Experimental investigation of high explosive detonation structure and dynamics near the failure diameter

In cylindrical charges of high explosives (HEs), the detonation speed decreases with diameter. This property is known as the diameter or size effect variation. Additionally, for a given HE, a self-sustaining detonation cannot be achieved below a certain diameter. The charge size at which this occurs is known as the detonation failure diameter. Despite the considerable effort that has been spent on characterizing failure diameters for a large range of HEs, the actual dynamics of failure are not well understood. In this study, we provide new insights into the dynamics of detonation behavior above and just below the failure diameter through a combination of rate-stick geometry testing and proton radiography imaging. Specifically, we examine the diameter effect variation around the failure diameter for the extensively studied 1,3,5-triamino-2,4,6-trinitrobenzene based insensitive high explosive PBX 9502. The rate-stick tests provide an accurate determination of the failure diameter, and allow high resolution determination of detonation front shapes and speeds for charge sizes slightly above the failure diameter. The proton radiography imaging provides an understanding of the mechanics of detonation failure events for charge sizes just below the failure diameter. One of our main findings is that for charge sizes just above the failure diameter, the curvature of the detonation front near the charge edges is lowered as the diameter decreases. Just below the failure diameter, as the detonation fails, its relative deflection progressively increases in time. Consequently, we believe failure appears to be associated with the initial generation of lower curvature shock fronts near the charge edge, likely due to a decoupling effect between the reaction zone and shock. We also examine the effect of density variations around the failure diameter, showing that near, but slightly above, the failure diameter, detonations in lower density charges propagate faster.