Explosive reactive armour (ERA) is used to protect the armoured vehicles against chemical energy warheads and long-rod penetrators. ERA generally consists of an ERA sandwich and an outer steel cover to protect it from unintended initiation. Explosively formed projectiles (EFPs) are used as antiarmour warheads, and their use in top attack antiarmour weapons is increasing. Interaction of EFP with the ERA sandwich alone has been studied in the past. This paper studies the effect of the cover plate and its thickness on the interaction of the EFP and the ERA sandwich. Numerical simulations and experiments have been used to study the interaction. It has been found that the penetration in the target is reduced as the thickness of the cover plate increases. Moreover, the decrease in penetration is directly proportional to the thickness of the cover plate. Also it has been found that the residual EFP after penetrating the cover plate has sufficient energy to initiate the ERA sandwich. The results of the numerical simulation and the experiments are in close agreement.
Explosive reactive armour (ERA) is widely used on modern armoured vehicles to protect them against attack from chemical energy warheads. Originally the ERA was developed to defeat shaped charge warheads [
In most practical scenarios, the ERA sandwich is protected with cover plates or is enclosed in protective metal boxes to protect them from elements, and from unintended initiation due to small calibre bullets, shrapnel, etc., this protection can further deteriorate the effectiveness of the EFP warhead.
This paper undertakes the study of the effect of the cover plate of the ERA sandwich on the penetration efficiency of the EFP. The effect of the cover plate has been studied by using both the numerical simulations using LS-DYNA and experiments. The results show that the penetration of the EFP is reduced in proportion to the thickness of the cover plate. The simulations and the experiments have good agreement, and the simulations can successfully be used to simulate the interaction under various conditions.
The layout used in this study is shown in Figure
Configuration of EFP warhead, cover plate, ERA, and target used for simulations.
The EFP used for the simulations is a 100 mm charge diameter EFP which has a charge height of 100 mm, liner is of hemispherical construction with the radius of curvature of 85 mm and liner thickness 4 mm. The EFP has 5 mm thick steel casing and is point initiated at the top. Similar EFP has also been used in the study previously carried out by Rasheed et al. [
The cover plate is an A36 steel plate for which the thickness was varied during the simulations and the experiments to analyse their effect.
The ERA sandwich consists of two A36 plates with C4 explosive sandwiched in between. The ERA plates are 150 × 150 × 5 mm in dimensions, and the C4 explosive is 3 mm thick. The configuration is referred as 5/3/5 ERA in this study. The inclination angle has been maintained at 30° throughout this study.
The target plate used is a 125 mm thick A36 steel plate.
The simulation consists of ALE formulation to model the explosive, air, and liner of the EFP, and this enables capturing the large deformations and material flow experienced during the EFP formation. The casing of the EFP has been modelled using the Lagrange formulation to cater for the confinement and breakup effect of the casing. Once the EFP is formed, it flies through the air towards the ERA and the target plate. The covering plate, ERA sandwich, and the target, all have been modelled using Lagrange formulation. The Euler–Lagrange interaction has been utilized to simulate the penetration and interaction with the Lagrange components. Lagrange formulation has been used to model the penetration problem, as in this formulation, failure and damage can be modelled without complicating the domain.
The 3D domain established for the simulation consists of a half symmetrical model as shown in Figure
Semisymmetric 3D domain including air, EFP, cover plate, ERA, and target.
In order to study the effect of the cover plate thickness on the residual penetration in the target plate, the model has been kept constant except varying the thickness of the cover plate. Thickness has been varied in successive simulations from no cover plate to 8 mm with 2 mm increment. The standoff between the target and the cover plate has been kept at 425 mm, distance between the cover plate and the ERA is 60 mm, and the distance between the ERA sandwich and the target is 100 mm.
The simulations used the following material and equation of state models to model the complete simulation [
The Steinberg model has been used for the EFP liner. The Steinberg–Guinan [
The Gruneisen equation of state has been used for all metallic materials in the simulation model. It defines the pressure for compressed material as
High explosive burn model and JWL equation of state [
Since the initiation of explosive in the ERA is not ensured, rather it has to be determined based on the shock pressures developed during the interaction of the EFP with the ERA sandwich, the ignition and growth model [ Ignition term in which a small amount of explosive reacts soon after the shock wave compresses it. Slow growth of reaction as this initial reaction spreads. Rapid completion of reaction at high pressure and temperature. The form of the reaction rate equation is
The Johnson–Cook material model has been used for the EFP casing, cover plate, ERA plates, and target in combination with the Mie–Gruneisen equation of state. The Johnson–Cook model expresses the flow stress as
Table
Material models and their properties used.
Material | Material model & equation of state property input data (units: cm, g, | ||||||||
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Composition A3 |
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RO | D | PCJ | |||||||
1.6500 | 0.8300 | 0.3000 | |||||||
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A | B | R1 | R2 | OMEG | E0 | VO | |||
6.1130 | 0.1065 | 4.4000 | 1.2000 | 0.3200 | 0.0890 | 1.000 | |||
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Air |
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RO | PC | MU | |||||||
1.29E-3 | 0.000 | 0.000 | |||||||
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C0 | C1 | C2 | C3 | C4 | C5 | C6 | E0 | V0 | |
−1.00E6 | 0.000 | 0.000 | 0.000 | 0.400 | 0.400 | 0.000 | 2.50E-6 | 1.000 | |
|
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Copper |
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RO | G0 | SIGO | BETA | N | GAMA | SIGM | RO | B | |
8.93 | 0.477 | 0.120E-2 | 36.0 | 0.450 | 0.00 | 0.640E-2 | 8.93 | 2.83 | |
BP | H | F | A | TMO | GAMO | SA | PC | SPALL | |
2.83 | 0.377E-3 | 0.10E-2 | 63.5 | 0.179E4 | 2.02 | 1.50 | −9.00 | 3.00 | |
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C | S1 | S2 | S3 | GAMA | A | E0 | V0 | ||
0.394 | 1.49 | 0.00 | 0.00 | 2.02 | 0.470 | 0.00 | 1.000 | ||
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4340 (EFP casing) |
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RO | G | A | B | N | C | M | TM | TR | |
7.830 | 0.770 | 0.00792 | 0.00510 | 0.26 | 0.014 | 1.03 | 1793.00 | 300 | |
EPSO | CP | D1 | D2 | D3 | D4 | D5 | |||
1E5 | 1.69 | 0.05 | 3.440 | −2.12 | 0.002 | 0.61 | |||
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C | S1 | S2 | S3 | GAMA | A | E0 | V0 | ||
0.3574 | 1.92 | 0.00 | 0.00 | 1.69 | 0.43 | 0.00 | 1.000 | ||
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Steel A36 (cover plate, ERA plates & target) |
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RO | G | A | B | N | C | M | TM | TR | |
7.890 | 0.750 | 0.00286 | 0.0050 | 0.2282 | 0.022 | 0.917 | 1811.00 | 300 | |
EPSO | CP | D1 | D2 | D3 | D4 | D5 | |||
1E5 | 1.69 | 0.4025 | 1.107 | −1.899 | 0.009607 | 0.30 | |||
|
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C | S1 | S2 | S3 | GAMA | A | E0 | V0 | ||
0.4569 | 1.49 | 0.00 | 0.00 | 2.17 | 0.43 | 0.00 | 1.000 | ||
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C-4 |
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RO | G | SIGY | EH | PC | FS | CHARL | |||
1.601 | 0.0440 | 2.00-3 | 0.00 | 0.00 | 2.00 | ||||
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A | B | XP1 | XP2 | FRER | G | R1 | R2 | R3 | |
6.0977 | 0.1295 | 4.50 | 1.40 | 0.667 | 0.25e-5 | 778.1 | −0.05031 | 2.223e-5 | |
R5 | R6 | FMXIG | FREQ | GROW | EM | AR1 | ES1 | CVP | |
11.3 | 1.13 | 0.3 | 1000 | 120 | 2 | 0.333 | 0.667 | 1e-5 | |
CVR | EETAL | CCRIT | ENQ | TMP0 | GROW2 | EN | FMXGR | FMNGR | |
2.78e-5 | 4.0 | 0.0 | 0.104 | 298 | 1000 | 2.0 | 0.5 | 0.02 |
The experiments were carried out to verify the simulation results. The experimental setup is shown in Figure
Experimental setup showing the EFP warhead on its stand with the ERA and the cover plate.
Figure
(a) EFP components and (b) the ERA plates used in the experiments.
For each experiment, the thickness of the cover plate was varied between 2 to 8 mm and the remaining setup was kept constant. The penetration depth was measured for each test and has been compared with the simulation results.
As described earlier, the simulations have been carried out to study the various aspects of formation and interaction of the EFP with the cover plate, ERA, and residual penetration. This was arranged in a manner to study the contribution of the cover plate in penetration reduction of EFP. Figures
Interaction of EFP with ERA having 2 mm thick cover plate at 0, 190, 230, 281, 316, and 444
Interaction of EFP with ERA having 4 mm thick cover plate at 0, 226, 320, and 450
Interaction of EFP with ERA having 6 mm thick cover plate at 0, 225, 321, and 405
Interaction of EFP with ERA having 8 mm thick cover plate at 0, 248, 300, and 481
From the simulation, it can be concluded that the ERA cover plate does in fact contributes towards the reduction in length of the EFP and it also deteriorates the nose shape of the EFP as well. However the residual EFP has enough energy to initiate the explosive in the ERA. This interaction with the ERA sandwich further reduces the length of the EFP. The residual length of the EFP after interaction with the cover plate and the ERA is shown in Figure
Residual length of EFP after interaction with cover plate and ERA.
The experiments were conducted as described above, and the penetration in the target plate was measured for each experiment. The results show that the penetration depth reduces with the thickness of the cover plate. The resulting penetration holes are shown in Figure
Penetration in steel target (a) without ERA, (b) without cover plate, (c) 2 mm, (d) 4 mm, (e) 6 mm, and (f) 8 mm cover plate.
The experiments show a decreasing penetration in the target plate as the thickness of the cover plate increases.
The first test was carried out without any ERA and cover plate to establish a base line as shown in Figure
Figure
As described earlier experiments were carried out to validate the simulation results. The simulations and the experiments both use the same configuration with 100 mm charge diameter point initiated EFP, steel cover plate, 5/3/5 ERA at 30° angle from the target surface, and the steel target. Table
Comparison of EFP residual penetration after cover and ERA interaction.
Test | ERA cover thickness (mm) | Simulated results | Experimental penetration (mm) | ||
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EFP residual length after cover plate (mm) | EFP residual length after ERA (mm) | Simulated penetration (mm) | |||
1 | No cover plate | 92 | 62 | 87 | 95 |
2 | 2 | 86 | 52 | 74 | 77 |
3 | 4 | 81 | 44 | 65 | 69 |
4 | 6 | 78 | 41 | 61 | 67 |
5 | 8 | 73 | 39 | 56 | 57 |
Experimental and simulated residual penetration in target.
As the simulation and experimental study match reasonably well, the study was further extended to analyse the effect of the inclination angle of the ERA plate on the penetration of the EFP, with varying cover plate thickness.
For this set of simulations, the setup was kept the same as in the previous simulations, and only the inclination angle of the ERA sandwich was changed from 30° to 45°. This has resulted in further reduction in penetration depth, as the increase in inclination angle reduces the residual length of the EFP resulting in reduction in penetration depth. Figures
Simulation of EFP against 4 mm cover plate, ERA at 45° at 0, 226, 320, and 450
Simulation of EFP against 8 mm cover plate, ERA at 45° at 0, 225, 329, and 450
The results of simulations with 30° inclined ERA and 45° inclined ERA sandwich with 4 and 8 mm cover plates are given in Table
Comparison of EFP residual penetration after cover and ERA interaction.
ERA cover thickness (mm) | ERA inclined at 30° | ERA inclined at 45° | ||||
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EFP residual length after cover plate (mm) | EFP residual length after ERA (mm) | Simulated penetration (mm) | EFP residual length after cover plate (mm) | EFP residual length after ERA (mm) | Simulated penetration (mm) | |
4 | 81 | 44 | 65 | 81 | 36 | 45 |
8 | 73 | 39 | 56 | 73 | 30 | 37 |
In this paper, the effect of cover plate thickness was studied on the interaction of EFP and ERA using experiments. A numerical model of the EFP warhead, cover plate, ERA sandwich, and target has been established to simulate the experiments conducted and was compared with the experimental results.
Since the simulations were able to simulate the experiments within reasonable accuracy, the simulations have been extended for ERA sandwich inclined at 45° using the same simulation setup, and the results show that the penetration is further reduced as the inclination angle of the ERA sandwich is increased.
Following conclusions can be drawn from the study presented in this paper: The cover plate contributes towards the reduction in the penetration of the EFP as it reduces the residual length of the EFP and also deforms the nose shape of the EFP. The reduction in length of the EFP and nose deformation caused by the cover plate does not affect the initiation of the ERA in the configurations tested. Despite the reduction in residual length of the EFP, it still possesses sufficient energy to initiate the explosive in the ERA sandwich. The reduction in penetration is directly proportional to the thickness of the cover plate, which is mainly caused due to reduction in overall length of the EFP after interaction with the cover plate. Also the study shows that the penetration is inversely proportional to the inclination angle of the ERA sandwich. Increasing the inclination angle decreases the penetration in the target. The formation, interaction, penetration, and initiation of the EFP with the plate, ERA, and target can be simulated using numerical simulation within reasonable accuracy. The simulations have been able to predict the penetration to within 10% of the experimental results.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.