Developing and validating thermomechanics models for explosives with experiments on commensurate scales

In both manufacturing and dynamic loading, the interplay between deviatoric stress, plastic strain, and heat generation at the mesoscale dictate the responses of plastic bonded explosives (PBX). In situ mesoscale insights are needed to quantify structure-property relationships, inform theory, and enable simulations. We have attempted such an effort and will present an overview of our progress so far.
Laser-driven shock, gas gun, and split-Hopkinson pressure bar experiments have been performed to span multiple orders of strain rate, using synchrotron and X-ray free electron laser radiation to measure time-resolved X-ray diffraction (XRD) and phase contrast imaging (PCI) in situ for single crystal and plastic bonded explosives. This range of strain rates enables investigation of coupling between crystal mechanics, thermal softening, and microsturcture that governs explosive response.
Multiphase single crystal plasticity models have been developed. They consist of non-linear thermo-elasticity, Orowan expressions for slip rate using the Austin-McDowell model for dislocation velocity, and multiphase equations of state (EOS) imposing phase transitions through Gibbs free-energy. Constitutive equations were parameterized with density functional theory and atomistic calculations for EOS and elastic constants along with experimental measurements of anisotropic deformation mechanisms and rates. These models are capable of predicting anisotropy, grain size, and pressure dependent effects remarkably well.
Combining the new capabilities, mesoscale thermomechanics can be investigated from the average lattice response up to PBX microstructures. For the first time, XRD quantify average lattice response and allows for direct comparison of experiments and simulations through measured and computed diagnostics. Using the experimentally validated models, simulation can be compared to PCI of heterogeneous micorstructure effects such as void collapse and grain boundaries.