There have been some questions, I´d like to adress with the memo below.
Thanks for directing my attention to the shot #6. Actually I've made a mistake in translation. It doesnt say "cracked bulge" but rather "a bulge with a crack partially separating it from the plate" which looks like the start of a plug ejection, just like one would expect to see in a penetration of a blunt headed projectile.
That´s one posibility, and one might add, when viewing from the perspective of expected plate failure modes here, a probable one.
In plugging/discing, the projectile punches out a large conical plug (or wide disc) of armor from the plate back when it hits with enough force to hole the plate large enough to allow the projectile to have a chance of penetrating the plate. If the projectile has not sufficient power to exact a sufficiantly large hole, the projectile rebounds, broken or not (compare f.e. 152mm hit#9).
In similar photos, but with a back of the plate shown, where the projectile did not quite penetrate (no through crack formed like here), there are obvious wide bulges surrounding the impact point before cracks are formed, in some cases with a ring-shaped crack surrounding the impact that, if the striking velocity had been higher, would have caused that circular area at the plate back to tear free.
http://tankarchives.blogspot.com/2013/0 ... m-gun.html
The projectile acts like a hammer, punching out the steel in front of it´s projectile nose as a cylindrical or cone-shaped ring around the projectile nose. With the blunt nosed WWII-era soviet APBC projectiles, or inferior sharp tipped AP which suffer break up, the nose tip virtually never sticks out of the plate back prior to the plug being torn entirely free of the plate (as would be the case with more ductile events).
The point remains that there was no back bulge, which is initially associated with plug formation even before cracks are formed (compare hit#16) and this suggest to me that this impact was very much below any actual Holing Limit. It´s therefore not representative for the protection ballistic limit.
How much more energy is required to punch out a plug is not clear to me from this hit alone (see more below).
MIL-S-12560 is not "relatively soft", in fact its very similar to the WW2 late german vehicle RHA. I've posted the specs earlier, you can see for yourself.
The document You referred to was revision -B to MIL-S-12560 and -that was my thought- may not be identic to the original specification. However, I have found recently that differences between revision-B and the older standart are minor and therefore happily stand corrected.
I see what you mean, but you cannot just that say that "the armour is good because it would've performed well in a controlled setting against the type of projectiles it was optimized to defeat". Most of the Allied WW2 AP shells would've broken their nose when hitting a 50°sloped plate, resulting in a blunt penetrator similar to soviet shell.
I think this, too, is a fair comment. The only ww2 projectile to have a chance of staying intact against this target (100mm @55°) is the 12.8cm Pzgr43.
Still regardless on how this armour would've performed at higher ambient temperatures, there is no denying that its bad feature to have in tank armour, when all it takes is to expose it to -10°C (average central european winter) for it to loose about 20% of its effective thickness. Were other WW2 armour steels just as bad?
I don´t think that the presumption is valid here. First of all, the difference would be 20% if the hit#6 resulted in a holing, which it did not. So the difference is smaller by an unknown fraction (more below). 2nd, I have some temperature strain rate/Charpy graphs and a difference in impact strength in order of 20% was not uncommen when room temperature was compared with -10°C (f.e. oil tempered PP792 had 7.9mkg/cm² at room temperature and 5.1 mkg/cm² at -10°C). It´s only after ww2 that standart testing temperature was lowered to -40°C in combination with metallographic studies of the fracture surface in order to reliably test for and expose poor lower temperature brittleness effects on AFV armor plate (except for the one and other research paper one jointly comes across to touch this question).
That beeing said I have invested the better of roughly five hours of work to examine the nose plate hits in Kubinka. All eight hits with AP and APCBC-HE (only 88mm Pzgr39) were in direct line to the target, which helps eliminating a source of variance. Because the target plate is therefore identic (100mm at 55° to the perpendicular) and presuming that the failure mode is identic, too (discing with broken up penetrators- which is likely true for all attacking projectiles,88mm pzgr39 included)), it may therefore be justified to run a regression analysis (see attachment).
I have choosen De Marre K functions as a proxy of plate resistence because this is how ARTKOM assessed armor tests in ww2 (see Krogfuss article of DeMarre in Artkom for the details). While we wouldn´t do it today (De Marre K has a significant scaling issue) it sheds light on how they did it in ww2.
What can be observed is that there testers intended to obtain a lower and higher bracket with the De Marre K scaling with T/D ratio (smaller calibre need more impact force to obtain a holing than larger ones). A number of hits can be viewed as fair hits on the plate, and a number as invalid hits too close to one another, or to a plate joint, or to a torn free plate joint. Notice that all these cases of invalid hits tend to degrade the plate resistence (discontinuities, free edge effect, communition effect, triaxial stress concentrations). Yet, not all resulted in a holing but rather in clean scoops with or without bulges.
The only holing event, which was the result of a fair hit is hit #9, a 152mm AP striking at 588.9m/s. The result was a disc thrown from the back plate and the broken up projectile rebound from the plate. the holing diameter was not large enough to let the 152mm projectile body pass (sub calibre hole). However, it certainly qualifys for A(BL). The 1st BR471 failed to make a hole at K=2820).It also shows why the 2nd BR471 was fired at K=3300, this is close enough to the upper border to represent a severe test for the armor. While the hit was very close to the joint with the 150mm upper glacis (10mm below the edge on the nose plate), which by virtue of the interlocked design and constructive angle gave peripheric support to conical plug formation, there again was only a scoop. This is somehow significant because the projectile (BR471) would first turn away from the plate, and then into the plate, which would tend to clear the actual penetration channel from the 150mm supporting glacis plate.
I personally, would have decided to repeat this hit, but the testers went on with the 100mm BS-3 which obtained a good, inverse straddle. This was probably enough to fix the trend by using the contemporary De Marre K procedure, the final 88mm hit just again confirmed the lower boundary (lower boundary with no holing events was confirmed by 122mm, 100mm and 88mm hits, making it a fairly robust baseline).
I have added a weighted, smoothed (Loess) interpolation line with probability brackets. The invalid hits are counted but weighted only for 50% individually in this graph.
However, in view of the limited data aviablility, one will need to stress that the trend described between both lines is a range of mixed results, while the trendline is the average, thus, the smoothed data interpolation.
in view of the so established trends one might notice, that for the 122mm BR471, a De Marre K=3100 would meet a 50% confidence level to give some holing events (Army(BL)) at K=3100, which equals a striking velocity =719.7m/s or 2361fps for what would be the US protective BL for this plate accordign to it´s respective smoothed interpolation. This certainly isn´t such a strong deviation from the US graph (say, approx.~10-12%).
Notice that the Army (HL) is different from a calibre sized Holing (where a lot of damage can be inflicted behind the plate, unlike shown here) and that the HL is different to the NB(L), where the complete passage of the projectile can be expected. In plugging /discing events, the delta between Holing limit and NB(L) -perforation limit- can be as much as twice as large as in ductile hole formation events (f.e. 15% instead of 8%).
hope it helps,
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