Armor Resistance Variations

[rexford]

Past posts and threads have discussed the degree to which armor varies in resistance, and whether a single plate could have different resistance based on where one hit it, or the impact angle.

The following lengthy and detailed analysis looks at the test results when the British fired 6 pdr AP at six plates which differed in thickness.

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The report covering British tests of German 75mm and 88mm APCBC against oblique armor included tests with 6 pounder AP at various angles. By comparing the 6 pounder AP results to estimates from the British National Physical Laboratory equation we can gain some insights into the penetration variability of British armor plate and shot during those tests.

1. When 6 pdr AP struck 56.8mm at 40°, the actual velocity for 50% success was 1939 fps and the predicted was 2147.

2. Against 61.5mm at 40°, actual velocity is 2395 and predicted is 2307

3. Versus 65.3mm at 40°, actual is 2341 and predicted is 2434

4. Attacking 75.9mm at 30°, actual is 2113 and predicted is 2225

5. Impacting against 75.9mm at 40°, actual is 2800 and predicted is 2790

6. Hitting 80.8mm at 30°, actual is 2373 and predicted is 2342

Following is analysis of variation in terms of 0 degree penetration variations using a DeMarre estimate (penetration difference proportional to velocity ratio raised to 1.428 power):

1. -13.5% below predicted resistance..56.8mm at 40 degrees

2. +5.5% above predicted resistance...61.5mm at 40 degrees

3. -5.4% below predicted resistance...65.3mm at 40 degrees

4. -7.1% below predicted resistance...75.9mm at 30 degrees

5. +0.5% above predicted resistance...75.9mm at 40 degrees

6. +1.9% above predicted resistance...80.8mm at 30 degrees

The average value of the absolute variation is 5.7%, and the standard deviation around zero variation is about 5.5% (67% within 5.5% variation, 83% within 7.1%). During U.S. tests with 37mm AP, the standard deviation was about 4%, so the British tests are somewhat consistent.

The interesting aspect of the data analysis is that when 6 pounder AP hits 75.9mm at 30 degrees the armor resistance appears to be -7.1% deficient, but when the same armor is hit at 40 degrees the resistance is 0.5% above expectations.

The inconsistencies in British armor resistance with angle changes continue when 50 degree hits are examined. 56.8mm plate was -13.5% deficient when struck at 40 degrees compared to 65.3mm plate being -5.4% deficient at same angle.

At 50 degrees slope, 56.8mm plate has a higher slope multiplier than 65.3mm by 2.01 to 1.96, which becomes even more significant when the difference in T/D ratio is examined (1.00 for 56.8mm, 1.15 for 65.3mm).

When 65.3mm is hit at 50 degrees, the slope multiplier is significantly less than for 56.8mm, which suggests a turnaround from the 40 degree case.

The reason for the unusual results with the same plate may be due to varying ballistic characteristics as hits strike different locations, or major projectile changes.

When 6 pounder APCBC attacked 65.3mm at 40 degrees, the actual resistance was -10.8% below expected, suggesting that 65.3mm plate was deficient on all hits during test.

Reference: British report No. M.69144/4 No.1, German 75mm and 88mm APCBC Ammunition at Oblique Angle, currently held by British Public Records Office as WO 194/749

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Could a welded hull Sherman have different resistance depending on where it was hit?

Yes, because the glacis was made up of many pieces, as many as eight welded together, and some could be good, some bad, some inbetween. The welded hull Sherman also tended to mix rolled and cast pieces on the glacis, where 2" cast is appreciably less resistant to 75mm and 88 hits.

Could a Panther glacis have areas that were bad and some that were good? Depends on how extensive the heat treatment problem areas are.

When 2" cast armor went bad, the entire piece would usually have a crystalline structure and be very brittle. It is possible that weak areas could be localized in nature and not appreciably influence hits on better armor spots.

U.S. tests on armor resistance decreases due to bad areas found the weakness causing factors could be restricted to small or large areas, which would result in a variety of results depending upon where on hit.

[kipanderson]

Rexford, hi,

As always, very interesting stuff. Your book on the subject is “the” work on the subject, in my view. Of course, that does not mean that we will all always agree on every detail. However, that is part of the fun.

One area I was wondering about, and I know you were looking into, is what is your view on face hardened armour and Russian APBC rounds? From the example of the Panther’s side armour and 76.2mm rounds in your book, plus the odd mention of the subject in a British 1947 report I have, I feel one must be careful in assuming Russian APBC rounds could not cope with German face hardened armour.

What is your latest view on the subject?

All the best,

Kip.