A new mechanism for deflagration-to-detonation in porous granular explosives

An investigation has been carried out into the differences between the defl agration-to-detonation (DDT) process as it occurs in low density [similar t o 30% theoretical maximum density (TMD)] columns of conventional grain size (similar to 180 mu m) pentaerythritol tetranitrate (PETN) and in ultrafine PETN with a grain size similar to 1 mu m. The principle technique for obse rving the process utilized charges confined within a steel housing fitted w ith a polycarbonate slit window. This allowed direct recording of the trans ition using high speed streak photography. The explosive was thermally igni ted using a pyrotechnic mixture with low gaseous emission to minimize any p repressurization of the charge. In addition to the photographic records of the events, the outputs of photodiodes along the length of the column were monitored in order to determine the rate at which the reaction proceeds. Th e results obtained show that the DDT process in the larger grain PETN at lo w density was similar in structure to the DDT process at higher densities. In contrast a different mechanism leads to detonation in columns composed o f the smaller grain size PETN when packed to densities less than 50% TMD. A fter ignition hot gases propagate along the column both compacting and igni ting material as they pass. After the gases have reached the downstream end of the column, the column continues to burn and the pressure and temperatu re increase. Some time later initiation takes place at a point along the bu rning column, and detonation waves propagate in both directions from this p oint. The detonation waves propagate from the initiation point at speeds th at would normally be associated with material compacted to around 60% TMD. The process appears to be in effect a deflagration-to-localized thermal exp losion detonation transition. (C) 1999 American Institute of Physics. [S002 1-8979(99)05115-4].