Heat generation in the vicinity of a void during shock compression plays a key role in the initiation of energetic materials. The shock response of a single β ‐HMX crystal with a single void is studied with simulations that include plasticity and heat transport. The numerical results are validated with an experiment in which a 500 μ m void is machined in an HMX single crystal and impacted. Experiments and simulations of the dynamical evolution of the morphology of the void during the collapse and the rate of the area are in very good agreement for weak shocks.
The initiation of high explosives (HEs) under shock loading lacks a comprehensive understanding: particularly at the particle scale. One common explanation is the hot spot theory, which suggests that energy in the material resulting from the impact event is localized in a small area causing an increase in temperature that can lead to ignition. This study focuses on the response of HMX particles (a common HE) within a polymer matrix (Sylgard‐184®), a simplified example of a polymer‐bound explosive (PBX). These PBXs consist of multiple HMX particles in a single polymer‐bound sample. A light gas gun was used to load the samples at impact velocities above 400 m/s. The impact events were visualized using X‐ray phase‐contrast imaging (PCI) allowing real‐time observation of the impact event. The experiment used two different types of samples (multi‐particle and two crystals) and found evidence of cracking and debonding in both sample types. In addition, it was found that the multiple particle samples showed similar evidence of damage at lower velocities than that of single particle samples. This is an expected result as the multiple particles add additional interfaces for stress concentration and frictional heating.
Imaging the collapse of a single void that creates a hot spot initiation site in an otherwise defect‐free explosive is challenging given the spatial and temporal scales involved in explosive systems. This work presents our attempt to examine a single hot spot mode (void collapse) in single‐crystal octahydro‐l,3,5,7‐tetranitro‐l,3,5,7‐tetrazocine (HMX) embedded in Sylgard. The hot spot heating mechanisms involved with pore collapse include adiabatic heating, jetting, and viscoplastic dissipation. Quantifying the dynamics of a pore collapse is a crucial step to understanding which mechanisms dominate during ignition events. Our experiments were conducted with a single‐stage, light‐gas gun at Argonne National Laboratory's Advanced Photon Source, applying the phase contrast imaging technique while collecting high‐speed video. The details of HMX single crystal production, defect (pore) engineering, and sample construction, along with experimental results are presented here. These results demonstrate that detailed collapse dynamics can be obtained from homogeneous, single‐crystal explosives with this approach. Qualitative comparisons are made with simulation data which show good agreement in the transition between a quasi‐symmetric pore collapse and an asymmetric collapse with jetting across the pore as measured with normalized pore area and pore circularity.
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