Shock-induced collapse of elongated pores: Comparison of all-atom molecular dynamics and atomistics-consistent continuum simulations

Microstructures of energetic materials (EMs) exhibit defects including pores, cracks, inclusions, and delaminated interfaces, all of which act as sites for energy localization under shock loading. Reactions are triggered at these sites and can couple with shocks, leading to detonation. Convoluted and elongated pores or cracks in energetic crystals can significantly enhance or mitigate EM sensitivity and must be factored into micro-structure aware reactive burn models. Here, we advance the state of modeling and physical understanding of the response of elongated pores in cyclotetramethylene-tetranitramine (HMX) to shock loading by employing: (1) updated atomistics-consistent models to show that continuum calculations with such models produce pore collapse and hotspots that closely reproduce molecular dynamics (MD) results; (2) high-order numerical methods to accurately capture shock and interfacial dynamics; and (3) grid resolution that resolves all relevant scales in the physics of elasto-viscoplastic deformation of the material under high strain-rate loading, down to a lower limit set by molecular/statistical-mechanical considerations. These high physical and numerical fidelity calculations demonstrate that continuum predictions are in agreement with atomistic calculations for various orientations of an elongated pore (penny-shape crack). Furthermore, such continuum simulations, particularly for micrometer-scale pores and cracks, can be performed at much smaller computational cost than MD calculations. This paper examines the emergence of shear bands and their impact on pore collapse and hotspot intensity for various orientations of a nm-scale pore. Then, the collapse of a micron-sized pore (inaccessible to MD) is studied to obtain insights into how the shear band and pore-collapse dynamics changes (or not) as the size of the pore increases by several orders of magnitude. The work provides confidence in the recently advanced atomistics-consistent model set for HMX and also provides new physical details of elongated pore-shock interaction that will be of interest to the energetic materials community.