摘要:

We report on large scale non‐equilibrium atomistic simulations of shock‐induced solid‐solid phase transformations. As an example the &agr; → &egr; transformation in iron and the A11 (GaI)→cI12 (GaII) transformation in gallium are discussed. The use of semi‐empirical descriptions of the inter‐atomic forces and today’s parallel computing resources allow for a quantitative comparison of the theoretically calculated data with the experimental results. The discussion will include the crystallographic orientation dependence on the transformation process in single crystals. Simulations containing several millions of iron atoms reveal that above a critical shock strength, many small close‐packed grains nucleate in the shock‐compressed bcc crystal. For shock waves in the [001] direction the initially small grains are growing on a picosecond time scale to form larger, energetically favored, grains. For the two other major crystallographic directions, here the annealing processes are slower and and have not finished within the time scales accessible with atomistic simulations (up to 50 ps). Furthermore, crystals shocked in [111] direction produce solitary waves ahead of the actual shock front. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780223

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

From the point of view of the modern statistical mechanics the problems on shock compression of solids require a reformulation in terms of highly nonequilibrium effects arising inside the wave front. The self‐organization during the multiscale and multistage momentum and energy exchange are originated by the correlation function. The theory of dynamic plasticity has been developed by the author on the base of the self‐consistent nonlocal hydrodynamic approach had been applied to the shock wave propagation in solids. Nonlocal balance equations describe both the reversible wave type transport at the initial stage and the diffusive (dissipative) one in the end. The involved inverse influence of the mesoeffects on the wave propagation makes the formulation of problems self‐consistent and involves a concept of the cybernetic control close‐loop. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780224

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

This paper presents experimental observation and modeling of time dependent energy localization occurring when a shock propagates along and through a grain boundary. This work is part of a larger program of investigation of the effect of grain boundaries on shock propagation in materials. The project is initially focused on the simplest of materials: a bicrystal. Our study covers the effects of grain orientation, the grain boundary angle, the boundary region, shock properties and the interplay between these in determining the characteristics of shock propagation. Ultimately, we will use this information as a basis for incorporation into models of polycrystalline materials. Some of the physics found in polycrystals is absent, but the use of simple, well‐defined samples allows thorough measurements to be made. Laser‐based experiments and diagnostics are used throughout, permitting us to perform the many experiments required in an economical way. NiAl was chosen as a suitable anisotropic material; single crystal and bicrystal samples were prepared. The EOS and single‐crystal elasticity were estimated withab fere initioquantum mechanics. Laser flyer impact and direct drive experiments, coupled with line‐imaging VISAR, were used to test and refine the EOS, and to measure crystal plasticity. Initial models of bicrystals under shock loading have been developed, shock experiments have been conducted on bicrystals and recovered samples have been analyzed. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780225

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The response of shocked materials in the pressure range 100 GPa reveals self‐similar features that could be linked with collective properties of typical mesodefects (dislocation substructures, microcracks, microshears). The statistical approach was developed that allowed us to specify the Ginzburg‐Landau theory for tensor order parameter of the mesodefect density and to establish the existence of self‐similar solutions as specific collective modes in mesodefect ensemble related to the plastic strain and damage localization. The subjection of relaxation properties of dynamically loaded materials to mentioned collective modes provides the self‐similar behavior of materials that experimentally were observed as the universality (fourth power law) of steady‐state plastic front and delayed failure (failure wave) phenomenon as the resonance excitation of blow‐up damage localization areas. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780226

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Calculations of the second‐order elastic constants of pentaerythritol (PE), and urea were performed using the output of the modeling program CRYSTAL98. CRYSTAL98 performs first principles calculations of periodic systems using Gaussian type basis functions. The elastic constants were obtained by performing a polynomial fit to the calculated total energy of the crystal as a function of elastic strain. When the application of strains resulted in a reduction of crystal symmetry, the crystal unit cells often had to be redefined to use a space group of lower symmetry. To reduce computation times, the new space group was chosen to maximize remaining symmetry. A rigid molecule model was employed, where the internal geometry of the molecule was kept unchanged. An optimization technique was developed for lattice relaxation and tested for both urea and PE. Results obtained using different functionals in the Density Functional Theory (DFT) method were compared to Hartree‐Fock (HF) calculations. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780227

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The intermolecular binding (lattice) energy is calculated for the molecular crystals RDX, PE,and PETN using the Crystal98 and Gaussian98 programs (with Gaussian basis sets) and the DMOL program (double‐numerical basis set) and the CASTEP program (plane‐wave basis set). These first‐principles theoretical results are compared with each other and experiment. The importance of the basis sets used (e.g., 6‐21G vs. 6‐311G*) and choice of Hamiltonian (Hartree‐Fock or density‐functional theory) is illustrated by these comparisons. The relevance of calculating the theoretical intermolecular binding energy, being the difference of two nearly‐equal numbers, as a tool for testing the intrinsic quality of a calculation is explained. The importance of optimization of the Gaussian basis sets for the Crystal98 program versus adding more terms to the Gaussian basis set is discussed. Work supported by the Office of Naval Research and the MURI program. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780228

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

A model of dynamic behaviour of quartzite under loading and release is developed, which describes polymorphic transformations and strength properties of this matereal. The model is based on recently obtained data of static, laboratory and large‐scale dynamic experiments. Main peculiarities of dynamic processes in quartzite are taken into account. They are related to considerable non‐equilibrium of polymorphic transformations under loading characterized by large deviation of transformation beginning from equilibrium curve and large length of transformation area; large hysteresis of transformation under loading and release; kinetic nature of transformation. Appropriate quantitative characteristics of transformations are estimated on adiabatic curves of loading and release. Relaxation times of transformations are estimated as well. Conclusions are drawn on the value of residual shear strength. Simulation results are presented for a series of shock experiments used to calibrate the model. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780229

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

A new multi‐scale simulation method is formulated for the study of shocked materials. The method combines molecular dynamics and the Euler equations for compressible flow. Treatment of the difficult problem of the spontaneous formation of multiple shock waves due to material instabilities is enabled with this approach. The method allows the molecular dynamics simulation of the system under dynamical shock conditions for orders of magnitude longer time periods than is possible using the popular non‐equilibrium molecular dynamics (NEMD) approach. An example calculation is given for a model potential for silicon in which a computational speedup of 105is demonstrated. Results of these simulations are consistent with the recent experimental observation of an anomalously large elastic precursor on the nanosecond timescale. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780230

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

For finite deformations of nonlinear viscoelastic solids, the speed of propagation of acceleration waves (i.e., ramp waves) generally depends not only on the current state of strain at the wave front but also on the prior strain history. Consequently, explicit formulas for the wave speed can be quite complicated. Simple formulas for the wave speed do exist for special classes of materials and/or special deformation histories, and in this regard we consider one‐dimensional motions of viscoelastic solids governed by single integral laws. Some of the relations obtained are universal in the sense that they hold for all materials in a given class and do not explicitly involve the relaxation kernel function in the hereditary integral defining these materials. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780231

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The numerical method for simulation of diffusion of electrons and holes in the inertial field of a crystal under shock loading is developed. To analyze this diffusion, a complete system of electro‐diffusion equations for charge carriers has been solved by means of the Poisson equation. Inertial forces were taken into account also by the drift‐diffusion and convection‐diffusion approximations. The equation system has been solved numerically by difference methods. Inertial currents in shocked Si are calculated for the shock wave with an amplitude of 1 GPa and different concentrations of doped impurity. It is shown that the shock wave traveling across the crystal creates an effective region of the electrical space charge that is moving along with the shock wave. The convection‐diffusion model, describing inertial effects, is able of explaining large electric currents in shocked metals while the drift‐diffusion model is not. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1780232

出版商:AIP    

年代:1904

数据来源: AIP