The Development of Armor-piercing Shells Part 2

The tensile strength of the projectiles in a lot shall not vary more than 20,000 pounds from the highest to the lowest.

CHEMICAL TEST

(Art. 30, O. D., U.S.A.)

A careful and complete chemical a.n.a.lysis shall be made of the metal of each heat from which the projectiles are manufactured under these specifications.

TESTS FOR THE DETECTION OF INITIAL STRAINS BORDERING ON RUPTURE

(Art. 31, O. D., U.S.A.)

After final treatment and before acceptance for the ballistic test, all A.P. shot must be cooled to a temperature of about 40 F., and then suddenly heated by being plunged into a bath of water at a temperature of from 180 F. to 212 F., as the Chief of Ordnance may direct. When thoroughly heated to this temperature each projectile must be plunged, with its axis horizontal, halfway into a bath of water at a temperature not greater than 40 F., and after a brief period shall be turned 180 for a like immersion of the opposite side, after which the projectile shall be removed from the bath.

This test shall be made in the presence of the inspector, and an interval of at least three days must elapse between the final treatment and the submission of the projectiles to this test. This test is not required for sh.e.l.l.

BALLISTIC TEST

(Art. 32, O. D., U.S.A.)

Each lot of projectiles shall be subjected to the following ballistic test:

After a final treatment and on presentation of the entire lot for the ballistic test, the inspector shall select three projectiles to represent the lot, which shall be finished, inspected and delivered in the same manner as required for the rest of the lot.

(a) Armor-piercing shot. Two capped shot, sandloaded to standard weight, shall be fired against a hard-faced Krupp armor plate from 1 to 1-1/2 calibres thick, secured to a timber backing in a manner satisfactory to the Chief of Ordnance, with about the corresponding velocity given by the following table, with the requirement that the shot shall perforate the plate unbroken and then be in condition for effective bursting.

If both projectiles fulfill the above test, the lot will be accepted.

If, one of the shots fails to pa.s.s the test here prescribed, a supplementary test shall be made by firing the third shot under the same conditions as the first two shot; if this pa.s.ses the test as prescribed above, the lot shall be accepted; if it fails to do this, the lot shall not be accepted.

--------------+-------------+----------------+------------- Calibre of Weight of Thickness of Velocity for shot shot plate penetration --------------+-------------+----------------+------------- 4-inch 33[3] { 4-inch 1,930 { 5-inch 2,295 4.7-inch 45[3] 5-inch 2,220 5-inch 58 { 5-inch 2,005 { 6-inch 2,320 6-inch 106 { 6-inch 1,950 { 8-inch 2,450 8-inch 316 { 8-inch 1,760 { 10-inch 2,100 10-inch 604 { 10-inch 1,745 { 12-inch 2,020 12-inch 1,046 12-inch 1,730 --------------+-------------+----------------+-------------

[3] Weight uncapped.

For intermediate thickness the velocity shall be determined by interpolation.

(b) Armor-piercing sh.e.l.l. Two capped sh.e.l.l, sandloaded to standard weight, shall be fired against a hard-faced Harveyized armor plate secure to a timber backing in a manner satisfactory to the Chief of Ordnance, of 3-inches thickness for 5-inch and 6-inch sh.e.l.l, 4-inches for 8-inch sh.e.l.l, 5-inches for 10-inch sh.e.l.l, and 6-inches for 12-inch sh.e.l.l, with a velocity[4] of about 1,420 f.s. for the 5-inch sh.e.l.l, 1,220 f.s. for the 6-inch sh.e.l.l and 920 f.s. for the 8-inch, 10-inch and 12-inch sh.e.l.l at impact, with the requirement that the sh.e.l.l shall go through the plate unbroken, and then be in a condition for effective bursting.

[4] The weight of powder charge to give the prescribed velocity will be determined shortly before the test, cast iron projectiles of proper weight being fired for the purpose; this weight of charge will be taken as giving the prescribed velocity to the projectiles undergoing test.

(c) 12-inch deck piercing sh.e.l.l. Two sh.e.l.l, sandfilled to standard weight, will be fired with a striking velocity sufficient to pa.s.s completely through a 4-1/2-inch nickel-steel protective-deck plate inclined so as to give an angle of impact of 60 degrees, and to be supported by a suitable backing of wood; or both sh.e.l.l shall be subjected to such alternate ballistic test as the Department may judge to be an equivalent to the above in its effect upon the projectile.

The nickel-steel protective-deck plate shall be manufactured by the open-hearth process and shall contain about 3-1/4 per cent of nickel, not more than six one-hundredths of one per cent of phosphorous; not more than four one-hundredths of one per cent of sulphur, shall be the best composition in all respects.

It shall be oil or water tempered and annealed, and the whole plate shall be subjected to the same treatment at the same time.

Tensile test will be made after final treatment. One longitudinal specimen for tensile test will be taken from each plate. Each shall show a tensile strength of at least 80,000 pounds per square inch and an elongation in 2 inches of at least 27 per cent.

Bending tests will be made as follows: A piece cut from the plate shall be doubled cold around a curve of which the diameter is not more than the thickness of the piece tested without showing any cracks. The ends of the piece are to be parallel after bending. These specimens shall be 12 inches long, 1-1/2 inches wide, and 1 inch thick.

At the discretion of the inspector, bending specimens 1/2 inch square taken with a hollow drill, may be subst.i.tuted. Such specimens must bend cold to 180 degrees flat, without sign of fracture on outer surface.

(d) 12-inch Torpedo Sh.e.l.l. Two sh.e.l.l, sandloaded to standard weight, will be fired from a gun or mortar into a sand b.u.t.t with a pressure in the powder chamber of about 37,000 pounds per square inch to test structural ability.

If the sh.e.l.l are found not seriously deformed by discharge from the piece and in a condition for effective bursting, the lot will be accepted.

If any of the sh.e.l.l fail to pa.s.s this test, the lot will be rejected.

The following extract from the "Circulars and Specifications of the Navy Department concerning Armor Plate and Appurtenances for Vessels of the U.S. Navy," (April 22, 1907) while pertaining to another subject, will be pardoned if introduced here for the purpose of demonstrating the seemingly paradoxical requirements a manufacturer is called upon to meet:

(Par. 60.) The ballistic test for acceptance of armor shall be made as strictly as practicable in accordance with the following tables, the Department reserving the right to use guns of other calibres than designated for any plate if it is deemed advisable.

In the test of armor of Cla.s.s A there shall be three impacts with striking velocities as given in the following table, capped armor-piercing projectiles being used:

-------------+-----------+-------------+----------- Wt. of sh.e.l.l Calibre of Thickness of Striking capped gun plates velocity Pounds Inches Inches Ft.-seconds -------------+-----------+-------------+----------- 105 6 5 1,451 105 6 6 1,648 105 6 7 1,836 165 7 6 1,464 165 7 7 1,631 165 7 8 1,791 260 8 7 1,459 260 8 8 1,603 260 8 9 1,741 510 10 9 1,458 510 10 10 1,568 510 10 11 1,676 870 12 11 1,424 870 12 12 1,514 -------------+-----------+-------------+-----------

The first impact shall be located near the central portion of the plate, and the other two impacts shall be located as directed by the Bureau; no impact, however, to be nearer another impact or an edge of the plate than 3-1/2 calibres of the projectile used.

On these three impacts no projectile or fragment thereof shall get entirely through the plate and backing, nor shall any through crack develop to an edge of the plate or to another impact.

From the above it is seen that a manufacturer supplying both armor-plate and sh.e.l.l to the Government is called upon to produce a sh.e.l.l with sufficient integrity to completely penetrate, and without breaking up, his armor-plate of sufficient thickness to resist that sh.e.l.l.

The capping of projectiles consists in placing over the point a cone or ma.s.s of metal of comparative softness. In the United States services soft steel is used for the purpose. Authorities disagree as to the exact function which the cap plays, some claiming it to act as a lubricating metal facilitating the pa.s.sage of the projectile, others claim that it gives an initial shock to the armor-plate before the sh.e.l.l proper has struck it, which latter then strikes the plate in a state of molecular unrest, and, therefore, of impaired resisting power.

Firing tests of sh.e.l.l at armor-plate at oblique angles have proven the capped sh.e.l.l superior, which would indicate that the cap in this instance at any rate is capable of securing a hold on the plate which the bare point of the sh.e.l.l cannot, in so much as uncapped sh.e.l.ls glance off. At any rate capped projectiles are, on the whole, superior to the uncapped and the practice of capping is recommended as an additional advantage when used in conjunction with the improvements here-in-after described.

At a specified distance from the base of the sh.e.l.l a groove or band-score is turned for the rotation band. For projectiles under 7-inches calibre, pure copper is usually employed, but for larger calibre an alloy of 97-1/2 per cent of pure copper and 2-1/2 per cent of nickel is used and is annealed before banding. The rough bands are in a form of solid rings cut from drawn tubes or cylindrical castings, and must be carefully hammered into the score or preferably pressed in by hydraulic pressure and finally turned to proper size, shape, and finish.

Their use has been previously described and the improvements in armor-piercing sh.e.l.ls hereinafter described are based upon a study of the stresses sustained by a projectile upon impact while rotating about its major axis at the high rotative velocity which the engaging of these bands with the rifling of the gun has imparted to the sh.e.l.l.

The following table compiled by the author gives the rotative velocities of various projectiles:

-------+----------+----------+-------+-------+------+------------ Muz. Engy. Calibre Wt., lbs. Muz. Vel. Ft. Type of Inches Projectile Ft. Secs. R.P.S. R.P.M. Tons Gun -------+----------+----------+-------+-------+------+------------ 3 12 870 139 8,340 63 Hotchkiss 3.2 13.5 1,685 253 15,180 266 Field '90 3.6 20 1,550 206 12,360 333 " 1891 3.6 20 650 86 5,160 59 Mortar 1890 5 45 1,830 176 9,560 1,045 Siege 1890 7 105 1,085 76 4,560 853 Howitzer '90 7 125 690 49 2,940 412 Mortar '92 -------+----------+----------+-------+-------+------+------------

U.S. SEA-COAST LAND SERVICE GUNS

-------+----------+----------+-------+-------+--------+---------- Muz. Engy. Calibre Wt., lbs. Muz. Vel. Ft. Type of Inches Projectile Ft. Secs. R.P.S. R.P.M. Tons Gun -------+----------+----------+-------+-------+--------+---------- 8 300 1,950 111 6,660 7,907 1888M 10 575 1,975 95 5,700 15,548 1888M 12 1,000 2,100 84 5,040 30,750 1902 16 2,370 1,975 59 3,540 64,084 -------+----------+----------+-------+-------+--------+----------

KRUPP GUNS

-------+----------+----------+-------+--------+-------+---------- Muz. Engy. Calibre Wt., lbs. Muz. Vel. Ft. Type of Inches Projectile Ft. Secs. R.P.S. R.P.M. Tons Gun -------+----------+----------+-------+--------+-------+---------- 6 2,600 192 15,520 3,000 222 13,320 8 2,200 133 7,980 10 2,250 108 6,480 12 2,250 90 6,400 -------+----------+----------+-------+--------+-------+----------