Logo from Scott Cunningham.
Logo from Scott Cunningham.
his page defines some terms that are used throughout the article. This article assumes that you have a good general knowledge of the different types of armour and ammunition. If you are new to this field, you may find it helpful to first read the following two articles:
Types of Armour. The manufacture of various types of armour, the different methods used to assemble armoured vehicles, a discussion of their relative merits, and a description of flaws in armour.
Types of Ammunition. All modern anti-tank projectiles have their roots in World War II. This article traces their development from before the tank up until the present day, with descriptions of their design, advantages and disadvantages.
Perforation can be defined in many ways, according to the stage at which defeat of the plate is considered to have occurred. Thus the ballistic limit of a plate is that velocity above which a given projectile will produce a cracked bulge and below which it will produce an uncracked bulge. The critical velocity corresponds to perforation with no residual velocity after the projectile has perforated the plate, i.e., the minimum velocity at which the projectile passes through the plate.1
The cracked bulge 14 is just barely showing, and so is at or very close to the ballistic limit. Higher velocity has produced the cracked bulge 25. The bottom middle penetration would correspond to the critical velocity if the projectile had passed through with no residual velocity — it hasn’t quite made it, but is close. The other two penetrations are beyond the critical velocity as the projectile passed through with greater than zero residual velocity. From the private collection of Chris Shillito.
The Thickness to Diameter (T/D) ratio refers to the thickness of the armour plate divided by the diameter of the projectile, or the diameter of the core in the case of a sub-calibre projectile. This is the crucial ratio in armour penetration for the types of projectiles and armour plate used during World War II. The armour plate thickness is determined perpendicular to the face of the armour, regardless of obliquity. For example, a 45mm thick armour plate hit by a 90mm projectile will have a T/D ratio of 0.5 whether the impact is at 70° to the projectile’s flight path or at 0°.2
The terms overmatch and undermatch refer to the T/D ratio. For example, a projectile is said to overmatch the armour plate if T/D<1 (i.e., the diameter of the projectile exceeds the thickness of the armour plate), and undermatch if T/D>1. Generally when a projectile overmatches a plate the projectile benefits, and vice versa.2
Normal is a mathematical and scientific term which means at right angles to (90°); it does not necessarily mean average or everyday. So “normal armour” means armour at right angles to an incoming projectile, i.e., vertical armour, rather than “ordinary armour”.
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