Effect of Continuous Damage Accumulation on Ignition of HMX-Based Polymer Bonded Explosives under Low-Velocity Impact

The ignition phenomena of polymer-bonded explosives (PBXs) due to hotspot formation from grit/friction, shear band formation, and friction between microcrack surfaces have not yet been clarified. Microcracks are known to play a critical role in PBX ignition under low-velocity impacts. Further, microcrack evolution determines whether or not a specimen ignites because of complex mechanical-thermal-chemical coupled processes. Although many studies have investigated the multiphysics processes of ignition under single impacts, few have investigated multiple-impact scenarios that are more likely in reality. The present study focuses on the influence of continuous damage accumulation during multiple impacts on the safety assessment of octahydro-1,3,5,7-Tetranitro-Tetrazocine (HMX)-based PBXs. The Visco-SCRAM model combined with restart technology is applied to conduct simulations with the standard confined Steven test. By controlling the first impact velocity, the damage accumulation could be designed. When the first impact velocity increases from 5 to 20 m/s, the second impact velocity threshold decreases from 34 to 22 m/s and specific mechanical energy decreases from 600 to 240 m²/s². Multiple impacts change the temperature distribution and ignition position. Further, different projectile shapes influence multiple impacts differently. Owing to continuous damage accumulation, the second impact velocity threshold decreases for flat and pin projectiles. Dimensionless analysis reveals that the trends of critical dimensionless parameters for flat and pin projectiles are similar, whereas oval projectiles show a different response.