Armour Trials at Indian Head, Maryland

Armour Trials at Indian Head, Maryland

June 25, 1918

I. Midvale 13.25 inch plate attacked by Hadfield’s 12-inch A.P.C. shell, Mark IV A.

This plate had been previously tested by 12-inch, 870 lbs. Firth-Stirling U.S. shell, had successfully resisted three rounds at the specified velocity 1568 f.s., and had consequently been accepted as satisfactory.

The plate, according to American practice described in previous reports, was erected backed by 3 inches of oak and 5/8 inch mild steel plate, the whole being well supported by heavy baulks of timber about 18 inches wide with about 2 feet space between each baulk. American practice is to allow 50 f.s. for this system of support above the striking velocity for an unbacked plate.

The results obtained were as follows:

Allowing 50 f.s. for backing, the limit of this 13-1/4" plate unbacked is about 1505 f.s., i.e., about 265 f.s. lower than the limit for British 480 lbs. Armour.

If this result for an American 13-1/4-inch armour plate held for a 12-inch plate and the limiting velocity were proportional to (thickness) 2/3 the limit for an American 12-inch plate, comparable to the British limit of 1770 f.s., would be 1390 f.s.

Test of 13-1/2 inch American Armour Plate, manufactured by Bethlehem Steel Co.

The object of this test was to ascertain:-

1. The effect of a British Hadfield 12-inch A.P.C. shell, Mark IV A on an American plate supported according to British practice.
2. The difference in resistance resulting from the difference in British and American methods of supporting the plate.

An American plate supported according to American practice had previously been attacked with Hadfield 12-inch Mark IV A and the results, which have been reported, indicated that the plate was distinctly inferior to a British plate. In an effort to account for this the hypothesis was raised that the British system of support allows the plate to give slightly on impact from the shell and the point of the projectile has consequently less opportunity to bite on the face. This is directly opposed to the ordinarily accepted hypothesis that the absence of backing lowers the resistance.

In the previous tests the plate was backed with oak backing and 5/8-inch mild steel plate, and strongly supported with stout timber baulks in the manner described in earlier reports. The plate now under report had been so tried with the following results:

Attack by 12-inch, 870 lbs. Crucible Steel Shell (1910)

The plate, therefore, satisfied the U.S. requirements and was accepted.

In addition a round was fired using Midvale 12-inch, 870 lbs. new type shell with the following result:

A smooth scoop 31" long, 6-1/2" wide, with 1-1/4" penetration was formed in the plate and a smooth bulge at the back. The plate thus satisfied specification requirements and no more rounds were fired.

Test of 13-1/2 Inch Armour Plate.

Plate manufactured by the Carnegie Steel Company for belt armour of "Colorado" class. It was tapered from 13-1/2 inches to 8 inches at the lower edge. The plate was backed with 3 inches of oak backing and bolted to 5/8" mild steel plate; this work is done by the armour plate manufacturers, the plate so backed is delivered to the Proving Ground ready for erection. The support to the plate is shown in the attached sketch and consists of 16" timbers well shored, the space between the timbers being 26 inches. The point of aim is between these supports, but the plate is so much more securely backed than in British trials that a direct comparison of ballistics would not do British armour justice unless strict account were taken of this difference in trial conditions.

Three rounds were fired at the centre of the plate, the distance between each round being 3-1/2 calibres, centre to centre. Attack was by 12" 870 lbs. capped shell at a portion of the plate actually 13-3/8" thick. The results were as follows:

The plate satisfied specification requirements; viz., to keep out shell at 1578 f.s. and no crack to extend right through plate. The back of the plate could not be examined, but the impression given was that the face was not so hard as that of British armour. The test, of course, was not severe. The surface of the plate was very rough and had a number of cracks, some about 1/16 inch wide and 5/8 inch deep. These were particularly noticeable at the end of the plate forged from the top of the ingot. I was informed that the plate was not planed, and was forged instead of being rolled, also that the face is plane during manufacture, the taper being formed by forging. At the end of the process of manufacture the plate is bent so that the back is plane:-

[diagram omitted]

The trial was attended by representatives of the Bethlehem Steel Works and the Midvale Steel Works, as well as representatives of the makers, the Carnegie Steel Company.

In the tests carried out on October 2nd the plate was unbacked and supported at the top and bottom edges with the sides lightly supported by tie-bolts to timber baulks.

The effect of the Midvale 12-inch, 870 lbs. New type Shell was as follows:-

After firing this round it was found that the gun jacket was cracked.

The effect of a Hadfield 12-inch, 850 lbs. A.P.C. Mark IV A shell was as follows:

The inferences drawn from these trials were:-

1. Previous results with Hadfield Mark IV A vs. American armour are confirmed and British armour is markedly superior.
2. The difference in resistance due to the difference in British and American methods of supporting the trial plate is so small that it cannot be measured from the results of rounds 4 and 6. In the two cases the damage was very much the same, the striking velocity being somewhat higher in round 4 and the penetration a little more.

It was interesting to note that the appearance of the damage done by rounds 5 and 6 was much more similar to that done to British armour than in any tests previously witnessed of American armour supported according to American methods. It seems probable that the presence of backing and supports, and the consequent increased resistance of the plate to bending, accounted for the smaller area of face damage, smaller lip at rear and fewer cracks round hole in rear, which have been previously considered to be characteristics of American armour.

These scans are courtesy of Dr. George Elder (GHE101@aol.com) and Warship Tech Spec. Unfortunately the quality of some of these images is poor.

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