Slope Effects for Thin Plates

[rexford]

The following web site has some penetration vs angle curves for German 380mm APCBC ammo, which brings up an interesting aspect of slope effect analysis.

http://www.warships1.com/W-INRO/INRO_Hood_p2.htm

Our base data for APCBC slope effects is limited to a range where the T/D ratio ranges from low to high figures, but does not really reach very low ratio's. I have read where thin plate gains added resistance from stretching, which suggests that our slope multiplier vs T/D ratio's for each angle may not work well at very low T/D.

Here is an analysis of 380mm APCBC slope effects vs T/D ratio with comparison to estimates using our equations;

380mm APCBC at 250 m/s

penetrates 200mm at vertical and 80mm at 57 degrees from vertical

2.50 slope multiplier when T/D = 0.211 (80mm/380mm)

If U.S. 75mm APCBC hit 16mm at 57 degree armor (T/D = 0.211), the slope effect would be about 1.88 from our equations.

380mm APCBC at 170 m/s

penetrates 120mm at vertical and 40mm at 67 degrees from vertical

3.00 slope multiplier when T/D = 0.105 (40mm/380mm)

If U.S. 75mm APCBC hit 8mm plate at 67 degrees (T/D = 0.105), the slope effect would be 2.13 from our equations.

380mm APCBC at 120 m/s

penetrates 80mm at vertical and 40mm at 57.5 degrees from vertical

2.00 slope multiplier when T/D = 0.105 (40mm/380mm)

U.S. 75mm APCBC against 8mm at 57.5 (T/D = 0.105) would have a slope effect of 1.63 from our equations.

It may be that our models don't consider hits on very thin armor where the stretching of the plate comes into play. Our base data may not go to really low T/D ratio's.

[rexford]

Robert Livingston has noted that scale effects may lead to greater resistance of an 80mm plate as opposed to 16mm armor.

U.S. analysis of firing test results theorized that the surfaces on a plate offer less resistance than the interior areas, since the outer surfaces are not tightly supported and are relatively free to move vertically and laterally. This would suggest that an 80mm plate would outperform a 16mm plate when both were hit at the same angle and T/D ratio.

The above explanation appears to offer a reasonable explanation of the deviations from our curves.

[Soddball]

How thin does the armour have to be to see this effect? Are we talking halftrack armour thin (8-16mm) or thicker or thinner? How much of a difference do you think this effect you're describing (I understood about 1 word in 3 ) could make?

[ShakyJake]

Rexford...as in Lorrin Rexford Bird? If so, I picked up your book "World War II Ballistics: Armor and Gunnery" some time back, and it is very well done. I've learned quite a bit about things I've had no clue about by reading through it. Consider this adulation.

[rexford]

Originally posted by ShakyJake:

Rexford...as in Lorrin Rexford Bird? If so, I picked up your book "World War II Ballistics: Armor and Gunnery" some time back, and it is very well done. I've learned quite a bit about things I've had no clue about by reading through it. Consider this adulation.

Same person. Thanks for nice words.

[rexford]

Originally posted by Soddball:

How thin does the armour have to be to see this effect? Are we talking halftrack armour thin (8-16mm) or thicker or thinner? How much of a difference do you think this effect you're describing (I understood about 1 word in 3 ) could make?

It seemed that 8mm armor would be less resistant than 40mm armor at the same Thickness/Diameter ratio, where 380mm rounds hit the 40mm and 75mm hit the 8mm.

So we're talking large ammo against really thin plates, but at a very long range. At normal combat ranges the lowered resistance of thin plates should not be a factor against big projectiles.

For small projectiles like machine guns and anti-tank rifles, the penetration data is against thin plates so no big change.

So the thin plate effect should not really have much impact on CMBO or CMBB.