| The coupled electromagnetic-thermo-mechanical response of RDX (cyclotrimethylene trinitramine)-polymer energetic aggregates under laser irradiation and high strain rate loads has been investigated to identify laser-induced hot spot formation and failure mechanisms at different physical scales. A computational approach was developed to investigate the coupled phenomena of high frequency electromagnetic (EM) wave propagation, laser heat absorption, thermal conduction, and inelastic dynamic thermomechanical deformation in heterogeneous energetic materials. The approach couples Maxwell's equations with a dislocation density-based crystal plasticity formulation with a nonlinear finite-element approach to predict and understand thermo-mechanical response due to the interrelated effects of dielectric heating, adiabatic heating, thermal decomposition, and heat conduction. The effects of heterogeneous microstructural characteristics, such as void distribution and spacing, grain morphologies and orientations, crystal-binder interactions, and dislocation densities were analyzed to determine their influence on hot spot formation and EM and mechanical energy localization.;The effects of beam intensity, incident wavelength, material electromagnetic absorption coefficient, and the heterogeneous microstructure on spatial and temporal behavior and mechanisms of laser-induced hot spot formation were characterized and related to the thermo-mechanical response. Different mechanisms for hot spot initiation under dynamic laser and pressure loads were identified, which are a function of shear strain localization and laser heat absorption. The predictions indicate that hot spot formation was accelerated by higher absorption coefficients and by localized plastic deformations that occurred in areas of significant laser heating. RDX crystalline interfaces and orientations, polymer binder, inelastic strains, dislocation-density evolution, and voids significantly affected the coupled EM-thermo-mechanical response. EM and thermo-mechanical mismatches at interfaces between RDX crystals, binder, and voids resulted in localized regions with high electric field and laser heat generation rates, which subsequently led to hot spot formation. The incident laser intensity and shear strain localization were the dominant mechanisms that led to hot spot formation.;The coupled electromagnetic-thermo-mechanical response of RDX-estane energetic aggregates under laser irradiation and high strain rate loads was also investigated. Temperatures induced by laser heating were above the glass transition temperature of estane, and therefore a finite viscoelastic constitutive relation was used to represent binder behavior. Local behavior mismatches at the crystal-binder interfaces resulted in geometrical scattering of the EM wave, electric field and laser heating localization, high stress gradients, dislocation density and crystalline shear slip accumulation. Viscous sliding in the binder was another energy dissipation mechanism that reduced stresses in aggregates with thicker binder ligaments and larger binder volume fractions.;Energetic aggregates with binders that had embedded crystals with a broad range of size distributions were investigated to account for physically representative energetic microstructures. The presence of embedded crystals in the binder ligaments restricted viscous sliding, and resulted in global hardening of the aggregate, which led to large stress gradients, localized plasticity, and dislocation density accumulation. The smaller crystals also increased scattering of the EM wave within the binder ligaments and increased the localization of EM energy and laser heat generation. Geometrically necessary dislocation densities (GNDs) were calculated to characterize how hardening at the binder interfaces can lead to strengthening or defect nucleation.;This investigation indicates how the interrelated interactions between EM waves, material microstructure, and mechanical behavior in RDX-polymer aggregates underscores the need for a coupled approach for accurate predictions of high strain rate deformation and laser irradiation of heterogeneous materials. The predictions indicate that the response is governed by laser EM energy and shear strain localization, and that controlling the aggregate microstructure and laser characteristics, such as beam intensity, material absorption coefficient, crystal size distributions, and porosity, will enable the prediction of hot spot formation, the enhancement of laser-based ignition and laser-based detection of energetic materials. |
