PERFORATION AND PENETRATION OF FIBER METAL LAMINATES TARGET BY HEMISPHERICAL PROJECTILE

This study aims to examine the phenomena that occur due to projectile penetration on fiber metal laminate. Ballistic testing was carried out experimentally according to National Institute of Justice standards (NIJ Standard 0101.06 level III-A) using a 9 mm full-metal jacket projectile with a normal angle of attack (90° to the target). The results showed that fiber metal laminate could withstand the projectile rate by penetrating the first layer (aluminum plate) and the second layer (aramid/epoxy), while the last layer was deformed to form a bulge. The pierced aluminum plate is characterized by petalling failure. Meanwhile, the aramid/epoxy was penetrated by the projectile with failure of the primary yarn to break the fiber.


INTRODUCTION
Recently, the development of bulletproof vest materials has been discovered using new materials, compositional combinations, and the manufacture of composites of both metals and ceramics, as well as fiber-or particle-reinforced polymers. Ballistic behavior with highspeed impact loads is a relevant issue in a variety of structural applications. Many studies have been conducted on impact ballistics both experimentally and numerically to analyze the target material's resistance and performance in terms of penetration and residual velocity of the projectile. However, characteristics such as depth of penetration, damage, failure and microstructure in ballistic impact areas have not been explored in depth.
A wide variety of materials and structures have been produced to increase the demand for the transport sector's light and strong properties. Fiber metal laminate (FML) has attracted a lot of attention due to its outstanding mechanical properties, including excellent fatigue properties and impact resistance [1][2][3][4]. Fiber metal laminate (FML) is a fiber-reinforced polymer matrix composite with metal on top and bottom [9]. FML is made by depositing and bonding adhesives on relatively thin layers of metal and fiber-reinforced polymer composites. Various types of metals and composites have been explored to construct FMLs. For the metallic coating, light alloys of aluminum [5], titanium [1,6], and magnesium [7] have been used. For composite coatings, various types of reinforcing fibers, including aramid [8], carbon [9], and glass [10] fibers in a thermosetting or thermoplastic matrix have been investigated.
Corderley et al performed ballistic experimental tests to determine the failure model of epoxy and carbon fiber reinforced titanium (FML). The test was carried out using a projectile with a speed of 2000 m/s. The test results show that one of the mechanisms for the failure of titanium is the formation of adiabatic shear bands. At the same time, the lamination sequence influences the failure model of composite materials [11]. Zu et al conducted a ballistic study of composite kevlar fiber reinforced rubber composite armor (KFRRCA) using practical methods to determine the depth of penetration of KFRRCA with a diameter of 56 mm. The results showed that KFRRCA can be used as an armor material because it has excellent protection capabilities, especially when the material is at 30° and 68° [12].
The emergence of very high modulus polyethylene fibers has increased the protective effectiveness of fabrics and generated interest in the protective ballistic performance of composites made of fibers. Many studies have been conducted on various versions of highstrength ceramic laminated fibers. But very few investigations have attempted to explore combinations of high strength metal and laminated fibers [9]. In the present work, the ballistic performance of a FML consisting of an aluminum constituent material and a composite made of an aramid fiber reinforced epoxy matrix was investigated in terms of ballistic impact on a target using scanning electron microscopy (SEM). This study used a 9 mm full metal jacket (FMJ) caliber projectile with a lead core protected by brass and a normal angle of attack (90° to the target).

MATERIALS AND METHODS
The FML studied in this research was made using a vacuum-assisted resin infusion method reinforced with woven aramid fibers with an angle orientation of 0°/90°. The aluminum used is the 5083 series which consists of two sheets on the outer surface. The epoxy matrix is used as a binder between aramid fiber sheets and a binder between aluminum and aramid fibers. Figure 1 shows a schematic of the stacking sequence used for sample preparation. FML is made with a 3-layer configuration, using 2 mm thick 5083 aluminum plate on the outer layer and aramid fiber as the core with a total of four layers. To analyze the ballistic behavior during the penetration process, material characteristics are very fundamental. The target material will initially experience an elastic response to a load. However, when subjected to extreme impact loads, the material will experience stress that exceeds the yield stress resulting in plastic deformation. The ballistic testing scheme is shown in Figure 2. The target material is impacted at an angle of attack of 90° with a 9 mm full metal jacket (FMJ) projectile. Ballistic testing based on NIJ 0101.06 type III-A standard from the U.S Department of Justice. Figure 3 shows the shape and dimensions of the projectile including the projectile core and mantle. After the ballistic impact, the microstructure near the impact area was observed using scanning electron microscopy (SEM).  Figure 4 shows the formation of a crater with cracks in the crater wall due to projectile penetration. The first layer of the aluminum plate and the second layer, namely aramid/epoxy, were penetrated by the projectile, while the last layer was deformed to form a bulge. The failure of the first layer is caused by the formation of ductile holes due to plastic deformation of the projectile while the failure of the second layer (aramid/epoxy) is caused by the failure of the fiber which is subjected to the compressive load of the projectile. The failure of the aluminum plate can be seen from the morphology of the fracture surface and the formation of petals due to projectile penetration. Whereas, the failure of aramid/epoxy is seen in fiber damage after impact. Figure 5 shows the exit side of the ballistically impacted aluminum plate bore. The surface layer appears to be crushed by the impact of the high-velocity projectile. Petal formation is also visible on the projectile exit from the target. During ballistic impact events and target piercing, the projectile is pushed outward laterally with circular and radial fracture failures. This causes the phenomenon of petalling. Several micro-cracks are also visible on the fracture surface. The fracture surface fragmentation area shows clear traces of aluminum melting caused by the temperature rise ( Figure 6) generated by the energy dissipation between the projectile and the aluminum plate.   Figure 6 shows the temperature profile of the aluminum plate starting from the center of impact and moving away from the impact point. The temperature increases at a distance of only 4 mm from the point of impact of 436.35 K and decreases to room temperature. The temperature at the point of impact is above the melting temperature of the aluminum creating a thin molten film. The aluminum plate also exhibits a ductile fracture surface morphology which is characterized by the spread of dimple fractures around the plate damage area. It can be seen that there are particles of projectile material scattered on the surface of the aluminum plate fracture.

RESULTS AND DISCUSSION
SEM/EDS evaluation of the fracture surface of the aluminum plate is shown in Figure  7. Figure 7a presents the fracture surface of the aluminum plate. Figure 8a also displays the position where the EDS was applied. The position at which the SEM/EDS is displayed in the shell area of the projectile bore is shown in Figure 7b. See Figure 7c, which allows identification of some of the projectile material particles, especially those formed by Pb and Fe (light dots in Fig.). The projectile that penetrated the aluminum plate caused the fragments of the projectile core to stick to the material. This can be seen in Figure 8e which shows the high Pb concentration of the projectile core.  Aramid/epoxy failure due to ballistic impact loads is shown in Figure 9. There appears to be a surface crack between the matrix and the fiber. In the area close to the impact of the projectile, it was seen that there was a tensile failure of the fibers which caused the aramid fibers to be pulled out of the matrix and cracked due to shear forces. The fiber is detached from the matrix (fiber pull out) causing the fiber to be pulled and then cut due to the compression of the projectile. In addition, fibers also stretch after ballistic impact events. The fibers simultaneously begin to stretch due to the longitudinal waves along the fibers. The mechanism of fiber tensile failure due to high levels of tension and high pressure causes the aramid fiber to break, fiber deformation (stretching and twisting). When the projectile hits the fiber in the webbing, the projectile is captured by the woven layer of the fiber so that the kinetic energy is transferred to a wider layer of the fabric and spreads outwards from the ballistic point of impact.

CONCLUSION
Based on the results obtained during the analysis of the ballistic impact of the 9 mm full metal jacket projectile on the FML, it can be concluded that: • The first layer (aluminum plate) was penetrated with failure of sheath formation at the rear and a spreading dimple fracture around the impact area, indicating the occurrence of a ductile fracture at the target. Meanwhile, the projectile penetrated the second layer (aramid/epoxy) with fiber fracture failure in the primary yarn and fiber pulling, fiber stretching, and fiber damage.
• SEM testing provides important information regarding the morphology of individual failures of the FML constituent materials after ballistic impact loads. The kinetic energy of the projectile is dissipated during the impact event in two different ways. The first part is to induce plastic deformation (enlargement of holes, shells, adiabatic shear bands), increasing the temperature of the target material. The second part, the penetration of the projectile and the target, provides a frictional effect in the form of heat.