Experimental Studies of Rod Impact on Bare/Uncovered PBX 9501 Explosive

We present an experimental investigation of the dynamics of rod impact on bare or uncovered PBX 9501 explosive. By weight, PBX 9501 contains 95% octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, commonly known as HMX. The plastic binder is Estane combined with a nitro-plasticizer made of bis(2,2-dinitropropyl)acetal and bis(2,2-dinitropropyl)formal. Experiments were conducted using our single stage gas-gun, allowing us to get very repeatable impact velocities. A stainless steel rod attached to the front of the projectile impacted the explosive target. Impact occurred while the projectile was still in the barrel, allowing for accurate centering of the rod on the target. Seven channels of Photonic Doppler Velocimetry (PDV) were used to measure shock waveprofiles after transmission through the explosive. Velocimetry measurement points were located at different radii from the center of the rod, allowing us to capture the 2-D characteristics of the flow. Also, multiple sets or sequences of experiments were performed. Each set of experiments used a constant rod diameter, impact velocity and PDV probe locations: the thing that was varied from experiment to experiment was the thickness of the PBX 9501 explosive sample. In this way, we built up a detailed experimental record of the impact and reaction dynamics. All experiments discussed used a rod impact velocity near 0.744 km/s resulting in an impact stress of ~ 4.5 GPa. One series of experiments with 17 mm diameter rods resulted in initiation with onset of detonation between 6 and 8 mm into the explosive. Another series of experiments with 8.5 mm diameter rods resulted in failure to detonate, reactions being quenched by rarefaction waves emanating from the edge of the rod.