Deflagration-to-Detonation Transition (DDT) in Granular HMX in Two Dimensions

There are sound reasons to question whether historical (quasi) 1-dimensional DDT tube tests, from which our best DDT models have been developed, have captured all of the physics required to precisely describe the complex process of deflagration-to-detonation transition, especially as it evolves in three dimensions.  Foremost is the fact that a burning column of explosive, mechanically constrained inside a tube, is prohibited from experiencing divergence in the stress forces and flows and, therefore, misses the effects of tensile fracture and flame intrusion into the deflagrating explosive part.  Consequently, tests with tube geometry likely preclude, or underrepresent, phenomena associated with crack burning and their effects on deflagration propagation and mass-burn-rate enhancement.   To investigate how fracture and flow divergence affect the DDT process in >1 dimension, we designed a suite of strongly confined 2-dimensional DDT tests containing thin (2 mm) HE slabs, where ignition location and slab shape (e.g., circle, ellipse and wedge) were varied.  The reaction vessels had transparent windows enclosing the explosive’s top surface and high-speed cameras were used to observe luminous flames, fracture and compaction of the explosive, as well as detonation transition and propagation.  Some tests included fluorescent particle trackers for measurement of bed strain.  In all cases, DDT was the result of constructive pressure wave interactions, either by reflection at the confinement interface, or at the intersection of colliding waves, to produce an amplified shock of sufficient strength to initiate detonation.  This detail offers a significant refinement to the final steps of the widely accepted mechanism for DDT.  We will present novel image data and analysis describing the important differences in the mechanism for DDT in multiple dimensions.  We will also discuss how our new DDT model may improve understanding of DDT in 1-dimensional DDT tube geometry.