We have not had a suitable opportunity up to the present to comment upon a leading review of Lieutenant Ackerman’s article in Number 73. The review will be found in full in the Engineer of May 24. Among other things it is stated there that “we are unable to accept his conclusions, which, in our Judgment, lead to the substitution of rather a complicated and confused theory, and one which does not accord with results recorded, instead of the present one which is clear and on the whole fairly established. The main subject of the paper is the action of hard armor on shot striking it. Hitherto it has been supposed that a hard and well-supported surface resisted the shot before its point got deep enough in to receive support, and thus it frequently caused fracture or distortion of the shot. In this way the chilled iron shot formerly employed with such success against wrought iron broke almost like snowballs against the first steel-faced armor, and had to give place to steel projectiles.”
The reviewer failed to note that Lieutenant Ackerman was not describing the action of chilled iron shot, but of the steel projectiles which took their place. In fact the effect of the typical face-hardened plate upon the projectile varies with the latter's quality. The conclusions of the essayist were based upon the results of hundreds of impacts of the best quality of modern armor-piercing projectiles, and to attempt to discredit those conclusions by citing the behavior of the inferior chilled iron shot seems absurd.
“Lieutenant Ackerman remarks that Captain Tresidder refers to ‘data which he has seen fit to withhold,’ which, he says, are widely different from the experience obtained in the States. It would be wrong to contradict this without knowing what experience is referred to, but we submit that Lieutenant Ackerman gives us no results in support of what he says. So far as we know, the results of trials in all countries bear out Captain Tresidder explanation; and Lieutenant Ackerman’s difficulty about what he calls the 'mashing of fragments in the indent' would, we think, be removed if he had had more experience with the earlier and inferior projectiles employed, when he would have seen what he regards as inexplicable realized ad nauseam.''
Here again the conclusions derived from numerous experiments with bona fide modern shells and armor are rejected in favor of those derived from the distantly analogous behavior of “the earlier and inferior projectiles employed.” The photographs and descriptions of the four shells opposite pages 42 and 50 in the essay certainly support the writer’s contentions. These shells are not isolated examples; they are typical of the behavior of all modern shells of their class and quality. Two of them are entirely too weak; it will be noticed, however, that the method of failure is not that described by Captain Tresidder and commented upon on page BO of the essay. It will be noted that in the photographs noted the principal cracks are longitudinal and spiral, the points are abraded or fused, and the surface of the ogival flaked away; but the planes of cleavage do not at all agree with those described by Captain Tresidder. On page 44 of the essay is given a description and sketch showing the method of the failure of an ordinary very hard armor-piercing shell, such as was found very successful against oil-tempered steel armor. Many of these heads of projectiles have been shaken out of the plate by subsequent impacts, permitting a close inspection.
“Lieutenant Ackerman adds that the usual action of the hard face, however, is ' that through its inability to bend or flow, it prevents the displacement of the more plastic metal beneath it towards the front, and thus brings the resistance of the whole thickness of the plate to bear before the projectile can advance.' That is to say, the shot is not able to squeeze the metal up in a lip around it as it enters. It is true that this lip is not formed in hard-faced armor. Lieutenant Ackerman has done good service in calling attention to this, which has not been sufficiently noticed, perhaps; but we lay very limited stress on it, because compound or steel-faced iron plate formed no swell or bulge round the point of impact, and yet this armor was easily perforated until it had its face treated on Tresidder's process, when the projectiles were defeated and broken up. Those who have followed the course of experiments in tills country can hardly fail to agree with Captain Tresidder's description of the phenomena exhibited in perforation.”
The results of experiments on bona fide face-hardened armor are again discredited in favor of those obtained with inferior projectiles and obsolete compound armor. As the steel face of compound armor referred to contained about 1 per cent, of carbon, little, if any, flow of metal could occur in it. This lack of elongation, however, made it incapable of absorbing much energy; it cracked through under impact, and being held to the soft back only by a defective weld, was easily displaced and flaked off. The abrupt combination of the brittle hard face and extremely soft back was disadvantageous, for the two parts failed separately; the face having cracked away before the back had elongated to the point of maximum tenacity. The same is true of the compound plate when water hardened, only in that case the superficial hardness had been greatly increased and the remaining metal in the steel face had been improved and perhaps toughened. The method of resisting was really the same; no change had occurred in the back, but the resistance of the face had been increased, and it required a better projectile to perforate it. As the front third of the plate was, as before, of high carbon steel, it could not flow anyway, so it can hardly be said that it bottled up the resistance of the plate by preventing a forward displacement of the metal. In both cases, however, of the water-hardened and the untreated compound plates, the rigid face, when not displaced or flaked off, increased the resistance of the wrougjt-lron back by preventing the metal displaced by the projectile flowing to the front. When the hard face was displaced, concentric waves or bulges of the iron back were almost invariably found around the impact, showing that the soft metal had moved to the front in the direction of least resistance. Water tempering undoubtedly improves the tensile and compressive strength of the face of modern face-hardened armor enormously; in addition, it has greatly improved and toughened the metal in the body of the plate. A great deal of space has been given by Lieutenant Ackerman in the essay to these points. It will be noticed, however, that he has explained the resistance of the plate by describing the combined effect of a face of very great compressive strength with a strong and tough body. Necessarily this creates a complicated and difficult theory. It is very easy to say with the reviewer that the hard face breaks the projectile up from the point, and that the present theory “Is clear, and on the whole fairly established”; but it may be asked, is not the present theory, requiring as it does not only a definition of hardness, but a full comprehension of the mutual behavior under impact of bodies of unknown relative hardness, far more difficult than the one proposed in the essay? P. Auerbach, in Annalen der Physik und Chemie, April, 1891, states that the subject of hardness “ in its broader bearings has not yet been attacked with success, nor has a rigorous definition of hardness been established. . . . The edge of a rapidly rotating, relatively soft disc is scarcely touched by a file or lathe tool, and that if the motion be rapid enough, it is the tool which suffers most. . . . Hardness, when determined by scratching, is much too complex a conception to be used as a basis for the definition of the property. Complications are introduced by the motional phenomena, the lateral sheer which accompanies scratching, and in short, by conditions which have nothing to do with hardness.” it is difficult to see how any theory involving a full comprehension of the property of hardness can be " clear” to any but the most easily satisfied and superficial of readers.
Again, "we believe that the real value of the cap is to prevent the fracture of the shot’s point, and this has been endorsed, so far as we know, by facts; the matter, however, is easily brought to the proof. If Lieutenant Ackerman is correct, the value of a cap simply depends on whether the plate has a hard face which will not bulge or flow, while we believe that a shot which is able to hold together on impact will not benefit by a cap.”
The four photographs of typical projectiles previously referred to support Lieutenant Ackerman’s contention. Even in the case of the capped projectile which perforated the plate, the striations produced by the fragments of the hard face are plainly visible. In shell No. 262 the hard face was carried into the plate, enveloping the excellent point which escaped injury. The upsetting of the shell and the intense compression farther back where the most of the friction occurred, caused flakes of metal to be sheared off and the shoulder to be deeply scored. This shell held together, and would doubtless have done better with a lubricating cap. In the case of shells No. 1G0 and No. 16, it is plainly apparent that the points would not have been benefited by caps. These shells penetrated deeply, but the plate was able to absorb all their energy and stop them. Their points have been fused, abraded and twisted, but they certainly were not pulverized from the point. The results of many armor tests in the United States have been published with the photographs of projectiles which had perforated face-hardened armor, and it is a very common thing for a shell to hold together and perforate, although the abrasive action of the hard face wears the ogival down to a hemisphere.
"If Lieutenant Ackerman is correct, however, results ought to be forthcoming in support of his theory. For example, we witnessed certain Holtzer projectiles fired at untreated nickel-steel at Indianhead in 1893. These yielded and set up in a bulge round the projectile, at about half-way down, where the walls were apparently weakest. These projectiles might probably be made to set up and behave in the same way against treated armor, and if they could be made thus to behave, they ought, according to Lieutenant Ackerman, to benefit by having iron caps put on them. We shall be greatly surprised if shot so behaving can benefit by the addition of a wrought-iron cap. By all means let such a result be produced, as it will go far to establish our author's theory.”
If the shell sets up or expands it will fail to perforate a face-hardened plate unless its energy greatly overmatches the resistance of the plate. In that case the shell would go through anyway. If the shell stops, however, the lubricant is of no use, for there is no friction. It does not appear that Lieutenant Ackerman states anywhere, inferentially or otherwise, that a cap would assist an expanded projectile in any other way than it assists one that retains its shape, that is, by “covering the asperities of the hardened metal.” To be of any value motion is necessary.
"If the metal of the cap could be substituted for so much rigid metal round it in the plate, we should see a great benefit; but seeing that however it may accommodate itself to the hard fragments, the shot has to cleave its path open to the full size of its caliber, it may be doubted if the cap has helped it much.”
This statement would seem to indicate a disbelief in the value of lubricants. From the table of tests with capped projectiles on page 52 it appears that the cap is of no appreciable value except when the uncapped shell would barely fail to perforate; the capped shell gets through merely because it is lubricated.
“We must now pass to Lieutenant Ackerman’s proposal to employ armor plates with ‘gashes’ and ridges made in them, so as to facilitate cementation, and 'permit deeper chilling in hardening.’ We may admire the inventor’s defense of his proposal, which is too elaborate to give here, but we know the result we expect to follow. Projectiles may be broken up by such plates under favorable conditions, but the plates will fly into fragments; nay, we are tempted to say into ‘smithereens,’ as more expressive. Whatever result may be produced on the first shot, then we should anticipate that the structure will be stripped bare of its armor very rapidly; in fact, in spite of having learned much from the United States, we do not anticipate any future for 'gashed’ plates.”
It would have been well tot the reviewer to have added: “The proof of the pudding, however, lies in the eating of it.” On August, the 13th last, a “gashed” plate 7 inches thick was tested at the Naval Ordnance Proving Ground. Fully 40 per cent, of the face was cut away at the points of impact by the gashes, which were about 1 inch deep. The plate did not represent in many important respects Lieutenant Ackerman’s wishes. In the first place it had been over-carbonized, and the ridges were brittle hard through and through. It had been his intention, moreover, to forge the ridges down in the case of such thin plates. There were also other objections to the plate which had been made to test the feasibility and advantage, if any, of the various changes in the manufacture proposed, rather than to represent the ballistic value of the process. As a result, the methods proposed for a thick plate were applied to a thin one; nevertheless the plate, in spite of its disadvantages, behaved excellently, and compared very favorably with service plates of the same thickness. Two excellent 6-inch 100-lb. armor-piercing shell were fired at the gashed portion of the plate, one with a velocity of 1816 ft. s., and the other with a velocity of 2100 ft. s. The first penetrated about 5".5, the second got its point through to the shoulder. Far from flying into fragments, the plate was not even cracked. A few fine hair cracks appeared in the hard surface in the immediate vicinity of the impacts, but the actual damage was confined to the closely circumscribing group of gashes.
The projectiles were excellent, as was shown by the fact that, although somewhat cracked, they were not crushed, remaining in the plate with their bases projecting.