The Hugoniot and shock initiation threshold of lead azide

Abstract

The Hugoniot of unreacted dextrinated lead azide has been determined using plate impact techniques in a 2.5 in. bore gas gun. The test specimens were compacted to a density of 3.4 gm/cc. Piezoresistive gauges were used to determine both the impact stress and the stress profile propagated through the sample. Each shot provided transmitted wave data for four sample thicknesses. The Hugoniot is linear up to 10 kbar. It can be represented by the equation σ=41.7 Up (σ in kilobars and Up in mm/μsec). The initiation threshold to long duration (3.5 μsec) shocks was 6.0 kbar based on evidence of reaction in the explosive inferred from increases in stress with propagation distance. Changes of wave speed were found to be noticeable only at higher stresses. Above 6 kbar the delay in initiation decreases with increasing stress levels. The initiation sensitivity to short duration (0.1 μsec) shocks was determined by impacting the lead azide with thin flyers accelerated by exploding foils. The impact velocity was determined by observation with a high-speed framing camera and the transmitted stress profiles were measured by x-cut quartz gauges. The short-pulse initiation threshold for polyvinyl lead azide is 4.2 kbar for a density of 3.6 gm/cc, and 2.1 kbar for dextrinated lead azide for a density of 2.9 gm/cc. Comparison of initiation threshold measurements suggests that there is a minimum thickness, or a run-up distance, before detonation occurs which is independent of pulse width with stresses up to 10 kbar. For long pulse (3.5 μsec) experiments with the gas gun, no evidence of detonation was detected for 1 mm thick samples. Detonation occurs with run distances in the range of 1–2 mm for impact stresses of 8.9 kbar. For stresses greater than 6.0 kbar, evidence of detonation was noted after a 2 mm run. In the thin flyer plate experiments at stresses of 8 kbar dextrinated lead azide displayed a 2 μsec initiation delay. Close to the initiation threshold the stress profiles in the explosive show a gradual transition from an unreactive shock to a stable detonation. It is postulated that reaction occuring behind the shock front produces pressure waves which travel through the explosive interacting with the unreacted explosive ahead of the reaction front causing a nonuniform rate of growth of the shock. This behavior is similar to that observed for heterogenous secondary explosives. The wave speed of the initial wave front propagating in the explosive changes abruptly from that appropriate to unreacted material to the detonation velocity.