Armour Plates

ARMOUR PLATES. The idea of giving extra protection to the hulls and decks of war vessels is a very ancient one. Archi medes, in building the famous "Syracusan"—that most palatial of ancient ships—for King Hiero, around 25o B.C. provided for "mats composed of stout ropes suspended by brazen chains." No doubt these were a provision against ramming and missiles; and the cables that were made taut around the hulls of Greek ships served at least a part of the same purposes. The "Syracusan," too, was completely surrounded by an iron balustrade ; we are not told its vertical dimensions. In Rome, there was an accepted technical distinction which divided all war vessels into apertae (with un covered deck) and cataphracts such as the quinqueremes, wherein the wall of the ship was carried up beyond the gunwale to the deck as a protection to the rowers. In Byzantine times, when combustible—and perhaps explosive—missiles came into use, leather curtains were adopted or else saturated woollen sheaths. By the 11th century the Scandinavian sea-kings had vessels that were armoured in the modern sense. In the Svarfdaela we read : "Ljot the Pale is in the east in the Swedish isles; he has . . . a `dragon' covered with iron above the sea; it goes through every ship." There was another such owned by Fridthjof's father, Thorstein, "and its sides were sheathed with iron" (Fridthjof's Saga, I.). The iron hands over the prows of the mediaeval ram ships are well known. A ms. of about 1430, describing the side wheeler, mentions a "covered" ship of that type as in use by Cat alonians; and there have come down other mentions of that formidable Catalonian ship. About 1535 the Knights of Malta possessed a certain "great ship" that is reported to have been completely sheathed with iron and that long remained the terror of the Turks and of the pirates.

Until the close of the Napoleonic wars ships-of-the-line were virtually armoured, for their sides of almost two feet of oak were nearly shot-proof to naval artillery of the day at all but short ranges. The increase of gun powder towards the middle of the century made metal armour indispensable.

Historically, the earliest of modern proposals to employ armour for ships of war (for body armour, etc., see ARMS AND ARMOUR) appears to have been made in England by Sir William Congreve in 1805. In The Tines of Feb. zo of that year reference is made to Congreve's designs for an armoured floating mortar battery which the inventor considered would be proof against artillery fire. Among Congreve's unpublished papers there is also a suggestion for armour-plating the embrasures of casemates. Nothing, however, seems to have come of these proposals, and a similar lack of appreciation befell the next advocate of armour, John Stevens of Hoboken, N.J., who submitted the plans of an armoured vessel to Congress in 1812. The Stevens family, how ever, continued to work at the subject, and by 1841 had determined by actual experiment the thickness of wrought-iron armour which was proof against the projectiles then in use.

In 1827 a proposal to apply iron for defensive purposes was put forward by Major General Ford, R.E., who suggested iron bars as a protection for the face of masonry in forts, but the result of the experiment was not encouraging. In 184o experiments were con ducted by the British Admiralty to ascertain the effect of shot on iron plates backed with various soft and elastic substances. These experiments were instituted not so much for the purpose of test ing the value of defensive armour against shot as to ascertain the value of iron as a material of construction for ships of war. The employment of thick iron plates as an external casing for the pro tection of ships of war appears first to have been carried out by the French, who in the Russian War employed three iron-cased floating batteries in the attack upon Kinburn on Oct. 17, 1855. These batteries were exposed to a heavy fire at a distance of 700 yards for about three hours, unsupported by the fleet, and al though some casualties occurred from shell and shot entering the large old-fashioned portholes yet the floating batteries themselves were practically uninjured. Early in the American Civil War the value of protective armour was again conclusively proved in the battle, March 9, 1862, between the "Monitor" and the "Merrimac" (q.v.), both armoured vessels. In the Civil War railroad iron was sometimes used for side armour, and turret-armour was built up of laminated one-inch plates. Thenceforth the utility of iron armour in protecting ships of war became apparent, and the present article describes the successive steps by which modern armour has been evolved.

The military application of armour for the protection of guns mounted in permanent fortifications followed. Its development, however, took rather a different course, and the question of armour generally is of less importance for the military engineer than for the naval constructor.

In 1857 experiments were made at Woolwich with a set of 4in. plates, some of iron and some of steel, and it was found that in some cases the plates offered a good resistance at 600 yards to 68-pdr. solid shot, but that with a repetition of blows the plates were broken up. Wrought-iron shot appeared much more destruc tive than cast iron, and as a defensive material steel plates were far inferior to wrought iron in point of resistance.

Tests were also made in 1857 at Woolwich on the resistance of blocks of cast iron. These blocks were 8ft. long, 2 f t. broad and 22ft. thick fitted together by tongues and grooves. They were fired at from a 68-pdr. with cast-iron solid shot, and wherever the shots struck radiating cracks were formed sometimes extending through the block, and the block was more or less displaced. With wrought-iron shot a greater effect was produced, these shot recoil ing unbroken whereas the cast-iron shot always broke up.

For the purpose of investigating thoroughly the application of iron to defensive purposes of war a special committee on iron was appointed in 1861 by the Secretary of State for War with the con currence of the Admiralty. The committee consisted of Captain Dalrymple Hay, R.N. (Chairman), Major Jervois, R.E., Brevet Colonel W. Henderson, R.A., Dr. Percy of the Museum of Geology, W. Fairbairn, Esq., and W. Pole, Esq., with Captain A. Harrison, R.A., as Secretary. this committee sat until 1864 and conducted a large series of investigations and experiments.

The committee came to the conclusion that a steel material, either alone or in combination with iron, was objectionable, and the most suitable material was simple wrought iron, the best kind of iron being that which combined in the greatest degree the qual ities of softness and toughness. In order to allow the energy of the shot to be absorbed in indenting and battering the plates without producing further fracture, rolled iron plates were on the whole found to be better than hammered plates, as hammered plates generally had the tendency to be hard and unequal, though at the same time rolled plates were frequently affected by unsoundness of welding.

Great interest had always attached to the question of backing most suitable for ship armour plates. When armour plates were first used they were fixed directly upon the hull of timber ships, and when first applied to iron ships it was thought expedient to imitate the former condition by placing a backing of Wood between the armour plate and the hull of the vessel. Many objections were raised to this, among them being the liability of the wood to decay and to be destroyed by fire and shells, but the committee were unable to recommend that wood backing could be dispensed with, as it appeared to perform important functions for which no thor oughly efficient substitute could be found. The wood backing was finally thinned down until its use was confined to fairing the surface of the ship so that the armour could be really fitted to it.

Many combinations of thin iron plates with layers of other sub stances interposed were tried, but never with such results as to warrant their adoption. Investigations were carried out into the chemical composition of the various plates tried, with the conclu sion that the purer the iron the more suitable was its quality, pro vided that it did not contain sufficient carbon to render it steel like. More importance was attached to soundness of manufacture than to freedom from impurities, and although the French plates tried were the more free from foreign matter yet the British plates sustained less damage at the trials.

Iron Armour.

The earliest armour was therefore made of iron ; both rolled and hammered plates being used. The French ship "Gloire," a wooden frigate protected by a complete belt of iron armour in thickness, completed in 1859, was the first armour-clad ship of war. The British ship "Warrior," completed in 1861, was the first armour-clad warship to be built entirely of iron, the armour belt consisting of iron armour 4-lin. thick. The increase in calibre of guns, and the use of steel shot, necessitated an increase in thickness of armour, and in the "Bellerophon," completed in 1866, a thickness of 6in. of armour was fitted. The introduction of Palliser's ogive-pointed chilled-iron shot and fur ther improvements in size and penetrative power of naval ordnance were met by further increases in thickness of armour, resulting in the "Inflexible," completed in 1881, carrying a belt of 24in. in thickness, worked in two thicknesses of 12in.

Plate II., fig. 2, shows a 41in. hammered iron armour plate, on a target with i8in. of wood backing, representing the side of H.M.S. "Warrior." The plate, which was 8ft. long by 4f t. wide, was made by the Thames Iron Works. It was tested at Shoeburyness on Jan. 8, 1864. Round 735 was an 8in. solid cast-iron shot, weight 661b., and rounds 736 and 737 were 8in. solid forged steel shot, weight 751b., all fired from a 68-pdr. gun, the striking velocity of the iron shot being about 1.400f t. per second and the steel shot about 1,3ooft. per second. The steel shot penetrated 4.8in. and the cast-iron shot 1.7 5in., showing the superiority of steel over iron shot. The steel shot at round 736 remained bedded in the plate, and when knocked out by the subsequent round was found to measure 82in. by Tin.

Compound Armour.

In the meantime, however, important experiments were being carried out in the use of steel for armour plates, among the most important being the Spezia trials in 1876 on a Schneider steel armour plate. Steel plates were harder than wrought iron, and offered greater resistance, but the steel was not tough enough and consequently cracked badly when struck two or three times. This defect in steel armour led to the adoption of compound armour, this armour consisting of a steel face with a wrought-iron back. As early as 1867 some experimental armour plates were made by welding steel-face plates to wrought-iron backs. The object of welding the steel and iron together was to prevent fissures extending through the armour, and as the ductility of the iron was superior to that of steel it was supposed that the welding of the plates would also hold the outer face of steel in its place after being broken by impact of projectiles. The first trial of this compound armour, however, showed that the welding of the two metals was imperfect and that they separated under im pact. Satisfactory compound armour plates were, however, intro duced by the two Sheffield firms, Cammell and Brown, the processes being known as the "Wilson" and "Ellis" respectively. In the Wil son process a wrought-iron plate, whilst red hot, was placed in a mould, and liquid steel was run in. The temperature of the liquid steel being much in excess of the welding heat of iron, the surface of the iron plate became partially fused by the liquid steel, and thus a complete union between the two metals was obtained. In this case, the weld is not limited to a simple line of junction be tween the iron and steel, but the change from iron to steel is grad ual. When the steel became solid, the plate was removed from the mould, and of ter reheating was rolled down to the thickness re quired. In Cammell's plate 18.9in. thick, tested at Spezia in 1882, the plate was rolled down from 3oin. thick, the final hard-steel face being about six inches.

In the Ellis process a steel plate for the face and a wrought-iron plate for the back were united by running molten steel between them, the combined plate being subsequently rolled down to the correct thickness. In Brown's 18.9in. plate tested at Spezia in 1882 the final hard face was about Sin. thick, the total thickness of steel being about six inches. As a result of trials, a proportion of about one-third steel and two-thirds iron was found to give the best results.

Compound armour was first used on the turrets of the "Inflex ible" and continued in general use in the British Navy up to the "Royal Sovereign" class (1892), in which ships the belt was of compound armour i8in. thick. The superiority of compound armour over iron was computed to be in the proportion of about three to two.

The introduction about 1886 of the Holtzer and other forged steel-armour piercing shot, which could not be shattered as the Palliser shot had been, presented fresh problems. The compound plate relied upon breaking up the shot, but if the shot pierced the hard face without breaking up, the soft iron back could not offer much resistance. Plate I., fig. 1, shows the face and Plate I., fig. 3, the back of a Io2in. compound plate manufactured by Messrs. Cammell and Co., tested in 1893. Rounds and 2,748 were Firth's 6in. armour-piercing shot, and rounds 2,746 and were Palliser 6in. shot, all rounds being fired at io yards range. The considerably increased penetrating power of the later armour piercing shot compared with the earlier Palliser shot is apparent from the photographs. It will be observed that the Firth shot per forated the steel face and caused considerable cracking, while the Palliser shot simply splashed on the face and broke up. It will be seen from fig. 3 that the wrought-iron back was not pierced by any of the rounds. The Firth shot caused considerable bulging, which was entirely absent in the case of the Palliser shot.

Steel Armour.

Considerable improvements in solid steel armour were made by the French firm of Schneider, and in 1888 the Sheffield firm of Vickers produced a very good all-steel armour plate. From 1882 to 189o, several international trials took place at which various makes of compound and all-steel armour were tested. and although compound armour was generally adopted in Great Britain yet on the Continent the superiority of this armour over all-steel armour was not universally acknowledged, and French ships of this period were protected by steel and compound armour in about equal proportions.

The attention of makers who favoured compound armour was thus directed to the necessity of increasing the hardness of the steel face, and in 1887 Captain T. J. Tresidder patented a method of rapidly chilling the steel face by means of jets of water under pressure. This method of chilling the face of armour plates is in general use at the present day.

The progress made in steel armour resulted in its adoption in 1891 for the secondary defence of battleships in Great Britain, the main belt being of compound armour. In the "Royal Sovereign" class the upper belt consisted of four inches of steel armour.

Pl. II., fig. i shows the face, P1. I., fig. 4 the back of a 14 in. nickel steel armour plate manufactured by Messrs. Vickers, tested in 1893. The plate was tested with Firth's 9.2in. armour-piercing shot, with a striking velocity of i,800ft. per second. The first shot, round No. 2,74o, penetrated the plate and made a star-shaped crack at the back, the shot being thrown back whole. The second shot holed the plate and caused cracking as shown, the shot being thrown back broken in three pieces.

Harveyed Armour.

The next important improvement was the introduction in 1891 of Harveyed armour. Mr. Harvey, in America, having obtained considerable success in water-hardening cemented steel in the case of small articles, turned his attention to the manufacture of armour plates by the same process. The method consisted in carburizing or cementing the face of a steel armour plate by keeping it at a high temperature in contact with finely divided charcoal, so that the heated surface absorbed a cer tain amount of carbon, which penetrated to some considerable depth. After cementation, the plate was allowed to cool to a dull red heat and was then chilled by the application of water, but by a less perfect method than that employed by Tresidder. Steel plates treated by the Harvey and Tresidder processes combined possessed about twice the resisting power of wrought iron, and about one and a half times the resisting power of the former steel armour and compound armour. The best American results had been obtained with Harveyed nickel steel, but there was considerable difference of opinion as to the value of introducing nickel, and its adoption was not universal in England. Harveyed armour was first used in the British Navy in the "Renown," laid down in 1893, where the belt armour was ten inches thick.

Plate I., fig. 2 is a Io2in. nickel Harveyed plate, made by Messrs. Vickers, tested in 1892. Rounds 1, 2 and 5 were 6in. Holtzer armour-piercing shot, and rounds 3 and 4, 6in..Palliser shot. The striking velocity in each case was 1,973f t. per second. The shot was broken up in each case, with only a slight bulge at the back of the plate. The Holtzer shot penetrated about four inches into the plate, and the Palliser shot about three inches. This plate on further trial stopped and broke up the 9.2in. Holtzer armour piercing shot at velocities of 1,698 and 1,8o8ft. per second. The plate was very severely cracked, but it was considered that it would still have offered considerable defence against further pro jectiles.

Krupp Armour.

The basic invention in all modern armour was Harvey's discovery as to carbonizing the front face of the plate, but the Harvey plate suffered from the defect that the back was not sufficiently tough, and after various experi ments with steel of different composition Krupp, of Germany, produced Krupp cemented plate in 1894, based on Harvey's in vention. This was a nickel chrome alloy, the great improvement being the introduction of differential heating. This consisted in heating the carburized face to a temperature sufficient to produce a glass-hard surface when water-quenched, at the same time keep ing the back to a much lower temperature, in order to produce a tough and fibrous back. Both face and back were water-quenched simultaneously. This process, with slight variations, brought about as a result of the development of more scientific principles in the casting and treatment of steel, is that usually adopted in modern armour making. Krupp cemented plates gave results which indi cated that they were equal to wrought-iron plates of about two and a half times their thickness. Krupp armour was first fitted in the "Canopus" class laid down in 1896. The Krupp method of differ ential heating was also applied to thinner plates which were not carburized. These were known as Krupp non-cemented plates.

Plate II., figs. 3 and 4, shows the face and back of a 14.4in. Krupp face-hardened plate tested in 1896. Round 1 was a 12in. St. Chamond armour-piercing steel shell, weight 7161b., striking velocity 2,159f t. per second. The shell was broken up and punched a cylindrical plug out of the plate. Round 2 was with a similar shell at 2,15 7 f t. per second. The shell punched back a cylindrical plug to a depth of 9.4in. but the plug remaied fast in the plate, the shell being broken up. Round 3 was a I2in. Krupp armour piercing shell with a striking velocity of 2,152ft. per second. The shell punched a piece out of the plate and remained stuck in the hole. It will be observed that the back of the plate is free from cracks. No important developments in heavy armour took place during or since the war.

Manufacture.

The manufacture of armour is a highly special ised process, involving great care in the selection of materials, and the subsequent casting and heat treatments. The details of com position and manufacture vary to some extent between different makers, but an average composition is 0•35 to o.5% C, 3.5 to 4% Ni, 1.5 to 2.5%Cr, o.4%Mn, o• 15 %Si. A general description is as follows : The ingot is slabbed down in the press and is then rolled to ap proximately the finished thickness required, with an allowance for machining and for wastage due to oxidation which takes place during the subsequent operations. In some cases, the plate is rolled direct from the ingot. The plate is then annealed, cooled and straightened, and is ready for carburizing. For this operation plates are usually taken in pairs, the two plates being placed face to face with a layer of finely divided charcoal between. The plates are then placed in the carburizing furnace, and are kept at a high temperature for a period of two to three weeks. After carburiza tion the plate is tempered by oil or water cooling, or sometimes by both, to give the required tensile strength and ductility. At this stage any holes required in the hard face are drilled. The plate is then subjected to the differential heat treatment for hardening, the face being brought to a high temperature while the back is kept to a comparatively low temperature. When the plate has reached the required temperatures on face and back, it is with drawn and cooled rapidly by being water-sprinkled simultaneously on face and back. An armour plate for a battleship may be as large as 121t. 6in. by 'oft. and 14in. thick, weighing when finished about 3o tons.

The long ranges at which fighting took place in the naval actions of the World War, and the consequent steep angle of descent at which projectiles struck at these ranges, brought out the desirabil ity of providing deck protection to resist the attack of modern projectiles, and a form of non-cemented armour was developed, the principal features of which were great strength and toughness, with the capability of suffering considerable deformation without rupture.

Another development of the war was a type of thin armour re quired to resist the fire of rifles and machine guns using armour piercing bullets. The plate had to be sufficiently soft in the un treated condition to permit of machining and hard enough after treatment to withstand perforation by the bullet, with sufficient toughness to prevent serious cracking. Various alloys were tried, but a nickel-chrome steel, with sometimes the addition of other metals, was found to give the best results.

Laws of Resistance.

The case of shell perforating armour plates involves so many unknown factors, both as regards the shell and the armour, that it cannot be regarded as an exact science. Many formulae have been evolved from time to time based on various theoretical considerations, the general form of the equa tion being V=C•Tt•D(.WW, V being the striking velocity of the shell, T the thickness of plate, D the diameter of projectile, W the weight of the shell, C a constant usually determined from actual trial results, and t, d and w indices, the numerical value of which varies somewhat in different formulae. From theoretical con siderations of similitude, there are grounds for supposing that the indices should be so related to one another that t+d-F3w=o. In the formulae which follow, the units adopted are V in feet per second, T and D in inches, and W in lbs.

The committee on iron, to which reference has been made, en deavoured to investigate the laws of resistance of plates of differ ent thicknesses to projectiles of different weight and with differ ent velocities. They arrived at the inference that with plates of equally good quality the resisting power might be approximately considered as proportional to the square of the thickness, but in attempting to compare this with the damaging power of the pro jectile, found the latter to depend so much on the material of which it was made that they were unable to deduce any rule.

Fairbairn, in 1861, derived from results of cast-iron shot on wrought-iron plates the formula for perforation V=1300 T•DI/Wi. The form of the expression is arrived at by consider ing the work done in punching a hole in a plate to be equal to the kinetic energy of the projectile. Experiments show that the force required to punch a hole in a plate varies as the plate's thickness if the diameter of hole is constant, and as the diameter of hole if the thickness of plate is constant. The force is therefore propor tional to the product of thickness and diameter. The distance through which the force must act to remove the plug of metal is proportional to the thickness of plate and consequently the work done is proportional to The kinetic energy of the shell is proportional to W.V. From the assumption made, therefore, oo or V oo TD4/W1 Fairbairn's law, even over the re stricted range of velocities in use at the time, required some variation in the value of the constant for any substantial change of velocity. A list of suitable values was tabulated and fairly satis factory results were obtained, within the limits of velocity then in use, but the formula failed on application to the higher velocities obtained later.

On the assumption that in high speed punching, such as is the case with a projectile perforating a plate, the force required to punch a hole in a plate is proportional to the product of the thick ness of plate and diameter of hole and inversely proportional to the velocity of punching. Tresidder obtains the formula for per foration of wrought iron c being a constant given by log c=8.8410. This formula may be applied to the perforation of hard faced armour plates by introducing a numerical co-efficient to T. This co-efficient is called the "figure of merit," and under any given attack is the ratio between the thickness of wrought iron and the thickness of the particular armour under consideration which will be perforated at that attack. The "figure of merit" of an armour plate will vary with the quality of the plate, the relation between thickness of plate and diameter of projectile, the quality of the projectile, and whether capped or uncapped. Experimental firing showed that a soft steel cap on the end of hard pointed armour-piercing projectiles held the point together on impact and gave deeper penetration. For a 6in. cemented plate and a 6in. uncapped projectile the figure of merit might be 2.7, for the same plate with the same projectile capped 2, and for a I tin. cemented plate with the same projectile capped and uncapped 1.8 and 2.3 respectively.

2 The Krupp formula for wrought iron is = WV' log c.D /3 With similar projectiles the weight will vary as the cube of the diameter, and in this case the Krupp formula is in agreement with Tresidder's.

The formula of Commandant Jacob de Marre for wrought iron 5 is log 2•9616.

A later formula adopted by Krupp for hard faced plates is WV' c = 6.3532. It is curious to note that on the assump c.D tion that W varies as this formula agrees in form with Fair bairn's formula for wrought iron. For any particular projectile and striking velocity, the thickness of wrought iron perforated according to Fairbairn's formula would be about twice the thick ness of hard-faced armour given by the Krupp formula.

A method of measuring the performance of armour and shell which is largely used is based on de Marre's later formula for 5 perforation of ordinary steel. This formula is T•7=-. c.D log c =3.00945. The criterion of performance is the ratio between the velocity required to perforate any given armour plate and the velocity calculated from the formula as being required to perf o rate an ordinary steel plate of the same thickness. This ratio is known as the "de Marre co-efficient." The numerical value of this co-efficient is not constant for any particular type of armour or projectile, but is affected by the same factors as have been referred to in the case of the figure of merit. As an example, a 1 tin. shell perforating a i 2in. plate at a velocity of 2,000f t. per second has a de Marre co-efficient of 1.56, and a 9.2in. shell per forating a 9in. plate of the same quality at a velocity of 1,95oft. per second a co-efficient of 1.51.

The various formulae for perforation may be used to obtain a rough comparison of performance between shell and plates of varying dimensions, provided the factors involved in the cases con sidered do not differ too widely. (W. J. B.)