Microstructural Behavior of Energetic Crystalline Aggregates
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Microstructural Behavior of Energetic Crystalline Aggregates D. LABARBERA (a), M.A. ZIKRY (a) (a) Dept. of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 276957910
ABSTRACT A dislocation-density based crystalline plasticity and specialized finite-element formulations were used to study the behavior of energetic crystalline aggregates. The energetic crystalline material studied was RDX (cyclotrimethylene trinitramine) with a polymer binder and different void porosities. The aggregate was subjected to different dynamic pressures, and the analyses indicate that maximum temperature increases, constrained dislocation densities, and plastic strain accumulations occurred around the void peripheries, which affected overall deformation behavior. These regions of extreme temperature rise and thermal decomposition can result in hot spot formation. INTRODUCTION Energetic crystalline aggregates consist of an energetic crystal that is mixed with a binder material, such as a polymer. The binder material is added to enhance the overall aggregate’s mechanical properties allowing it to be molded into any unique shape. Energetic crystalline aggregates can have microstructural defects, such as porosity, dislocationdensities, and cracks. The behavior of energetic crystalline aggregates is not well understood on the microstructural level (for example, see, Czerski and Proud [1]and Borne and Franck [2]). It has also been noted that for RDX (cyclotrimethylene trinitramine) the presence of only three unique slip systems leads to a possible failure mechanism of brittle fracture on cleavage planes[3]. The objective, therefore, of this study is to examine the behavior of energetic crystalline aggregates and how the behavior is interrelated to microstructural defects. A dislocationdensity based crystalline plasticity and specialized dynamic finite-element formulation have been utilized, and it was used to predict dislocation-density evolution, temperature evolution, slip behavior, and wave propagation in RDX-polymer aggregates. DISLOCATION-DENSITY BASED CRYSTAL PLASTICITY FORMULATION Constitutive formulations for rate-dependent multiple-slip crystal plasticity, which are coupled to evolutionary equations for the dislocation-densities, have been used. For a detailed presentation see the work by Shanthraj and Zikry [4], and Ashmawi and Zikry [5]. The velocity gradient is decomposed into a symmetric deformation rate tensor Dij and an antisymmetric spin tensor Wij. Dij and Wij are then additively decomposed into elastic and inelastic components as Dij = Dij* + Dijp ,
Wij = Wij* + Wijp .
(1a-b)
The inelastic parts are defined in terms of the crystallographic slip-rates as Dijp = Pij(α )γ˙ (α ) ,
Wijp = ω (ijα )γ˙ (α ) ,
(2a-b)
where (α) is summed over all slip-systems, and Pij(α ) and ω (ijα ) are the symmetric and antisymmetric parts of the Schmid tensor in the current configuration respectively. The rate-dependent constitutive description on each slip system can be characterized by a power law relation f
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