Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
ARMOUR PLATES
Online EncyclopediaEncyclopedia Home :: APO-ARN
del.icio.us it!
|
See also:ARMOUR PLATES . The earliest recorded proposal to employ armour for See also:ships of See also:war (for See also:body armour, &c., see ARMS AND ARMOUR) appears to have been made in See also:England by See also:Sir See also: 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. For the employment of armour in See also:ship construction and in permanent See also:works on See also:land, see the articles See also:SHIPBUILDING; FORTIFICATION AND SIEGECRAFT; the See also:present See also:article is concerned solely with the actual armour itself. The earliest armour, both for ships and forts, was made of wrought iron, and was disposed either in a single thickness or in successive layers sandwiched with See also:wood or See also:concrete. Conatruc Such armour is now wholly obsolete, though examples tion See also:aaa of it may still be found' in a few forts of See also:early date, testing. The See also:chief application of armour in See also:modern land defences is in the See also:form of See also:shields for the protection of guns mounted en See also:barbette. Examples of such shields are shown in See also:figs. 1 and 2. Fig. 1 shows a 4.5-in.•steel See also:shield for the U.S.A. government, See also:face-hardened by the See also:Harvey See also:process, to which reference is made below. It was attacked by 5-in. and 6-in. armour-piercing shot, and proved capable of keeping out the 5-in. up to a striking velocity of nearly 1800 ft. per second, but was defeated by a 6-in. capped A.P. shot with a striking velocity of 1842 ft. per second. The mounting was not seriously damaged by the firing, but could be operated after the impact of one 3.2-in., five 5-in. and three 6-in. projectiles. Fig. 2 shows a See also:gun-shield, manufactured by Messrs Hadfield of See also:Sheffield, after attack by 4•r-in., 4.7-in. and 6-in. armour-piercing and other projectiles. The limit of the shield's resistance was just reached by an uncapped 4.7-in. A.P. shell with a striking velocity of 2128 ft. per second. The shield (the See also:average maximum thickness of which was 5.8 in.) showed See also:great toughness, and although subjected to a severe battering, and occasionally outmatched by the attacking projectiles, See also:developed no visible crack. It is chiefly remarkable for the fact that it was See also:cast and not forged. As is evident from the fringing around the hole made by the 6-in. A.P. shell, the shield was not face-hardened. A more highly developed form of the gun-shield is to be found in the armoured See also:cupola, which has been employed to a very considerable extent in permanent fortifications, and whose use is still strongly advocated by See also:continental See also:European military See also:engineers. The See also:majority of the cupolas to be found in continental forts are not, however, of very See also:recent date, those erected in 1894 at See also:Molsheim near See also:Strassburg being comparatively modern instances. Any cupolas constructed nowadays would be of See also:steel, either forged or cast, and,would probably be face-hardened, but a large number of those extant are of See also:compound or even of iron armour. Many of those on See also:sea-fronts are made of chilled cast iron. Such armour, which was introduced by Gruson of See also:Magdeburg in 1868, is extremely hard, and cannot be perforated, but must be destroyed by fracture. It is thus the See also:antithesis of wrought iron, which, when of See also:good quality, does not break up under the impact of the shot but yields by perforation. Armour of the Gruson type is well adapted for curved surfaces such as cupolas, which on See also:account of their shape are scarcely liable to receive a See also:direct See also:hit, except at distant ranges, and its extreme hardness would greatly assist it to throw off shot striking obliquely, which have naturally a tendency to glance. Chilled iron, on account of its liability to break up when subjected to a continuous bombardment by the armour-piercing steel projectiles of guns of even See also:medium calibre, was usually considered unsuitable for employment in inland forts, where wrought iron, mild steel or compound armour was preferred. On the other See also:hand, as pointed out by the See also:late See also:Captain C. Orde See also: The manufacture of steel, however, continued to improve, so that in 1890 we find steel plates being made which were comparatively See also:free from liability to through-cracking, while their power to resist perforation was somewhat greater than that of the best compound. The difference, however, was at no See also:time very marked, and between 188o and 1890 the resistance to perforation of either steel or compound as compared with wrought iron may be taken as about 1.3 to I. Compound armour required to be well backed to bring out its best qualities, and there is a See also:case on See also:record in 1883 when a 12-i11. Cammell plate weighing 10~ tons, backed by See also:granite, stopped a 16-in. Palliser shot with a striking See also:energy of nearly 30,000 See also:foot tons and a calculated perforation of 25 inches of wrought iron. As steel improved, efforts were made to impart an even greater hardness to the actual surface or skin of compound armour, and, with this See also:object in view, Captain T. J. Tresidder, C.M.G., patented in 1887 a method of chilling the heated surface of a plate by means of jets of See also:water under pressure. By this methodit was found possible to obtain a degree of hardness which was prevented in See also:ordinary plunging by the formation of a layer of See also:steam between the water and the heated surface of the plate. Compound plates face-hardened on this See also:system gave excellent results, and forged-steel armour-piercing projectiles were in some cases broken up on their surfaces as if they had been merely chilled iron. Attempts were also made to increase the toughness of the back by the substitution of mild See also:nickel steel for wrought iron. The inherent defect of compound armour, however—its want of homogeneity,—remained, and in the year 1891 H. A. Harvey of See also:Newark, N.J., introduced a process whereby an all steel plate could be face-hardened in such a way that the advantages of the compound principle were obtained in a homogeneous plate. The process in question consisted in carburizing or cementing the surface of a steel plate by keeping it for a fortnight or so at a high temperature in contact with finely divided See also:charcoal, so that the heated surface absorbed a certain amount of See also:carbon, which penetrated to a considerable See also:depth, thus causing a difference in chemical See also:composition between the front and back of the plate. After it had been See also:left a sufficient time in the cementation See also:furnace, the plate was withdrawn and allowed to cool slowly until it reached a dull red See also:heat, when it was suddenly 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, which shortly became combined, possessed about twice the resisting power of wrought iron. The figure of merit, or resistance to penetration as compared with wrought iron, varied with the thickness of the plate, being rather more than 2 with plates from 6 to 8 in. thick and rather less for the thicker plates. In 1889 Schneider introduced the use of nickel in steel for armour plates, and in 1891 or 1892 the St Chamond works employed a nickel steel to which was added a small percentage of See also:chromium. All modern armour contains nickel in percentages varying from 3 to 5, and from 1•o to 2.0% of chromium is also employed as a general See also:rule. Nickel in the above quantities adds greatly to the toughness as well as to the hardness of steel, while chromium enables it to absorb carbon to a greater depth during cementation, and increases its susceptibility to tempering, besides conducing to a tough fibrous condition in the body of a plate. Alloy steels of this nature appear to be very susceptible to thermal treatment, by suitable variation of which,, with or without oil quenching, the See also:physical condition of the same steel may be made to vary to an extraordinary extent, a peculiarity which is turned to good account in the manufacture of the modern armour plate. The See also:principal modern process is that introduced by See also:Krupp in 1893. Although it is stated that a few firms both in Great See also:Britain and in other countries use See also:special processes of their own, it is probable that they differ only in detail from the Krupp process, which has been adopted by the great majority of makers. Krupp plates are made of nickel-chrome steel and undergo a special heat treatment during manufacture which is briefly described below. They can either be cemented or, as was usual in England until about 19o2 in the case of the thinner plates (4 in. and under) and those used for curved structures such as casemates, non-cemented. They are in either case face-hardened by chilling. Messrs Krupp have, however, cemented plates of 3 in. and upward since 1895. Although the full process is now applied to plates of as little as '2 in. in thickness, there is some difference of See also:opinion between manufacturers as to the value of cementing these very thin plates. The See also:simple Harvey process is still employed to some extent in the case of plates between 5 and 3 in. in thickness, and excellent results are also stated to have been obtained with plates from 2 to 4 in. in thickness, manufactured from a special steel by the process patented by M. Charpy of the St Jacques steel works at Montlucon. A Krupp cemented (K.C.) plate is not perhaps harder as regards surface than a good Harveyed plate, but the depth of hard face is greater, and the plate is very much tougher in the back, a quality which is of particular importance in the thicker plates, The figure of merit varies, as in Harveyed plates, with the thickness of the armour, being about 2.7 in the case of good 6-in. plates. while for the thicker plates the value gradually falls off to about 2.3 in the case of 12-in, armour. This figure of merit is as against uncapped armour-piercing shot of approximately the same calibre as the thickness of the plate. The resisting power of the non-cemented Krupp plates is usually regarded as being consider-ably less than that of the cemented plates, and may be taken on an average to be 2.25 times that of wrought iron. Figs. 3, 4 and 5 are illustrations of good cemented plates of the Krupp type. Fig. 3 shows an 11.8-in. plate, tried by Messrs Krupp in 1895, after attack by three 12-in. steel armour-piercing projectiles of from 712.7. to 716.1 lb in See also:weight. In the third See also:round the striking velocity of the projectile was 1993 ft. per second, the calculated perforation of wrought iron by Tresidder's See also:formula being 25.9 in. The attack was successfully resisted, all the projectiles being broken up without effecting perforation, while there were no serious cracks. The figure of merit of the plate was thus well in excess of 2.2. The great toughness of the plate is perhaps even more remarkable than its hardness; its width was only 6.28 ft., so that each shot See also:head formed a See also:wedge of approximately one-See also:sixth of its width. The excellence of the See also:metal which is capable of withstanding such a See also:strain is apparent. Fig. 4 is of a 9-in. K.C. plate, made by Messrs See also:Armstrong, See also:Whitworth & Co. for the See also:Japanese government, after undergoing an unusually severe See also:official test. The See also:fourth round was capable of perforating 22 in. of wrought iron, so that the figure of merit of the plate must have been considerably in excess of 2.45, as there were no through-cracks, and the limit of resistance was far from being reached. Fig. 5 shows the front of an excellent 6-in. cemented plate of Messrs Beardmore's manufacture, tried at Eskmeals on the 11th of See also:October 1901. It withstood the attack of four armour-piercing 6-in. shot of Too lb weight, with striking velocities varying from 1996 to 2177 ft. per second. Its limit of resistance was just passed by the fifth round in which the striking velocity was no less than 2261 ft. per second. The projectile, which See also:broke up in passing through the plate, did not get through the skin plate behind the wood backing, and evidently had no surplus energy left. The figure of merit of this plate was between 2.6 and 2.8, but was evidently much closer to the latter than to the former figure. A sixth round fired with a See also: At a See also:low estimate its figure of merit against 3-in. A.P. shot may be taken as about 2.6, which is exceptionally high for a non-cemented, or indeed for any but the best K.C. plates. The plate also withstood the attack of a 4.7-in. service See also:pattern steel armour-piercing shell of 45 lb weight striking the unbacked portion with a velocity of 1599 ft. per second, and was only just beaten by a similar shell with a velocity of 163o ft. per second. The effect of all the above-mentioned rounds is shown in the photograph. The same plate subsequently kept out two 6-in. See also:common shell filled up to weight with See also:salt and plugged, with striking velocities of 1412 and 1739 ft. per second respectively, the former being against the unbacked and the latter against the backed See also:half of the plate,—the only effect on the plate being that round 6 caused a fragment of the right-hand See also:top corner of the plate to break off, and round 7 started a few surface cracks between the points of impact of rounds 1, 2 and 3. Within the limitations referred to below, the resisting power of all hard-faced plates is very much reduced when the armour-piercing projectiles used in the attack are capped, the average figure of merit of Krupp cemented plates not being more than 2 against capped shot as.compared with about 2.5 against uncapped. So See also:long ago as 1878 it was suggested by Lt.-See also:Col. (then Captain) T. See also:English, R.E., that armour-piercing projectiles would be assisted in attacking compound plates if caps of wrought iron could be fitted to their points. Experiments at See also:Shoeburyness, however, did not show that any advantage was gained by this See also:device, and nothing further was heard of the cap until 1894, when experiments carried out in See also:Russia with so-called " magnetic " shot against plates of Harveyed steel showed that the perforating power of an armour-piercing projectile was considerably augmented where hard-faced plates were concerned, if its point were protected by a cap of wrought iron or mild steel. The conditions of the Russian results (and of subsequent trials in various parts of the See also:world which have confirmed them) differed considerably from the earlier English ones. The material of both projectiles and plates differed, as did also the velocities employed—the low velocities in the earlier trials probablyi contributing in large measure to the non-success of the cap. The cap, as now used, consists of a See also:thimble of comparatively soft steel of from 3 to 5 % of the weight of the projectile, attached to the point of the latter either by See also:solder or by being pressed hydraulically or otherwise into grooves or indentations in the head. Its See also:function appears to be to support the point on impact, and so to enable it to get unbroken through the hard face layers of the plate. Once through the cemented portion with its point intact, a projectile which is strong enough to remain undeformed, will usually perforate the plate by a true See also:boring action if its striking velocity be high enough. In the case of the uncapped projectile, on the other hand, the point is almost invariably crushed against the hard face and driven back as a wedge into the body of the projectile, which is thus set up so that, instead of boring, it acts as a See also:punch and dislodges or tends to dislodge a coned plug or disk of metal, the greatest See also:diameter of which may be as much as four times the calibre of the projectile. The disproportion between the maximum diameter of the disk and that of the projectile is particularly marked when the calibre of the latter is much in excess of the thickness of the plate. When plate and projectile are equally matched, e.g. 6" versus 6", the plug of metal dislodged may be roughly cylindrical in shape, and its diameter not greatly in excess of that of the projectile. In all cases the greatest width of the plug or disk is at the back of the plate. A stout and rigid backing evidently assists a plate very much more against this class of attack than against the perforating attack of a capped shot. Fig. 7 shows the back of a 6-in. plate attacked in 1898, and affords an excellent See also:illustration of the difference in action of capped and uncapped projectiles. In round 7 the See also:star-shaped opening made by the point of a capped shot boring its way through is seen, while rounds 2, 3, 4 and 5 show disks of plate partially dislodged by uncapped projectiles. The perforating action of capped armour-piercing projectiles is even better shown in fig. 8, which shows a 250-mm. (9.8 in.) Krupp plate after attack by 150-mm. (5.9 in.) capped A.P. shot. In rounds 5 and 6 the projectiles, with striking velocities of 2302 and 2281 ft. per second, perforated. Round 7, with a striking velocity of 2244 ft. per second, just got its point through and rebounded, while round 8, with a striking velocity of 2232, lodged in the plate. In many cases a capped projectile punches out a plug, usually more or less cylindrical in shape and of about the same diameter as the projectile, from a plate, and does not defeat it by a true boring action. In such cases it will probably be found that the projectile has been broken up, and that only the head, set up and in a more or less crushed condition, has got through the plate. This peculiarity of action can best be accounted for by attributing either abnormal excellence to the plate or to that portion of it concerned—for plates sometimes vary considerably and are not of See also:uniform hardness throughout, —or See also:comparative inferiority to the projectile. Whichever way it may be, what has happened appears to be that after the cap has given the point sufficient support to get it through the very hard surface layers, the point has been flattened in the region of extreme hardness and toughness combined, which exists immediately behind the deeply carburized surface. The action from this point becomes a punching one, and the extra strain tends to break up the projectile, so that the latter gets through wholly or partially, in a broken condition, See also:driving a plug of plate in front of it. At low striking velocities, probably in the neighbourhood of 1900 ft. per second, the cap fails to See also:act, and no advantage is given by it to the shot. This is probably because the velocity is sufficiently low to give the cap time to expand and so fail to grip the point as the latter is forced into it. The cap also fails as a rule to benefit the projectile when the See also:angle of incidence is more than 300 to the normal. The See also:laws governing the resistance of armour to perforation have been the subject of investigation for many years, and a considerable number of formulae have been put Laws of forward by means of which the thickness of armour resistance. perforable by any given projectile at any given striking velocity may-be calculated. Although in some cases based on very different theoretical considerations, there is a general agreement among them as far as perforation proper is concerned, and Tresidder's formula for the perforation of wrought iron, t2=wva/da, may be taken as typical. Here t represents the thickness perforable in inches, w the weight of the projectile in pounds, v its velocity in foot seconds, d its diameter in inches and A the constant given by See also:log A=8.841o. For the perforation of Harveyed or Krupp cemented armour by capped armour-piercing shot, this formula may be employed in See also:conjunction with a suitable constant according to the nature of armour attacked. In the case of K. C. armour the formula becomes 12=wva/4dA. A useful rough rule is 1/d=v/1900. Hard armour, such as chilled cast iron, cannot be perforated but must be destroyed by fracture, and its destruction is apparently dependent solely upon the striking energy of the projectile and See also:independent of its diameter.- The punching of hard-faced armour by uncapped projectiles is intermediate in See also:character between perforation and cracking, but approaches the former more nearly than the latter. The formula most used in England in this case is Krupp's formula for K.C., viz. t2=wv2/dA', where t,w,v and d are the same as before, and log A'=6.3532. This, if we assume the sectional See also:density (w/da) of projectiles to be constant and equal to 0.46, reduces to the very handy rule of thumb t/d=v/2200, which, within the limits of striking velocity obtainable under service conditions, is sufficiently accurate for See also:practical purposes. For oblique attack up to an angle of 3o° to the normal, the same formula may be employed, t sece being substituted for t, where 0 is the angle of incidence and t the normal thickness of the plate attacked. More exact results would be obtained,' however, by the use of Tresidder's W.I. formula, given above, in conjunction with a suitable figure of merit, according to the nature and thickness of the plate. It should be remembered in this connexion that the figure of merit of a plate against a punching attack falls off very much when the thickness of the plate is considerably less than the calibre of the attacking projectile. For example, the F.M. of a 6-in. plate may be 2.6 against 6-in. uncapped A.P. projectiles, but only 2.2 against 9.2-in. projectiles of the same character. In the case of the perforating action of capped projectiles, on the other hand, the ratio of d and t does not appear to affect the F.M. to any great extent, though according to Tresidder, the latter is inclined to fall when d is considerably less than t, which is the exact opposite of what happens with punching. Another method of measuring the quality of armour, which is largely employed upon the See also:continent of See also:Europe, is by the ratio, r, between the velocity requisite to perforate any given plate and that needed to See also:pierce a plate of mild steel of the same thickness, according to the formula of Commandant See also:Jacob de Marre, viz. v=Ae° a°76/p°'5 where e= the thickness of the plate in centimetres, a= the calibre of the projectile in centimetres, p = the weight of the projectile in kilogrammes, v = the striking velocity of the projectile in metres per second, and log A=1.7347. Converted into the usual English See also:units and notation, this formulabecomes v= A't°'?d°'75/w°'b, in which log A1=3'0094; in this form it constitutes the basis of the ballistic tests for the See also:acceptance of armour plates for the U.S. See also:navy. Common shell, which are not strong enough to remain undeformed on impact, derive little benefit from the cap and usually defeat a plate by punching rather than by perforation. Their punching power may be taken roughly as about j that of an uncapped armour-piercing shot. Shells filled with high See also:explosives, unless special arrangements are made to deaden the bursting See also:charge and so obviate detonation upon impact, are only effective against the thinnest armour. With regard to manufacture, a brief account of the Krupp process as applied in one of the great English armour plate works (omitting confidential details of temperature, &c.) will illustrate the great complexity of treatment a ure. which the modern armour plate has to undergo before its remarkable qualities of combined hardness and toughness can be developed. The composition of the steel probably differs slightly with the manufacturer, and also with the thickness of the armour, but it will usually contain from 3 to 4 % of nickel, from 1.0 to 2.0 % of chromium and about 0.25 to 0.35 % of carbon, together with from 0.3 to 0.7 % of See also:manganese. After being cast, the See also:ingot is first heated to a uniform degree of temperature throughout its See also:mass and then generally forged under the See also:hydraulic See also:forging See also:press. It is then reheated and passed through the rolls. After See also:rolling, the plate is allowed to cool, and is then subjected to a thermal treatment preparatory to surfacing and cutting. Its surface is then freed from See also:scale and planed. After planing, the plate is passed into the cementation furnace, where its face remains for some See also:weeks in contact with specially prepared carbon, the temperature being gradually raised to that required for cementation and as gradually lowered after that is effected. After cementation the plate is heated to a certain temperature and is then plunged into an oil See also:bath in See also:order to toughen it. After withdrawal from the oil bath, the plate is cooled, reheated to a See also:lower temperature, quenched again in water, reheated and passed to the bending press, where it is See also:bent to shape while hot, proper See also:allowance being made for the slight See also:change of See also:curve which takes See also:place on the final chilling. After bending it is again heated and then allowed to get See also:cold, when the final machining, drilling and cutting are carried out. The plate is now placed in a furnace and differentially heated so that the face is raised to a higher temperature than the back. After being thus heated for a certain period the plate is withdrawn, and both back and face are douched simultaneously with jets of cold water uhder pressure, the result being that the face is left See also:glass-hard while the back is in the toughest condition possible for such hard steel: The cast-steel armour made by Hadfield has already been alluded to. That made by Krupp (the only other maker at present of this class of armour) is of face-hardened nickel steel. A 5.94n. plate of this material tried in 1902 had a figure of merit of more than 2.2 against uncapped 5.9-in. armour-piercing projectiles of are lb in weight. The See also:main advantage of cast armour is that it is well adapted to armoured structures of complicated See also:design and of varying thickness, which it would be difficult or impossible to forge in one piece. It should also be cheaper than forged armour, and, should time be a See also:consideration, could probably be turned out more quickly; on the other hand, it is improbable that heavy castings such as would be required could be as See also:regular in quality and as free from flaws as is possible when forged material is used, and it is unlikely that the average resistance to attack of cast-steel armour will ever be equal to that of the best forged steel.
Of recent years there has been a considerable demand for thin steel plating proof against small-See also:arm bullets at See also:close ranges. This class of steel is used for See also: too „ 0.167 „ 1865 „ 560 „ 0.080 „ 1080 „ The weight of the o•o8-in. plating is only 3.2 lb per sq. ft. The material is stated to be readily adaptable to the ordinary operation of bending, machining, drilling, &c., and is thus very suitable for the purposes indicated above. (W. E. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] ARMORICA (AREMORICA) |
[next] ARMOUR, PHILIP DANFORTH (1832-1901) |