Class A Armor
1211. Class A armor
The general term Class A armor is applied, in the American Navy, to all face-hardened armor, whether Krupp cemented or Krupp non-cemented.
1212. Ballistic test
During manufacture the processes are watched to insure uniformity in procedure and product. Chemical analyses are made to insure homogeneity in the metal, and test specimens are taken out of the ends of the plates to determine the uniformity of the physical properties. Finally, after water hardening, fragments called coupons are broken off diagonally opposite corners, in order that a microscopical examination may be made of the internal structure. (See Plate V.)
Plates are arranged in “groups,” each containing from 600 to 1,200 tons, and after water hardening a plate is selected from the group, sent to the Proving Ground, and there tested ballistically. If successful, the group is passed to completion; if unsuccessful, the group is rejected for retreatment. The ballistic test consists, at present, in subjecting the plate to the normal impact of two major caliber A. P. projectiles at specified velocities, the velocity depending on the gauge of the plate and size of the projectile. This relation will be discussed under the heading of “penetration.”
In testing and experimenting with armor, great energies must be absorbed. For instance, a 16-inch projectile striking normally at 2,000 f. s. delivers about 60,000 foot-tons. Assuming that the plate is 10 ft. X 20 ft. X 15 in., that gives about 300 foot-tons per square foot of plate area, or about 1,100 foot-tons per ton of plate. The structure to which the plate is attached must, therefore, be of great strength. Such structures are called plate-butts. Similar structures used for testing the action of projectiles against plates are called projectile-butts.
Particular attention must be given in conducting all impact tests to eliminate, so far as is possible, all movement of the attacked plate, or, in other words, to preserve the rigidity of the structure as a whole. This is particularly true when plates are attacked at other angles than normal, for in such cases there may be most powerful end thrusts, which, if uncontrolled, will permit end movement and therefore subject the projectile to uncontrolled and unknown “whip,” thereby vitiating the value of the test.
Plate VI shows an armor plate after undergoing test.
The two shots required for the routine ballistic test are for the purpose of determining whether the manufacturer’s current manufacturing methods, in general, and the group represented, in particular, are up to the required standard. That condition having been established, and the group’s acceptability settled, it is customary to attack the plate with one or two additional shots to determine its precise ballistic limit. To determine this ballistic limit the velocity of impact is increased until penetration is just or nearly secured. Experienced personnel can, with ordinary armor, generally secure this result (after two ballistic test shots) in two more shots, and sometimes even with one shot.
In experimenting with and testing armor it is the ballistic limit which is the significant figure. How it is expressed will be shown later. But it is readily seen that by always working for it, a record can easily be compiled, in time, which will show not only the general average of performance, but also the peak of performance. And by always carefully investigating the plates which give the greatest resistance, the general standards may be raised.
1213. Penetration
Considerable discussion took place in early days of ship armor as to whether armor could best be defeated by “racking” or “punching.” Racking was produced by very large solid ball shot at low velocity and resulted in knocking plates off the ship’s side, thus exposing her vitals; and punching was produced by elongated projectiles, at high velocity, and resulted in perforation and the immediate attack of the ship’s vitals. This discussion was settled during our Civil War very emphatically, and later in the Huascar-Cochrane and Blanco Encalado engagement; for the shots which secured decisive effects, practically without exception, were piercing shots. This discussion finds its echo in more recent controversies concerning the relative merits of armor-piercing and high-capacity projectiles. Here, again, war decided emphatically, for in the World War it was conclusively shown that the decisive hits were those made by armor-piercing projectiles.
The principal function of armor is, therefore, to prevent, so far as possible, the penetration of a projectile into a ship. But the “racking” effect cannot be neglected. This is at present, primarily a matter of securing; but the use of guns with projectiles of great weight, the demand for increased speed and consequent sacrifice in protection, and the probable obliquity of impact, are gradually forcing attention back to the “racking” or “smashing” attack.
The penetration of a pointed projectile into a homogeneous or simple plate was very accurately expressed, in the early days of armor, by various empirical formulas. The one which has survived the various developments, and is still the basis of armor calculation, is that of a Frenchman, J. DeMarre.
This formula, as used in the late eighties and as applied to the penetration of cylindrical projectiles with ogival heads into plain wrought iron, is as follows:
Problem I.-How far will a 10-inch 500-pound projectile penetrate to a wrought-iron plate at a striking velocity of 1,772 f. s.?
Answer: 23.1 inches.
1214. Penetration of nickel-steel armor
Subsequent to the adoption of nickel-steel armor, the formula was revised and, when applying to that material, is used in the following form:
Problem II.-How far will a 10-inch 500-pound projectile penetrate into a nickel-steel plate at a striking velocity of 1,772 f. s.?
Answer: 15.77 inches.
Problem III.-What is the ratio between penetration in wrought and nickel-steel?
In nickel-steel armor, the projectile encounters a homogeneous material and therefore a more or less constant resistance. In face-hard-armor the projectile encounters media of varying hardness and strength, and its retardation follows laws which can be but imperfectly stated. For purposes of comparison, therefore, the limit of resistance of face-hardened plates is generally referred to a basis of penetration of nickel-steel.
1215. Penetration of face-hardened armor
The factor of performance.-Upon the introduction of face-hardened armor, therefore, the DeMarre formula was modified by introducing a coefficient of reduction (K), thus;
or in the form in which it is used:
Problem IV.-Assuming K as 1.23 for a particular Class A plate, what would be the penetration of a 6-inch A. P. projectile weighing 105 pounds at 1,836 f. s. striking velocity?
Answer: 7 inches.
The most useful and frequent application of the formula in this connection is, however, to determine the comparative value of plates. If we determine by test the velocity at which a plate just resists penetration or just permits penetration by a particular projectile, we may consider the coefficient of reduction, mentioned above, as a factor of performance, and then solve the equation to determine the factor of performance. The formula then becomes:
This formula is very useful for normal impacts, but, like all empirical formulas, it has its limitations. It is most accurate and useful at striking velocities between 1,400 f. s. and 2,000 f. s. and when the ratio between the diameter of the projectile and thickness of plate lies between 1.2 and 0.7; but it is liable to give erratic or misleading results when those limits are exceeded. The formula (in addition) assumes similarity of projectile quality and penetrative ability.
Problem V.-Find the maximum factor of performance, or, as it is generally expressed, the DeMarre coefficient, of a particular K. C., 12-inch plate.
Solution: Let us assume that our recent experience in testing armor has shown that the average DeMarre coefficient is about 1.16. Experience has also dictated that the DeMarre coefficient of a plate can be most reliably and usefully determined when even caliber projectiles are used. (By even caliber, we mean that the caliber of the attacking gun and thickness of the plate are the same.) We therefore decide to use the 12-inch gun, with an 870-pound projectile, with which to secure our data.
By solving formula (3) for the assumed conditions, we get 1,463 f. S. We fire a shot at this velocity and learn that the projectile penetrated by a small margin, the amount of margin being approximated by its travel after penetration.
We fire a second shot at a velocity of 1,400 f. s. and the projectile fails to penetrate, but it is evident that a slightly greater velocity would have caused penetration. We conclude that a velocity of 1,420 f. S. would have just caused penetration. With this figure we solve formula (4) and determine:
The maximum factor of performance of this plate is 1.12, or, in other words, its limit of resistance is represented by a DeMarre coefficient of 1.1165.
Problem VI.-A 14-inch Class A plate was attacked as follows:
First shot, 12-inch 870-pound projectile, at 1,500 f. s. Penetration, 10 inches.
Second shot, similar projectile at 1,545 f. s. Penetration, 13 inches.
Third shot, similar projectile at 1,590 f. s. Penetration complete.
What DeMarre coefficient expresses the limit of resistance of plate?
Answer: About 1.108 at v= 1,570 f. s.
Problem VII.-A 13-inch Harveyized nickel-steel plate was just defeated by a 10-inch 500-pound projectile, at a velocity of 1,620 f. s., while a 12-inch K. C. plate was just defeated by a similar projectile at a velocity of 1,685 f. s. What is the relative value of the two plates?
Answer: Harvey plate is to Krupp plate as 1 is to 1.216.
1216. Oblique attack
-It will be accepted, without proof, that by inclining a plate to the line of fire its resisting power is enhanced. We may state this in two ways: first, as the angle of obliquity increases, with a given plate, the projectile will require a greater velocity to penetrate; or, as the angle of obliquity increases, there is a decrease in thickness of the plate necessary to defeat a given projectile at a constant velocity.
The angle of obliquity is the angle between the normal to the face of the plate and the axis of the projectile at the point of impact.
Considerable experimentation has shown that while the ratio is obscure throughout the entire quadrant (angle of obliquity 0° to 90°), a much clearer relation is shown in each of the three sectors: angle of obliquity 0° to 30°, from 30° to 60°, and from 60° to 90°. That is to say, having determined the velocity necessary to defeat a plate at, say 45°, the velocity to defeat the plate at angles of obliquity between 30° and 60° may be fairly accurately determined by applying a given ratio to velocity which caused defeat at 45°. Or, having determined the defeating velocity of a plate with given projectile at 15°, one may predict with reasonable accuracy, the defeating velocity with that projectile and plate at angles of obliquity between 0° and 30°.
It should be borne in mind that there is a real difference in the action of the plate and projectile in the three sectors mentioned above, the change from sector to sector being gradual. However, in the most oblique sector, that from 60° to 90°, there is a decided difference from other two, for in this sector we are dealing with the glancing blow in most pronounced form. This difference has given rise to a specific class of armor, designed to care for this particular condition.
Experiments have indicated that the velocity necessary to defeat a plate varies about as the fourth power of the secant of the angle of obliquity.
This ratio can be reduced to figures, and when so used is generally referred to as an “angle multiplier.” In order to plan an attack on a given plate we fix the normal velocity by the DeMarre formula (4) and apply the “angle multiplier” as determined by the above formula (5) to fix the velocity for the specified test.
In the sector from 0° to 30°, the ratio is more or less correct, while in the middle sector, from 30° to 60°, it can be used as a basis of comparison.
Or, if we know with reasonable certainty the velocity necessary to effect penetration at a given angle on a particular plate and desire to calculate the velocity required to penetrate a plate of different thickness and equal ballistic resistance, we would divide the velocity by the angle multiplier, then use the DeMarre formula (5) to calculate the relative velocities on the basis of normal impact, and finally secure our desired result by applying the “angle multiplier” to our calculated normal velocity.
In our discussions of Class A armor penetration and the use of the DeMarre formula, we have thus touched on two special factors: the factor of performance for normal impact, and the angle multiplier for oblique impact. In experimental work it is well to keep these two factors separate and distinct, and to plan and work out our problems so that the ratios between different plates are kept related to these two separate factors.
Back to top
