Length scale and grid resolution effects in the simulation of shear and energy localization during pore collapse in shocked energetic crystals

Previous works [Herrin et al., J. Appl. Phys. 136(13), 135901 (2024), Nguyen et al., J. Appl. Phys. 136(11), 114902 (2024)] obtained atomistics-consistent material models for two common energetic crystals, HMX (1,3,5,7-Tetranitro-1,3,5,7-tetrazocane) and RDX (1,3,5-Trinitro-1,3,5-triazinane) such that pore collapse calculations adhered closely to molecular dynamics (MD) results on key features of energy localization, particularly the appearance of shear bands, shapes of the collapsing pores, and the transition from viscoplastic to hydrodynamic collapse. However, only one pore size (of 50 nm diameter) was studied and some important aspects such as temperature distributions in the hotspot were found to be inconsistent with the atomistic models. One potential issue was noted but not resolved adequately in those works, namely, the grid resolution that should be employed in the meso-scale calculations for various pore sizes and shock strengths. Conventional computational mechanics guidelines for selecting meshes as fine as possible, balancing computational effort, accuracy, and grid independence, were shown not to produce physically consistent features associated with shear localization. Here, we examine the physics of pore collapse, shear band evolution and structure, and hotspot formation for both HMX and RDX; we then evaluate under what conditions atomistics-consistent models yield “physically correct” (considering MD as “ground truth”) hotspots for a range of pore diameters, from nm to micrometers, and for a wide range of shock strengths. The study provides insights into the effects of pore size and shock strength on pore collapse and hotspots, identifying aspects such as size-independent behaviors, and proportion of energy contained in shear as opposed to jet impact-heated regions of the hotspot.