Chapter 17: Assessment of the Constant Shock-Velocity Assumption Used in the Analysis of One-Dimensional Overdriven Detonation Plate Impact Experiments

When a pressure higher than the Chapman-Jouguet (CJ) pressure is generated in an explosive, an overdriven detonation state occurs with the wave having a velocity greater than the CJ detonation speed. Overdriven states in explosives are commonly studied via high-velocity plate impact gas-gun experiments. During their analysis, it is common practice to assume a constant shock velocity determined by the explosive length and the time for the overdriven wave to pass through it. However, this ignores any induction time for the non-reacted shock wave entering the explosive to rapidly react and reach the final overdriven detonation state. In this paper the validity of the constant shock-velocity assumption used in the interpretation of the overdriven experimental results is assessed. This is carried out by comparing measured overdriven data, for the HMX-based explosive PBX 9501 and the TATB-based explosive PBX 9502, with corresponding steady-state velocities obtained from modelling relevant gas-gun shots in a one-dimensional hydrocode with the CREST reactive burn model. The CREST models for PBX 9501 and PBX 9502 included overdriven detonation data in their respective calibrations. For both explosives, the CREST model predicts an initial non-linear shock velocity prior to stabilising at steady state. By comparing the CREST-calculated and experimentally measured overdriven Hugoniots, constant shock-velocity assumption is found to be valid except for low-pressure overdriven PBX 9502 data. Therefore, when calibrating models in the overdriven detonation regime, experimental data points require validation as to whether the recorded shock velocity accurately represents the steady-state condition, and to calibrate each point accordingly.