DETONATION OF HIGH EXPLOSIVES OR BY THE IMPACT OF BULLED). 455 



be that the distribution of the pressure in time is not materially different in the two 

 cases. If that were so, the maximum pressure developed on the surface of the gun- 

 cotton would be 80 or 100 tons per square inch. 



It is hoped that by the use of special steels it may be possible to give greater 

 precision to these estimates of the amount and duration of the pressure produced by 

 the detonation of gun-cotton in the open. Meanwhile the iiitbrmutioii already 

 obtained as to the order of magnitude of these quantities is sufficient to throw some 

 light v on the nature of the fractures produced. The general result obtained may be 

 expressed by saying that a gun-cotton cylinder 1^ inches x !$ inches produces at ite 

 surface, when detonated, pressure of the average value of 100,000 Ibs. per square 

 inch lasting for TToTooo second. Probably figures of the same sort of magnitude will 

 describe the blow produced by the detonation of a slab 1^ inches thick, one of whose 

 faces is in contact with a steel plate. It may be that the pressure is greater and the 

 duration correspondingly less, but this does not affect the point that the pressure is 

 an impulsive one in its effect on the plate. That is, the effect of the pressure is to 

 give velocity to the parts of the plate with which the gun-cotton is in contact but the 

 pressure disappears before there has been time for much movement to take place. 

 For instance, if the plate be 1 inch thick (mass 0'28 Ibs. per square inch) a pressure 

 of 100,000 Ibs. per square inch acting on it for 5^jo second will give a velocity of 

 about 230 feet per second, and while the pressure is being applied it will move 

 0'028 inches. 



The parts of the plate not covered by the gun-cotton are left behind and the strain 

 set up by the forced relative displacement is the cause of the shattering of the plate. 

 The magnitude of this strain, and of the consequent stress, depends (speaking 

 generally), on the relation between the velocity impressed on the steel by 

 the explosion and the velocity of propagation of waves 

 of stress into the material. For instance, if the sec- 

 tion AB (fig. 15) be given instantaneously a velocity 

 of 200 feet per second and this velocity be maintained, 

 the state of the plate after the lapse of 10 o!ooo 

 second will be that represented diagrammatically by 

 fig. 15. The section AB has moved forward rela- 

 tively to the remainder by 0'002 feet. As soon as 

 this section started moving a wave of shear stress 

 started out from A into the parts of the plate to 

 the left which had been left at rest by the blow. This wave travel 

 11000 feet per second and will therefore in 100 1 OU "OOOd get 

 AC = O'll feet. To the left of C the metal has not moved, the wave not hu 



reached it ; therefore the average shear in the section AC is - 



forces of this durationl even mild steel has nearly perfect elasticity up to very 



