British Terms Used in Relation to Armor, WWII Tactical and Technical Trends, No. 18, February 11, 1943 (Lone Sentry)

ANGLE OF IMPACT

This is the angle between the plate and the line of flight of the projectile on impact.

In the British and American Armies, this angle is measured from a line at normal (perpendicular) to the surface; some foreign countries, including Germany, measure it from the plane of the plate. Thus 30° British or U.S., equals 60° German.

At 10° to 15° angle of impact, the reduction in AP performance of a projectile is generally small compared with normal impact, but beyond about 20° the reduction is increasingly marked, and must be taken into account; 30° angle of impact is used for proof of all AP projectiles in Britain.

GERMAN AP 40 (Pzgr. 40) (Sketch No. 3)

A special type of AP ammunition used with most German tank and antitank guns in addition to the more conventional types of AP projectile.

The AP 40 projectile consists of a mild steel body, a light alloy or plastic ballistic cap, and a cemented tungsten carbide core. The weight of this type of projectile is only about 50 percent to 65 percent that of the normal AP shell. The muzzle velocity is high, but the velocity falls off rapidly with increasing range, so that increased penetration is obtained at short ranges only.

ARMOR-PIERCING CAP (Sketch No. 1)

A hard steel cap fitted to AP projectiles to assist in penetrating face-hardened armor.

A projectile so fitted is known as "APC." All modern German AP projectiles of 50 mm and over have a piercing cap; in calibers of 75 mm and over it is of blunt shape, making a ballistic cap necessary.

BALLISTIC CAP (Sketch No. 2)

A long and pointed cap fitted to a projectile to reduce the air resistance in flight. (Where both AP and ballistic caps are fitted, the projectile is designated APCBC.)

In the case of normal AP projectiles, the presence of a ballistic cap, although in itself slightly impeding penetration, actually increases it at medium and long range due to the reduced deceleration caused by air resistance, and the consequent higher striking velocity.

BRINELL NUMBER see HARDNESS

CARBURIZING (also known as cementing or case hardening)

The process of increasing the carbon content of the surface of steel by heating the metal in contact with carbonaceous material.

This enables the surface to develop a much greater hardness than the interior when the carburized steel is heat-treated. In "gas carburizing," a gas rich in carbon is used instead of a solid material. See also CYANIDING.

CASE HARDENING (see CARBURIZING above)

CEMENTING (see CARBURIZING above)

CYANIDING (see CARBURIZING above)

Another process of carburizing in which the metal is heated in a molten bath of sodium cyanide and other salts.

DISKING (see FLAKING and DISKING)

FACE-HARDENED ARMOR

Armor with a hard face, but tough base.

This armor is usually made by carburizing one surface of the plate so that this surface becomes much harder than the body of the plate when the plate is heat-treated. Face-hardened armor may also be produced by flame-hardening.

FLAKING OR DISKING

Terms used to describe the type of plate failure which is accompanied by an approximately circular piece, of much greater diameter than the projectile, coming off the back of the plate. It occurs when the plate has insufficient shearing strength in a direction parallel to its surface. Flaking or disking is dangerous in single-skin armor.

FLAKE-HARDENING

A face-hardening process which consists in using an oxyacetylene flame to heat the surface layer of metal above the critical temperature, and then quenching rapidly by a spray of water falling behind the flame.

GUERLICH PRINCIPLE (Diagram No. 4)

This principle involves the use of a tapered-bore barrel and a skirted projectile, the skirts of which are squeezed down as the projectile travels through the bore. A high-muzzle velocity is obtained by the use of a light projectile with a large effective base area at the commencement of shot travel. It is a means of obtaining a high armor-piercing performance without necessitating a heavy weapon. Owing to the relatively light weight of the projectile, this performance is obtained at short ranges only. The thickness of armor penetrated is considerable in relation to the weight of the gun. The hole made is, of course, small. Barrel wear is high.

German antitank guns so far known to be working on this principle are the 28-mm tapering to 20-mm (see Tactical and Technical Trends, No. 5, p. 14), and the 42-mm tapering to 28-mm (see Tactical and Technical Trends, No. 7, p. 3). The projectiles for those two guns have a tungsten carbide core.

HARDNESS

The hardness of a material is its resistance to deformation.

Various systems are employed for measuring hardness, including the Brinell, Vickers Diamond, and Rockwell systems. The Brinell number of a given specimen is derived from the diameter of the impression obtained by pressing a steel ball on to the surface of the material.

HOLLOW-CHARGE SHELL (Diagram No. 5)

A type of shell with a shaped cavity in the forward end of the HE filling.

The effect on impact is to concentrate a jet of blast in a forward direction. The underlying principle is to pierce armor by blast perforation, instead of the projectile forcing its way bodily through the armor by its weight and striking velocity. The penetrative power of hollow-charge AP projectiles is therefore independent of the striking velocity. Their use in low-velocity weapons, such as howitzers or infantry guns gives these weapons an improved performance against tanks, within the limits of their accuracy.

[Armor Piercing Projectiles]

HOMOGENEOUS ARMOR

Armor which has approximately the same composition and hardness throughout its thickness.

HOMOGENEOUS HARD ARMOR (HOMO HARD)

A homogeneous armor of a hardness too great to be conveniently machined by ordinary commercial methods; i.e., of a Brinell harness greater than 400, usually 440 to 480.

IMMUNITY THICKNESS

The thickness of plate specified as providing protection against a given attack. When the plate is up to the average in quality, this figure is usually the thickness that will so withstand the specified attack that the bulge caused at the back of the plate is uncracked.

IZOD NUMBER (see TOUGHNESS)

MACHINABLE QUALITY ARMOR (MQ)

A homogeneous armor sufficiently soft to be machined by ordinary commercial methods; i.e., of a Brinell hardness less than 400.

PANZERGRANATE (Pzgr)

The German generic term for all types of AP projectiles, whether shot or shell, capped or uncapped.

PETALING

This is said to occur when metal displaced on perforation forms a ring of petals round the hole in the plate. With thin plates, petals are formed on the back only. With thick plates, they may form on the front as well. Petaling is the most desirable type of back damage on penetrated plate, since with perfect petaling, none of the armor is projected into the vehicle, as is the case when the penetration of the plate is accompanied by flaking or plugging. There are, however, intermediate conditions under which the petals formed on the back during penetration do not remain attached to the plate.

PLUGGING

A plug is said to be formed when the pressure of the head of the projectile causes the separation by pure shear of an approximately cylindrical plug of the plate metal. Next to petaling, it is the least undesirable form of failure, since less metal is projected into the tank than with flaking.

SHELL

A projectile having a cavity which may be filled with HE, smoke, or chemicals, and generally fitted with a fuze.

SHOT

A solid projectile.

SPACED ARMOR

When two or more plates are used with a space between them, the arrangement is known as "spaced armor."

There is little data on the effectiveness of spaced armor. It has been stated that the space should be greater than "the length of the projectile plus one inch." The shot may then be tipped while passing through the outer plate and turned in the space, thus striking the rear plate sideways and being stopped by it. This effect is likely to be more pronounced at oblique attack, for at normal the shot may sometimes keep straight and so penetrate the rear plate. A smaller gap may, however, be effective if the projectile shatters during its passage through the outer plate, as would appear to be the case with projectiles having tungsten carbide cores, or if the outer plate is face-hardened.

TOUGHNESS

The toughness of a material is its capacity to absorb energy before fracturing. It is commonly measured by the Izod Impact Test in which a notched bar test piece of the material is held in a vice and then broken by a heavy pendulum. The Izod value is the energy in foot pounds required to break the test piece and is calculated from the continued movement of the pendulum before commencing to swing back.

Toughness and hardness are largely opposing properties, the relation of which has to be balanced if a satisfactory ballistic performance is to be obtained.