MD- and crystal-plasticity guided material models for HMX and RDX under shocks

Meso-scale computations of the shock response of energetic materials with resolved complex microstructures (e.g., a PBX) often rely on isotropic models for computing the elasto-plastic response. Although robust and computationally efficient, these models rely on empirical model constants, which are not predictive outside of calibration intervals and have large uncertainties even for relatively well-characterized high explosives such as HMX and RDX. We develop isotropic models from single crystal plasticity computations of HMX and RDX under shocks. All-atom MD is used to guide the model and serves as the fundamental metric for validation. The continuum simulations are performed in an Eulerian framework that incorporates thermally activated and drag-regulated dislocation motion as well as dislocation multiplication, trapping and annihilation for heterogeneous nucleation. Ensembles of anisotropic single-crystal plane-strain simulations of RDX and HMX are performed to compute the von Mises stress behind the shock for different orientations and loading velocities, which are then averaged to calibrate Johnson-Cook and Steinberg-Guinan-Lund models for HMX and RDX. Meso-scale computations performed using those models are validated against MD simulations under different shock strengths.