February 15, 191 2] 



NATURE 



535 



it double without fracture, yet as the effect of the blow 

 delivered by the gun-cotton it is broken with very little 

 bending, almost as though it were cast iron or very hard 

 steel. Time will not permit of my going further into the 

 interesting question — of course a very important one in 

 connection with our subject — of the effect on the character 

 of the fracture produced of very big stresses lasting for a 

 very short time. This case of the fracture of mild steel 

 by gun-cotton shows, however, that one result may be that 

 the property of ductility largely disappears under the action 

 of a sufficiently violent blow. The mild steel, in fact, 

 behaves very much like sealing-wax or pitch. The stick 

 of sealing-wax which I hold in my hand has been bent by 

 the continued action of a small force acting for several 

 days, and the same force, had it continued to act, would 

 ultimately have bent it double without breaking it. Yet 

 under the application of a force many times as great it 

 snaps like a piece of glass. 



The pressures produced by the detonation of gun-cotton 

 are of the same order of intensity as those developed in 

 ordinary blows. We saw that in the impact of billiard 

 lialls the average pressure over the area of contact may 

 reach a value of 27 tons per square inch, and with steel 

 balls moving at quite small velocities, such as 2 or 3 feet 

 per second, it is easy to get pressures of 100 tons per 

 square inch or more. These pressures, however, are very 

 local, the area over which they act being a few hundredths 

 of an inch in diameter only. By means of gun-cotton 

 similar pressures may be applied over any desired area, 

 but the intensity is no greater. About 120 tons per square 

 inch is probably the limit of simple static gaseous pressures 

 produced by known practical explosives. Probably greater 

 pressures are produced with fulminate, but that cannot be 

 used except on a very small scale. For the production of 

 destructive effects on hard steel greater pressures than this 

 are required, and in order to develop them on any con- 

 siderable scale we must again have recourse to the dynamic 

 action of collision. 



We have already seen that a lead bullet moving at 

 1800 feet per second probably generates a pressure of 200 

 tons per square inch or more. We went on to consider the 

 impact of rods of hard metal, and it appeared that two 

 rods of steel colliding end on with a relative velocity of 

 40 feet per second would develop a pressure of about 

 15 tons per square inch over the whole section of either. 

 The theory on which that conclusion is based has been 

 subjected to experimental test — indirect, it is true, but 

 sufficiently searching — and is certainly correct for veloci- 

 ties and pressures of that order. According to the theory, 

 the pressure is simply proportional to the relative velocity 

 of the two rods, so that if they collided at 2000 feet per 

 second, that is, fifty times as fast, the pressure would be 

 750 tons per square inch, assuming that the theory con- 

 tinues to hold under these very different conditions. 



One of the fundamental assumptions on which the theory 

 is based, however, would certainly break down long before 

 such a velocity was reached. That assumption is that the 

 pressure leaves no permanent effect on the material. I 

 do not know what is the strongest steel for this purpose 

 which has been produced, but I think it may safely be 

 asserted that no known substance would stand an end 

 compression, such as results from the blow of the colliding 

 rods, of more than 300 tons per square inch. If it were 

 ductile it would flow so rapidly under this pressure that 

 there would be appreciable deformation even in the very 

 short time during which the pressure lasts. If it were 

 very hard it would be instantly shattered. In both ca<?es 

 the circumstances of pressure transmission would be com- 

 pletely altered. It is, however, fairly certain that in 

 neither would the pressure exceed that calculated on the 

 hypothesis of perfect elasticity, and that in both it would 

 be greater than that calculated (as for the lead rifle bullet) 

 on the hypothesis of no elasticity. 



I am afraid, therefore, that at present our theories can 

 throw but little light on the interesting question of the 

 pressure developed when a hard steel armour-piercing shell 

 strikes a hard steel plate with a velocity of 2000 feet per 

 second. But a consideration of the visible effects of such 

 a blow is suggestive in many ways, and by the kindness 

 of Sir R. Hadfield I am able to describe and show some 

 of them to you to-night. 



You see before you specimens of modern armour-piercing 

 shot. The shell is made of a special steel of great strength 

 and considerable ductility, and after manufacture the point 

 is hardened by thermal treatment, the base and most of the 

 body of the shell remaining more or less ductile. In 

 recent years it has become the practice to fit a cap of soft 

 steel over the hardened point. I will speak of the func- 

 tions of this cap later, and for the present we will consider 

 the shell without it. 



I first show the effect of firing an uncapped shell at a 

 plate of wrought iron or mild steel. In this case the metal 

 of the plate is so soft that pressures that are quite without 

 effect on the hardened point of the shell are able to make 

 it flow very rapidly. The shell simply ploughs its way 

 through, pushing out the wrought iron before it, and 

 emerges quite unscathed. It will be noticed that on the 

 striking side there is a rim or lip of wrought iron which 

 has been squeezed out in a direction opposite to the move- 

 ment of the shell. .\ similar lip is formed if a hole is 

 blown in a lead plate by means of a gun-cotton primer, 

 and there seems to be a good deal of analogy between the 

 two cases. 



Completely to stop a 14-inch shell, such as that which 

 you see before you, would require a thickness of at least 

 2\ feet of wrought iron, and almost as great a thickness 

 of mild steel. I believe that some ships twenty-five years 

 ago were fitted with armour of this sort of thickness, but, 

 of course, the weight is almost prohibitive. Modern 

 improvements in armour, whereby the same eflfective 

 resistance is obtained with less than half the thickness, are 

 based on the use of special steel having sufficient ductility 

 to enable it to be worked and fixed in place on the ship, 

 while possessing greater strength than wrought iron or 

 ordinary structural steel. Even such a special steel, how- 

 ever, is handicapped as against the shell by the hard point 

 of the latter, which is able to force the softer material 

 aside, though itself undamaged. This disability, however, 

 has been overcome by hardening the face of the plate, so 

 that it now possesses a composite structure, the back being 

 tough and ductile, but the face as hard as it is possible 

 to make it. When such a plate is struck by the shell it 

 is a case of Greek meeting Greek, and this is the result 

 (photograph). Both the shell and the hardened face of the 

 plate are shattered by the pressure, sufficient of which is 

 transmitted through the substance of the plate to crack it 

 right through, though, of course, none of the shell has 

 penetrated it. . . . ,. j r u 



It would seem that when it acquired the hard face the 

 armour plate more than overtook the shell in the race. 

 Though the shell might by sheer energy pierce a somewhat 

 thinner plate, I am told that it was apt to be smashed 

 to pieces in the process. The balance has of recent years 

 been more than restored by the addition to the shell of the 

 soft steel cap. I have already shown you the effect of 

 firing an uncapped shell ; I will now direct your attention 

 to that of firing the same shell with cap at the same plate. 

 The shell goes through minus its cap, but otherwise so 

 completely uninjured that I am told it might in many 

 cases be used again. It punches a clean hole m the plate. 

 The fate of the cap is interesting. The shell punches n 

 hole in it, as of course it must do before it reaches the 

 plate, and the cap forms a ring, which is held up by the 

 plate and through which the shell passes. The fragments 

 of the cap are found on the front side of the plate, and in 

 some instances they have been collected and put together, 

 forming a ring. I have one such ring here. Its largest 

 diameter is that of the shell, its smallest about an mch 

 less and it looks as though the ring had Rot intact as far 

 as "the shoulder of the projectile, but had then burst into 



^^The u^'ual explanation of this remarkable eflfect of a soft 

 steel cap is that it supports the point of the projectile. As 

 I pointed out in connection with billiard balls, the destruc- 

 tive effect of pressure depend* on the difference of pressures 

 in different directions, and not on their absolute amounts, 

 and it is obvious that by the exercise of a sufflcient 

 lateral pressure the point might be completely protected. 

 The difficulty is to sec how the comparativelv weak 

 material of which the cap is made can exert the very 

 larc^ prr<;sures which are ner.»s<«ary for effective support. 

 1- hardly possibk- ' - h pressures could bo 



NO. 2207, VOL. 88] 



