Mechanism-informed constitutive modeling of molecular crystal cyclotetramethylene tetranitramine (β-HMX)

Abstract

Molecular crystals are of importance in many applications including energetic materials and pharmaceuticals. Here we focus on the energetic material cyclotetramethylene tetranitramine (β-HMX) whose plastic deformation plays an important role in reaction initiation and detonation. While constitutive modeling of monatomic crystals is well established, the development of models for molecular crystals is less advanced. We present a mechanism-based constitutive formulation for β-HMX crystals, and an equivalent Reuss averaged homogenized model, which are based on a wealth of information about the physics of plastic deformation collected from the literature and developed in this work. We use atomistic models to evaluate the dislocation velocity vs. resolved shear stress relation in multiple slip systems and at various pressures up to 20 GPa, while accounting for slip asymmetry. We use this data to evaluate the strain rate sensitivity on the crystal scale and calibrate all constitutive parameters of the thermally activated and dislocation drag regimes. The resulting crystal plasticity model is then Reuss averaged to produce a homogenized version. The model is perfectly plastic, strongly pressure-dependent and rate sensitive. Further, we observe that strain localization takes place at large enough strain rates and propose a criterion for the occurrence of localization. The formulation can be used to advance the state of the art of continuum modeling of microstructure-dependent initiation in HMX.