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    Semi-Infinite Targets


There is a perfect definition in Wikipedia:

Penetration into a semi-infinite or massive target is penetration (in the strict sense of the word) of targets so thick that the level of penetration is not affected by the target's thickness. There is a transition region between semi-infinite penetration and perforation, in which the target is not perforated but the projectile, as it nears the back face of the target, meets reduced resistance and is capable of penetrating a greater distance than it would in a semi-infinite target. This effect is variously named the back or rear surface, plate, or face effect and is also present when perforation occurs.

https://en.wikipedia.org/wiki/Penetration_(weaponry)


The mentioned transition from penetration to perforation shows that there are high demands on the target dimensions. For testing tank ammunition, semi-infinite targets can hardly be realized: The dimensions of target blocks with the required high hardness exceed the technical feasibilities. Therefore penetration tests are carried out with a stack of armor plates.

The difference between true semi-infinite targets and plate arrays will be shown with the help of test results. An experimental rod was used in a 140 mm smoothbore gun. With 7 tests, perforation limits were determined in the velocity range of 1.5 to 1.9 km/s at NATO angles of 26 to 62°. With 3 tests, penetration depths were determined in a plate array, consisting of four 152 mm thick plates.



The oblique targets and the plate array had similar Brinell Hardness. Therefore the two target types can be compared.

The graph above leads to the following conclusions:

In the plate array, the depth of penetration is massively overestimated compared to the real semi-infinite target. The closer the rear side of the last plate has been reached, the more the overestimation increases. The penetration in function of the impact velocity is therefore linear and doesn't follow the well-known s-shaped penetration curve. The typical linear curve is often found in laboratory tests with single plates of insufficient thickness. The related papers incorrectly refer to semi-infinite penetration.

The penetration depth is also dependent on the number of plates involved: penetrated as well as intact plates. If two 304mm plates were used instead of the four 152mm plates, the depths of penetration would be lower. Consequently, plate array tests with different conditions are not comparable.

Perforation limits which were determined with different boundary conditions can, however, be compared with each other. Different obliquities, impact velocities, plate hardness e.t.c. can be normalized to standard values with the help of the perforation equation. In the simple case of different NATO angles and target hardness, the test results can be normalized using the following equation:



Favorable normal values are:

60° obliquity and BHN 237 corresponding to an ultimate tensile strength of 800 MPa (mean value of thick RHA plates)





 
   
   © September 2019 by Willi Odermatt