Effect of kenaf fibers on trauma penetration depth and ballistic impact resistance for laminated composites

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1 Original article Effect of kenaf fibers on trauma penetration depth and ballistic impact resistance for laminated composites Textile Research Journal 0(00) 1 15! The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / trj.sagepub.com Suhad D Salman 1,2, Z Leman 1, MTH Sultan 3, MR Ishak 3,4 and F Cardona 3 Abstract Combat helmets have been utilized to provide protection against a variety of ballistic threats, by reducing traumatic head injuries and fatalities. Nevertheless, head protection from injury is critical to function and for survivability. Soldiers and civilians incur Traumatic Brain Injury (TBI) most commonly from exposure to homemade bombs or improvised explosive devices. Although the Personal Armor System for Ground Troops (PASGT) helmet is expensive, environmental issues are some technical advantages that encourage using natural/synthetic hybrid laminated composites. The effects of different configuration patterns of kenaf fibers on the Backface Signature and energy absorbed by a military helmet (PASGT) were investigated. The ballistic behaviors of the 19 layers of aramid composite and plain woven kenaf composite were compared to hybrid laminated composites. The ballistic impact tests were performed using a 9 mm full metal jacket bullet and fragment simulating projectiles at various impact velocities, using a powder gun on fabricated square panels and helmets. The results showed the positive effect of hybridization in terms of energy absorbed (i.e. penetration), Backface Signature and damage mechanisms for ballistic impact and NIJ (National Institute of Justice) tests. Hybridization of plain woven kenaf/kevlar laminated composites will open new avenues to reduce the dependency on the ballistic resistance component (Kevlar) in the helmet shell. Keywords ballistic, trauma, aramid, kenaf, helmets The Personal Armor System for Ground Troops (PASGT) helmet is classified as a conventional ballistic helmet, having been used for head protection for centuries. The primary goal of the PASGT is to protect the head from a variety of threats. Over the years, the PASGT material has changed relatively little since the introduction of aramids (in the 1970s), whereas there are continually growing demands for lower costs due to the ever-increasing lethality and diversity of threats. 1 Synthetic fibers such as aramid (e.g. Kevlar and Twaron) are widely used for protective systems and many engineering applications; 2 however, they are relatively expensive to produce as production is a complex process. 3,4 Hence, research avenues in substituting traditional synthetic fibers with natural fibers for different applications are progressively being explored to replace or reduce the use of expensive and non-renewable synthetic fibers, and concurrently to preserve the environment. 5 Despite this growing interest in the natural hybrid composite field, only scarce attention has been devoted 1 Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Malaysia 2 Materials Engineering Department, University of Mustansiriyah, Iraq 3 Aerospace Manufacturing Research Centre (AMRC), Universiti Putra Malaysia, Malaysia 4 Laboratory of Bio-Composites Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Malaysia Corresponding author: Suhad D Salman, Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia. suhaddawood2007@yahoo.com.

2 2 Textile Research Journal 0(00) to ballistic impact resistance, with inadequate reports detailing the effects of natural fabric composites on trauma depth. The results of combining the advantages of synthetic and natural fibers into new materials for ballistic impact have been verified by Wambua et al. 6 Radif et al., 7 Ali et al., 8 Azrin et al. 9 and Risby et al. 10 The results indicated that natural fibers couldbeusedasapotentialreinforcing material for high-velocity impact impedance, such as body armor, in order to decrease the use of synthetic fibers while taking advantage of natural resources. Randjbaran et al. 11 studied experimentally the effects of stacking sequence layers of hybrid composite materials on ballistic energy absorption of Kevlar/carbon/glass composites. They concluded that the energy dissipation increased with increasing hybrid thickness and projectile diameter. Even though the helmet prevents the projectile from penetrating (ballistic protection), the helmet wearer can still suffer head injuries from the impact of the delaminated back plane with the skull and brain, such as blunt trauma injuries. 12 Cheeseman and Bogetti 13 studied several fabric composites that were fabricated to improve energy absorption, but did not describe how they were able to contribute towards blunt trauma reduction. Investigations by Ahmad et al. 14 showed that higher ballistic limit results of Kevlar 29 composites indicated higher energy absorption and improved the damage resistance after ballistic impact. Sabet et al. 15 experimentally studied the projectile perforation of glass-fiber reinforced plastic (GFRP) plates containing sand filler under a high-velocity impact. They found that delamination was the major failure mode in highvelocity tests and the main energy absorbing mechanism in thick-walled plates. There is an urgent need to develop a head protection helmet in such a way that it becomes widely available and cheap, as the military and civilians who are exposed to different threats require adequate protective gear to safeguard themselves. To that end, an alternative material is required to reduce the dependency on the ballistic resistance component (Kevlar); therefore, further understanding of its impact properties should be taken into consideration in order to be safely and economically driven into the ground. The level of protection and cost should be taken into account when choosing the materials for the ballistic protection helmet (the key determinants). 16 Therefore, to develop innovative materials for the PASGT helmet, hybridization of plain woven kenaf/kevlar composites may be possible to create a material with the combined advantages. This study is motivated by designing suitable sequence configurations of the hybrid composites panels, and experimentally evaluating their responses to high-velocity impact tests. Further, a series of helmet prototypes has been produced to take advantage of the new materials and fabrication/manufacturing technologies, as well as to meet the continually growing demands for lower cost. In addition, the trauma performance was assessed and the damage properties of the tested samples were analyzed. Consequently, helmet armor was developed that is cheaper and more readily available. The potential harmful effects of the petroleum products were reduced, without jeopardizing ballistic-resistant capabilities. Materials and methods Materials Two types of woven fabrics are used: plain woven kenaf and Heracron Õ aramid fibers coated with 12% Polyvinyl Butyral (PVB) phenolic, double side coated. Table 1 shows the physical characterization of plain woven kenaf, 17 supplied by ZKK SdnBhd, Malaysia. As well as Heracron Õ aramid fibers, PVB film and PVB-phenolic are used, which are always used to manufacture traditional PASGT helmets. The hot hydraulic press technique was adopted to fabricate hybrid laminates of different kenaf fiber weight contents with PVB film and Heracron Õ aramid fabric coated with PVB-phenolic film, as shown in Figure 1. Table 2 illustrates the various configuration layers and stacking sequence of the hybrid laminates and helmets. In order to reduce the number of aramid layers and to identify the effect of the layering sequence, Table 1. Physical and mechanical properties of materials (manufacturer data sheet). Material Thickness (mm) Areal density (g/m 2 ) Density (g/cm 3 ) Average breaking strength (MPa) Average maximum strain (%) Woven kenaf Heracron Õ aramid coated with PVB- phenolic PVB film PVB: Polyvinyl Butyral.

3 Salman et al. 3 Figure 1. Ballistic hybrid laminate fabrication processes. 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 purposes of comparison. To fabricate a square flat laminated panel, the PVB film stacks between woven kenaf layers and coated aramid/ PVB-phenolic were cut into mm sheets. A special kind of mold release agent was sprayed onto the mold surfaces before any molding process to prevent adhesion, as well as to obtain a smooth sample surface. The stacks of 19 layers of different laminates were centered between two stainless steel molds and hot plates of a compression molding press. Subsequently, the platens were closed, the hot press plates were heated to 165 C for 20 min, and the compressive pressure was set to 8 MPa. Once the temperature of the platens reached 165 C, the compressive 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 molding frame. The same procedure was followed for fabricating the best hybrid to produce the novel helmet shell, but with another molding hot hydraulic press. The dimensions and mass of the hybrid laminates were measured to calculate the density and the areal density of the hybrid materials. Hybrid laminated composites of mm size and helmets were used for the ballistic testing and blunt trauma assessment. In order to assess potential trauma injury mechanisms prior to fielding new helmets, the deformation depth for the ballistic velocity was measured using Roma Plastilina clay; in this way, the new helmet performance results were validated. Ballistic impact determination The ballistic experiments were conducted in an indoor firing range at the Science and Technology Research Institute for Defence, Ministry of Defence, Malaysia (STRIDE). The high-velocity impact tests were performed using.22 caliber (diameter of 7.62 mm) fragment simulating projectiles (FSPs), using a powder gun on fabricated square panels with various impact

4 4 Textile Research Journal 0(00) Table 2. Specifications of the laminated hybrid composites. Specimens descriptions Sample code Stacking sequence Specimens thickness (mm) Areal Density (kg/m 2 ) Fiber volume fraction (%) Kevlar Kenaf 19 Kevlar 29 KV Kevlar/ 2 kenaf H Kevlar/ 3 kenaf H Kevlar/ 3 KENAF HH Kevlar/ 4 kenaf H Kevlar/ 4 KENAF Alt. H3A Kevlar/ 4 kenaf HH Kevlar/6 kenaf H Kevlar/ 8 kenaf H Kevlar/10 kenaf H kenaf KF velocities, to estimate the ballistic limit velocity V 50 and hybrid failure mode, as shown in Figure 2. The highimpact tests were performed according to three standards of helmets: the V 50 requirement of the US military specification for the Personal Armor System Ground Troops (PASGT) Helmet, 18 and military standards MIL-STD-662F 19 and MIL-H-44099A. 20 These tests were performed on both flat panels and helmets with partial lateral support positioned at 5 m forward from the muzzle of the test barrel to produce impacts of 90 obliquity. The general method for characterizing a material s ballistic limit is to perform a V 50 ballistic test, the velocity at which there is an equal probability of a partial (target not defeated) or complete perforation (target defeated) for the given armor and threat. To carry out a V 50 (ballistic limit) test, the propellant charge was placed inside the projectile, then closed by two pairs of sabots around a small steel fragment

5 Salman et al. 5 Figure 2. Actual set-up for ballistic impact test. 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 in the standard. 20 Two Chronographs and a Doppler Radar Antenna combined with a computer were used to measure the projectile velocity; one Chronograph positioned at 2 m in front the targets and another behind them. During ballistic impact, most of the kinetic energy of the bullet is transformed and absorbed by the composite material plate, which prevents the projectile from completely penetrating it. Projectiles, which pass through the panel, are considered to be a complete penetration (CP), whereas the others are defined as being partial penetrations (PP), following the United States Department of Justice s National Institute of Justice (NIJ). The impact striking velocities (V s ) and residual velocity (V r )of the projectiles were recorded, whereas the ballistic limit (V 50 ) and energy absorption (E abs ) were calculated by the following equation 21 E abc ¼ 1 2 mvs2 1 2 mvr2 where m is the mass of the projectile in kg. ð1þ Depth of trauma penetration tests During non-penetrating projectile impacts, a potential reaction mechanism of the composite helmet shell is interior surface delamination and deformation, which transfers large forces to the head, causing blunt trauma injury. Blunt trauma is measured by the depth of back face deformation or deflection of the material, after projectile impacts that should not be more than 44 mm (1.73 inches) as specified by National Institute of Justice (NIJ) STD and STD ,23 The hybrid panels and helmets were tied using elastic straps on a square box filled with standard modeling clay backing material (Roma Plastilina), which gave the indentation depth (by forming a crater) from each shot, as shown in Figure 3. This clay has mass properties similar to those of a human body and does not spring back after ballistic impact. A calibration process was used to qualify the clay for ballistics testing, which it must be maintained at a controlled temperature of 37 C. By using a Vernier caliper from the plane defined by the front edge of the clay box fixture, the indentation depth (PD) was measured. The gun-to-target distance was the same as the ballistic limit determination but for four shots for each sample. All NIJ tests were performed in an indoor firing range at the Science and Technology Research Institute for Defence, Ministry of Defence, Malaysia (STRIDE).

6 6 Textile Research Journal 0(00) Figure 3. Clay-backed ballistic testing and trauma depth. Table 3. Energy absorption of ballistic resistance results. Specimens descriptions Sample code Striking energy (J) Energy absorption (J) Experimental results and discussion High-velocity impact results Thickness (mm) 19 Kevlar 29 KV Kevlar/2 kenaf H Kevlar/3 kenaf H Kevlar/4 kenaf H Kevlar/4 kenaf [Alt.] H3A Kevlar/6 kenaf H Kevlar/8 kenaf H Kevlar/10 kenaf H kenaf KF The effect of kenaf hybridization on ballistic energy absorbing capacity has been studied under high-velocity impact loadings. Initial impact velocities, impact energy and residual velocities were measured experimentally to calculate the energy absorbing capacity of the hybrid laminates. The striking energy was determined based on the impact velocity of the projectiles, while energy absorption was evaluated as the whole energy absorbed by the hybrids at the end of an impact condition. Based on Table 3 and 4, additional layers of kenaf fibers increased the thickness of the hybrids, whereas using different layering sequences had an insignificant effect on the energy absorption of the hybrid composites. It is clear that the energy absorption of both hybrids and kenaf composites is lower than that of the Kevlar composite. In order to quantify the influence of impact velocity on the residual energy capability of the hybrids, impact energy for all hybrid composites was calculated and compared to the residual energy for each hybrid during the ballistic tests. Figure 4 shows the linear relationship between striking energy and the corresponding residual energy. The impact energy at zero residual energy represents the ballistic limit of the hybrid V 50, meaning zero residual velocity. The results show that Hybrid H1 has superlative energy absorption of J, signifying that this hybrid configuration provides a better penetration resistance compared to other hybrid materials, as shown in Figure 5. In the H2 hybrid, although laminated hybrid thickness increased by 5%, the ballistic impact energy absorption decreased by 8% (to J). It was also reported that many factors influenced the energy absorption of hybrid composite such as target geometry, strike velocity and target material properties (density). 24 Hybrid H3A, alternate layers of kenaf and Kevlar, exhibited less energy absorption compared to hybrid H3, for same number of layers and thickness. Both bulged out and higher delamination occurred in the interlaminar surface for fabricated alternative Kevlar layers with kenaf layers. As reported by Babu et al., 25 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. The results indicated that placing woven kenaf together and Kevlar 29 layers separately is more efficient in the laminated hybrids. When the number of kenaf layers increased (i.e. a thickness increase), the difference in ballistic impact energy

7 Salman et al. 7 Table 4. The penetration depth (PD) for each laminated hybrid. Shot No. 1 Shot No. 2 Shot No. 3 Shot No. 4 Sample code Vs (m/s) BFS (mm) Vs (m/s) BFS (mm) Vs (m/s) BFS (mm) Vs (m/s) BFS (mm) Mean BFS (mm) KV H H HH H HH H H H BFS: Backface Signature. Figure 4. Residual energy versus striking energy of the projectile. Figure 5. Energy absorption of all hybrid composites during ballistic tests.

8 8 Textile Research Journal 0(00) absorption gradually increased compared to the Kevlar 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 absorb J energy. Energy absorption as a function of volume fraction of kenaf and Kevlar content for all hybrid laminated composites is presented in Figure 6. It can be clearly seen that with an increasing volume fraction of kenaf, the energy absorbed decreases. Similarly, the energy absorption curve increases as the Kevlar volume fraction increases. The overall results for the high-velocity impact tests indicate that approximately a 30% volume fraction of both kenaf and Kevlar fibers is more effective for the energy absorbed. This can be explained by the fact that 30% fibers content presents better interfacial surface properties, which lead to an increase of the surface area for energy dissipation. All the hybrid materials showed a similar energy absorption relationship as the impact velocity increases: an increase in the energy absorbed by the hybrids from the bullet s kinetic energy until the ballistic limit of each hybrid, as shown in Figure 7. Beyond the ballistic limit, the projectile completely penetrated the hybrid, leading to the increase in the residual velocity, therefore reducing the energy transferred to the hybrid material by the bullet. Similar results were also reported by Tan et al. 26 and Wambua et al.; 6 a decrease in energy absorption at Figure 6. Energy absorption volume fraction curves of kenaf and Kevlar hybrid composites. Figure 7. Energy absorption striking velocity of the hybrid composites.

9 Salman et al. 9 Figure 8. Specific energy absorption and energy density of all hybrid composites. velocities slightly higher than V 50 with a subsequent recovery in absorption levels of fabric systems shot with different-shaped projectiles. This reduction in energy absorption can be explained as a reduction of the time of residence of the bullet in the material. This means that the energy is not able to dissipate throughout the material, which generates an area of less deformation, for all the thicknesses. 27 Figure 8 is a comparison between specific energy and the energy density for each hybrid. Specific energy absorption was evaluated by dividing the energy absorption by the mass of each hybrid, whereas the energy density was determined by dividing the total energy absorption by the density of each hybrid. The results demonstrate the diminishing effectiveness of specific energy absorption when increasing the kenaf fibers layers compared to Kevlar composite, due to the difference in the mass and density of the fibers. Among the hybrid composites, H1 recorded the highest specific energy absorption compared to the others, a decrease of 32% of the total energy absorbed of the Kevlar composites. The difference between the H3 and H3A hybrid composites in terms of energy absorbed may be attributed to the large coefficient of friction when stacking the same fiber layers, which resists the formation of a complete shear plug and increases the energy absorbed. The hybrid laminate configuration plays a crucial role in the penetration resistance owing to various stiffness and friction coefficients between the layers, as stated by Pandya et al. 28 However, for the H3A hybrid composite, the reduction in energy absorbed is 44% of total energy absorbed of Kevlar composites, which is lower than the H4 hybrid composite (58%). The KF composite reported the lowest specific energy absorption owing to the low interfacial adhesion of fibers with PVB film compared to the Kevlar composite, leading to transverse matrix damage and shear plug formation corresponding to the delamination. To evaluate the effect of kenaf fibers, percentage changes in energy absorption (%E abc ) of the hybrids were calculated using the following equation 29 %E abc ¼ E K E H E K 100% ð2þ where E K is energy absorptions of Kevlar composite and E H is energy absorptions of the hybrid. Figure 9 shows the effect of kenaf fiber hybridization on the Kevlar composite with reference to the energy absorption. A significant increase in changing absorbing energy with the highest values in H6 was recorded, at which the high percentage of kenaf fibers was used. The total change in energy absorption of the hybrids ranges between 21 45% compared to the energy absorption of the Kevlar composite. This reduction might explain why the use of woven kenaf layers of lower than 30% content does not have much effect on the energy absorbed by the hybrid. Depth of trauma penetration results The NIJ level and Backface Signature (BFS) are two main parameters used to evaluate the ballistic impact properties of hybrid composites. The penetration depth (PD) for each laminated hybrid was measured and compared to 19 coated Kevlar layers, which had the highest ballistic limit results, as given in Table 2. However, since the impact velocities were high, H1, H2, H3, H4, H5 and H6 were sufficient to provide resistance to penetration from the 9 mm bullets, according to the NIJ Standard. 22 Figure 10 shows the effect of impact velocity on penetration depths for all hybrid laminated composites. Delamination area is highest on the back layers of the panel for all thickness values compared to the helmet s shell. At similar impact velocities, the KV (19 Kevlar

10 10 Textile Research Journal 0(00) Figure 9. Percentage changes in energy absorption of hybrid composites. Figure 10. Penetration depths and impact velocity for all hybrid laminated composites. layers) and hybrids H1 and H2 fabric systems gave marginal backface deformation from mm. As reported earlier, contrasting results of the ballistic resistance effect in the blunt trauma on the backing material were also obtained when using two fabric systems. 30 Fabric hybrid H5 has 19 layers of fabrics including eight kenaf layers, and gave a good performance in terms of reducing the backface deformation compared to fabric hybrids H4 and H6. However, it has a higher number of kenaf layers than fabric hybrid H4. There are insufficient Kevlar fabric layers in hybrid H4 to contribute and participate in the energy absorption and blunt trauma depth upon ballistic impact at velocities between m/s. The contributions of ramie fiber to ballistic performance were also shown by Ali et al., 8 where composites hybridized with Kevlar/Polyester gave lower blunt trauma compared to Kevlar composite. In accordance with hybrids H2 and H3 and hybrid helmets HH2 and HH3, the results indicated that NIJ levels were not affected at the same hybrid volume and thickness, while Backface Signature were decreased, as shown in Figure 11. The possible explanation is that, owing to the arc shape fabric for HH2 and HH3, the fabric layers are closer to one another and have stronger bonding, resulting in better interaction among the fabric layers. As a result, the impact energy is transferred to each fabric layer much faster than the panel fabric system, which resulted in higher ballistic impact resistance. The positive effect in terms of NIJ levels and

11 Salman et al. 11 Figure 11. The effect of different composite configurations on penetration depth of all hybrid composites. Figure 12. Penetration depth volume fraction curves of kenaf and Kevlar hybrid composites. penetration depth compared to the Kevlar composite shows that hybridization contributes to the same performance in high-impact penetration tests. This is attributed to the performance of kenaf as reinforcement in PVB composites being satisfactory and comparable to other natural fibers owing to its mechanical properties. Figure 12 illustrates the penetration depth and volume fraction curves of kenaf and Kevlar contents for all hybrid laminated composites. It can be clearly seen that with increasing volume fraction of kenaf, the penetration depth increases, whereas it exhibited opposite trends of decrease as the Kevlar volume fraction increased, under high impact velocities of up to 358 m/s. Under a velocity of up to 426 m/s, the curve of penetration depth exhibited similar trends of decreasing then increasing as the Kevlar and kenaf volume fraction increased. The overall results in the high-velocity impact tests indicate that approximately a 25 30% volume fraction of both kenaf and Kevlar fibers is more effective in the penetration depth value. This improvement may be explained by the fact that these fibers contents present better interfacial surfaces properties, which leads to an increase of the surface for energy dissipation, then a decrease of the penetration depth. With these percentages, fewer number of Kevlar layers are needed in the helmet hybrid for comparable or better blunt trauma performance compared to pure Kevlar composites. Kenaf fabric composites (Kf), which have pure 19 kenaf layers, were not able to stop the bullet in one of the four shots, despite giving higher energy absorption in all the shots. The pure

12 12 Textile Research Journal 0(00) Figure 13. Optical pictures of ballistic failure modes of hybrid composite laminates, 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 comprising alternating layers of kenaf and Kevlar layers and (d) Kenaf composite. kenaf composite is considered a failure in the highimpact velocity tests, but might be suitable for lowerimpact velocities. Damage mechanisms The hybrid composite plays a critical role in absorbing energy owing to different interlaminar and intralaminar failure mechanisms such as delamination, bulge, fiber breakage and matrix cracking. 31,32 Therefore, predictions of damage, energy absorption and penetration resistance are critical to determine the proper thickness of the hybrid composite. Post-test examination of selected samples was conducted to analyze the damage mechanisms during ballistic impact tests. The failure modes of the cross-sections, front and back surfaces of selected samples surfaces are illustrated in Figure 13, to examine the failure modes after the

13 Salman et al. 13 Figure 14. (a) Exterior helmet shell impacted, (b) interior helmet shell impacted, (c) impact depth using ROMA PLASTILINA clay, at different velocities. ballistic impact tests. Interestingly, the region of the affected samples that was affected by the projectiles seems to become most localized at high-impact velocities for ballistic penetration of woven fabric. The conical-shaped impact formation can be seen more clearly, owing to the involved stress wave propagation through the material, leading to localized damage. Damage of shear plugging at the front of the Kevlar 29 composite damaged the surfaces depicting the failure mechanism, with de-fibrillation and Kevlar fiber tension at the rear surfaces, as shown in Figure 13(a). While bulging out and peeling out were depicted in the kenaf composite damaged surfaces, a conical shape was created when the full perforation was completed, as shown in Figure 13(d). As described earlier, Lee et al. 33 and Greenhalgh et al. 34 also reported that the ballistic perforation failure modes were highly localized around the perforation point for laminated hybrid composites. Figure 13(b) shows that the damaged surfaces of the hybrid consisted of placing woven kenaf together and Kevlar 29 layers separately: bulging and fiber failure were observed. A combination of delamination and bulging out between alternative layers was shown in the hybrid comprising alternating layers of kenaf and Kevlar layers, as shown in Figure 13(c). The delamination area is a maximum on the back side where woven kenaf layers alternate with Kevlar 29 fabric layers, owing to debonding occurring in the interlaminar and intralaminar areas of different fabrics. This suggests that separate laminates are more effective in dissipating energy than alternative fabric in the application of a combat helmet. Analyzing the damage in these hybrids, it was possible to note that a greater number of woven kenaf layers show the compression-shear region to be higher than the tension-shear region. All woven fabric hybrids showed the same visible failure behavior: a cone-shaped delaminated area. The energy absorption and Backface Signature of the laminated hybrids was influenced significantly by the layering sequence of the composite constituents. As reported, a scale ratio between reinforcements of different aspect ratios may play a role as a controlling factor in the damage mechanisms of the laminated composites. 35,36 Figure 14 presents images of an impacted helmet and trauma deformations generated in its shell. It is important to remember that the maximum trauma depth permitted by the standard is 20 mm. These results demonstrate that the presence of the woven kenaf and PVB film affect positively the posterior deformation of a helmet shell, even though its configuration allows flexibility. Figure 14(a) shows the external damage in the hybrid composite helmet, and the failures that occurred due to the loads imposed by the bullets (i.e. fiber shear by compression-shear). The interior failures in these hybrids were due to the fiber tension-shear, as shown in Figure 14(b). Figure 14(c) shows the impact depth using Roma Plastilina clay, at various velocities, a local deformation area in cavity shape. Generally, Kevlar and kenaf fibers ruptures, as well as punches out, were major failure modes on the back ply of partial penetration, whereas matrix cracking and delamination of fabric plies occurred for completely penetrated perforation. Similar behavior of other types of helmet materials has been documented in other works: 37,38 the main damage mechanisms were fiber tensile failure and matrix cracking caused by helmet back face deformation. This study will be useful for characterizing and predicting damage in natural/synthetic hybrid composites, and represents an important step towards understanding the effects of layering sequences during high-velocity penetration. Conclusions The potential of using plain woven kenaf fibers to replace Kevlar 29 fabric for future environmentally friendly helmets and energy absorption structures was

14 14 Textile Research Journal 0(00) studied, such that a balance in cost and performance could be achieved. Based on the results, the following conclusions are made: a. A new combat helmet PASGT shell provides enhanced fragmentation and ballistic protection (fourth level of NIJ III-A), reducing cost and the harmful effects of the petroleum products, as well as being more readily available, compared to previous military helmet shells. b. Hybrid laminated composites absorbed more penetration energy compared to kenaf/pvb composites, and they absorb over three-quarters of the initial impact energy of the bullet through plastic deformation. c. The absorbed energy was significantly affected by the impact velocity, increasing with initial velocity amounting to about 10 15%. However, the energy absorption per unit thickness first decreased slightly and then increased significantly. d. Placing woven kenaf layers alternate with Kevlar 29 fabric layers provides lower energy absorption than placing woven kenaf layers together and Kevlar 29 layers separately for the same hybrid volume and thickness. e. Compared to kenaf composites, the hybridization of kenaf/kevlar affects positively the energy absorption changes. f. The tested samples were optically observed; the delaminated area had a conical shape for completely penetrated perforation, a punched-out effect on the back ply of partial penetration, with Kevlar and kenaf fibers breakages, Kevlar and kenaf fibers stretching, shear, Kevlar and kenaf fibers and matrix rupture and matrix cracking, in both tests. This approach is expected to develop helmet armor that is less costly and more readily available compared to conventional helmet armor associated with reducing the potential harmful effects of petroleum products, without jeopardizing ballistic resistant capabilities. This research will open a new avenue for use in military utilities, aerospace, marine and civilian structures, to reduce the use of Kevlar fabric in ballistic laminate composites, while meeting the prescribed baseline performance specifications. Acknowledgements 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 their scientific assistance and financial support. Declaration of Conflicting Interests The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding This work is supported by UPM under GP-IPS/2014/ and UPM under GP-IPB grant, References 1. Tham C, Tan V and Lee H. Ballistic impact of a KEVLAR Õ helmet: Experiment and simulations. Int J Impact Eng 2008; 35: Tarim N, Findik F and Uzun H. Ballistic impact performance of composite structures. Compos Struct 2002; 56: Sayer M, Bektas NB and C allioğlu H. Impact behavior of hybrid composite plates. J Appl Polymer Sci 2010; 118: Salman SD, Hassim WSM and amd Leman Z. Experimental comparison between two types of hybrid composite materials in compression test. Manuf Sci Technol 2015; 3: Salman SD, Leman Z, Sultan MTH, et al. Kenaf/synthetic and Kevlar Õ /cellulosic fiber-reinforced hybrid composites: A review. BioResources 2015; 10: Wambua P, Vangrimde B, Lomov S, et al. The response of natural fibre composites to ballistic impact by fragment simulating projectiles. Compos Struct 2007; 77: Radif ZS, Ali A and Abdan K. Development of a green combat armour from rame-kevlar-polyester composite. Pertanika J Sci Technol 2011; 19: Ali A, Shaker Z, Khalina A, et al. Development of antiballistic board from ramie fiber. Polym-Plast Technol Eng 2011; 50: Azrin Hani Abdul R, Roslan A, Jaafar M, et al. Mechanical properties evaluation of woven coir and Kevlar reinforced epoxy composites. Adv Mater Res 2011; 277: Risby M, Wong S, Hamouda A, et al. Ballistic performance of coconut shell powder/twaron fabric against nonarmour piercing projectiles. Def Sci J 2008; 58: Randjbaran E, Zahari R, Abdul Jalil NA, et al. Hybrid composite laminates reinforced with Kevlar/carbon/glass woven fabrics for ballistic impact testing. The Scientific World Journal 2014; 2014: Freitas CJ, Mathis JT, Scott N, et al. Dynamic response due to behind helmet blunt trauma measured with a human head surrogate. Int J Med Sci 2014; 11: Cheeseman BA and Bogetti TA. Ballistic impact into fabric and compliant composite laminates. Compos Struct 2003; 61:

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