On the critical threshold for detonation

Abstract

This paper considers the stress threshold required to shock an explosive so that it runs to full detonation. At present it is assumed that a reaction zone composes localised burning regions that increase in density as pressure is increased. However, a series of burn rate experiments and investigations into the shock response of explosives indicate a critical stress level at which response changes. The stress at which the theoretical shear strength is exceeded under shock loading is called the Weak Shock Limit. This is the gateway to the strong shock region in which a compression front travels faster than an elastic wave. In this region experiments suggest that the response becomes independent of microstructure and that there is a transition in an energetic crystal's electronic state. This results in pre-explosion conductivity and luminescence, consistent with a reduced bandgap in the homogeneous flow. The rate-limiting steps in explosive decomposition are generally accepted to follow Arrhenius kinetics, and it is hypothesised here that the activation energy then decreases above the weak shock limit. This allows reaction to move from temperature-dependant burn at mesoscale hot spots, to temperature-dependant, homogeneous reaction behind the shock wave. The shock can then quickly build to detonation even through the stress for this transition is an order of magnitude less than the Von Neumann spike pressure. This hypothesis has a range of implications that may be tested in further experiments. If explicitly proven, the weak shock limit will allow the description of the transition to, and propagation of, detonation within continuum codes. It will thus represent a vital quantity to consider for the safe handling of energetic materials.