Ballistic resistance of double-layered armor plates

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Abstract

In this paper, the ballistic resistance of double-layered steel shields against projectile impact at the sub-ordnance velocity is evaluated using finite element simulations. Four types of projectiles of different weight and nose shapes are considered, while armor shields consist of two layers of different materials. In a previous study of the same authors, it was shown that a double-layered shield of the same metal was able to improve the ballistic limit by 7.0–25.0% under impact by a flat-nose projectile, compared to a monolithic plate of the same weight. Under impact by a conical-nose projectile, a double-layered shield is almost as capable as a monolithic plate. The present paper extends the analysis to double-layered shields with various metallic material combinations. The study reveals that the best configuration is the upper layer of high ductility and low strength material and the lower layer of low ductility and high strength material. This configuration results in some 25% gain in the ballistic limit under moderate detrimental impact. This research helps clarify the long standing issue of the ballistic resistance of the multi-layered armor configuration.

Introduction

An optimization of metal shields against projectile impact has long been of practical interest in military and civilian applications. This paper proposes a double-layered configuration that consists of two parallel layers as a potential improvement over a monolithic plate. Although a lot of investigations have been done on the perforation resistance of monolithic plates experimentally, numerically, and theoretically. Limited studies on multi-layered metal shields were reported in the open literature.

On the basis of a series of tests, Marom and Bodner [1] found that multi-layered beams were more effective in resisting perforation than monolithic beams of the same weight under projectile impact. Corran et al. [2] showed experimentally that a double/triple-layered shield was superior in ballistic resistance to a monolithic plate if the total thickness exceeded a critical value. An opposite conclusion was obtained by Radin and Goldsmith [3]. They performed normal impact tests on multi-layered shields of a wide range of thickness. The ballistic limit of a monolithic plate was always higher than that of a multiple-layered shield of the same total thickness. This finding was confirmed by Almohandes et al. [4] through an extensive experimental program on steel plates of varies configurations impacted by standard 7.62 mm bullet projectiles. Dey et al. [5], [6] recently reported a comprehensive experimental and numerical study on the perforation resistance of double-layered steel armor plates. They found that in the case with a blunt-nose projectile, the ballistic limit of a double-layered shield was 30% higher than that of the monolithic case.

The above literature review indicates that the protection effectiveness of multi-layered shields remains a subject of debate. Note that in those studies various projectiles and target materials were considered. In this connection, two questions can be posed: Under what type of projectile impact would a multi-layered shield be superior in the perforation resistance to a monolithic plate of the same weight? Between ductility and strength, which property is more important for the perforation resistance against projectile impact?

Previous research has revealed that different failure modes may be developed in a target by changing impact conditions, e.g. see Teng and Wierzbicki [7], and Børvik et al. [8], [9]. A failure mode with higher energy absorption can significantly improve the ballistic resistance of a shield. By replacing a monolithic plate with a double-layered plate, the bending action can be enhanced and thus the double-layered plate may undergo considerable deformation before fracture. By defining two different grades of metals of various strength and ductility for two layers, the performance of a double-layered shield can be further improved.

The objective of the present paper is to evaluate the effectiveness of the double-layered configuration against projectile impact. In practical applications, a variety of projectiles including heavy fragments generated from improvised explosive devices (IEDs) and light bullet projectiles typical for small arms may be encountered. Since a shield would behave differently under the impact of various projectiles, four types of projectiles of different weight and nose shapes are considered in this paper.

For each projectile–target system, a thorough parametric study should be conducted to determine the ballistic resistance of the shields. A corresponding experimental study could be overly expensive. As an alternative, commercial finite element codes are able to fulfill this task equipped with a suitable fracture model. In this paper, all of perforation cases are simulated using ABAQUS/explicit. Numerical modeling provides an insight into failure mechanisms and the number of necessary tests is reduced. The paper concludes by pointing out the advantage of the double-layered configuration over the monolithic plate.

Section snippets

Problem formulation

The objective of the present paper is to assess the ballistic resistance of an armor plate consisting of two layers of metallic materials with different mechanical characteristics. The performance of such a system will be compared to that of a monolithic plate. For that purpose several impact scenarios are considered by varying the nose shape and mass of the projectile, the impact velocity and the relative strength and ductility of the constituent layers. Fig. 1 shows a double-layered target

Plasticity model

Consider the class of materials obeying the following hardening rule:σ¯=C1f(ε¯pl)=C1[A+Bε¯pln]1+Clnε¯˙plε¯˙01-T-T0Tm-T0m,where σ¯ is the von Mises stress; ε¯pl is the effective plastic strain; A, B, n, C, and m are five material constants and need to be calibrated from tests; ε¯˙pl and ε¯˙0 are the current and reference strain rate; Tm and T0 are the melting and room temperature, respectively. The above equation describes the isotropic hardening, strain rate effects and temperature rise due to

Fracture model

The material failure will be formulated in terms of a macroscopic ductile fracture model recently proposed by Xue [14], Bao and Wierzbicki [15], Bai and Wierzbicki [16]. According to this model, the unknown equivalent strain to fracture at a material point is determined from the equationD(ε¯f)=1,where the damage indicator function D(ε¯f) is defined byD(ε¯f)=0ε¯fdε¯plε¯f*(ξ,η).In the above equation, the denominator ε¯f*(ξ,η) is referred to as a 3-D fracture locus, calibrated from fracture tests

Computational model

The case with the monolithic plate will be considered as a reference. Two materials of different ductility and strength are designed for the two layers of the double-layered shields. In our simulations, the radius of the circular plate is 250 mm and the equivalent total thickness is 12 mm. Clamped boundary condition are assumed at the periphery of the plate. However, for the assumed dimensions of the plate, the local perforation process will not be affected by the far field boundary conditions.

Simulation with varying either strength or ductility

The perforation resistance of the monolithic plate varies with strength and ductility of the material. At first, the fracture strain is maintained to be the same, while the stress–strain curve is scaled with the baseline stress–strain curve. Another limiting case is scaling the fracture strain and maintaining the stress–strain curve the same. Fig. 11, Fig. 12 show, respectively, the time history of the velocity of the projectile for the two cases. A plot of the normalized residual velocities of

Heavy conical-nose projectile

In this section, a conical-nose projectile of the mass M0=200g and the diameter d=24mm is considered as the striker. At first, the monolithic plate of the same material, Weldox 460 E steel or Domex Protect 500 steel, is studied. Fig. 14, Fig. 15 show the final stage of the perforation processes of the monolithic plates of high ductility and low ductility materials, respectively, impacted by the heavy conical-nose projectile at V0=400m/s.

As shown in Fig. 15, shear plugging is the predominant

Two layers with gap

In the present study, the two layers made of the two different metals are assumed to be perfectly bonded. The displacement field across the interface is continuous and all the stress components can be transferred between the two layers. Recent investigations by Teng et al. [17] and Dey et al. [5] reveal that the perforation resistance of a target can be improved by about 7–25% by simply laminating the two layers without any adhesion. The two layers were made of the same metal and there was no

Conclusions and discussions

At low impact velocities, the double-layered shield with the upper layer of high ductility metal and the lower layer of low ductility and high strength metal is always the best configuration for the perforation resistance among the four configurations, while the double-layered shield with the upper layer of low ductility material and the lower layer of high ductility material is the worst one.

Under high velocity impact by the heavy flat-nose projectile, which is the most detrimental projectile,

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