The deflagration-to-detonation transition process has been observed in pressed granular columns of the explosive pentaerythritol tetranitrate. Charges were confined within a steel housing which had been fitted with a polycarbonate window to allow direct recording by high-speed streak photography. The explosive was thermally ignited by a gasless pyrotechnic mixture to minimize pre-pressurization of the charge. The results indicate that upon ignition, early choked flow of the combustion gases prevents the continued propagation of the combustion via a convective heat transfer mechanism and that the propagation of reaction becomes governed by a leading compaction wave which causes ignition through the mechanical formation of hot spots. Detonation finally occurs when the leading front of an accelerating plug of highly compacted material (density close to the theoretical maximum) formed between the reaction front and the leading compaction wave, attains the critical pressure necessary for shock-to-detonation transition.

The channel effect: Coupling of the detonation and the precursor shock wave by precompression of the explosive

The deflagration-to-detonation transition (DDT) has been studied in prepared columns of granular secondary explosive. The secondary explosives 2- (5-cyanotetrazolato) pentammine cobalt (III) perchlorate (CP) and pentaerythritol tetranitrate (PETN) were chosen for the study due to their known propensity to undergo DDT within a few millimetres of ignition. Confinement of CP columns within polycarbonate and PETN within metallic confinement fitted with slit windows allowed direct high-speed streak photography of the events. Deflagration and detonation velocities and the run-to-detonation lengths were measured as a function of charge pressed density. Ignition of the explosive column was attained thermally through a copper barrier with a gasless pyrotechnic. Deflagration and detonation velocities were seen to depend strongly upon pressed density with both explosives. There appeared to be a maximum density conducive to DDT with both explosives but no minimum with CP. Studies of DDT continue to have interest for the safe storage and use of reactive materials, and for the development of a detonator based on a secondary explosive.

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