The mechanism of the initiation and propagation of detonation in solid explosives

Abstract

Although half a century has elapsed since the publication of the classical treatise of Berthelot upon explosives, the detailed mechanism of the initiation and propagation of detonation in liquid and solid explosives is still obscure. Detonation is a phenomenon exhibiting a number of specific characteristics which differentiate it quite definitely from the explosive combustions of such substances as gunpowder and cordite. It is well known that the latter are governed by laws relating the rate of reaction to the surface area, the temperature and pressure of the surrounding gases, etc., and that heat is the chief medium of initiation and propagation, whereas in the case of detonation, the reaction wave-front travels directly through the explosive medium in the same sense as does a sound wave, and the velocity of propagation is a very definite characteristic of the phenomenon. This stability of the detonation velocity is well demonstrated for solid explosives by the photographs in a recent paper by E. Jones ; the speeds are usually much greater than any exhibited by explosive combustions, and range from 1500 to 10,000 metres per second. Finally, the initiation and propagation of detonation appear to be associated much more intimately with mechanical shock than with flame. The weight of evidence strongly indicates that the difference between detonation and explosive combustion is fundamental and not merely of degree, and the term “high explosive” is reserved for substances capable of the former property. The theoretical treatment of detonation as a shock wave traversing the medium and maintained by the accompanying chemical reactions has been developed by several investigators. These writers have built up a quantitative theory from thermodynamical reasoning and have been able to calculate velocities of propagation, which in some cases are correct, but in practice it has been found that the thermodynamical conditions, while necessary, are not sufficient. Thus, a great number of compositions possessing all the thermodynamical qualifications of a high explosive cannot be made to detonate; others permit detonation to be initiated successfully but without propagation, and the reaction degenerates into a mere deflagration, or even dies out completely. It is indeed very difficult to judge whether a particular composition is a true detonating explosive without the opportunity to test the sample in reasonable quantity. The violent decompositions of small samples or single crystals furnishes no a priori evidence of detonation, and innumerable examples may be quoted of such material in bulk being unable to propagate the local and violent initial activity.