An investigation of the effect of a Kevlar-29 composite cover layer on the penetration behavior of a ceramic armor system against 7.62 mm APM2 projectiles

Highlights

• Ballistic tests of bilayer ceramic/composite armor structures with a Kevlar-29 composite cover layer were conducted at different velocities.

• 3D finite element models were developed to elucidate the covering effect through the definition of a cohesive surface at the interface between the cover layer and the ceramic tile.

• Ballistic resistance of covered and uncovered armor structures was compared through numerical simulation.

• Two major mechanisms in covered armors were identified to have a positive effect on their ballistic resistance.

Abstract

High hardness ceramics are commonly used in lightweight armor systems to defeat the intrusion of high-speed armor piercing (AP) projectiles. However, bare ceramic tiles are intrinsically brittle, and fragments of various sizes can be generated when subjected to impact, which can cause secondary impact to the wearer and surroundings. In a typical armor design, the ceramic tile is usually covered with a compliant thin sheet to mitigate fragments splattering. In this study, the effect of a Kevlar-29 composite cover layer on a bilayer ceramic/composite armor system is investigated through a combined approach of experimental testing and finite element (FE) simulation. In the experiments, 7.62 mm APM2 projectiles were used to impact single Kevlar-29 composite layer covered ceramic/composite armor systems at three different velocities: 884 m/s, 1070 m/s and 1164 m/s. The restraining effect of the cover layer on the ceramic fragments was clearly observed. A 3D FE model was further developed to analyze the covering effect through the definition of a cohesive surface behavior at the interface between the cover layer and the ceramic tile. The FE model successfully captured the peeling behavior of the cover layer and dominant fracture patterns in the ceramic tile. It shows that, the cover layer can improve the ballistic resistance of the armor system through two major mechanisms: the energy dissipation by itself and its effect on fracture process of the ceramic tile. A high bond strength at the interface can also improve the energy dissipating capability of the armor system.

Introduction

Ceramics of high compressive strength and high hardness are broadly used in today's lightweight armor systems to defeat various projectile threats [1], [2], [3], [4], [5]. Typical ceramics are being used include alumina, silicon carbide and boron carbide, etc. In a layered ceramic armor, the preeminent role of the ceramic is to blunt and shatter the high-speed intruding projectiles, which are usually made of high hardness steel. A good example is the 7.62 mm APM2 projectile [6]. Its steel core gives the APM2 projectile a strong penetration capability, which necessitates an even harder shield to accomplish the desired protection level, making ceramics an ideal choice due to their high hardness and low weight. However, ceramics are inherently brittle and of low tensile strength. When a bare ceramic tile is subjected to a high-speed projectile impact, fragmented ceramics tend to splatter outward from the impact site which could cause secondary impact to the surroundings [7,8].

A common practice in lightweight body armor design is to integrate a ceramic tile with a backing plate to form a layered structure. The backing plate can be made of metallic materials such as aluminum alloys, or high-performance fiber reinforced composites [9]. Compared to metallic materials, the advanced fiber reinforced polymer composites can further reduce the weight of the armor and improve the wearer mobility. A major purpose of the backing plate is to absorb the residual kinetic energy of the projectile after it has penetrated through the ceramic layer. Phoenix et al. [10] pointed out that the ballistic resistance of a woven fabric is inversely related to the area density of the projectile. When the pointed tip of a APM2 projectile is eroded by a ceramic layer, it becomes much easier for a composite backing to fully arrest it due to increased cross-sectional area. Another major purpose of the backing plate is to constrain the ceramic fragments thus prolong the interaction time with the projectile.

While a backing plate is sufficient to prevent the fragments from splattering toward the armor wearer, a front covering layer is needed to constrain the fragments from splashing so as to reduce impairments to the surroundings. As advanced armor design has been driven by rapidly growing demand for personnel protection, the effect of adding a thin front covering sheet on the ballistic performance of ceramic tiles has been investigated in several studies [11], [12], [13], [14], [15], [16]. Nunn et al. [11] found that the ballistic resistance of boron carbide tiles can increase 40% when restrained by several layers of polymeric composites. Reddy et al. [12] found that the energy absorption capability of alumina tiles could be doubled when it is wrapped by a few layers of polymer fabrics. However, Crouth [13] observed that a cover layer does not have a significant effect on the recovered projectile length when impacted by a Mild Steel Cored ammunition. In these studies, the ceramic tiles were usually backed by a composite plate that is much thicker than the cover layer. Other studies focused on the effect of a cover layer on the ballistic resistance of a bare ceramic tile, where no backing plate is used. Particularly, Sarva et al. [14] conducted a series of testing to study the effect of different cover materials on the ballistic resistance of bare alumina and silicon carbide tiles. A significant improvement in the ballistic efficiency was noticed when the tiles are covered. Very recently, Rahbet et al. [15] studied the position effect of four layers of glass fiber composite covers on the projectile fragments at near muzzle velocities. A reduction in the residual mass of the largest fragment was observed when the four composite layers are placed on the back side. However, no remarkable difference in terms of projectile residual velocities was noticed. Through a finite element simulation, Batra et al. [16] found that at the impact velocity of 180 m/s, adding a thin polyether-ether-ketone (PEEK) layer on the surface of a SiC ceramic tile can reduce the normal force and impulse transmitted from the tile to the backing gelatin block.

In the above referenced studies, the testing configurations and target designs were all different, i.e, different ceramic materials, ceramic thickness, projectile types, backing conditions, cover materials. The impact velocity ranges from 200 m/s to 900 m/s. These factors all exert influences on the testing outcomes, and the differences might explain some of the inconsistent conclusions obtained. In this study, we focus on the effect of a Kevlar-29 composite covering layer on the ballistic impact behavior of a bilayer armor system, made from an alumina front layer and a Kevlar-29 composite backing plate, when impacted by an APM2 projectile at very high velocities. In addition, since it is impossible in experiments to precisely isolate any single factor that could contribute to the impact outcome, we further developed a 3D finite element (FE) model to analyze the effect of the cover layer. The numerical simulation helps elucidate the mechanisms attributed to the variances of penetration behavior due to an additional cover layer. It also allows the interface bond strength effect to be parametrically studied.