Read Ebook: The Development of Armor-piercing Shells (With Suggestions for Their Improvement) by De Zafra Carlos

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TEST FOR DETECTION OF HOLES, CRACKS, ETC.

TENSILE TEST

After forging, the projectiles shall be annealed at a temperature of at least 1,200? F.; and after being annealed, tangential test specimens shall be taken from the base or base prolonged of 2 per cent of the projectiles from each lot selected at random by the inspector.

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

A careful and complete chemical analysis 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

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 shell.

BALLISTIC TEST

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.

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 pass 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 passes the test as prescribed above, the lot shall be accepted; if it fails to do this, the lot shall not be accepted.

Weight uncapped.

For intermediate thickness the velocity shall be determined by interpolation.

Armor-piercing shell. Two capped shell, 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 shell, 4-inches for 8-inch shell, 5-inches for 10-inch shell, and 6-inches for 12-inch shell, with a velocity of about 1,420 f.s. for the 5-inch shell, 1,220 f.s. for the 6-inch shell and 920 f.s. for the 8-inch, 10-inch and 12-inch shell at impact, with the requirement that the shell shall go through the plate unbroken, and then be in a condition for effective bursting.

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.

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 substituted. Such specimens must bend cold to 180 degrees flat, without sign of fracture on outer surface.

If the shell 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 shell fail to pass 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," 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:

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 Class A there shall be three impacts with striking velocities as given in the following table, capped armor-piercing projectiles being used:

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 shell to the Government is called upon to produce a shell with sufficient integrity to completely penetrate, and without breaking up, his armor-plate of sufficient thickness to resist that shell.

The capping of projectiles consists in placing over the point a cone or mass 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 passage of the projectile, others claim that it gives an initial shock to the armor-plate before the shell 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 shell at armor-plate at oblique angles have proven the capped shell 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 shell cannot, in so much as uncapped shells 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 shell 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 shells 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 shell.

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

U.S. SEA-COAST LAND SERVICE GUNS

KRUPP GUNS

From the above table it will be noted that the R.P.M. are exceedingly high in some cases. Upon the impact of a shell with armor-plate the physical phenomena occur instantaneously and the resultant forces are so great that it is impossible to mechanically record their action. A study of the stresses in the shell can, however, be made on a theoretical basis.

In the first place, if the projectile were twenty calibres in length and of a material offering less resistance to torsional stress than steel and rotated at the high velocities indicated we would find that upon impact the torsion would be plainly evident as per the following:

Assume a projectile A of length twenty calibres, about to penetrate an armor-plate B of thickness sufficient to prevent complete penetration by the shell in question.

The tendency of the impact is to stop the rotation of the projectile, owing to the friction between the surfaces in contact, but owing to the length of the projectile the point receives this retarding influence before it can be transmitted throughout the body of the shell to its base. The consequent result is that the head will finally come to a stop while the base is still rotating, however slightly that may be.

The objection to the present method of forging shells is as a result, the grain or fibre of the metal lies parallel with the major axis of the forging, the forging process causing an elongation of the ingot and the metal grain following the direction of elongation. Consequently any flaws occurring in the material will extend parallel to the grain or major axis. If a flaw remains undiscovered in a finished projectile--as is sometimes the case--the projectile is not only weakened thereby, but the element of weakness lies in such a direction that the compression forces and counterforces produce very much the same results as would a wedge driven into a niche, i.e. the separation of adjacent material. The author is in possession of a shell in which a longitudinal flaw was revealed in the ogive by the cutting away of a longitudinal quarter section, Fig. 28.

There are, therefore, two great forces with which to contend in the design of projectiles, to one of which, compression, has been given the greatest attention because of its recognized tendency to cause the base of the shell to crowd upon the head and cause the shell to break up about the ogive. The other force, torsion, seems not to have been considered prior to the present instance, at any rate so far as the author has been able to ascertain, not because thought to be unimportant, but because of oversight or failure on the part of investigators to take into consideration in this instance, an element of reaction commonly considered in mechanical engineering practice, as in shafting for vessels and for power transmission in shops, etc.

In a projectile making one complete revolution about its major axis in every twenty-five calibres flight, any one elementary unit area or mass in that shell likewise makes one complete revolution in the same distance of travel, and the path traversed by that unit area or mass is that of a spiral of radius equal to the distance of that unit area or mass from the major axis of the shell, the diameter of which spiral would be the diameter of the shell in question--and the pitch twenty-five calibres--if said unit area were on the surface of the body of the shell.

Furthermore, should any flaws be present in the ingot, their size would be reduced by the twisting, as are the spaces between the strands of a rope when twisted in the proper direction for so doing. Also, with a flaw in a finished projectile, and lying in a spiral direction the result of the compression stresses would be to jump across the flaw or to decrease the gap instead of acting wedgelike along the flaw causing it to open as before mentioned. Finally, an increase in integrity means an increase in penetrability, or in the percentage of complete penetration, with the ultimate necessity of increasing the thickness of armor-plate to successfully exclude the improved armor-piercing shell.

No. 863,248. PATENTED AUG. 13, 1907.

C. DE ZAFRA.

PROJECTILE.

APPLICATION FILED DEC. 10, 1906.

Carlos de Zafra Inventor

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