforced Polyvinyl Butyral Laminated Hybrid Composite
Suhad D. Salman,a,c,* Zulkiflle Leman,a Mohamed T. H. Sultan,b Mohamad R. Ishak,b,d and Francisco Cardona b
Traditionally, the helmet shell has been used to provide protection against head injuries and fatalities caused by ballistic threats. In this study, because of the high cost of aramid fibres and the necessity for environmentally friendly alternatives, a portion of aramid was replaced with plain woven kenaf fibre, with different arrangements and thicknesses, without jeopardising the requirements demanded by U.S. Army helmet specifications. Furthermore, novel helmets were produced and tested to reduce the dependency on the ballistic resistance components. Their use could lead to helmets that are less costly and more easily available than conventional helmet armour. The hybrid materials subjected to ballistic tests were composed of 19 layers and were fabricated by the hot press technique using different numbers and configurations of plain woven kenaf and aramid layers. In the case of ballistic performance tests, a positive effect was found for the hybridisation of kenaf and aramid laminated composites.
Keywords: Kenaf fibres; PVB film; Kevlar fibres; Ballistic properties; Helmet
Contact information: a: Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; b: Aerospace Manufacturing Research Centre (AMRC), Level 7, Tower Block, Faculty of Engineering, 43400 UPM Serdang, Selangor, Malaysia; c: Materials Engineering Department, Faculty of Engineering, The University of Mustansiriyah, Baghdad, Iraq; d: Laboratory of Bio-Composites Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia;
* Corresponding author: suhaddawood2007@yahoo.com
INTRODUCTION
The global market for personal protection systems alone is worth between 300 and 400 million euros per year (Rahner 2012), with an annual growth rate of more than 5%. The ballistic helmet shell type known as the Personnel Armour System-Ground Troops (PASGT) is a standard infantry combat wear used by the U.S. military that has changed relatively little since the 1970s. The shell is a one-piece structure composed of multiple (at least 19) layers of Kevlar® (an example of an aramid) ballistic fibre. The primary goal of the PASGT helmet shell is to protect the soldier from a variety of prevailing threats by limiting the perforation of fragments or bullets through the helmet (Walsh et al. 2005). Kevlar® fibres are among the high-performance fibres used as the reinforcement in many high-velocity impact applications against projectiles and fragments (Salman et al. 2015a).
Recently, (Bandaru et al. 2016) have investigated the ballistic impact response of Kevlar® fabric and polypropylene (PP) composite armors having different fabric architecture against ballistic test standard NIJ-STD 0106.01. It was observed that good interfacial compatibility was acheived between PP and Kevlar® laminates with reduced density as compared to that of the thermoset-based laminates. The effect of hybridization on the hybrid composite armors under ballistic impact has been studied using hydrocode simulations by Bandaru et al. 2015. The hybrid composite armor is constructed using different combinations and stacking sequences of woven kevlar, glass, and carbon fibres reinforced composites, for a fixed thickness of the target. The results indicated that at a fixed thickness of the hybrid composite, stacking sequence of hybridized layer shows a significant effect on the ballistic performance. The energy absorption and ballistic limit velocity were sensitive to projectile geometry. These armors, however, give rise to practical issues such as their high cost and their potential for harmful effects including eye, skin, and upper respiratory irritation (Tham et al. 2008).
Ballistic helmet protection is generally used by those in the armed and civilian police forces and entrepreneurs, so there is an urgent need to develop a head protection helmet that can become more readily available and affordable. Environmental regulations, availability, cost, and lightweight factors encourage researchers to develop new hybrid composites with natural fibres to reduce the dependency on the ballistic resistance component. Kenaf (Hibiscus cannabinus L.) bast fibre is the most suitable natural fibre for hybridisation with Kevlar®, as determined using the analytical hierarchy process (AHP) (Yahaya et al. 2014). It has the highest priority among 13 natural fibre alternatives (Salman et al. 2015c). Hybridisation of kenaf and synthetic fibres has several advantages, including that it reduces dependency on petroleum, which is the source of the synthetic fibres used in PASGT helmets (Salman et al. 2015d).
A few attempts have been made to study the response of either Kevlar®/cellulosic or kenaf/synthetic composites under ballistic impact conditions (Wambua et al. 2007). The first serious discussions and analyses were carried out by Sohaimi (2003), who evaluated the potential of using coconut shell powder/Twaron®-reinforced epoxy (COEX) for ballistic applications. The higher ballistic limit has been obtained from using Twaron® layers on the exterior and coconut layers in the interior. In 2009, Radif et al. (2011) conducted a ballistic impact test on different types of laminated hybrid composites of Kevlar®/ramie-reinforced polyester resin for application in body armour. It was concluded that the panel geometry has the greatest effect in increasing the impact ballistic limit, the greatest energy absorption, and the life time rupture.
Further research on optimising the layered configurations and environmental effects on the Kevlar® 29/ramie-reinforced polyester resin was carried out by Ali et al. (2011). The results indicated that the developed personnel armour composite was improved in terms of the impact response with increasing relative humidity in the range of 50 ± 20%. Abdul et al. (2011) carried out experiments to determine the ballistic limit of coir yarn and Kevlar®/epoxy hybrid composites for ballistic applications. They examined the effect of stacking sequences of Kevlar® fibre on residual velocity and failure mechanism of the composites. They concluded that delamination and fibre fracture were the major failure modes in high-velocity impact tests, whereas matrix fracture and fibre sliding failures were the main mechanisms in quasi-static tests for thick-walled panels.
Not much experimental work has been reported on the high-impact response of kenaf/synthetic hybrid laminated composites. Davoodi et al. (2012) performed experiments on the potential of using hybrid kenaf/glass-based epoxy hybrid material in a passenger car bumper beam. The impact properties can be improved by optimising the thickness of the beam and improving the material, such as through epoxy toughening, to modify the ductility behaviour to improve energy absorption.
In the present study, an attempt was made to design plain woven kenaf/aramid-reinforced polyvinyl butyral (PVB) hybrid composites; their responses to ballistic impact tests, as well as their damage characteristics, were experimentally evaluated. Poly-vinylbutyral (PVB) film is now employed in a wide array of industrial and commercial applications because of its impressive mechanical performance and outstanding versatility. PVB offers advantages like longer shelf life and a stable ballistic performance over a broad temperature range, as well as it can be shaped into all manner of protective equipment. Identifying the proper configuration of woven kenaf fibre, resin, and reinforcement architecture for specific requirements of the PASGT shell material to provide equivalent protection at a reduced cost was the goal of this study.
EXPERIMENTAL
Two types of woven fabrics were used: plain woven kenaf and Heracron® aramid fibres double-side coated with 12% PVB-phenolic. Table 1 shows the physical characteristics of plain woven kenaf (Salman et al. 2015e), supplied by ZKK Sdn Bhd, Malaysia. The PVB-phenolic is a vinyl polymer with an additional 5% phenolic. It is commonly used to manufacture traditional PASGT helmets because of its low cost, easy fabrication, long durability, and good mechanical and chemical properties (Torki et al. 2012). Films of PVB are one of the most popular interlayer materials used for laminated safety glass. They are commonly used in the automotive and architectural fields bonded between two panels of glass. The polymer interlayer of PVB is tough and ductile, mostly used for applications that require strong binding, adhesion to many surfaces, toughness, and flexibility.
Table 1. Physical and Mechanical Properties of Materials (Manufacturer Data Sheet)
Fabrication of Composite Samples
The hot hydraulic press technique was used to fabricate hybrid laminates of different kenaf fibre weight content with PVB film and aramid fabric coated with PVB-phenolic film, as shown in Fig. 1.
Fig. 1. The hybrid composite specimens prepared using the hot press technique
Table 2 illustrates the various configuration layers and stacking sequences of the hybrid laminates and helmets. To reduce the number of aramid layers and to identify the effect of layering sequence, plain woven kenaf layers were placed in eight different locations. Studies were also carried out on 19 layers of aramid/PVB-phenolic composite and plain woven kenaf/PVB composite for the purpose of comparison.
To fabricate a square flat laminated panel, the PVB film stacking between woven kenaf layers and coated aramid/PVB-phenolic were cut into 335 × 335 mm sheets. A mould release agent was sprayed on the mould surfaces before any moulding process to prevent adhesion as well as to obtain a smooth sample surface. The stacks of 19 layers of different laminates were centred between two stainless-steel moulds and hot plates of a compression moulding press.
Table 2. Specifications of the Laminated Hybrid Composites
(a) Hot press (b) Processing cycle
Fig. 2. Compression moulding hot press and temperature profile
Subsequently, the platens were closed, the hot press plates were heated to 165 °C for 20 min, and the compression pressure was set to 8 MPa, as shown in Fig. 2. Once the temperature of the platens reached 165 °C, the compression pressure was increased to 8 MPa and held constant for 15 min. After this compression cycle, the platen temperature was reduced to room temperature (25 °C), while the pressure was maintained at 8 MPa until the temperature reached 25 °C. Once the platen temperature reached 25 °C, the hybrid composite laminates were taken out of the compression moulding frame. The same procedure was followed for fabricating the hybrid to produce the helmet shell (arc shape), but with another moulding hot hydraulic press, as shown in Fig. 3. The dimensions and mass of the hybrid laminates were measured to calculate the density and the areal density of the hybrid materials.
Fig. 3. Ballistic hybrid helmet fabrication processes
High-Velocity Impact Test
The high-velocity impact tests were performed using a 9-mm full metal jacket bullet with a powder gun on fabricated square panels with various impact velocities. The ballistic limit velocity V50, hybrid failure mode, and NIJ levels were evaluated. NIJ standards are used for law enforcement armors by using a standard set of test methods under ARMY MIL-STD-662F. This approach defines each armor program and can select a unique series of projectiles and velocities as required in the armor standards. The ballistic experiments were conducted in an indoor firing range at the Science and Technology Research Institute for Defence, Ministry of Defence, Kajang, Selangor, Malaysia (STRIDE). The high-impact tests were performed according to four standards of helmets: NIJ-STD-0106.01 (Standard-0106.01 1981), the V50 requirement of the U.S. military specification for the PASGT helmet (0108.01 1985), military standard MIL-STD-662F (MIL-STD-662F 1997), and MIL-H-44099A (ML-H-44099A 1986). Using a powder gun, two types of bullets were fired: 9-mm, 8.0 g full metal jacket (FMJ) bullets to investigate the NIJ levels, and .22 calibre (diameter of 7.62 mm) fragment simulating projectiles (FSPs) to determine the V50 ballistic limit (shown in Figs. 4a and 4b). These tests were performed on both flat panels and on helmets with partial lateral support positioned 5 m in front of the test barrel’s muzzle to produce impacts of 90° obliquity (illustrated in Fig. 5).
The general method for characterising a material’s ballistic limit is to perform a V50 ballistic test, the velocity at which there is an equal probability of a partial (target was not defeated) or a complete perforation (target was defeated) for the given armour and threat. In this study, the NIJ methods were used to determine minimum performance requirements for ballistic resistant protective materials levels.
To carry out a V50 (ballistic limit) test, the propellant charge was put inside the projectile, then closed by two pairs of sabots around a small steel fragment to produce fragment simulating projectiles (FSPs). The amount of propellant for different impact velocities was determined through an empirical method by adjusting the mass of the propellant. The propellant fill for the shots and next shot were adjusted until three partial and three complete penetrations had been achieved within a velocity spread of not more than 38 m/s, as recommended by the standard ML-H-44099A 1986. Both chronographs and Doppler radar antenna, combined with a computer, were used to measure the projectile velocity. One chronograph was positioned 2 m in front of the target and another behind it, as shown in Fig. 5. Projectiles, which passed through the panel, were considered to be a complete penetration (CP), while the others were defined as partial penetrations (PP), following the NIJ definitions. The impact striking velocities (vs) and residual velocity (vr) of the projectiles were recorded, while the ballistic limit (V50) was calculated.
(a) (b)
Fig. 4. (a) Fragment simulating projectiles for V50 tests, (b) 9-mm projectile for NIJ tests
Fig. 5. Actual set-up for ballistic impact tests
RESULTS AND DISCUSSION
The effects of hybridisation on ballistic limit and NIJ levels was studied for high-velocity impacts. Initial impact velocities and residual velocities were measured experimentally to evaluate the ballistic limit (V50) and NIJ levels of laminates. The NIJ results showed that the H1, H2, HH2, H3, H3A, and HH3 passed the fourth level (III-A), while H4, H5, and H6 passed the third level (II) (shown in Table 3). The positive effect in terms of NIJ levels compared with the aramid composite showed that hybridisation contributed to the same performance in high-impact penetration tests. In accordance with hybrid H3A (alternating woven kenaf layers with aramid 29 fabric layers) and hybrid H3 (placing woven kenaf layers together and aramid 29 layers separately), the results indicated that NIJ levels were not affected at the same hybrid volume and thickness. Based on Table 3, additional layers of kenaf fibres increased the thickness of the hybrids, while using different layering sequences had an insignificant effect on the NIJ Level of the hybrid composites. It is clear that the ballistic limit (V50) of the hybrid composites was lower than that of the helmet shell composite. Signifying that this arc-shaped geometry provides a better penetration resistance compared to the flat panels. In the case of a flat panel that has the same configuration and sequence of shells shape, shells exhibited smaller deflections than a flat panel. The reason for this is due to decrease the strain distribution at the bottom surface of the arc-shaped geometry, as reported by (Choi 2016).
Table 3. Ballistic Resistance Results
The ballistic limit velocity (V50) was estimated using experimental data on the basis of whether the projectile penetrated the hybrid composite completely or partially. It is the most common assessment tool to determine the ballistic performance of a material; the accuracy of the estimation, however, increases with increasing number of ballistic tests (Boccaccini et al. 2005). Figure 6 shows a plot between the initial velocity and the residual velocity for the hybrid laminated composites. An increase in initial velocity resulted in the increase in the residual velocity (which was zero up to certain initial value) for all the hybrids.
Fig. 6. Residual velocities as a function of impact velocities
Figure 7 shows the ballistic properties of kenaf/aramid hybrid composites in terms of ballistic limit velocity (V50) compared with aramid /PVB-phenolic and kenaf/PVB composites. Hybrid H3A, alternate layers of kenaf and aramid, exhibited less ballistic limit compared to hybrid H3, for the same number of layers and thickness. Both types of hybrids bulged out and higher delamination occurred in the interlaminar surface for fabricated alternative aramid layers with kenaf layers. As reported by (Babu et al. 2006), when using different material surfaces (different flexibility and deflection), the magnitude of friction forces will be affected, leading to a decrease in the impact energy absorption mechanisms due to greater delamination. When the number of kenaf layers increased (i.e. a thickness increase), the difference in ballistic limit gradually increased in comparison to the aramid composite. Although the KF composite exhibits total perforation to the specimens at the highest impact velocities, the KF composite with a plate thickness of 17 mm was found to have 417.8 m/s ballistic limit.
Fig. 7. Ballistic limit (V50) of all composites
According to two ballistics test standards for helmets, NIJ-STD-0106.01 Type II, IIA, III, IIIA, and MIL-H-44099A, the V50 requirement of the U.S. military specification for PASGT helmets were calculated. There was a distinct difference between the ballistic limits of aramid /PVB phenolic and kenaf/ aramid hybrid composites. Hybrid HH2 and HH3 composites recorded higher V50 than the H2 and H3 composites, which consisted of the same number of kenaf layers, denoting the positive effect of the arc shape of the helmet (Salman et al. 2015b). The low values of ballistic limit in H3A hybrid composites compared to the H3 hybrid, which consisted of the same number of kenaf layers, denoted the negative effect of the alternative arrangement. This result can be explained by the fact that an alternating arrangement led to delamination between the inner surfaces, presenting less traveling distance for the projectile within the target (Zhang et al. 2014). As a result of less travel distance, there was less surface for energy dissipation (Sabet et al. 2009). Figure 8 shows the ballistic limit (V50) versus volume fraction curves of kenaf and aramid hybrid composites. The kenaf volume fraction and aramid volume fraction had an important effect on the ballistic limit velocity.
Generally, these curves showed a bilinear behaviour and the line slope changes (increases) as the number of kenaf layers increases. It can be clearly seen that with an increasing volume fraction of kenaf, the ballistic limit decreased. Similarly, the ballistic limit curve increased as the aramid volume fraction was increased. The overall results for the high-velocity impact tests indicate that approximately a 30% volume fraction of both kenaf and aramid fibres was more effective for the ballistic properties. This can be explained by the fact that 30% fibres content presents better interfacial surface properties, which leads to an increase of the surface area for energy dissipation.
Fig. 8. Ballistic limit (V50) versus fibre volume fraction curves of kenaf and aramid hybrid composites
This approach is expected to develop a helmet armour that, when compared to conventional helmet armour, is less costly, more readily available, less associated with the potentially harmful effects of the petroleum products, and will not jeopardise ballistic resistance. This research will open a new avenue for its use in military utilities, aerospace, and marine and civilian structures to reduce the use of aramid fabric in ballistic laminate composites, and meets the prescribed baseline performance specifications.
Damage Mechanisms
Post-test examination of selected specimens was performed to analyse the failure mechanisms during ballistic impact tests. Cross-sections of selected samples at the impact region were cut along the thickness direction to observe the damage failure modes after testing. Interestingly, the region of the specimens affected by the projectiles appeared to become more localised at higher impact velocities, as shown in Fig. 9. Aramid fibre failure is depicted in the damaged surfaces of the aramid 29 composite (Fig. 9a), while the kenaf composite is depicted as bulging and peeling out (Fig. 9d). Figure 9b shows the damaged surfaces of the hybrid that consisted of placing woven kenaf together and aramid 29 layers separately; bulging and fibre failure were observed. The hybrid with alternating layers of kenaf and aramid layers showed a combination of delamination and bulging out of the kenaf layers (Fig. 9c).
Similar behaviour of other types of hybrid materials has been documented in a Pandya research paper (Pandya et al. 2011). The main damage mechanisms in these hybrid materials were fibre tensile failure and matrix cracking. It is postulated that delamination only starts at an advanced stage of the loading, resulting in a small rhombic region of delamination just before the specimen is perforated during high-velocity penetration. When the impact velocity is increased, the stress wave propagation occurs, causing randomly broken fibres (Lin and Fatt 2006; Sultan et al. 2012; Pandya et al. 2013). Generally, the rupture of aramid and kenaf fibres as well as matrix fracture were major failure modes in the high-impact tests.
Fig. 9. Optical pictures of ballistic failure modes of hybrid composite laminates at V50, cross-sectional surface, impacted surface and rear surface for: (a) Kevlar composite, (b) Hybrid consisted of placing woven kenaf together and Kevlar 29 layers separately, (c) Hybrid comprised of alternating layers of kenaf and Kevlar layers and (d) Kenaf composite
CONCLUSIONS
1. The effects of hybridisation and the stacking sequence of hybrid composite materials under high-velocity ballistic impacts were investigated.
2. The arrangement of fibre layers was found to highly affect the ballistic performance of the hybrid composites. Placing woven kenaf alternating with aramid 29 fabric layers provided a lower ballistic limit velocity than placing woven kenaf together and aramid 29 layers separately for the same hybrid volume and thickness.
3. Nineteen aramid /kenaf layers reinforced by PVB were successfully developed to meet the fourth production level of the NIJ standard, with four layers of woven kenaf.
4. The proper configuration of woven kenaf fibre, resin, and reinforcement architecture were identified for specific requirements of the PASGT shell material to provide equivalent protection at a reduced cost.
5. The tested samples were optically observed and demonstrated the following: the delaminated area formed a conical shape for completely penetrated perforation; and there was a punched-out effect on the back ply following partially penetrated perforation, aramid and kenaf fibre breakages, aramid and kenaf fibre stretching, shear, and aramid fibre, kenaf fibre, and matrix rupture and cracking.
ACKNOWLEDGEMENTS
This work was supported by UPM under GP-IPS/2014/9438714 and GP-IPB grants, 9415402. The authors would like to express their gratitude and sincere appreciation to the Mechanical and Manufacturing Engineering Department and Aerospace Manufacturing Research Centre of the Universiti Putra Malaysia. Our appreciation and gratitude also extend to the Ministry of Higher Education & Scientific Research of Iraq and to the Material Engineering Department, College of Engineering, at the University of Mustansiriyah, for scientific assistance and financial support.
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Article submitted: April 26, 2016; Peer review completed: June 12, 2016; Revised version received and accepted: June 28, 2016; Published: July 14, 2016.
DOI: 10.15376/biores.11.3.7282-7295