Characterization of hypervelocity metal fragments for explosiveinitiation

The fragment impact response of two plastic-bonded explosive (PBX) formulations wasstudied using explosively driven aluminum fragments. A generic aluminum-capped detonatorgenerated sub-mm aluminum particles moving at hypersonic velocities. The ability of thesefragments to initiate reaction or otherwise damage two PBX materials was assessed usinggo/no-go experiments at standoff distances of up to 160 mm. Lower density PBX 9407(RDX-based) was initiable at up to 115 mm, while higher density PBX 9501 (HMX-based) wasonly initiable at up to 6 mm. Several techniques were used to characterize the size,distribution, and velocity of the particles. Witness plate materials, including copper andpolycarbonate, and backlit high speed video were used to characterize the distribution ofparticles, finding that the aluminum cap did not fragment homogeneously but rather withlarger particles in a ring surrounding finer particles. Finally, precise digitalholography experiments were conducted to measure the three-dimensional shape and size ofthe fastest-moving fragments, which ranged between 100 and 700 μm andtraveled between 2.2 and 3.2 km/s. Crucially, these experiments showed variability in thefragmentation in terms of the number of fragments at the leading edge of the fragmentfield, indicating that both single and multiple shock impacts could be imparted to thetarget material. These types of data are critical for safety experiments and hydrocodesimulations to quantify shock-to-detonation transition mechanisms and the associatedrisk-margins for these materials.