Anti-tank, anti-armor, and armor-piercing ammunition needs to have a specification describing its penetration. Now, any scientific test would be buried in disclaimers and details. What muzzle velocity, what distance, what angle, what atmospheric conditions. But there are certain norms. It’s customary to convert ambient temperature and pressure during the test to an international standard atmosphere, 59ºF and 29.95 inches of mercury. It’s customary to convert slanted armor to its thickness equivalent along the axis of the shot. And it’s customary to describe penetration as distance, millimeters or inches, in a specific medium, RHA.
RHA is Rolled Homogeneous Armor and it’s the most common of three types of steel armor that was commonly used in World War II. The others were Cast Homogeneous Armor and Face-Hardened Armor. In general RHA was the gold standard at the time, with CHA and FHA used for specific purposes. There are some terminological differences, of course: the British called RHA machineable armour, because it could be practically cut with machine tools; FHA was very difficult to cut on its armored face, due to heat-treating giving it a very hard, but brittle if overstressed, surface. RHA, conversely, is strong but ductile, which enables it to shuck off more and harder hits. The British breakout of FHA, which Americans call face-hardened armor, is flame-hardened armour.
The US transitioned to mostly RHA early, as did the USSR (all T-34 hulls were entirely RHA, and both RHA weldments and CHA castings were used for turrets). British and German tank production started the war using face-hardened armor, and changed midwar. All armies used cast homogeneous armor for some purposes. For example, the Germans used it in the commander’s cupola and in the mantlet or gun shield of all Panzer V Panther tanks. The US made Shermans with cast turrets and with both cast hulls and welded RHA hulls. The initial Panther model, Ausführung D, was made with face-hardened armor for the hull and turret (apart from the two cast parts mentioned above). In July 1943, they changed to RHA for the glacis (the upper front plate), and a year later they began using RHA on the sides.
As a rule of thumb, RHA is the best of these steel armors, with the best protection from penetrating and HE attacks, but it has some deficiencies. It is hard to form in anything but flat plates. CHA can be cast in almost any shape.
Heat treating is used to bring RHA to a specific hardness. The hardness of steel armor is measured on the Brinell hardness scale. As a general rule, the thicker the armor the lower the Brinell scale value, and therefore hardness of the metal, will be. The FHA plates of WWI armored vehicles, which were 1/8 to 1/4 inch thick, had a Brinell hardness of 420-650. The RHA for the WWII generation vehicles ranged from 220-390 or so. For example, these are the German specified values (for both RHA and CHA):
|Thickness Range (mm)||Brinell Hardness|
The reason for this decline in hardness with increase in thickness was the state of the production art, and it was fairly universal across the belligerents’ RHA armor. The Russians’ armor was the hardest by Brinell measurement.
FHA was hardened to a higher level (Brinell in the 500 range), but only a few mm deep. The idea was to have more resistance to penetration on the surface, but more ductility in the rest of the armor to prevent brittleness and fractures, or spalling of chunks off the inside of the armor. Spalling was the kill mechanism of the British HESH (High Explosive Squash Head) round of late-war was designed to produce. The US later produced a version called HE-Plastic or HE-P.
The more you study armor penetration, the stranger it gets. For example, a long rod penetrator like the APFSDS rounds used in modern tank guns can actually perforate armor thicker than it can penetrate, by causing failure in the armor plate; it can also perforate the armor deeper, through that failure mechanism. That’s completely counterintuitive, but penetration and perforation curves from live testing demonstrate it.
Late in the war, shaped-charge warheads became a problem. Using WWII-era understanding of lining materials and explosives, effective shaped charges tended to be larger than most tank main gun calibers. Instead, they were deployed by short-range rockets like the German Panzerschreck and the US 2.36″ rocket launcher, and other infantry weapons, such as Russian drogue grenades, the British PIAT and the Japanese lunge mine (which is exactly what it sounds like, a shaped charge on a stick for a suicidal human attack on tanks. They were used on Okinawa and were made in the hundreds of thousands for the anticipated defense of the home islands).
The hardness of armor had much less influence on shaped charge penetration. But as a shaped charge has an optimum standoff distance, detonating it early reduces its ability to burn its way through armor. This led to various kinds of appliqué armor, some factory and some improvised. The Germans were a step ahead here. They had already added stand-off plates called Schürzen to many combat vehicles (including the Panzer III, IV and Panther) as a countermeasure against Russian anti-tank rifles. The Schürzen were homogeneous, but not very hard — only Brinell 105 or so. Schürzen were ineffective against conventional tank and antitank guns, but would sometimes fragment or deflect the steel or tungsten-cored 14.5mm Russian anti-tank rifle projectile, which otherwise could penetrate the side armor of those tanks at close ranges (~100m). The effectiveness of Schürzen against shaped-charged warheads was an unexpected but welcome bonus.
As we said, armor penetration is a weird science. The Schürzen, for instance, had almost zero effectiveness against the 14.5 if struck absolutely square on, at a 90º angle, but got much more effective as the angle increased even a few degrees.
Farrand, Magness, and Burkins. Definition and Uses of RHA Equivalences for Medium Caliber Targets. Interlaken, Switzerland: 19th International Symposium of Ballistics, 7–11 May 2001. Retrieved from: http://ciar.org/ttk/mbt/papers/symp_19/TB151159.pdf (That site has the whole proceedings of the symposium).
Green, Michael & Gladys. Panther: Germany’s Quest for Combat Dominance. Botley, Oxford: Osprey, 2012. (A very worthwhile book, rich in technical detail, with excellent notes and index and a wealth of photographs).
Uncredited, Armor-Piercing Ammunition for Gun, 90-MM, M3. Washington: Office Of The Chief Of Ordnance, January 1945. Retrieved from: http://www.lonesentry.com/manuals/90-mm-ammunition/index.html (Lone Sentry is a former W4, and is highly recommended for this sort of period material).
Numerous other sources were used en passant but the bulk of the information in this post is from Farrand et. al. and the Greens.