The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and through subjective observations. In the work presented here, the explosive was loaded into a heated gun assembly where we subjected a 300 mg charge to a cook-off schedule and a range of static and inertial confinements. Static confinement was controlled using rupture disks calibrated at 34.5 and 138 MPa. The use of 3.15 and 6.3 g projectile masses provided a variation in inertial confinement. This was a regime of strong confinement; a significant fraction of the explosive energy was required to rupture the disk, and the projectile mass was large compared to the charge mass. The state variables pressure and volume were measured in the breech. From these data, we quantified both the reaction enthalpy change and energy release rate of the explosive on a microsecond time scale using a thermodynamic analyisis. We used these values to unambiguously quantify explosion violence as a function of confinement at a fixed cook-off schedule of 190 C for 1 h. , a measure of critical shock energy required for shock ignition of an adjacent explosive was also computed. We found variations in this confinement regime to have a weak effect on enthalpy change, power, violence and shock energy. Violence was approximately 100 times lower than detonating trinitrotoluene, but the measured shock energy approached the critical shock energy for initiating secondary high explosives.
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This is the value representative of diatomic species, likely prevalent in reaction products.
