A long-held belief is that shock energy induces initiation of an energetic material through an energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a Fermi’s ...golden rule-based 3-phonon theoretical analysis of energy up-pumping in RDX is presented that considers possible doorway pathways through which energy transfer occurs. On average, modes with frequencies up to 102 cm–1 scatter quickly and transfer over 99% of the vibrational energy to other low-frequency modes up to 102 cm–1 within 0.16 ps. These low-frequency modes scatter less than 0.5% of the vibrational energy directly to modes with significant nitrogen–nitrogen (NN) activity. The midfrequency modes from 102 to 1331 cm–1 further up-pump the energy to these modes within 5.6 ps. The highest-frequency modes scatter and redistribute a small fraction of the vibrational energy to all other modes, which last over 2000 ps. The midfrequency modes between 457 and 462 cm–1 and between 831 and 1331 cm–1 are the most critical for vibrational heating of the NN modes and phenomena, leading to initiation in energetics. In contrast, modes stimulated by the shock with frequencies up to 102 cm–1 dominate vibrational cooling of the NN modes.
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Void strengthening in crystalline materials refers to the increase in yield stress due to the impediment of dislocation motion by voids. Dislocation dynamics (DD) is a modeling method ...well suited to capture the physics, length scales, and time scales associated with void strengthening. However, previous DD simulation of dislocation–void interactions have been unable to accurately account for the strong image forces acting on the dislocation due to the void’s free surface. In this article, we employ a finite-element-based DD method to determine the obstacle strength of voids, defined as the critical resolved shear stress for a dislocation to glide past an array of voids. Our results demonstrate that the attractive image forces between the dislocation and free surface significantly reduce the obstacle strength of voids. Effects of surface mobility and stress concentrations around the void are also explored and are shown to have minimal effect on the critical stress. Finally, a new model relating void size and spacing to obstacle strength is proposed.
Simulations of High-Pressure Phases in RDX Munday, Lynn B; Chung, Peter W; Rice, Betsy M ...
The journal of physical chemistry. B,
04/2011, Letnik:
115, Številka:
15
Journal Article
Recenzirano
Using a fully flexible molecular potential in equilibrium molecular dynamics simulations, we study the α- and γ-polymorphs of the energetic molecular crystal hexahydro-1,3,5-trinitro-s-triazine ...(RDX), their respective properties, and the conditions that contribute to the stress-induced γ → α solid−solid phase transition mechanisms. We find the pressure-dependent atomic structure, mechanical properties, and transition behavior to be described reasonably well. Uniaxial deformation of α-RDX along the crystal axes is shown to result in three different crystal responses where compression of the c-axis results in the α → γ transition, compression of the b-axis causes a transition with resulting structure similar to stacking faults observed by Cawkwell et al. J. Appl. Phys. 2010, 107, 063512 , and no transitions are observed for compression of the a-axis.
Plastic deformation induced by stress concentrations near crystal defects occurs through the generation of prismatic dislocation loops (PDL). The production of PDLs leads to void growth and particle ...decoherence. In this work we use dislocation dynamics simulations to characterize two mechanisms for PDL formation. The first mechanism corresponds to a classical model of PDL generation from dislocation nucleation. The second mechanism considers PDL generation through cross-slip of a screw dislocation intersecting the particle. We systematically study the effect of the crystal lattice and defect type on PDL generation for both mechanisms as a function of pressure. The simulations show image stresses produced by the dislocation's interaction with the free surface of a void suppresses PDL generation. The highest PDL generation rates are found for a dislocation nucleated from a void in a body-centered cubic lattice. Our simulations also show helical coiling of screw dislocations produces a continuous emission of PDLs without the need for dislocation nucleation at pressures as low as 1.0 GPa.
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Prismatic punching is a process where voids grow through the nucleation and emission of prismatic dislocation loops (PDLs). In this work we employ dislocation dynamics to determine the effect of ...image stresses produced by the void’s free surface on PDL formation in a face-centered cubic lattice. We find that image stresses cause PDL formation to fall into two distinct pressure regimes. In the low pressure regime, image stresses dominate dislocation cross-slip, reducing the PDL’s size and formation rate.
The acoustic fluid‐structure interaction (FSI) formulation is a practical numerical approach for the seismic analysis of fluid‐filled tanks. However, there are no verification and validation studies ...reported in the literature that demonstrate the ability of an acoustic FSI numerical model to predict responses important to structural and mechanical design for intense translational and rotational earthquake inputs. Herein, an acoustic FSI formulation is implemented in the open‐source Multiphysics Object‐Oriented Simulation Environment (MOOSE), and is formally verified and validated using analytical solutions and code‐to‐code verification, and experimental data, respectively. The analytical solutions are for small amplitude, unidirectional seismic inputs. The code‐to‐code verification utilizes a previously verified and validated Arbitrary Lagrangian‐Eulerian (ALE) numerical model in the commercial finite element code LS‐DYNA. The validation studies utilize a comprehensive data set assembled from results of 3D earthquake‐simulator tests of a fluid‐filled vessel. The acoustic numerical model in MOOSE is verified and validated for hydrodynamic pressures and support reactions except for cases that involve significant convective response. For small amplitude inputs, numerically predicted wave heights match those of the analytical solutions. The numerical model is not verified and validated for wave height calculations under intense 3D seismic inputs. The run times for the acoustic FSI simulations in MOOSE are an order of magnitude, or more, shorter than for the corresponding ALE simulations in LS‐DYNA. The utility of the MOOSE acoustic FSI implementation is demonstrated by seismic analysis of a building equipped with a fluid‐filled, advanced nuclear reactor.
The generalized stacking fault (GSF) energy surfaces in the organic energetic molecular crystal, hexahydro-1,3,5-trinitro-s-triazine (RDX), were studied through atomistic simulations. Using a fully ...flexible molecular potential in highly damped molecular dynamics simulations, we determined quenched 0 K GSF energy surfaces and structures for a set of planes in the α-polymorph RDX crystal and subsequently compare predictions of slip or cleavage with available experimental observations. To account for the steric contributions and elastic shearing due to the presence of flexible molecules, a modified calculation procedure for the GSF energy surface is proposed that enables the distinction of elastic shear energy from the energy associated with the interfacial displacement discontinuity at the slip plane. Comparisons of the unstable stacking fault energy with the surface energy are used to differentiate cleavage planes from likely slip planes, and the calculations are found to be largely in agreement with available experimental data.
The acoustic fluid-structure interaction (FSI) formulation is a practical numerical approach for the seismic analysis of fluid-filled tanks. However, there are no verification and validation studies ...reported in the literature that demonstrate the ability of an acoustic FSI numerical model to predict responses important to structural and mechanical design for intense translational and rotational earthquake inputs. Herein, an acoustic FSI formulation is implemented in the open-source Multiphysics Object-Oriented Simulation Environment (MOOSE), and is formally verified and validated using analytical solutions and code-to-code verification, and experimental data, respectively. The analytical solutions are for small amplitude, unidirectional seismic inputs. The code-to-code verification utilizes a previously verified and validated Arbitrary Lagrangian-Eulerian (ALE) numerical model in the commercial finite element code LS-DYNA. The validation studies utilize a comprehensive data set assembled from results of 3D earthquake-simulator tests of a fluid-filled vessel. The acoustic numerical model in MOOSE is verified and validated for hydrodynamic pressures and support reactions except for cases that involve significant convective response. For small amplitude inputs, numerically predicted wave heights match those of the analytical solutions. The numerical model is not verified and validated for wave height calculations under intense 3D seismic inputs. The run times for the acoustic FSI simulations in MOOSE are an order of magnitude, or more, shorter than for the corresponding ALE simulations in LS-DYNA. The utility of the MOOSE acoustic FSI implementation is demonstrated by seismic analysis of a building equipped with a fluid-filled, advanced nuclear reactor.
A long-held belief is that shock energy induces initiation of an energetic material through an indirect energy up-pumping mechanism involving phonon scattering through doorway modes. In this paper, a ...3-phonon theoretical analysis of energy up-pumping in RDX is presented that involves both direct and indirect pathways where the direct energy transfer dominates. The calculation considers individual phonon modes which are then analyzed in bands. Scattering is handled up to the third order term in the Hamiltonian based on Fermi's Golden Rule. On average, modes with frequencies up to 90 cm-1 scatter quickly and redistribute the energy to all the modes. This direct stimulation occurs rapidly, within 0.16 ps, and involves distortions to NN bonds. Modes from 90 to 1839 cm-1 further up-pump the energy to NN bond distortion modes through an indirect route within 5.6 ps. The highest frequency modes have the lowest contribution to energy transfer due to their lower participation in phonon-phonon scattering. The modes stimulated directly by the shock with frequencies up to 90 cm-1 are estimated to account for 52 to 89\% of the total energy transfer to various NN bond distorting modes.
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