2015 - Ballistic Test of Multilayered Armor With Intermediate Epoxy Composite Reinforced With Jute Fabric

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Multilayered armors with a front ceramic followed by aramid fabric (Kevlar™) are currently used against high velocity ammunition. In these armors, a front ceramic layer that shatters and spalls the bullet is followed by an intermediate layer, usually plies of aramid fabric, which dissipates both the bullet and ceramic fragments energy. In the present work, the intermediate aramid fabric layer was replaced by an equal thickness layer of 30 vol% jute fabric reinforced epoxy composite. Ballistic impact test with 7.62 caliber ammunition revealed that both the plain epoxy and the jute fabric composite have a relatively similar performance of the Kevlar™ and also attended the NIJ standard for body protection. The energy dissipation mechanisms of jute fabric composite were analyzed by scanning electron microscopy and found to be the rupture of the brittle epoxy matrix as well as the interaction of the jute fibers with the post-impact fragments. This latter is the same mechanism recently disclosed for aramid fabric. However, the lightness and lower cost of the jute fabric composite are additional advantages that favor its substitution for the aramid fabric.
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  DOI: http://dx.doi.org/10.1590/1516-1439.358914  Materials Research . 2015; 18(Suppl 2): 170-177© 2015*e-mail: snevesmonteiro@gmail.com 1. Introduction Bullet proof vests for personal protection may use a single layer armor with just one ballistic resistant material as that of a composite formed with plies of aramid fabric; the well known Kevlar  ™[1,2] . This protection, however, is limited to relatively low impact velocity (v i  < 600 m/s) projectiles, such as a 9 mm ammunition. Protection against high impact velocity (v i  > 700 m/s) ammunition, such as a 7.62 mm caliber bullet requires a multilayered armor system (MAS) 3,4 . To stand the 7.62 mm fast bullet with perforating power, a conventional MAS possesses a front ceramic tile, which absorbs most of the impact energy. This energy dissipation mechanism combines not only the bullet spalling but also the fragmentation of the brittle ceramic 5,6 . A high velocity impact  projectile will also produce a blast of fragments, both from the bullet and the ceramic, with sufcient energy to inict lethal personal trauma. Consequently, a second layer is added after the front ceramic to decrease even further the energy of the blast of fragments associated with the post-impact shock wave. In usual MAS conguration, Kevlar  ™  acts as the second layer and may be followed by a ductile metal sheet, like aluminum, to restrict the penetration of fragments in a body. Eventually, a spall shield covers the MAS front,  before the ceramic, to avoid ight way fragments 6 . The use of Kevlar  ™  as the second intermediate layer is indicated by the aramid ber strength around 4,000 MPa 7 , which is much higher than any conventional material and comparable to carbon ber. It is reported that the Kevlar  ™  ballistic energy dissipation occurs by yarn rupture and elastic stretching as well as friction and pullout in the aramid fabric 2,8 .Another factor affecting the energy absorbed in a MAS is the shock impedance at the interface between layers 9 . Upon projectile impact in the front layer, a compressive- transverse wave travels inside the ceramic and suffers multiple reections as it crosses the other interfaces. The nature of the reected waves depends on the impedance of the interface. A lesser dense second layer, such as Kevlar  ™ , is associated with relatively lower shock impedance and causes the compressive component of the wave to reect as a tensile wave 10 . This contributes to an effective fragmentation of the  brittle ceramic. Furthermore, the proceeding compressive wave travelling in the other MAS layers will carry a comparatively lower transmitted energy. Thus, the lesser dense is the second layer, the more efcient is the energy absorption. Another possible second layer to be used is substitution for the Kevlar  ™  with even lower density could be a natural ber reinforced polymer composite. Natural bers, especially those extracted from plants, are used since the beginning of our civilization in simple items, such as ropes, baskets and fabrics. From the middle of last century, several natural bers were incorporated into different matrices to be applied as engineering materials 11 . In past decades, an exponential growth in both research works 12-22  and industrial applications 23-25   has confer to natural ber composites a prominent position owing to their technical properties (lightness, lower abrasion to molding equipments, toughness and strength) as well as economical, societal and environmental advantages 20 . Ballistic Test of Multilayered Armor with Intermediate Epoxy Composite Reinforced with Jute Fabric Fernanda Santos da Luz  a  , Edio Pereira Lima Junior  a  , Luis Henrique Leme Louro a  ,  Sergio Neves Monteiro a *  a  Departamento de Ciência dos Materiais, Instituto Militar de Engenharia – IME,  Praça General Tibúrcio, 80, Praia Vermelha, Urca, CEP 22290-270, Rio de Janeiro, RJ, Brazil  Received: November 21, 2014; Revised: July 28, 2015 Multilayered armors with a front ceramic followed by aramid fabric (Kevlar  ™ ) are currently used against high velocity ammunition. In these armors, a front ceramic layer that shatters and spalls the  bullet is followed by an intermediate layer, usually plies of aramid fabric, which dissipates both the  bullet and ceramic fragments energy. In the present work, the intermediate aramid fabric layer was replaced by an equal thickness layer of 30 vol% jute fabric reinforced epoxy composite. Ballistic impact test with 7.62 caliber ammunition revealed that both the plain epoxy and the jute fabric composite have a relatively similar performance of the Kevlar  ™  and also attended the NIJ standard for body  protection. The energy dissipation mechanisms of jute fabric composite were analyzed by scanning electron microscopy and found to be the rupture of the brittle epoxy matrix as well as the interaction of the jute bers with the post-impact fragments. This latter is the same mechanism recently disclosed for aramid fabric. However, the lightness and lower cost of the jute fabric composite are additional advantages that favor its substitution for the aramid fabric. Keywords: ballistic test, multilayered armor, jute fabric composite, bullet penetration depth  Ballistic Test of Multilayered Armor with Intermediate Epoxy Composite Reinforced with Jute Fabric 2015; 18(Suppl 2)171 Among the natural bers, that extracted from the stem of the jute plant ( Corchorus capsularis ) is worldwide cultivated and has been extensively investigated in this decade as composite reinforcement 26-32 . The jute ber was reported 20  to present density, 1.30-1.45 g/cm 3  and tensile strength, 393-800 MPa, convenient to replace synthetic bers in polymer composites. In particular, both recycled and new fabrics made from jute ber yarns, were found to signicantly increase the Charpy and Izod impact energy of a polyethylene matrix 33,34 . As for the possibility of using jute ber composites as  ballistic resistant material, Wambua et al. 35  investigated the performance of polypropylene matrix reinforced with 46 vol% of jute plain woven. Their composites were either faced or sandwiched by mild steel sheets. In terms of energy absorption, they concluded that the sandwiched (steel/composite) have advantage over neat steel and plain  jute composite. Moreover, the composite dominant failure modes included delamination as well as ber rupture and shear cut-out. Despite these relevant information, it was not the direct scope of their work  35  to assess the ballistic performance of jute composites as armor for personal protection. This is for the rst time conducted in the present work. Based on the aforementioned considerations, the ballistic  performance of armors composed of a front ceramic and an intermediate composite as well as a back aluminum layers was investigated in terms of depth of penetration into clay witness simulating a personal body. Ballistic tests were conducted in MAS with a front Al 2 O 3 -based ceramic tile. As the following intermediate layer, lighter jute fabric reinforced epoxy composite plates were compared (same thickness), to plain epoxy plates and Kevlar  ™ . The contribution of each separated material was also assessed by individual  ballistic tests. The fracture aspects of the different types of intermediated layer materials were analyzed by scanning electron microscopy. 2. Material and Methods Figure 1 illustrates schematically the side view of the multilayered armor system (MAS) arrangement used in this investigation. The front layer (A), rst to be hit by the  projectile, was a 10 mm thick hexagonal tile with 31mm of side dimension and made of 4 wt% Nb 2 O 5  doped Al 2 O 3  impact resistant ceramic. Ceramic tiles were fabricated by sintering Al 2 O 3  powder (0.3 µm of particle size) supplied by Treibacher Schleifmittel as commercial purity (US$ 1.59/kg) mixed with  Nb 2 O 5  powder (0.69 µm of particle size) supplied by the Brazilian rm CBMM as 99% pro-analysis (US$ 16.13/kg). Sintering was carried out at 1,400 ºC for 3 hours under air. Before tests, the clay witness was subjected to thermal treatments and evaluated according to the norm. The intermediate layer (B) with 10 mm in thickness and square sides with 150 mm was either: (i) Kevlar  ™  with 16 plies of aramid fabric, or (ii) 30 vol% of jute fabric reinforced epoxy matrix composite (jute fabric composite for short) plate, or (iii) plain epoxy plate. The Kevlar  ™  was supplied by the Brazilian rm LFJ Blindagem Com. Serv. S.A. (US$ 47.69/kg). The jute fabric was supplied by a Brazilian textile industry Lealtex Ltda. in the form of a roll (US$ 8.38/kg). Pieces of fabric were taken from the roll, dried at 60 ºC in a laboratory stove for 2 hours, and placed with the correct amount inside a steel mold. The Dow Chemical  produced diglycidyl ether of the bisphenol-A (DGEBA) epoxy resin and the trietylene tetramine (TETA) as hardener were supplied by the Brazilian rm Resinpoxy (US$ 19.35/kg). Still uid epoxy resin mixed with phr stoichiometric fraction of the hardener was poured in between the fabric pieces onto the mold. A pressure of 5 MPa was applied and the composite plate cured for 24 hours. Figure 2a illustrates a  plate of jute fabric composite. In a similar procedure, plain Figure 1. Schematic diagram of the multilayered armor. Figure 2. Plate of jute fabric composite (a) and the actual front view of a typical complete MAS investigated (b).  Luz et al. 172  Materials Research DGEBA/TETA epoxy plates were also fabricated. The back layer (C) was a 150 × 150 mm 5052-H34 aluminum alloy sheet with 5 mm in thickness, supplied by the Brazilian rm Metalak (US$ 12.43/kg). These layers were bonded in the composite with commercial Sikaex ™  adhesive supplied  by the Brazilian rm Sika Co. Figure 2b shows the actual front view of a typical jute fabric composite second layer MAS target. The density of each distinct MAS component: ceramic tile (3.51 ± 0.60 g/cm 3 ); jute fabric composite (1.13 ± 0.07 g/cm 3 ); aramid fabric (1.08 ± 0.03 g/cm 3 ) was determined by precise measurements of volume and mass in 20 samples, using Weibull statistics. In direct contact with this metallic back layer, a block of clay witness simulated a body protected by the MAS. The clay witness was a commercially supplied CORFIX ®  plastilene. The trauma in the clay duplicates the plastic deformation imposed by the projectile impact on the aluminum back layer. The corresponding depth of penetration was measured with a special Mitutoyo caliper with an accuracy of 0.01 mm, as shown in Figure 3. At least 15 measurements were performed for each test and the average/deviation values determined  by means of the Weibull statistics. Eventual random and systematic errors were identied and treated as per this statistical methodology. The ballistic tests were conducted at the Brazilian Army shooting range facility, CAEX, in the Marambaia peninsula, Rio de Janeiro. All tests, 10 for each type of MAS target, were carried out according to the NIJ 0101.06 standard using 7.62 × 51 mm NATO military ammunition (7.62 mm for short) with a 9.7 g projectile propelled from a gun barrel. Figure 4 shows, schematically, the exploded view of the  ballistic test setup. A dashed straight line indicates the  projectile trajectory. A steel frame was used to position the target, which was held in place by spring clips. The gun, located 15 m from the target, was sighted on its center with a laser beam. The exact velocity of the projectile at two moments: leaving the gun and immediately before impacting the MAS target was measured by an optical barrier, Figure 4, and a model SL-52 OP Weibel xed-head Doppler radar system. Tests in which the target was totally perforated, allowed the residual velocity of the outcoming projectile to  be measured. Fractured samples of each MAS component after the ballistic test were analyzed by scanning electron microscopy (SEM) in either a model FSM 6460 LV Jeol or a model QUANTA FEG250 Fei microscopes operating with secondary electrons at 20 kV. 3. Results and Discussion All ballistic tests conducted in MAS targets failed to  perforate the materials and the impact energy was dissipated inside the armor in association with a penetration depth in the clay witness, as shown in Figure 3. To evaluate the individual ballistic behavior of each distinct intermediate layer, tests were separately performed in the ceramic tile, Kevlar  ™ , jute fabric composite plate and plain epoxy plate. In these tests, contrary to the MAS tests, the target was always  perforated. Therefore, in addition to the impact velocity (v i ), the projectile residual velocity (v r  ) after perforation could also be measured.Table 1 presents the average depth measured in the clay witness, Figure 3, for the different MAS targets investigated. In this table, some points are worth discussing. The three materials, tested as the intermediate layer that follows the front ceramic layer, showed corresponding penetration depth  below the NIJ 0101.06 standard limit of 44 mm for serious  body trauma. The Kevlar  ™ , with 23 ± 3 mm, displays the deepest average penetration value in confront to both the  jute fabric composite, with 21 ± 3 mm and the plain epoxy, with 20 ± 1 mm. However, by considering the corresponding deviations, the three different MAS targets display similar  ballistic performance. For application in armor vest, it is important to mention that the jute fabric composite is lighter and signicantly cheaper than the Kevlar  ™ . These are factors that are further discussed and might play practical advantages in considering the substitution of jute fabric composites for Kevlar  ™  in a MAS for personal protection against high velocity projectiles. The projectile impact against the rst MAS ceramic layer, which is responsible for most of the energy dissipation 5,6  is associated with spalling of the brittle ceramic tile. In order to investigate its fracture, ceramic particles collected after the tests were observed by SEM after gold sputtering to  provide an electrical conducting coating. Figure 5 shows the expected intercrystalline brittle fracture surface of a collected Figure 3. Measurement of the depth of penetration in the clay witness caused by the projectile impact. Figure 4. Schematic exploded view of the ballistic experimental setup.  Ballistic Test of Multilayered Armor with Intermediate Epoxy Composite Reinforced with Jute Fabric 2015; 18(Suppl 2)173 macroscopic ceramic particle, which is almost splitting into microscopic fragments associated with grains. Grain  boundary embrittlement was caused by the 4 wt% Nb 2 O 5  addition to the Al 2 O 3 . This contributes to dissipate more energy by increasing the fractured surface. As indicated by Medvedovski 6 , a 7.62 mm projectile causes different kinds of cracks to be formed during the impact. This complex  pattern of propagating cracks associated with intercrystalline fracture is observed in Figure 5. Figure 6 shows the damage region of the Kevlar  ™  caused  by blast of fragments resulting from the projectile impact against the front ceramic tile. The general features in Figure 6a corroborate evidences, pointed by arrows of ber pullout from the fabric yarn, ber stretching and ber rupture 2,8 . Additionally, Figure 6b reveals a massive participation of  bright and white particles of Al 2 O 3  attached to the bers. This indicates that the Kevlar  ™  contributes in the energy dissipation by collecting ceramic fragments. Indeed, in a recent publication 36 , the main energy absorption mechanism of the aramid fabric in a MAS was found to consist on its bers capacity to collect fragments by mechanical incrustation and van der Waals forces. A surprising conclusion was that the superior strength and stiffness of the Kevlar  ™  in a MAS is not as important as its fragment collecting capacity. Figure 7 shows the fracture region of a jute fabric composite after penetration by fragments resulting from the  projectile impact suffered by the front ceramic tile. In this Figure 7a , with higher magnication, it is observed the jute fabric separation in thinner brils, which is a characteristic of its mechanical rupture 34 . This certainly contributes to absorb the impact energy. Moreover, the fracture of the brittle epoxy matrix in Figure 7b is another source of energy dissipation. Similar to what was recently reported 36  and here found in the damaged Kevlar  ™ , Figure 6b, the capture of fragments also impregnated the jute fabric composite (ber and epoxy matrix) by bright and white particles shown in Figure 7.Figure 8 shows the fracture of a plain epoxy after hit by fragments (projectile/ceramic) resulting from the projectile impact in the front ceramic tile. In addition to collected fragments, an important mechanism for energy dissipation in this gure is the nucleation and propagation of cracks in a typical “river pattern” characteristic of brittle polymers. Consequently, the plain epoxy rupture might also be efcient in reducing the energy or stopping fragments from the ballistic  post-impact. This explains the similar depth, Table 1, as compared with the Kevlar  ™  and the jute fabric composite. The contribution of each MAS component was assessed  by individual ballistic tests using the same ammunition, 7.62 Figure 5. Fracture surface of a particle from the Al 2 O 3  after the  ballistic test. Table 1. Average depth of penetration in the clay witness backing different multilayered armors. Intermediate Layer Material Depth of Penetration (signicant gures) from Measurements and Weibull Analysis (mm) Kevlar  ™ 23 ± 3Epoxy composite reinforced with30% of jute fabric21 ± 3Plain epoxy plate20 ± 1 Figure 6. Damaged Kevlar  ™  by fragments (projectile/ceramic) after the ballistic impact: (a) lower magnication and (b) higher magnication.
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