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Armor Hardness and Penetration Resistance

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We recently received excerpts from a WW II Aberdeen Proving Grounds reports on the penetration of 90mm T33 APBC rounds as a function of armor thickness, angle and brinell hardness.

The Army Protection Limit varies with armor hardness in the following manner at angles from 60° and 65° against 3" plate:

60° Impact

2629 fps against 280 Brinell, 2554 fps against 360 Brinell and 2348 fps against 400 Brinell

65° Impact

3026 fps against 280 Brinell, 2948 fps against 300 Brinell and 2870 fps against 320 Brinell

What does this mean for WW II penetration range estimates against tanks with high hardness armor, like the T34, T34/85, SU 85 and SU 100?

When 90mm T33 APBC strikes 3" armor the thickness/diameter ratio (T/D) is 0.85, and 400 Brinell Hardness armor at 60° resists with about 85% of the 0° resistance of 280 Brinell plate((2348 fps/2629 fps) raised to 1.4 power). Since penetration tests are conducted against armor with about the same resistance as 280 Brinell plate, this means that 400 Brinell armor is going to lose substantial resistance when the T/D ratio is 0.85 or less.

When 45mm high hardness armor (400 Brinell) is hit at a 60° angle by 75m German rounds, the T/D ratio is 0.60 (45mm/75mm) and the armor would be expected to resist with less than 85% of the resistance offered by 280 Brinell plate. Based on our research (which is presented in WW II BALLISTICS: Armor and Gunnery, by L. Bird and R. Livingston), 45mm armor at 400 Brinell would resist like 34mm of 280 Brinell plate when it is hit by 75mm rounds (slope effects based on the actual 45mm thickness).


The 90mm T33 tests at 70° showed that a 2" plate at 280 Brinell presented 86% of the resistance offered by 3" at 360 or 400 Brinell plate.

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The U.S. tests against high hardness armor were conducted with "decent" quality armor: defeated armor in tests that described armor failure modes did not suggest poor armor characteristics.

The estimates in our book for high hardness armor resistance are based on equations drawn from U.S. test data. When our penetration range estimates for T34 against 75L43 APCBC are compared with actual combat reports in Thomas Jentz' Panzertruppen books, the estimates present reasonable agreement with reports. This was one of the factors we considered when we assumed that U.S. high hardness armor was similar to Russian.

Reports on T34 armor also show undesirable elements as well.

During 1942, the Germans reproduced T34 armor with the same composition and heat treatment, and fired 37mm and 50mm tank guns against the plate at various angles (100m range).

High hardness armor is going to be less resistant than 250 Brinell plate when the projectile diameter exceeds the armor thickness, due to low impact resistance and brittle structure (high hardness requires crystalline structure which is brittle and subject to rapid failure, 250 Brinell will be fibrous and ductile when carefully mixed and produced).

The German 50mm AP round fully penetrated 47.2mm at 60.5 degrees, and 42.1mm at 65 degrees, during the tests. Partial penetrations were obtained against 47.7mm at 59° and 47.1mm at 59.5°.

The abovementioned results suggest a 1-in-3 complete penetration ratio when 50mm AP struck 47.3mm at about 60°, which requires a penetration/armor resistance ratio of about 0.98.

If 47.3mm of 240 Brinell plate at 60° is estimated as equivalent to 123mm at 0° on 50mm AP hits, then the penetration/armor resistance ratio is only 0.80. However, if 47.3mm of high hardness armor resists with 83% of the resistance of 240 Brinell armor when it is hit by 50mm rounds, then the 1-in-3 penetration ratio is supported.

WW II BALLISTICS: Armor and Gunnery, by L. Bird and R. Livingston, estimates that 45mm high hardness plate resists with 83% of 240 Brinell armor against 50mm hits.

Granted we are talking about a handful of firing test results, but the German tests are consistent with the equations prepared using U.S. high hardness armor, and are reasonably consistent with reported penetration ranges in Jentz' works.

Interestingly enough, when 50mm AP hit 40.9mm high hardness plates at 40°, two of five hits failed even though the armor resistance was well below the penetration of 50mm AP at 100m. 40.9mm at 40° should resist 50mm AP like 63mm, and less when high hardness deficiencies are considered.

It appears that 50mm AP rounds were shatter failing against the 40.9mm plates even though they possessed more than enough penetration to defeat the armor.

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Great information…as usual. Interesting that the T33 APBC – in spite of its lack of a penetration cap – shows decreasing ballistic limits vs. increasing target plate hardness. Isn’t this somewhat ass backwards from what one might normally expect? T33 is a pretty large caliber AP round by WWII standards. Are you attributing this decrease in ballistic limit relative to increasing target plate BHN purely to T/D?

Somewhat tangent to your posts but as you brought up APBC I have found yet another independent source discussing Soviet “artificial penetration cap” action for APBC. “Return of the breakaway nose and circumferential grooves on the ogive”. The following snippet seemingly implies Soviet APBC was not truly APBC. At least not in the same since that the US Army’s T33 round was. Soviet APBC is seemingly a hybrid between APBC and APCBC. The breakaway nose being monolithic with the penetrator can’t really be seen in the same light as true APCBC in which the penetration cap was a separate piece of steel…in some cases only lightly welded to the penetrator. However, clearly the intent of the flat-nose on Soviet APBC was an attempt at penetration cap action.

From: Arthur G. Volz, “Soviet Artillery Weapons, Part III, 1941-1945.

<blockquote>quote:</font><hr>Soviet ammunition problems lay less in lack of design capabilities than in industrial capacity. German armor-piercing projectiles employed armor-piercing caps. Soviet designers were well aware of this feature, which had been developed much earlier on the suggestion of the Russian admiral Makarov who was killed in the Russo-Japanese war. Manufacture of projectiles with armor-piercing caps was more complicated than the Soviet wartime practice of machining circumferential grooves on the ogive. The latter facilitated breaking away of the head of the projectile while leaving the remainder undamaged, thus giving something like a penetrative cap action and preventing the shattering of the projectile at high-impact velocities.<hr></blockquote>

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We have a German test report dated 1942 where 50mm AP penetrated 47mm T34 type armor at 100m and 60° slope. From the 90mm APBC tests and the 50mm AP performance against T34 type armor, it appears that the high hardness of T34 armor works against the tank, and the brittle plate makes a T34 much more vulnerable.

If T34 used 250 Brinell Hardness armor, which takes some care to produce properly, the 45mm at 60° front hull would have the same penetration resistance as a 122mm thick vertical plate. This will stop PzKpfw IVH hits at 500m.

But T34 used 400+ Brinell high hardness armor, and the 45mm at 60° resistance drops to a 93mm vertical plate due to brittle behavior. A PzKpfw IV will have no trouble with this armor at 1000m at any angle.

The German test results that we have show 37mm AP penetrating 54mm of high hardness armor like the T34 carried at 100m.

Like the Sherman which suffers an 85% quality multiplier in CMBO, T34 in CMBB may have a 76% quality factor against 75mm hits due to the frailities of very high hardness.


American APBC is uncapped pointed nose shot with a ballistic cap, Russian APBC is a flat nose round with a ballistic cap.

When the ballistic limit falls with increasing hardness, the results are strongly suggesting that the armor brittleness is causing premature failure. High hardness armor is crystalline in structure, and is like a house of cards in that the structure is dependent upon a weird set up where molecular pieces are hanging by their edges. Once a sharp impact is made and one of the edges lets go, the armor fails suddenly and in a big way.

Softer armor is much more giving, and will bend alot and absorb tons more energy before the round gets through. Soft armor stops the hit by making the projectile push its way through the thickness. Hard armor defeats hits by causing the round to absorb its own energy through nose and body damage, since hard armor cannot deflect much without falling apart.

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<blockquote>quote:</font><hr> When the ballistic limit falls with increasing hardness, the results are strongly suggesting that the armor brittleness is causing premature failure. High hardness armor is crystalline in structure, and is like a house of cards in that the structure is dependent upon a weird set up where molecular pieces are hanging by their edges. Once a sharp impact is made and one of the edges lets go, the armor fails suddenly and in a big way.<hr></blockquote>

This of course assumes the round doesn’t shatter on face-hardened armor before penetrating. Presumably you are referring to homogenous hardness throughout the target plate resulting in brittle failure of the plate.

As I know you are aware the British apparently had a very tough time of it in N. Africa during the initial years of the war in the desert. The penetrator on the 2-pdr was not initially fitted with a penetration cap, making it that much more difficult to penetrate FHA on the Afirka Korps MkIII’s and MKIV’s. By 1943 the cat was pretty much out of the bag what with most Anglo-American tank killing rounds being fitted with penetration caps. FH armor on German Panzer wasn’t really any sort of an advantage like it had been in 1941 and 1942.

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