# 1922 Encyclopædia Britannica/Armour Plate

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​ARMOUR PLATE (see 2.578). The history of armour plate during the years 1909–21 differs from that of most other mate- rials used in warfare, inasmuch as the period of greatest progress and activity occurred before the World War and was followed by a period of rest amounting almost to stagnation. The actual years of the war, which constituted a period of intensive culture as regards guns, shell, airships, aeroplanes, tanks, etc., added no stimulus to progress in the manufacture of armour plate. The efforts of British shipbuilders were devoted to the building of light, fast cruisers and destroyers for which there was urgent and immediate need, rather than to heavily armed battleships which would take three years to complete.
During the years immediately preceding the war, however, the manufacture of armour plate had made steady progress, and the improvement in quality was marked. There were no radical alterations such as the employment of a new alloy steel, or the introduction of a new process of manufacture; but in the application of scientific principles to the details of manufacture, and the various heat treatments through which the plate passes, immense improvements had been made and were apparent in the quality of the finished plate. In this connexion it can be recorded that a long series of trials have proved beyond doubt that British armour made immediately before the war was greatly superior in ballistic qualities to that manufactured in Germany, in spite of the fact that the process of armour-plate manufacture originally came from Germany. For example, a German 12-in. plate was found to be no better than a British 9-in. under the same test, while a German 10-in. plate was only equal to the British 8-in. The plates tested were taken from the ex-German battleship “Baden,” and are therefore thoroughly representative of the German product.
Table I. British and German Armour.
| Thickness of plate in ℔. per sq. in. | Index representing limiting velocity of penetration | Index representing limiting velocity of penetration |
| 320 ℔. armour 400 " 480 " 560 " | British 1,000 1,000 1,000 1,000 | German 940 less than 855 less than 835 915 |
In Table I the average limiting velocity of penetration for British plates is taken to be 1,000 ft. per second in each case; ​
the third column shows the comparative figures for German plates. The projectiles used at these trials were of similar mark and quality to those used in testing British plates of the same thickness.
In the case of the 400 and 480-℔. plates the actual limiting velocities were not reached, the projectiles, at the velocities indicated by the index figures, passing through the plates in a practically undamaged condition. Tests carried out on turret roof plates of 160-℔. and 200-℔. thickness also showed a marked superiority in favour of the British plates. These results may be accounted for to some extent at least by the fact that the manufacture of armour in Germany was a monopoly, and to all intents and purposes a State monopoly, whereas in Great Britain there were five rival firms of manufacturers and an Admiralty always asking for something better.
The necessity for improvement has been constant owing to the introduction of larger and more powerful guns the 13.5-in. in the ships of the 1909–10 programme and the 15-in. in those of the 1912–3 programme.
Tables II and III, compiled from information contained in a paper read by Sir Eustace d'Eyncourt before the Institution of Naval Architects, show the increase in the thickness of armour on British and German battleships in answer to the challenge of the new guns.
Table II. British Armour v. German Guns.
| Programme | Guns on German ships | Main armour on British ships | Gun-houses |
| 1906–7 | 11 in. 45 calibre | 10 in. and 8 in. | 11 in. |
| 1907–8 | 11 in. 45 calibre | 10 in. and 8 in. | 11 in. |
| 1908–9 | 12 in. 50 calibre | 11 in. and 8 in. | 11 in. |
| 1909–10 | 12 in. 50 calibre | 12 in., 9 in. and 8 in. | 11 in. |
| 1910–11 | 12 in. 50 calibre | 12 in., 9 in. and 8 in. | 11 in. |
| 1911–2 | 12 in. 50 calibre | 12 in., 9 in. and 8 in. | 11 in. |
| 1912–3 | 12 in. 50 calibre | 13 in. tapering to 8 in.  bottom and 6 in. top. | 13 in. |
| 1913–4 | 15 in. | 13 in. and 6 in. | 13 in. |
Table III.—German Armour v. British Guns.
| Programme | Guns on British ships | Main armour on German ships | Gun-houses |
| 1906–7 | 12 in. 45 calibre | 11.8 in. tapering to 6.3 in. | 11 in. |
| 1907–8 | 12 in. 50 calibre | 11.8 in. tapering to 6.3 in. | 11 in. |
| 1908–9 | 12 in. 50 calibre | 11.8 in. tapering to 6.7 in. | 11¾ in. |
| 1909–10 | 13.5 in. | 13.8 in. tapering to 9 in. | 11¾ in. |
| 1910–11 | 13.5 in. | 13.8 in. tapering to 9 in. | 11¾ in. |
| 1911–2 | 13.5 in. | 14 in., 10 in. and 7.9 in. | 14 in. |
| 1912–3 | 15 in. | 14 in., 10 in. and 7.9 in. | 14 in. |
| 1913–4 | 15 in. | 13¾ in. tapering to 10 in. | 13¾ in. |
Any increase in the thickness of armour presents very serious problems to the naval architect on account of the great additional weight to be carried, and it is therefore of vital importance that the quality of the armour should be of the best. It is not only in regard to increase in thickness, however, that progress has been made. The superficial area of plates has also been increased, and plates measuring 15 to 20 ft. in length and 10 to 12 ft. wide are now not uncommon. Large plates are in fact a necessity in modern battleship construction. The striking energy of a large shell is so great that the resistance opposed to it must be distributed over as large an area as possible. As an example of the forces involved the striking energy of a is-in. shell at a range of 10 m. is 30,000 foot tons, or in other words its energy is equivalent to that of an express train weighing 250 tons and travelling at 60 m. an hour. There is grave danger, therefore, that a small plate, even if it succeeds in stopping the shell, may be driven bodily into the ship. Moreover, it is as true to-day as ever it was that the weakest point in any armour is the joint. A heavy shell striking a plate near an edge or corner is liable to break off and carry away a piece of the plate with disastrous results, and it is therefore desirable to eliminate this risk as far as possible by reducing the number of joints to a minimum, that is to say by increasing the size of the individual plates. At the present time the size of plate capable of being placed on a ship is only limited by the carrying capacity of the railways.
No substantial alteration in the process of manufacture of armour has taken place since 1910, and the description given in the earlier article in this Encyclopaedia requires neither modification nor addition. In other respects, however, much has been learned, and some of the views held in 1911 require revision. For example, the statement that “plates sometimes vary considerably and are not of uniform hardness throughout” can scarcely be said to be true to-day, in spite of the great increase in size of modern plates over those made in the years previous to 1911.
It is impossible to discuss improvements in armour plate without at the same time taking into consideration the improvements which have been made in armour-piercing shell, and also the changes which have taken place in the nature of the attack. Conditions during recent years have been constantly changing. The introduction of capped projectiles, and the substitution of “unbacked” for “backed” trials, each presented problems for the armour-plate manufacturer. Moreover it has only been possible to solve these problems by the laborious process of trial and error, for there is no exact knowledge on the subject, and theories (of which there are many) have proved sadly misleading. For example, the action of the cap has been, and still is, a subject for discussion. It was for some time believed that the action of the cap was only effective at velocities over 1,700 ft. per second, whereas actual experiment has proved that it is equally effective at velocities of 1,000 ft. per second and even less. Constant alterations in the size, weight, and design, as well as in the quality of steel used in the manufacture of the cap, have further complicated the problem from the armour-plate manufacturer’s point of view.
Interesting as the theoretical aspect of the subject undoubtedly is, there are at present too many unknown factors, both as re- gards shell and armour, to enable it to be regarded as an exact science; and recent experience has only served to confirm the statement made by an early authority, Maj.-Gen. Inglis, R.E., in 1880, that “in no subject that has ever been raised has mere opinion unsupported by practical experience proved so worth- less as in this.”
Bullet-proof Armour.—While there was a lull in the activity of armour-plate manufacture for naval purposes during the war, there was greatly increased activity in the production of light or bullet-proof armour. When the armies on the western front dug themselves in and the fighting resolved itself into trench warfare there was an insistent demand for some means of protection for the men who had to face rifle fire at close quarters. Innumerable suggestions were made and a vast number of experiments carried out with a view to producing a bullet-proof material of reasonable weight. The ordinary service bullet, consisting of a cupro-nickel (or in some cases a mild steel) case filled with lead, breaks up fairly easily on a plate of hard steel; but the Germans soon discovered that if the bullet is removed from the cartridge and reversed (i.e. so that the bullet travels with the base or blunt end in front instead of the pointed end) it did not break up but punched a hole in the plate.
Every effort was made to defeat this attack, but it was found that even with the use of the best quality of alloy steel available a minimum thickness of half an inch was necessary to stop the reversed bullet at short range. All sorts of materials were employed, but steels were found to be the most efficient, and of these nickel, chrome, manganese, vanadium, molybdenum and zirconium, both singly and in combination, were all tried. The best results, however, were obtained from nickel-chrome plates, sometimes with an addition of one of the rarer metals.
While these experiments were being carried out in England the Germans were busy endeavouring to produce something ​more satisfactory than the reversed bullet which was only effective at short range. In this they were completely successful, and they produced the K or armour-piercing bullet. This con-. sists of an outer envelope of mild steel of the same size and dimen- sions as the ordinary bullet. In the centre of the envelope is the bullet proper, made of hardened tungsten steel 30 mm. long, 6 mm. in diameter, and pointed at one end. The space between the envelope and the hard bullet is filled with lead. On striking a hard steel plate the outer envelope breaks up, but it and the lead lining appear to perform the function of a cap, and the hardened steel bullet perforates the plate.
At ranges up to 60 yd. with a good rifle, and more than this with a rifle in which the rifling has been worn, the armour-piercing bullet is not effective, owing to unsteadiness in flight, but at longer ranges nothing less than half an inch of the best steel is of any use as a protection against a direct hit at the normal. The action of the armour-piercing bullet, however, differs from that of the reversed bullet. The former is a clean penetration of the plate, whereas the latter punches a hole and removes a portion of the plate in the form of a small cylinder. Both at long and short ranges, therefore, a plate of at least half an inch in thickness was found to be necessary to give any real protection, and as plates of this thickness weigh 20 Ib. per sq. ft. it was obvious that a soldier could not carry his own means of protection in addition to a rifle and the other impedimenta which he took into action. It became necessary then to devise some mechanical method of carrying protection, and the combined efforts of many minds in this direction finally resulted in that weapon of offence and de- fence which was afterwards known as a " tank " (see TANKS).
From the nature of the requirements it will be seen that the practice of the armouring of the tanks was by no means an easy one. In the first place the plates in an untreated condition had to be soft enough to be easily machinable, while after treatment they were required to withstand the penetration of the armour- piercing bullet and the punching action of the reversed bullet. This necessitated a very hard plate, but on the other hand it was essential that they should be sufficiently tough to prevent crack- ing or the breaking off of portions of the plate even when struck near an edge or corner. In addition the plates were to be capable of being riveted to the body of the tank or to one another, and finally they must be of the minimum thickness, as the question of weight was all-important.
These requirements were met by the use of nickel-chrome steel, which possesses properties of hardness and toughness to a remarkable degree. Steel containing 0-3 to 0-5% of carbon with 3 to 4% of nickel and 1-5% to 2-0% of chromium was largely used, and in some cases improved by the addition of one of the rarer metals.
In view of the work which the tanks were designed to carry out it was of the utmost importance that they should be perfectly bullet-proof, and it is perhaps not generally known that every plate was tested by firing trial against the German bullet before it was built into the tank. Under this severe but necessary test a very high degree of excellence in the quality of the plates was attained. The manufacturers had their own rifle ranges where the plates were tested under Government supervision before they were dispatched. (E. F. L.)