Development of composite armour resistant to armour piercing small arms ammunition

Due to people's survival instinct, the need for protection from external factors has emerged. Over time, they have tried to protect themselves against external threats with various materials. These materials have developed parallel to or even ahead of humanity's scientific and technological levels. Because the instinct to live has led people to research and develop new, effective and safe materials to protect themselves. The process, which started with materials such as leather, wood and stone, developed over time with the discovery of metals such as copper, brass, iron and steel. In today's world, ceramic materials, composite materials and alloy steels have an important place in armor technology. Steel materials continue to have the largest share in armor production. In addition, composite materials aim to develop a more efficient material by combining the positive aspects of different materials. Especially in the personal protection of people, ceramic and composite materials have the biggest share due to their lightness. For example, Kevlar-like fibers are used extensively both in ballistic protection and in the field of occupational health and safety. Especially in the personal protection of military personnel, its lower weight and less movement restriction compared to steel plates constitute very important advantages on the battlefield. Although its production costs are high, its intensive use continues due to its many advantages. In this study, it was aimed to investigate the differences in the performance of composite armour consisting of different materials between the composite armour obtained by bonding only steel materials. The composite armour was designed as 3-element and 4-layer. The materials used were selected from readily available products that do not require special processing. Commercially hardened Ck75 steel sheet was used in the front and back layers, Al 7075 sheet was used in the first intermediate layer, and high density polyethylene (HDPE) materials were used in the second intermediate layer. The relatively high hardness and toughness of the steel sheet used will allow the projectile and the steel core to absorb the energy of the bullet and the steel core by forcing the material to shear and bend. The steel material has an average yield strength of 1245 MPa. The average hardness of the material was measured as 42 HRC. Al 7075 material has T6 heat treatment quality and stands out among aluminum with its high strength and medium toughness values. The purpose of this material is to help absorb the energy of the bullet with its higher toughness compared to steel. In addition, it will support to stop the bullet with its hardness. The aluminum material has an average yield strength of 470 MPa. HDPE acts as a damping element in armor due to its low density and high resistance to impacts. Epoxy adhesive was preferred for bonding the materials. The selected epoxy adhesive was chosen to be strong enough to hold the layers together during the impact and at the same time to flex together with the materials during bending. The armor-piercing ammunition to be used in the tests is the 7.62 mmx51 M61 cartridge. This cartridge is used in 7.62 mmx51 infantry rifles and machine guns and is fired at lightly armored targets. The bullet of this ammunition is FMJ (Full Metal Jacket) type. There is a hardened steel core with a lead jacket inside an outer jacket of copper or mild steel. The tip of the steel core is pointed to increase its armour penetration capability. In order for the prepared armor to be considered successful, the steel core must leave the armor at a speed higher than zero. For this reason, armor that was partially penetrated or even completely penetrated was considered successful if the bullet did not leave the armor. This is also the success criterion in military specifications. As the first experiment, Ck75 sheets were bonded in different thicknesses and it was tested which thickness could stop the bullet. Here, tests were carried out starting from 15 mm thickness and reducing by 1 mm. he 11 mm thick armour stopped the steel core and the 10 mm thick armour was penetrated. For this reason, 11 mm was taken as a reference for steel armor. In this case, the areal density of the armor is calculated as 8.58 g/cm2. Then, 3 different experimental sets were prepared with 4-layer composite structures. In these test sets, the effects of the front layer, first intermediate layer and back layer thicknesses on armor performance were observed. In the first set of experiments, the first three layers were kept constant and the effect of the back layer was examined. Tests were carried out by keeping the front layer 4 mm steel, the first intermediate layer 7 mm Al7075, the second intermediate layer 10 mm HDPE constant and reducing the thickness of the back layer from 4 mm steel by 1 mm. The sample with the back layer of 2 mm was successful. In this case, the areal density of the armor is calculated as 7.58 g/cm2. In the second set of experiments, the effects of different thicknesses of the first intermediate layer on the armor performance were examined. Tests were carried out so that the front layer was 3 mm steel, the first intermediate layer was 11.2 mm Al7075, decreasing by 1.4 mm, the second intermediate layer was 10 mm HDPE and the back layer was 3 mm steel. The sample with the second intermediate layer of 8.4 mm was successful. In this case, the areal density of the armor is calculated as 7.98 g/cm2. In the third set of experiments, the performance of the armor was examined by keeping the front layer at different thicknesses and the other layers constant. Tests were carried out by starting from 4 mm steel in the front layer and decreasing it by 1 mm, with the first intermediate layer was 7 mm Al7075, the second intermediate layer was 10 mm HDPE and the back layer was 4 mm steel. The sample with a front layer of 3 mm was successful. In this case, the areal density of the armor is calculated as 8.37 g/cm2. The areal density of each sample was calculated and compared with composite armor consisting only of steel. The most successful sample was the one with a front layer of 4 mm and a back layer of 2 mm The importance of the front layer has been revealed in the tests performed here. The amount of steel material in the total armour is not only important, but also the distribution between the front and back layers affects the performance of the armour. The use of thick steel in the front layer disrupts the tip sharpness of the bullet and increases the resistance applied to the projectile. When the area densities were compared in the tests, it was seen that the most successful sample provided 11.6% weight saving compared to the steel sample only. This rate is not meaningful for personal protection, but it is a value that can be meaningful in the armouring of vehicles or buildings.