1912] on the Pressure of a Blow. 2ft7 



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 l)low. 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 there 

 can be no doubt that 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 balls 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 de- 

 sired 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 considerable 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 34 feet per second would develop a pressure of about 13 

 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 velocities 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 sixty times 

 as fast, the pressure would be 780 tons per square inch, assuming 

 tliat the theory continues to hold under these very different conditions. 

 ii^^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 per- 

 manent 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 



