penetration limits and definition

[critical mass]

please find attached the criteria and confidence levels of various nations penetration definition (for medium calibre projectiles: 50mm to 203mm).
(edit: upload http errors encountered )

[critical mass]

Providing that the data of successful penetration and unsuccessful penetration are distributed normally without skewness, than an harmonization might be attempted on the US/British 50% probability criterion at least on 30° impact condition:

USSR: 50% limit=(IP@30°+CP@30°)/2
Brit: as given
US: as given
german: 50% limit=(G(d)+(G(d)/0.92))/2

As with all such data, the deviation should be noted, data permitting. Such normalizations would still contain a margin of error, but likely this error in total variance is smaller than other processes.
The limiting condition for the german 50% limit is the envelope of correctness of G(d), which requires no projectile break up. Lower quality charges had identic limits with the higher quality charges but a lower, critical velocity where projectile break up sets in. High quality 75mm and 88mm Pzgr39 AP will be intact at 30° impact out to ca. 1240-1260m/s striking velocity (which entails the potential of excellent penetrative performances), while poor quality charges of 88mm Pzgr39, f.e. experience projectile break up already at or below 1000m/s. Some more data are given below:

[+] uncapped 50mm Pzgr Gg AP at 550m/s,
[+] capped 75mm K.Gr. rot Pz Gg at ca. 500m/s,
[+] capped 88mm Pzgr Gg (large filler AP) about 800m/s in average,
[+] uncapped 10cm Pzgr Gg (large filler AO) at about 400m/s

It must be noted that just adding a cap to the projectile doesn´t help much. Only if the projectile nose and shoulder receive sufficiently good hardness treatment does a cap help in pushing the critical velocities noticably out.

These critical velocities, where the failure mode of the projectile changes from intact to broken (or even shattered in some cases), is the most difficult character of the projectile to get any data on.

[Thoddy]

just to note durchschlag means projectile through plate projectile in condition fit to burst = Heilbleiben
sometimes a 3/3 success (mostly for experimental projectiles and plates) was used as projectile and plate tolerances did not allow 5/5 success

additional criterion Grenzdurchschlag this is used if the projectile breaks up at least 50% of projectile mass or remains of plate can be found behind plate(capable of damaging delicate equipment and cause injuries)

in naval fireeffect tables the Grenz criterion was usually at ~92 % of the Heilbleiben criterion

[Mobius]

Also, note there is a difference in what constitutes a complete penetration.
1. Russians is at least 75% of the mass passes behind the armor.
2. British is 20% or more of the mass passes behind the armor.
3. US is at least 50% of the mass passes behind the armor.
4. German is the shell remains intact behind the armor less maybe cap and driving bands.

[critical mass]

The number of consecutive successes required for G(D) is depending on the calibre according to Teltz:

Large. 210mm and up: 3 out of 3
medium. 75mm, 88mm and 105mm: 5 out of 5
small. 20mm and 37mm: 10 out of 10

this applies for generating graphs for penetration, not for service acceptance of projectiles.

Additionally "Durchschlagskurven" do not specify whether or not the projectile breaks up. The specification was that the G(D) velocity is the velocity at which the projectile just passes the plate with no residual velocity left. In practice, and because the reference obliquity is only 30° off the perpendicular, this often (but not always) means intact penetration. However, exceptions do exist to this rule.

The "Grenzkurven" could show not only break up but also intact cases, in the latter, the intact projectile was stuck in plate, while in the former, the fragments were to be found behind the plate.

At low obliquities, generally up to and including 60° (30° in US terminology), the G(D) was indeed intact (similar to what the Navy called "Heilbrleibkurve"), but not at high obliquity, when the projectile generally breaks up. Also, older projectiles could experience break up at elevated velocities even at normal obliquity in their "Durchschlagskurven" (shown to flatten out once break up sets in). Finally, HEAT and APCR projectiles had their "Durchschlagskurven", too, but couldn´t carry a burster behind any plate and were destroyed/eroded in the process of penetration.

For service acceptance, the criterium was that the projectile had to completely penetrate and also to be in an intact state fit to burst in min. 2 out of 3 cases at specification velocity and specification obliquity (generally 30°, except late in 1944, when some projectiles, like 75mm Pzgr39 and 88mm Pzgr 39-43 were raised to 45°).

"Durchschlag" is the oldest penetration term, stemming back from the 19th century (the oldest reference, I have come across was 1872, though it might not be the oldest actually), at a time when little emphasize was paid to penetration in an intact condition fit to burst.

[Peasant]

I wonder if anybody can explain to me, where does the soviet 20% limit comes from? AFAIK They had a PTP-limit, which can be roughly translated as "limit of back face integrity" which I always understood as no round can pass through since if it would, it'd definitely do more than crack the plate.
?

[critical mass]

The PTP is the limit of back strength. It does not mean, as You realized, that a projectile can pass. It´s rather the velocity, where the plug / disc formation is sstarted to be torn free. The limit is more difficult to be seen in ductile penetration modes, but can be associated with the development of a bulge plus star cracks.

The PTP is not shown in the drawing but would be very close to the Army BL.
IP20 and IP80 are later, soviet penetration limits, for which the old battlefiedl.ru site compiled a lot of data.

One thing I have to add to my initial outline is that the german delta of 8% velocity spread between G(d) and G(s) does only apply under conditions where the projectile stays intact and the hole expansion is ductile or intermediate (both of which was typically the cas ein german 30° obliquity standart using RHA and their respective, capped Pzgr39).

[Peasant]

The PTP is the limit of back strength. It does not mean, as You realized, that a projectile can pass. It´s rather the velocity, where the plug / disc formation is sstarted to be torn free. The limit is more difficult to be seen in ductile penetration modes, but can be associated with the development of a bulge plus star cracks.

The PTP is not shown in the drawing but would be very close to the Army BL.
IP20 and IP80 are later, soviet penetration limits, for which the old battlefiedl.ru site compiled a lot of data.

One thing I have to add to my initial outline is that the german delta of 8% velocity spread between G(d) and G(s) does only apply under conditions where the projectile stays intact and the hole expansion is ductile or intermediate (both of which was typically the cas ein german 30° obliquity standart using RHA and their respective, capped Pzgr39).

Is the G(s)=0,92*G(d) holds true for other obliquities? As I understand these two limits get closer together as the obliquity increases. Would this imply that they are more spread out on the other end(e.g. near the 0°)? And is there some study done on this matter that you know of?

[critical mass]

I have not enough information to jump to a conclusion. But judging from the fact that the G(D) and G(S) are their respective minima and maxima, the total spread is also correlated to variance in projectile quality. If You exceed the limit of intactness for a shell, which will be between 40° and 50° for Pzgr39, You will encounter larger variances (some projectile stay intact while others break up initially, changing the penetration mode), so one might expect an increase in the delta to account for the mixed zone before the spread narrows down again due to all projectiles breaking up at obliquities >50/55°.

[Peasant]

There seems to be not much info about British plate penetration/defeat definitions around, so here I repost a comment I copied from an older thread:

The following is copied verbatim from Appendix F to PRO document WO 194/662, “Report on firing trials held at Inchterf range, 01/01/1942 - 31/12/1942”:

"PENETRATION CODE USED IN FIRING TRIAL REPORTS

A. = Slight or no impression.

B. = Deep impression.

C. = Deep impression and bulge at back.

D. = Deep impression and bulge at back and crack.

E. = Deep impression with flake off back of plate, or with bulge and crack sufficient to see daylight through the plate (after removal of shot if necessary) or with any portion of the projectile protruding through or visible from the back of the plate.

P. = Pinhole penetration.

R. = Penetration; core or shot not clean through the plate.

W. = Penetration; core or shot clean through.

(L)= Core or shot lodged in the plate.

Note :- (L) can qualify any letter of the code from B to R, and is used as in the following examples.

D(L) = Deep impression and bulge at back and crack. Core in plate.

P(L) – Pinhole penetration. Core in plate.

The C/D Limit is the average of four or more velocities within a range of 70 f.s. half of which give a “D” or worse damage and half of which give a “C” or less damage. It shall not be considered valid if such average exceeds any “D” by more than 35 f.s.

The Ballistic Limit is the average of four or more velocities within 70 f.s., half of which give “E”s or worse damage and half “D”s or less damage. It shall not be considered valid if such average exceeds any “E” by more than 35 f.s.

The W/R Limit is the average of four or more velocities within 70 f.s., half of which give “W”s and half “R”s or less damage. It shall not be considered valid if such average exceeds any “W” by more than 35 f.s.

“C” Angle Immunity is the smallest angle at which the projectile at the velocity specified causes only “C” damage. The velocity specified for 2-Pr. is that obtained by full service charge at 100 yards.

NOTE It will be noted from these definitions that it is possible for any of the above limits to coincide with any of the others and this is quite common in attack at angle."

I'd argue that therefore the "Ballistic Limit" shown in the British penetration charts is quite misleading as it's virtually identical to the US Army BL rather than the complete penetration or W/R Limit most assume. Therefore, unless it's explicitly indicated that the numbers are the W/R limit, the values must be converted first to N.B.L before an accurate comparison can be made.

[Mobius]

I believe that was early war British penetration.

The March 1944 British Tank School Definition of CV.
The Critical Velocity is an average velocity at which a 50% projectile success rate occurs. A projectile success being an impact velocity at which at least 20% of the projectile (or the less subjective "significant proportion of the projectile nose") passes completely through the plate. A projectile "failure" is any impact velocity which does not meet the definition of projectile "success".

At least this is what I copied.

[Peasant]

The Critical Velocity is an average velocity at which a 50% projectile success rate occurs.

Mobius, this doesn't contradict what was said previously. Both limits can have their own different Critical velocities, in this instance the W/R limit is being used an an example with some clarification on what constitutes "shot clean through the plate."

And the OB still used the therm "ballistic limit" as a distinct thing from the W/R Limit, as far as during this test from 28 Feb. of 1945:

[Mobius]

What gun were they testing, 17 pdr? A.P. in 1945?

[Peasant]

What gun were they testing, 17 pdr? A.P. in 1945?

6 pdr vs Panther side armour.

Btw, you might want to quote some part of the post you're referring to, like I do, to make the forum send a notification otherwise you're gonna wait for days until the person will check the thread.

[Mobius]

What gun were they testing, 17 pdr? A.P. in 1945?

6 pdr vs Panther side armour.

Btw, you might want to quote some part of the post you're referring to, like I do, to make the forum send a notification otherwise you're gonna wait for days until the person will check the thread.

There are no 6 pdr CV graphs that match those striking velocities with those ranges. Whelp posted a number of them awhile ago. Even the MV= 2925 f/s AP has 2636 f/s @ 350 yds.