Allied tank armor, quality control

Discussions on the vehicles used by the Axis forces. Hosted by Christian Ankerstjerne
critical mass
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Re: Allied tank armor, quality control

Post by critical mass » 25 Apr 2021 12:30

Peasant,

at 0° obliquity (=90° german definition, or perpendicular to the plates surface), softer armor generally resists less well than harder armor, if the plate is of decent quality (assuming no temper brittleness in the soft plate). If the plate resist with its strength vs plastic deformation, as is the case against pointed and high quality AP, then a soft plate resists less well to plastic deformation. Even at beyond 30° obliquity there is an advantage for the harder plate but it starts to drop to 0 at in between 40°-50°(depending on the cal/plate ratio as well as on the impact velocity) and becomes a definitive disadvantage at high obliquities (45-55° and up, depending on cal/D ratio, failure mode and velocity). From what I see, up to 30° obliquity, German period reports assumed that the hardness differences can be normalized by sqr(h1/h2). Thus, Sqrt(335/295) = 1.0656 (figure of merit in critical G(D) velocity (!) of the avg. 335BHn plate vs an avg. 295BHN RHA plate). At higher obliquities, this formula deviated from empirical results and was beyond its use. However, it seems to fit reasonably with russian post ww2 trials on RHA and HHA using German Pzgr.39 at 0-30°, too, as long as the plate is thick compared to the attacking projectile and therefore does not exhibit lower energy shear plugging failure (at cal/D ratios of 1.0 or below, this must be reckoned with).

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Re: Allied tank armor, quality control

Post by Peasant » 25 Apr 2021 19:35

critical mass wrote:
25 Apr 2021 12:30
-- snip --
You are speaking about plastic and plugging mode of failure of a plate as if they were mutually exclusive. Correct me if I'm wrong, but a given plate can exhibit both at the same time, plastic deformation early on and plugging at the final stage of penetration, even when attacked by pointed AP.

My understanding is that once the plate reaches a certain threshold level of hardness, it's rear face starts producing plugging type of failure under given conditions of attack, and increasing it's total thickness does not change this behavior (as long as the attacking projectile can withstand increasing striking velocity). At this point increasing the hardness of the plate would still increase it's resistance to plastic deformation at the initial stages of penetration but will decrease the depth at which the low energy failure(plugging) will occur. This generally, more than offsets the increase of energy absorbed during the plastic stage and the total energy required to perforate the plate will decrease with hardness from now on (unless at some point the projectile starts to exhibit damage due to increased hardness of the plate).

The simple rule of increase of energy required being proportional to the square root of hardness/tensile strength is only valid when both the target and the reference plate exhibit completely ductile behavior.

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Re: Allied tank armor, quality control

Post by Yoozername » 25 Apr 2021 21:42

An interesting image showing the rear interior view of the 'powertrain' unit of a M4.

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critical mass
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Re: Allied tank armor, quality control

Post by critical mass » 26 Apr 2021 11:09

Peasant wrote:
25 Apr 2021 19:35
critical mass wrote:
25 Apr 2021 12:30
-- snip --
You are speaking about plastic and plugging mode of failure of a plate as if they were mutually exclusive. Correct me if I'm wrong, but a given plate can exhibit both at the same time, plastic deformation early on and plugging at the final stage of penetration, even when attacked by pointed AP.

My understanding is that once the plate reaches a certain threshold level of hardness, it's rear face starts producing plugging type of failure under given conditions of attack, and increasing it's total thickness does not change this behavior (as long as the attacking projectile can withstand increasing striking velocity). At this point increasing the hardness of the plate would still increase it's resistance to plastic deformation at the initial stages of penetration but will decrease the depth at which the low energy failure(plugging) will occur. This generally, more than offsets the increase of energy absorbed during the plastic stage and the total energy required to perforate the plate will decrease with hardness from now on (unless at some point the projectile starts to exhibit damage due to increased hardness of the plate).

The simple rule of increase of energy required being proportional to the square root of hardness/tensile strength is only valid when both the target and the reference plate exhibit completely ductile behavior.
Sure. That´s the limit of the formula but within those limits, a softer plate will provide less resistence than a hard one -at low obliquity. Generally, You want a plate as hard as feasible without undue loss in ductility under the preferred condition of impact. The effect You explained is known as work hardening, where the plates hardness changes locally under impact, creating thin shear bands. However, a lot of excess hardness is required for plugging to be the dominant factor combined with appreicably loss of resistence caused by adiabatic shear failure. Mixed ductile-brittle plate responses, as shown by plates treated to the first ductile-brittle transition point, are compromised for optimum levels of ballistic resistence to penetration. Plugging isn´t necessarely a bad feature for them as it does not correlate in this hardness range with inferior ballistic resistence (i.e. BRUMMBÄR plate tests carried out by the british after ww2). Increasing the resistence to platsic deformation still does, as You indicate, help the plate somewhat, by transmitting shock through the projectile nose (hopefully destroying it, at which point even a *too* hard RHA/HHA plate may retain its ballistic superiority vs low quality AP) but excess hardness against an underformed projectile nose wwill cause lower energy shear failure, which is a major reduction of ballistic resistence (f.e. 5cm Pzgr.39 vs 45mm MZ2, HHA). Thickness and cal/D ratio actually do appear to have an effect here. Too thin plates can´t produce plugs or discs. They will either give in, creating star cracks and then petals (ductile) or shatter, leaving a large, typically oversized and irregular calibre holes. In too thick and hard plates, another effect may set in. Here higher hardness works to the plates adavantage for longer and more of the impacting energy is transmitted to the sheared out plug from the plates back riding on its nose (like a Billard ball hitting another). Theoretically, there should be a point where the projectile is stopped cold and all its energy is transferred to the plug but the high cal/D ratios required make it somewhat unlikely that intact penetration could be obtained anyways.

critical mass
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Re: Allied tank armor, quality control

Post by critical mass » 29 Apr 2021 08:13

addition to the above:
In too thick and hard plates, another effect may set in. Here higher hardness works to the plates adavantage for longer and more of the impacting energy is transmitted to the sheared out plug from the plates back riding on its nose (like a Billard ball hitting another). Theoretically, there should be a point where the projectile is stopped cold and all its energy is transferred to the plug but the high cal/D ratios required make it somewhat unlikely that intact penetration could be obtained anyways.
That is for low cal/D ratios. The plate is "thick" in relation to the attacking shot (i.e. 37mm vs 75mm plate) . If You scale the projectile size up together with the plate thickness (to, say, 100mm vs 200mm plate), the scaling effect works massively to the hard plates disadvantage by emphasizing shear failure, but this is more of a problem with major calibre naval projectiles, rather than anti-tank bullets we are discussing here.

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Re: Allied tank armor, quality control

Post by Peasant » 17 Jun 2021 23:18

"British Battle Tanks: British-made tanks of World War II"(2017) by David Fletcher
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Re: Allied tank armor, quality control

Post by Peasant » 17 Sep 2021 22:33

Found some cool stuff:

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Re: Allied tank armor, quality control

Post by Peasant » 31 Dec 2021 20:56

The highest PP is at 2079fps, the lowest CP is at 2109fps. If we take the BL(Navy) as equal to 2094fps, the equivalent thickness of vertical armour defeated by M62 shell at this velocity is 3,85in(98mm) (1500yrds.).

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Source: http://the.shadock.free.fr/sherman_minu ... osite.html (I've brightened and equalized the brightness of both pictures)

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Re: Allied tank armor, quality control

Post by Peasant » 03 Oct 2022 16:25

Interesting comparison. Both sets of curves are for 50% probability of "success", but while the US Navy Limit defines it was "whole projectile passes through the armour in condition fit to burst", the UK standard requires as little as 20% of the mass of projectile flying behind the armour. As we can see the UK testing standard gives the BL consistently about 100fps lower than the US one.
Alternatively this discrepancy can be explained by lighter US shell, but from my research I've seen that the M86 shell, filled and fuzed, weights almost exactly 7lb(3,17kg), not significantly different than 3,23kg for the UK 6pdr APCBC shot. Makes sense, as both countries would benefit from having the ammo for their 57mm guns being interchangeable.
If the US made shells were more prone to shatter we would see greater difference at higher striking velocities and less at low velocities, but the difference is about the same.
And finally the last possible explanation that I can see is that the UK armour testing plate being just a bit worse than the US one.
What do you people think?

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Re: Allied tank armor, quality control

Post by Peasant » 09 Oct 2022 10:02

Interesting read on the topic: https://apps.dtic.mil/sti/pdfs/ADA954822.pdf

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Re: Allied tank armor, quality control

Post by critical mass » 07 Nov 2022 13:55

The US NBL did not require the projectile to pass intact. It only required the projectile body -if intact- or body pieces -if broken up- that are all elements of the projectile sans nose coverings, driving bands and fuze adaptor to be found behind the plate.

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Re: Allied tank armor, quality control

Post by Peasant » 07 Nov 2022 15:38

critical mass wrote:
07 Nov 2022 13:55
The US NBL did not require the projectile to pass intact. It only required the projectile body -if intact- or body pieces -if broken up- that are all elements of the projectile sans nose coverings, driving bands and fuze adaptor to be found behind the plate.
That's only for solid shot. APHE projectiles were required to pass through in condition fit to burst, just like the navy projectiles did in order to seriously damage a ship, which is why it's named "Navy" limit.
On a tangent: I know people often use "in condition fit to burst" as interchangeable with "intact (sans windshield and driving bands)" but from penetration curves in TB Vol3 the US projectiles were clearly not required to survive impact literally intact, especially at obliquities over 20°.

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Re: Allied tank armor, quality control

Post by critical mass » 08 Nov 2022 21:24

No. This is not how Ordnance pamphlet 8 defines the US NBL. This is because historically, it was devised when the De Marre formula was adopted in the USN and no US AP bullet could stay in condition fit to burst when striking a severe target at this timeframe.
The NBL is just a perforation limit, and may or may not involve projectile break up.
The ABL on the other hand was a penetration limit (hole or crack through the plate without projectile passage).
F.e. for major calibre US APCBC-HE (12in to 16in), the NBL against FH class A armor in the period 1921-1936 was usually intact at 0-15 deg and broken beyond 15 deg.
Against RHA the projectile generally stayed intact up to 25 deg and brokeup beyond, when striking at NBL velocity.
OP8 gives NBL velocities up to 60 deg for these projectiles.

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Re: Allied tank armor, quality control

Post by Don Juan » 04 Dec 2022 22:43

Peasant wrote:
17 Jun 2021 23:18
"British Battle Tanks: British-made tanks of World War II"(2017) by David Fletcher

UK plate.png
The firm in question here was the South Durham Steel and Iron Co. Ltd. in Middlesbrough, who only made their first armour plates in May 1942, these being the plates referred to here, which were actually mainly fitted to Cavaliers. These tanks were also demarcated with a "U" at the end of their serial numbers, this episode actually being a very rare occurence in British tank armour production. By 1944 this firm were making outstanding quality armour plate that often performed well above specification.
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"The demonstration, as a demonstration, was a failure. The sunshield would not fit the tank. Altogether it was rather typically Middle Easty."
- 7th Armoured Brigade War Diary, 30th August 1941

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Re: Allied tank armor, quality control

Post by Don Juan » 05 Dec 2022 16:52

Peasant wrote:
15 Aug 2020 13:26
Hope this hasnt been posted here yet.

British armour flaking problem.png
Source: "Mr. Churchill's Tank: The British Infantry Tank Mark IV" by David Fletcher
This quote actually shows that British armour quality control was working very well. The "IT" numbers quoted by the author were not related to izod testing, but were the standard British nomenclature for armour, with the IT 80 series being rolled homogenous armour plate of various grades, and IT 90 being cast armour of various grades. The reason the cast Churchill turret was thicker was that the British, and also I think the Americans, considered cast armour to be about 10% less resilient than RHA.

This passage is inadvertently valuable, because the usual story that is trotted out is that the British stuck to riveted and bolted construction later than other nations because of a reluctance on the part of the tank building firms to change their processes. However, this passage seems to indicate that there was in fact a technical reason - it was not possible to produce high quality thick weldable plate in the necessary volumes. I will have to review the British armour trial reports and battle casualty reports I have on German tanks, as iirc there is quite a lot of internal flaking identified in them. I also seem to remember that early war Soviet welded plate tended to flake a lot. Therefore it may have been the case that the earlier German and Soviet adoption of welding might have been due to their willingness to take more risks with internal flaking than the British and Americans, rather than being due to technical advancement per se.
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"The demonstration, as a demonstration, was a failure. The sunshield would not fit the tank. Altogether it was rather typically Middle Easty."
- 7th Armoured Brigade War Diary, 30th August 1941

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