Wednesday, 17 June 2015

Reactive Armour and Edge Effect

Penetration mechanics of even simple steel are complicated enough, but can be somewhat simplified for most uses, such as penetration tables and video game calculations. However, when reactive armour is thrown into the mix, how does one evaluate its increase in protection? How does the edge effect, present in only a small percentage of hits on conventional armour, affect it? An article in the Armoured Journal (Bronetankoviy Vestnik) by A.I. Anisko, S.V. Bodrov, D.A. Rototayev, and A.A. Scinarenko attempts to answer this question.

"Reactive armour is used to protect tanks from HEAT shells. Usually, these are removable containers with thin steel plates, sandwiching an explosive element. In order to place these elements correctly on the hull, one must first consider the protection of the reactive armour when used with the main armour. One methodology counts it as an equivalent thickness added to the main armour in certain conditions of HEAT shells striking the center of the container. Consider the dependence of the protection of the reactive armour and the time of reaction of the HEAT jet and protective plates. When the point of impact is moved from the center to the edge of the plate, the penetrative power of the HEAT shell decreases by 30-60%. However, the contribution of the edge effect to the protection of reactive armour has not been explored. 

A contained with two reactive armour elements were tested. The test was performed by detonating the warhead of a 93 mm AT grenade at an angle of 60 degrees. The warhead was placed to imitate a hit to various parts of the container (fig. 1). The peripheral points were 10-15 mm away from the side of the element."

Fig. 1. Locations of hits to the container.

The following is the data obtained when detonating the warhead, averaged over 5 trials for each point. Remaining penetration is defined as penetration through a plate of medium hardness steel after passing through the reactive armour. Q is defined as the contribution of the reactive armour to protection, obtained by subtracting remaining penetration from penetration through the same steel plate when no reactive armour is present. This table contains both the predicted and experimentally obtained Q.

Point Remaining penetration Q (calculated) Q (experimental)
A 211 266 258
B 37 452 462
C 274 214 225
D 292 179 207
E 86 420 413
F 376 126 123

When hit in the center, the reactive armour has the greatest effectiveness, adding over 450 mm to the thickness of the armour. When hit in the sides, the effectiveness is about half as much, even less when the HEAT jet hits the corners of the plate. This is caused by the various sizes of the plate fragments that interact with the HEAT jet, as well as the number of fragments it is likely to pass through. 

The scientific model is pretty good, as you can see from the small differences between the predicted and observed values of Q. The model predicts that about 25% of the surface area of the reactive armour element will give more than 400 mm of additional protection, about 50% contributes more than 350 mm, and about 75% contributes more than 300 mm to the tank's armour.

The equation that governs this distribution is Q=123.85+3.4x1+3.77x2-0.027x12-0.016x22 and looks like this when plotted.

Fig. 2. A line equal to the protection contribution Q in a coordinate plane defined by x1 and x2.

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