Microstructural Modeling of Coupled Electromagnetic-Thermo-Mechanical Response of Energetic Aggregates to Infrared Laser
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Microstructural Modeling of Coupled Electromagnetic-Thermo-Mechanical Response of Energetic Aggregates to Infrared Laser Radiation and Dynamic Fracture J.A. Brown1*, D.M. Bond1, M.A. Zikry1 1 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695-7910, U.S.A. * Present address: Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. ABSTRACT A dislocation-density based crystalline plasticity, a finite viscoelasticity, and a nonlinear finite-element formulation were used to study the high strain-rate failure of energetic crystalline aggregates. The energetic crystals of RDX (cyclotrimethylene trinitramine) with a polymer binder were subjected to high strain-rate tensile loading, and the predictions indicate that high localized stresses and stress gradients develop due to mismatches along crystalline-crystalline and crystalline-amorphous interfaces. These high-stress interfaces are sites for crack nucleation and propagation, and the predictions are used to show how the cracks nucleate and propagate. INTRODUCTION Energetic aggregates, such as plastic bonded explosives (PBX) and solid rocket propellants, are heterogeneous materials consisting of crystals held together by a polymer binder. When subjected to high-energy sources, such as laser irradiation and high strainrate loading conditions, these materials can undergo extensive fracture, plastic deformation, and localized hot spot formation [1,2]. These mechanisms are governed by the heterogeneous microstructure and defects, such as non-uniform crystal size distributions, crystal-binder interfaces, and dislocation densities [3-6]. Both interfacial fracture between crystal-crystal and crystal-binder interfaces and crack nucleation within individual crystals have been observed [7-9]. The fracture behavior is governed by the energetic crystalline structure and the complex defect interactions that occur at the interfaces, which can play a significant role under dynamic loads [9,10]. In this paper, we have investigated dynamic fracture in energetic aggregates and the effects of crystal-crystal and crystal-binder interfacial interactions, dislocation density evolution, viscoelastic polymer binder deformation, and crystalline orientation. A coupled thermo-mechanical formulation for high strain rate loading conditions was obtained and incorporated within a nonlinear finite element framework that couples fracture evolution with formulations for a dislocation-density based crystalline plasticity behavior for the energetic crystals, finite viscoelastic behavior for the polymer binder, and thermal effects for the energetic composite. THEORY The constitutive formulation for the RDX crystals consists of a multiple-slip dislocation-density based crystal plasticity formulation coupled to computational schemes
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