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RESEARCH PAPERS: Terminal Ballistics

The Role of Kelvin–Helmholtz Instabilities on Shaped Charge Jet Interaction With Reactive Armor Plates

[+] Author and Article Information
Andreas Helte

Defence & Security, Systems and Technology Division, FOI, Swedish Defence Research Agency, Grindsjön, SE-147 25 Tumba, Swedenhelte@foi.se

Ewa Lidén

Defence & Security, Systems and Technology Division, FOI, Swedish Defence Research Agency, Grindsjön, SE-147 25 Tumba, Sweden

J. Appl. Mech 77(5), 051805 (Jul 01, 2010) (8 pages) doi:10.1115/1.4001738 History: Received August 12, 2009; Revised November 25, 2009; Posted May 10, 2010; Published July 01, 2010; Online July 01, 2010

Reactive armor panels have been used for many years as very efficient add-on armor against shaped charge warheads. The main features of the defeat mechanisms of the armor are therefore well known. The origin of the irregular disturbances on the shaped charge jet, which leads to the severe fragmentation and scattering of the jet, is however not described in literature. As this scattering of the jet provides the main protection mechanism of the armor, it is of interest to understand the details of the interaction and the origin of the disturbances. Some experimental observations have been made showing that the backward moving plate often displaces the jet relatively smoothly while it is the interaction with the forward moving plate that causes the disturbances that leads to fragmentation and scattering of the jet. In this work, a mechanism for the interaction is proposed based on the theory of Kelvin–Helmholtz instabilities, which explains the origin of the disturbances on the jet due to the interaction with the forward moving plate. Numerical simulations have been performed to show the difference in the mechanisms of backward and forward moving plates when interacting with the jet. The impact angle of the plate seems to be the dominant parameter for the onset of instabilities. A parametric study has also been performed on how different interaction and material parameters influence the development of instabilities of the interface between the jet and the armor plate. The parametric study shows that low-strength jets promote development of instabilities, a tendency that is amplified by frictional forces between the materials. The influence of the plate strength is more complex due to the influence of the structural stability on the contact forces. The effect of friction and melting of the metals in the boundary layer to the development of the instabilities is discussed. A microscopic study of the edge of the penetration channel has been made, which shows that the materials have been melted during the interaction between the plate and the jet.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Kelvin–Helmholtz instabilities observed in (a) cloud formations (http://en.wikipedia.org/wiki/File:Kelvin_Helmholz_wave_clouds.jpg) and (b) in the contact surface between two materials explosively welded (5)

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Figure 2

Shaped charge jet penetrating reactive armor panels: (a) explosive reactive armor and (b) inert reactive armor. The appearance of the jet 110 μs after impact is shown in (c) for explosive reactive armor and in (d) for inert reactive armor.

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Figure 3

A shaped charge jet penetrating single-plate reactive panels: (a) backward moving plate and (b) forward moving plate

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Figure 4

Shaped charge jet penetrating inert reactive armor. The time sequence is composed from three different experiments with the same set-up.

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Figure 5

Microscope pictures of the edge of the penetration channel in the plate in different enlargements (the white line in the lower right corner of the pictures corresponds to 200 μm, 20 μm, and 50 μm). In the least enlarged picture (a), the jagged surface can bee seen. The melt zone can clearly be seen in (b). A disturbance on the surface is shown in (c).

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Figure 6

Numerical simulation of the interaction between a shaped charge jet and inert reactive armor using multimaterial Eulerian approach. The colors in the jet correspond to transverse jet velocity: red color corresponds to more than 50 m/s downward, blue color to more than 50 m/s upward, and green color means low transverse velocity. Jet interaction with forward moving plate starts at 17 μs after impact (a) and with backward moving plate at 39 μs after impact (b).

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Figure 7

Schematic drawing of the interaction geometries in the numerical study. Above, the backward moving plate. Below, the forward moving plate.

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Figure 8

Time sequence from numerical simulations showing the pressure during the interaction between (a) a jet and a forward moving plate and between(b) a jet and a backward moving plate. Red color corresponds to more than 2 GPa and blue color to less than 0. The time interval between the pictures is 0.2 μs.

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Figure 9

Simulated disturbances on the jet after interaction with a forward moving plate using different jet strength without (left) and with (right) friction

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Figure 10

Simulated disturbances on the jet after interaction with a forward moving plate using different plate strength

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Figure 11

The effect of mesh resolution on the predicted development of disturbances on the jet

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Figure 12

Comparison between experimental registered and numerical simulated disturbances on the jet after interaction with a forward moving plate (30 μs after impact and 13 μs after initial contact). The backward moving plate has not yet started to interact with the jet.

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