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.
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.
The cover picture shows both X‐ray phase contrast images of gas gun impact experiments on single particle HMX as well as simulations of the experiment that have been visualized for clearer viewing. Both the experimental and simulational images are from the same experimental setup, materials, impact velocity, and times. Details are discussed in the article by Nicolas E. Kerschen et al. on page 447 ff.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.