Domain partitioning material point method for simulating shock in polycrystalline energetic materials

Heterogeneous energetic materials (EMs) subjected to mechanical shock loading exhibit complex thermo-mechanical processes which are driven by the high temperature, pressure, and strain rate behind the shock. These lead to spatial energy localization in the microstructure, colloquially known as hotspots where chemistry may commence and possibly culminating in detonation. Shock-induced pore collapse is one of the dominant mechanisms by which localization occurs. In order to physically predict the shock sensitivity of energetic materials under these extreme conditions, we formulate a multiplicative crystal plasticity model with key features inferred from molecular dynamics (MD) simulations. Within the framework of thermodynamics, we incorporate the pressure dependence of both monoclinic elasticity and critical resolved shear stress into the crystal plasticity formulation. Other fundamental mechanisms, such as strain hardening and pressure-dependent melting curves, are all inferred from atomic-scale computations performed across relevant intervals of pressure and temperature. To handle the extremely large deformation and the evolving geometry of the self-contact due to pore collapse, we leverage the capabilities of the Material Point Method (MPM) to track the interface via the Lagrangian motion of material points and the Eulerian residual update to avoid the mesh distortion issue. This combination of features enables us to simulate the shock-induced pore collapse and associated hotspot evolution with a more comprehensive physical underpinning, which we apply to the monoclinic crystal beta-HMX. Treating MD predictions of the pore collapse as ground truth,head-to-head validation comparisons between MD and MPM predictions are made for samples with identical sample geometry and similar boundary conditions for reverse-ballistic impact speeds ranging from 0.5 km s-1 to 2.0 km s-1. Comparative studies are performed to reveal the importance of incorporating a frictional contact algorithm, pressure-dependent elastic stiffness, and non-Schmid critical resolved shear stress in the mesoscale model.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Heterogeneous energetic materials (EMs) subjected to mechanical shock loading exhibit complex thermo-mechanical processes, and shock-induced pore collapse is one of the dominant mechanisms by which spatial energy localization occurs. In order to physically predict the shock sensitivity of EMs, a multiplicative crystal plasticity model is formulated with key features inferred from molecular dynamics (MD) simulations. The Material Point Method (MPM) is used to handle large deformation and evolving geometry, allowing for comprehensive simulations of shock-induced pore collapse and hotspot evolution. Comparative studies are performed to reveal the importance of frictional contact algorithm, pressure-dependent elastic stiffness, and non-Schmid critical resolved shear stress in the mesoscale model.