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).

Peasant
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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.

Image

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.

Peasant
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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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