rexford Posted February 1, 2001 Author Share Posted February 1, 2001 Paul, Data from TM-9-1907 At 1850 fps, 75mm M61 penetrates 2.5" at 30° 50% of time. At 1100 fps, 75mm M61 penetrates 1.25" at 30° 50% of time. If 75mm M61 strikes first 1.25" @ 30° and penetrates 100% of time, and then continues on and penetrates second 1.25" @ 30° half the time, the required velocity is 1665 fps. So two spaced 1.25" @ 30° plates have an equivalent single plate resistance of 2.17" at 30° (this is penetrated at 1665 fps). A single 2.5" plate at 30° offers 15% more resistance than two 1.25" spaced plates hit at 30°. Germans put spare tracks on a target and calculated resistance at three angles. At low impact tracks lowered resistance, at intermediate tracks did nothing and at steep angle the tracks added. This may mean that at low angles tracks did not add full thickness to resistance, and at steep angles track addition to resistance was greater than track thickness. Will look deeper into plates in contact hit at angle. Link to comment Share on other sites More sharing options...
rexford Posted February 1, 2001 Author Share Posted February 1, 2001 Previous post was for homogeneous armor. Against face-hardened 75mm M61 penetrates 2.5" at 30° at 1710 fps, and 1.25" at 30° at 1060 fps. 1604 fps penetrates both 1.25" at 30° spaced face-hardened plates 50% of time, which is equivalent to a single face-hardened 2.25" plate at 30°. So one 2.5" face-hardened plate at 30° is 11% more resistant than two 1.25" face-hardened spaced plates hit at 30°. When round went thru first plate at 30° it probably would be turned towards the second plate, so 30° impact on first spaced plate and less than 30° on second, so a single plate hit at an angle is even more resistant than previously calculated. There is little available data on plates in contact hit at an angle, and this really is an important issue. Crusader had sandwich armor with a very hard plate underneath, and it is said that the armor performed better than a single plate of lower max hardness. With face-hardened armor the hard layer is on the outside, Crusader carried hard layer on inside. When armor is hit at 0° and 40°, two different types of failure would be active. Against near vertical armmor, projectile may penetrate plates in contact by pushing nose through armor. As nose pushes through first plate, there is less resistance to pushing armor away from line of travel since back-up plate does not apply shear stresses as first plate moves outward. This is theory behind concept that homogeneous plates in contact are less resistant. On 40° hits, plate defeat by plugging. Projectile hits first plate and tries to push a plug downward, second plate resists plug movement. It is possible that on angled hits plates in contact are better than one plate with same overall thickness. This needs further research. Link to comment Share on other sites More sharing options...
Paul Lakowski Posted February 2, 2001 Share Posted February 2, 2001 I have papers on Tungsten penetrators hitting aluminum targets that are a series of blocks at normal and slanted impact [30-45°]. The curious thing is that , as one would expect, they penetrate hugh amounts of aluminum , but in almost every case the penetrator hits the second or third block and either 'turns' an additional 30° instantly and stops penetrating or fractures into several piece, again right away and stops penetrating right away. I'll re read the paper to see if I can get any more out of it and I'll take another look at TM 9-1907. Link to comment Share on other sites More sharing options...
Guest Andrew Hedges Posted February 2, 2001 Share Posted February 2, 2001 I like reading these Rexford threads, and, to the extent that they are correct, I hope CM models them. But I'm sure happy that I don't have to check the equations or do the coding. In fact, I would happily pay someone $45 plus $8 shipping do that for me. Oh, wait. Link to comment Share on other sites More sharing options...
rexford Posted February 2, 2001 Author Share Posted February 2, 2001 Found some WW II tests for 40mm AP against two plates in contact, 30 degree impact. Two plates in contact resisted with 8% less resistance than a single plate with same overall thickness. Bovington provided penetration data for German guns at 30 degree impact, with following figures for Panther 75: 30 degree penetration 100 yards-141mm 1000 yards-121mm 0 degree penetration (estimated) 0 yards-190mm 100 yards-186mm 1000 yards-157mm Above figures are what we use for Panther 75 penetration. Following British data appears to be actual test data since weight of armor driven out of plate is given: --------------------------------------- 88L56 APCBC-HE 500 yards 30 degrees-114mm 0 degrees-148mm (my estimate) 1000 yards 30 degrees-106mm 0 degrees-136mm (my estimate) 1500 yards 30 degrees-97mm 0 degrees-123mm (my estimate) 0 yards 0 degrees-163mm (my estimate) ----------------------------------- 88mm Flak 41 500 yards 30 degrees-159mm 0 degrees-210mm (my estimate) 1000 yards 30 degrees-149mm 0 degrees-195mm (my estimate) 1500 yards 30 degrees-139mm 0 degrees-181mm (my estimate) 0 yards 0 degrees-226mm (my estimate) A DeMarre estimate for 0m penetration of 88 Flak 41 from 88L56 yields 234mm, which is close to 226mm (3.5% difference). Link to comment Share on other sites More sharing options...
Paul Lakowski Posted February 2, 2001 Share Posted February 2, 2001 Originally posted by rexford: 0 yards 0 degrees-226mm (my estimate) A DeMarre estimate for 0m penetration of 88 Flak 41 from 88L56 yields 234mm, which is close to 226mm (3.5% difference). Heres my estimate based on erosion modeling My adpated [Anderson ]Formula is V[striking velocity] x 0.52 - In of L/d x 0.15 [natural log of the projectile lenght to diameter] raised to a value for projectile sharpness [ratio of tip diameter to body diameter] x scaling factor [ from Ordnance board 1950 Penetration of armor plate] . 88 L71 mm 1000m/s 0.52- 0.18 = 0.336 ^ 0.38 = 0.66 x 1.1 = 0.727x 309mm = 224.5 mm @ 0° [3.5:1 L/d Steel APC with 6.5:1 T/B @ 1000 m/s Vs 0° RHA 300 @ muzzle ] Hogg figures are 207mm @ 0° @ 500 m suggests 224m @ 0° @ muzzle ; I don't yet have a good penetrator dimentions so it could be slightly higher. Looks like we're both heading in the same direction. Link to comment Share on other sites More sharing options...
rexford Posted February 3, 2001 Author Share Posted February 3, 2001 Paul, Re-checked WW II tests where 40mm AP fired at two plates in contact, impact angle is 30 degrees. Two plates in contact at 30 degrees are 11% more resistant than a single plate of same overall thickness hit at 30 degrees. Same result in both cases. So the result you noted in an earlier post, that plates in contact at an angle react differently from vertical case, appears to apply to WW II steel ammo. This suggests that 89+89+13 around Churchill MG port is one heck of a tough target with any kind of lateral angle. Link to comment Share on other sites More sharing options...
Paul Lakowski Posted February 3, 2001 Share Posted February 3, 2001 Originally posted by rexford: Paul, Re-checked WW II tests where 40mm AP fired at two plates in contact, impact angle is 30 degrees. Two plates in contact at 30 degrees are 11% more resistant than a single plate of same overall thickness hit at 30 degrees. Same result in both cases. So the result you noted in an earlier post, that plates in contact at an angle react differently from vertical case, appears to apply to WW II steel ammo. This suggests that 89+89+13 around Churchill MG port is one heck of a tough target with any kind of lateral angle. In addition to that if the projectile impacts in any kind of Yawed manner this effect will be even more pronunced, mind you it takes alot of YAW to reduce AP penetration [ as opposed to APFSDS]. I'll do some digging and since I have you address I can always make copies and send you some papers that may be of interest. Link to comment Share on other sites More sharing options...
Jeff Duquette Posted February 3, 2001 Share Posted February 3, 2001 Rex Said: There is little available data on plates in contact hit at an angle, and this really is an important issue. Crusader had sandwich armor with a very hard plate underneath, and it is said that the armor performed better than a single plate of lower max hardness. This seems ass-backward. Why would the Brits place the hard plate on the interior side of sandwiched plates. Even if penetration doesn’t occur there is still the possibility of interior high velocity spalling from the hard plate. Link to comment Share on other sites More sharing options...
rexford Posted February 3, 2001 Author Share Posted February 3, 2001 Hard armor on outside of Crusader, softer steel on inside. Layered armor on Crusader performs well, according to stories. Link to comment Share on other sites More sharing options...
Paul Lakowski Posted February 3, 2001 Share Posted February 3, 2001 Originally posted by rexford: Hard armor on outside of Crusader, softer steel on inside. Layered armor on Crusader performs well, according to stories. There was a paper that I sent around awhile ago , that showed this...must say it came as a suprise to us too. It seems that alot changes as you move from plate to plate and the penetrator can 'turn' or 'shatter'when it hits the next plate, especially when the next plate is harder. In fact this kind of effect is supposed tobe embodied in Chobham armor[ called "Interlayer Defeat"]. Hard outer layer is supposed to shatter AP projectile tip, however if the AP projectile has a ballistic cap , the shattering tends not to happen. In a series of test back in the 60s it was shown that the penetrator and plate under went 'work hardening', immediatly after penetration and this could make the penetrator too brittle , and that could lead to shatter. As always we need to do more digging. Link to comment Share on other sites More sharing options...
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