CHAPTER-4-B

GENE SLOVER'S
US NAVY PAGES

NAVAL ORDNANCE AND GUNNERY, VOLUME 1
CHAPTER 4
ARMOR AND PENETRATION

HOME INDEX
CHAPTER 4 ARMOR AND PENETRATION
A. TYPES OF ARMOR
B. PENETRATION

B. Penetration

4B1 Introduction

The same advances in metallurgy which contributed to the development of armor plate have proved to be equally useful in the manufacture of guns and of projectiles, particularly those designed to penetrate armor. The increased toughness effected by alloying steel with chromium and nickel, as well as improved methods for producing and forging large ingots, have resulted in better guns and in a race for supremacy between the designers of protective armor and the designers of projectiles to defeat the armor.

Armor plate is carburized to extreme surface hardness, whereas guns and projectiles, which must combine toughness with elasticity and heat resistance, are not. Steel used in the manufacture of guns usually contains molybdenum, an alloying element which imparts strength at high temperatures.

4B2. Projectile steel

Steel used in projectiles designed to penetrate armor is of the same general formula as Class A armor but with a higher carbon content. After rough forging, the projectiles are annealed, then rough-finished and again heat-treated. Decremental hardening is achieved by dipping the noses in melted lead and cooling them with water, this process being repeated twice. The result is a very hard nose and a tough, ductible body, this last characteristic being necessary to keep the projectile from being broken up by the violent transverse stress caused by crashing through armor plate at an angle.

4B3. Armor-piercing projectiles

This term is used to designate the projectile designed to be used against armor plate of about one-caliber thickness. It must penetrate this plate with its bursting-charge cavity intact so that, when detonated by its delay fuze, it may produce high-velocity fragments within the ship.

For stabilization in flight, the center of gravity of a projectile must be just abaft the midpoint of its axis; but to effect proper penetration, an armor-piercing projectile should have the great mass of its weight immediately behind its blunt nose. Figure 4B1 shows how these two conflicting characteristics are reconciled by fitting a light, tapered false ogive or wind-shield over the heavy front end.

Within the false ogive, soldered and peened to the nose of the projectile proper, is an armor-piercing cap. Made of the same steel as the projectile, the cap is hardened, but by a single immersion in molten lead. This cap serves several purposes: it is so shaped that it increases the biting angle; that is, the angle at which the projectile will penetrate instead of ricocheting; it spreads the shock of impact over the periphery of the nose instead of allowing the initial contact to batter the nose tip; and it pre-stresses the armor plate upon impact before the cap shatters away and allows the projectile to penetrate the weakened plate.

Projectiles of this type are not efficient against lightly armored ships, because of the relatively small bursting charge that they are able to carry. Because of the delay feature incorporated in their fuzes, they have been known to pass entirely through unarmored craft without bursting.

4B4. Common projectiles

The common projectile is for use against lightly armored ships, being designed to pierce plate of 1/3 to 1/2 caliber thickness. It resembles the armor-piercing projectile except that it has thinner walls and can, therefore, carry a larger bursting charge. It has, instead of an armor-piercing cap, a hood, which provides a means of attaching the windshield without weakening the projectile body by cutting threads. The hood, like the cap, is soldered and peened to the projectile nose.
See figure 4B2.

4B5. Ballistic tests

Tests of both armor plate and projectiles consist of firing the latter against the former at measured striking velocities and at specified angles of obliquity. The projectile to be tested must be tried against armor plate of known resistance; or, if the armor plate is to be tested, the characteristics of the projectile must be known. The penetration test in each case is a measure of the striking velocity at which the element being tested will defeat the standard element. In testing, armor plate must withstand a maximum velocity; projectiles must penetrate at a minimum velocity.

Certain terms must be defined to provide a basic picture of test procedure:

Ballistic limit or limit velocity is the striking velocity which will permit the projectile to pass completely through the plate and emerge from the back with zero residual velocity. Ballistic limit measures the true penetration resistance of the plate.

Residual velocity is the velocity of the center of gravity of the projectile at the instant the projectile emerges completely from the back of the plate.

Complete penetration is obtained when the projectile passes through the plate and emerges. Incomplete penetration describes any result less than complete penetration, and partial penetration describes the case of a projectile that breaks up, only a part of it passing through the plate.

The ballistic limit of a plate may be determined by any 1 of or a combination of the 3 following methods:

The bracket method consists of firing at varying velocities until an incomplete and a complete penetration are obtained within the desired small velocity difference or bracket. The ballistic limit is taken as the mean of the two velocities forming the bracket.

The residual velocity measurement method consists of firing one round at a velocity a little above the estimated ballistic limit and measuring the residual velocity, the ballistic limit being then computed by reference to established relationships between the residual velocity and the limit velocity.

The penetration method works in the opposite direction. A single round is fired at a velocity below the estimated ballistic limit and the depth of penetration measured, from which data the true ballistic limit is computed.

Armor plate is also subjected to shock tests, which measure the resistance of the plate to shattering or breaking up from the shock of projectile impact.