Journal of KONES Powertrain and Transport, Vol. 21, No. 1 2014

INFLUENCE OF DESTRUCTOR CASE TYPE ON BEHAVIOUR OF FRAGMENTS IN MILITARY VEHICLES

ACTIVE PROTECTION SYSTEM

Jacek Nowak, Robert Panowicz, Marcin Konarzewski

Military University of Technology Department of Mechanics and Applied Computer Science

Kaliskiego Street 2, 00-908 Warsaw, Poland tel.: +48 22 683-98-49, fax. +48 22 683-93-55

e-mail: jnowak@wat.edu.pl, rpanowicz@wat.edu.pl, mkonarzewski@wat.edu.pl

Abstract

Military vehicles active protection systems against cumulative missiles are designed to destroy the attacking missile before it hits the vehicle armour. The article presents the results of numerical studies of one of the elements of active protection system, which is the fragmentation destructor. A typical directed fragmentation warhead consists of a few parts: metallic or composite case, explosive material and fragmentation elements in the form of spheres, cylinders.

The authors of this study evaluated the influence of the destructor case type – in particular material – on the effectiveness of the destructor. The effectiveness was evaluated on the basis of the maximum speed of balls. Evaluation was performed for the selected balls from each layer. Numerical calculations were performed for two materials of the case: steel and aluminum. It was assumed in simulation that the detonating material is the plastic explosive C4.

The numerical analyses were based on the finite element method with the explicit time integration method implemented in the Ls-Dyna program. The interaction of solid and gaseous medium has been modelled using ALE coupling. Mechanical properties of the case were described using a simplified Johnson-Cook type material. The detonation process was described using programmed burn model approximations, and the behaviour of detonation products was described with the JWL (John, Wilkins, Lee) equation.

Keywords: finite elements method, simulation, military vehicles, directed fragmentation warheads 1. Introduction

During modern armed conflicts, asymmetric conflicts and terrorist activities, military vehicles are vulnerable to destruction or damage by four basic weapons: kinetic projectiles, mines and IEDs (mechanisms) explosives, missiles with cumulative warheads, smart missiles fired from mortars or dropped from the trays.

Different types of kinetic projectiles belong to the first group of ammunition threatening not only the light armoured vehicles, but also tanks. The real threat, against which there is no protection, also for the last of these vehicles are sub-calibre bullets fired from tank guns (calibre 100 mm).

These bullets manufactured on the basis of heavy metals, mainly tungsten and depleted uranium, have a mass of about 9 kg and can gain speeds of the order of 1575 m/s Their kinetic energy is equal to about 5.7 MJ which allows penetration of the front armours of most of the tanks.

A typical armour (so-called passive protection system) is not able to weaken the action of the projectile. Therefore, more and more popular protection systems are the active systems.

The protection defence should consist of three basic systems: − the detection system, − the decision-making system, − the counter measure system.

Detection system may consist of one or more sensors. The most common detection systems are solutions based on radar or optical technology. The decision-making system assesses the risk of

ISSN: 1231-4005 e-ISSN: 2354-0133 ICID: 1134093 DOI: 10.5604/12314005.1134093

J. Nowak, R. Panowicz, M. Konarzewski

the destruction of the vehicle on the basis of the information sent from the detection system. The counter measure system neutralizes the threat of destruction without contact with the vehicle. It is possible through interacting with a cumulative pressure wave on the missile head. Operation of active protection system, exemplified with Caslon system, is shown in Fig. 1.

Fig. 1. Operation of active protection system, exemplified with Zaslon system [5]

Destructors using fragments to neutralize the threat (Fig. 2) give better opportunities. Their effectiveness is higher and they are used to protect a greater area and, therefore, their applications do not require highly accurate detection systems. There are many solutions of fragmentation destructors. The simplest destructors are launched from the classic barrels. More advanced destructors contain explosives fragments to allowing gaining much higher speeds.

The task of the destructor is damage to the explosive shaped charge or creates a short circuit in the detonator. For fragments of destructor, there should be estimated their density which reduces the effective zone of destruction. It is usually assumed that the missile should hit 1-2 fragments characterized with sufficiently high energy, and thus providing appropriate effectiveness.

Fig. 2. Example of directed fragmentation warheads

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Influence of Destructor Case Type on Behaviour of Fragments in Military Vehicles Active Protection System

2. Description of the computational model

Figure 3 presents a numerical model of the tested fragmentation destructor (due to the symmetry of the model, a half of the destructor was adopted). Directed fragmentation warhead consists of a few parts: steel or aluminium cover, five layers of fragmentation elements (steel balls) and the explosive material. Fragmentation elements were embedded in the resin layer. As the detonating material, explosive C4 was adopted. Numerical analyses were performed using the LS-Dyna system. The interaction of solid material and gas was modelled using a coupling type of ALE. Tab. 1 shows the parameters for description of the casing material of the models used in materials for the Johnson-Cook. Material parameters for the cases are presented in Tab. 1.

Fig. 3. Numerical model of directed fragmentation warhead: a) side view – 5 layers of elements of fragmentation, b) front view – a half of the model

Tab. 1. Material parameters for cases

Parameter unit steel aluminumkg/mm3 7.89e-6 2.8e-6

E GPa 210 69v - 0.3 0.33A GPa 0.365 0.369B GPa 0.51 0.684N - 0.9 0.73C - 0.0936 0.0083

f - 0.7 0.7

Mechanical properties of the fragmentation elements (steel balls) were described using a bilinear model of material. Numerical calculations were based on criteria of plastic deformation for the balls of the level of 70%.

The detonation process was described using programmed burn model approximations [4, 6], and the behaviour of detonation products was described with the JWL (John, Wilkins, Lee) equation:

1 2

1 2

1 1RV R V

p A B ERV R Vω ω ωρ

− −

= − + − + (1)

where: V = 0 / ,

0 – initial density, – density of detonation products,

A, B, R1, R2, – constant .

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J. Nowak, R. Panowicz, M. Konarzewski

The values parameter of the JWL equation are presented in Tab. 2.

Tab. 2. The values constant of the JWL equation for C4

Parameter unit C4 kg/mm3 1.6e-6

D mm/ms 8000 PCJ GPa 28 A GPa 609 B GPa 12.95 R1 - 4.5 R2 - 1.4

- 0.25 3. Numerical analyses

The behaviour of the destructor of steel and aluminum case is shown in Fig. 4. The comparison was came out for time t = 0.1s from the time of the explosive detonation. There is a clear difference in the way of collapse of aluminum and steel case. To investigate the effect of this phenomenon on the scatter velocity of balls, there were compared the velocities of selected balls, located closest to the centre of the destructor. For comparison there were selected balls located in successive layers, the ball from the layer nearest to the explosive was denoted with A, and the ball from the surface layer of the destructor was denoted with E (Fig. 5).

Fig. 4. Behaviour destructor of steel case (a) and aluminum case (b) after the time t = 0.1 seconds, counting from

the time of explosive detonation

Fig. 5. Designations balls adopted in Tab. 2

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Influence of Destructor Case Type on Behaviour of Fragments in Military Vehicles Active Protection System

Table 2 lists the maximum values of the resultant velocity of the individual layers of balls for the test cases. In all cases, higher speeds are gained by balls of the destructor with steel casing.

Tab. 2. The results of numerical analysis

Fragmentation element Aluminum case Max. velocity [m/s]

Steel case Max. velocity [m/s]

A 460 743

B 806 972 C 840 963 D 697 779 E 585 595

4. Summary

The authors of this study evaluated the effect of the destructor case type, in particular material, on

the destructor effectiveness. The effectiveness was evaluated on the basis of the maximum speed of the fragments. On the basis of numerical analyses can, it can be stated a clear difference is observed between the results of the speed of balls for different destructor cases. However, quantitative evaluation of this phenomenon is extremely difficult due to the behaviour of the fragments (disintegration of balls in many fragments, the rotation of individual fragments of the destructor). References [1] Bezel, J. B., Stewart, D. S., Jackson, T. L., Program burn algorithms based on detonation

shock dynamics: discrete approximations of detonation flows with discontinuous front models, J. Comput. Phys. 174, pp. 870-902, San Diego 2001.

[2] Hallquist, J. O., Ls-Dyna Theory Manual, Livermore Software Technology Corporation, Livermore 2005.

[3] Jach, K., et al., Komputerowe modelowanie dynamicznych oddzia ywa metod punktów swobodnych, PWN, Warsaw 2001.

[4] Kapila, A. K., Bezel, J. B., Stewart, D. S., On the structure and accuracy of programmed burn, Combustion Theory and Modelling, Vol. 10, Is. 2, 2006.

[5] Army Guide. Zaslon [online], http://www.army-guide.com/eng/product3706.html. [6] Masahiko, O., Yamato, M., Kenji, M., Yukio, K., Shigeru, I., A Study on Shock Wahe

Propagation Process in the Smooth Blasting Technique, 8th International LS-DYNA Conference, Detroit 2004.

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