X‐Ray Phase Contrast Imaging of the Impact of a Single HMX Particle in a Polymeric Matrix

Abstract

A complete understanding of the mechanisms by which high explosives (HEs) are shock initiated, especially at the particle scale, is still in demand. One approach to explain shock initiation phenomenon is hot spot theory, which suggests that distributed energy in energetic material is localized due to shock or impact to generate the high temperatures for ignition. This study focuses on the impact response of a HE polycrystalline particle, specifically HMX, in a polymer matrix. This represents a simplified analog of a traditional polymer‐bonded explosive (PBX) formulation. A light gas gun, together with high‐speed x‐ray phase contrast imaging (PCI), was used to study the impact response of a single particle of production‐grade HMX in a Sylgard‐184® matrix. The high‐speed x‐ray PCI allows for real‐time visualization of HE particle behavior. The experiments revealed that, at impact velocities of ∼200 m s−1, the energetic particle was cracked and crushed. When the impact velocity was increased to 445 m s−1, a significant volume expansion of the particle was observed. This volume expansion is considered to be the result of chemical reaction within the HE particle.

X‐ray Phase Contrast Imaging of the Impact of Multiple HMX Particles in a Polymeric Matrix

“…Some of the lines forming at the edges appeared curve, likely pointing to open interfaces due to debonding, however, most appeared linear and spread through the sample, evidencing cracks. These samples acted similarly to the production‐grade single particles previously examined (particle expansion due to cracking and open interfaces) , but at lower impact velocities, likely due to increased stress concentrations from the particle‐particle interactions occurring as the particles expanded into one another (i. e. damage seen in single‐particle experiments at 440 m/s was seen in multiple particle experiments at 400 m/s). For the two crystal samples, evidence of cracking was observed originating from the crystal‐crystal interface, most often spanning from the bottom left to the top right.…”

“…As the crystals primarily separate after impact, increased stress concentrations are likely more influential than any frictional heating in this sample type (versus samples with many more particles present). Additionally, to help give an idea of the pressure in the samples, 1‐D shock matching calculations were performed . While these are simplified and do not account for attenuation, it gives an estimate of approximately 1 GPa for the samples discussed in this paper.…”

“…More recent technological advancements have made the direct observation of particles under high-speed impact possible using X-ray phase-contrast imaging (PCI) [15][16][17][18][19][20]. The study discussed in this paper builds upon previous work performed on similar samples, but with only one HMX particle contained in each PBX [15].…”

In situ characterization of foreign object damage (FOD) in environmental‐barrier‐coated silicon carbide (SiC) ceramic

“…A detailed description of the combined light-gas gun and in situ dynamic X-ray imaging facility can be found elsewhere [30,33]. Only a brief overview is offered here.…”

Capturing Laser Induced Dynamics of Reactive Materials via Ultrafast Transmission Electron Microscopy

“…The response of these materials to impact and ultrasonic excitation has been heavily studied in the past years to explain the initiation and heat dissipation mechanisms 4,5 . It was experimentally proven that energetic materials can react below their desired threshold and hot-spot formation has been dominantly accepted as the underlying mechanism 6 . This theory suggests mechanical energy localization under impact due to formation of stress concentration points.…”