Influence of entrapped gas on initiation of explosion in liquids and solids

Further evidence has been obtained for the view that the initiation of explosion by impact in liquids is due to the compression and adiabatic heating of trapped gas. It is shown that the sensitivity of an explosive is very dependent upon the pressure ratio. Flat impact experiments on nitroglycerine spread as a ring on a flat anvil show that the explosion efficiency, which is high when the initial gas pressure is 1 atm., is reduced to zero when the initial gas pressure is c. 30 atm. A high explosion efficiency is still observed when the initial air pressure is less than 10-5 mm., and it is suggested that under these conditions the initiation is due to the compression of the nitroglycerine vapour itself present at a pressure of c.10-3 mm. It is further suggested that the explosion begins in the vapour phase. The behaviour of some solid secondary explosives such as P.E.T.N. and cyclonite shows many similarities to that of the liquid explosives. When spread as a ring rather than as a uniform film of crystals, an increase in explosion efficiency is again observed. When the impacts are carried out at an initial gas pressure of 100 atm. there is a considerable reduction in explosion efficiency. The striker must be sufficiently hard to make the explosive flow plastically so that gas pockets can be sealed off. Again it would appear that the adiabatic compression of trapped gas initiates the explosion. Even when spread as a continuous film of crystals, the air spaces which are present between the crystals may be sealed off by local melting or plastic flow during the impact and the adiabatic compression of these gas spaces may initiate the explosion.