Finite element models for the high-rate deformation of explosives*

The high-rate deformation of explosives controls energy localization, which ultimately leads to the initiation of chemical reactions. Most high explosive molecular crystals are highly anisotropic and exhibit complex deformation mechanisms that depend on the applied strain rate and crystal orientation. We have developed a set of well-validated models for the high-rate deformation of explosive single crystals that we have subsequently applied to the study of deformation and temperature localization in plastic bonded composites (PBX). The finite element software Abaqus has been used to model the high-rate deformation of: (1) single crystal cyclotetramethylene tetranitramine (HMX), and (2) polymer bonded cyclotrimethylene trinitramine (RDX). A thermomechanical model developed by Luscher et al. is advanced with a phase-field twinning model for the purpose of modeling the high-rate response of HMX. Simulation results of deformation twinning behavior as a function of crystal orientation and distance from the impact surface are presented. We found that twinning in HMX does not affect the interface velocity, which implies that other experimental techniques are needed for its quantification. For purpose of modeling the high-rate response of a PBX, we develop 1D and 2D finite element models and solve the heat transfer equation in addition to the momentum balance. The 2D models are meshed with Eulerian elements. The RDX material model has been recast in a form that minimizes the number of advected tensorial variables, while retaining the main features of the original RDX hyperelastic model. We report the simulated temperature distributions in function of particle size, and loading pressure. These simulation results are discussed in relation to the experimental observations of shock sensitivity.