摘要:

Recent experimental developments to examine fundamental processes and mechanisms governing shock wave induced physical and chemical changes are reviewed and summarized. The emphasis is on time-resolved measurements at various length scales and representative examples are provided to discuss achievements in this decade. Directions for future work are indicated. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303412

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

VISAR predecessors are described, including the displacement and velocity interferometer techniques for shock instrumentation. The advance to the VISAR in 1972 made laser interferometry applicable to a very wide range of shockwave experiments. With a 1974 refinement of the VISAR data reduction equation, the VISAR was shown to produce velocity measurements with better than 1&percent; accuracy, and with time resolution to about 2 ns. The power of the VISAR was demonstrated in a plate impact study of the 13 GPa phase transition in iron. Rate effects in shock compressed iron were measured and correlated with theory, and the unloading stress-volume path was determined, revealing the reverse phase transition stress to unprecedented accuracy. Later improvements in VISARs are reviewed, including the lens delay VISAR (Amery), the push-pull VISAR (Hemsing), the ORVIS (Bloomquist and Sheffield), the line VISAR (Hemsing), the fixed-cavity VISAR (Sweat, et al.), the “never-search-for-fringes” VISAR (Barker), and the multi-beam VISAR (Barker). ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303413

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Recent computer simulation methodologies are reviewed that attempt to bridge length scales from atomistic scales to macroscopic scales describable by continuum mechanics. Basic concepts of the quasicontinuum method are introduced. Its use of atomistic descriptions at the element level in a finite element code makes possible a seamless transition from atomistic length scales to scales that are orders of magnitude larger. The atomistic description allows crystal geometry to be introduced directly and the generation and propagation of defects to be a natural consequence of the loading. Computational efficiency, relative to molecular dynamics, is obtained by adaptive meshing so that atomistic-scale refinement is used only in regions where the physical fields have steep gradients. Mesh size dependence is essentially eliminated in order to preserve dependence on physical length scales by using all atom positions within the cutoff radii of atomic potentials, even when some of these atoms are in adjacent elements. To model the interaction of many dislocations, discrete-dislocation models are described in which dislocation segments interact through their long range elastic fields. In these models, dislocation dynamics concepts are applied at the level of connected dislocation segments. Bridging of length scales in dynamic fracture is described using a cohesive surface formulation. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303414

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The mesoscopic processes of consolidation, deformation and reaction of shocked porous energetic materials are studied using shock physics analysis of impact on a collection of discrete “crystals.” Highly resolved three-dimensional CTH simulations indicate that rapid deformation occurs at material contact points causing large amplitude fluctuations of stress states with wavelengths of the order of several particle diameters. Localization of energy produces “hot-spots” due to shock focusing and plastic work near internal boundaries as material flows into interstitial regions. Numerical experiments indicate that “hot-spots” are strongly influenced by multiple crystal interactions. Chemical reaction processes also produce multiple wave structures associated with particle distribution effects. This study provides new insights into the micromechanical behavior of heterogeneous energetic materials strongly suggesting that initiation and sustained reaction of shocked heterogeneous materials involves states distinctly different from single jump state descriptions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303415

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

At the level of atoms, using large-scale molecular-dynamics simulations, we have begun to elucidate the response of materials to shockwave loading and unloading. In dense monatomic, chemically unreactive fluids, the profile or structure of a shock wave is rather boring, being well described by viscous flow. In solids, on the other hand, the structure is far more complex, being dominated by plastic flow mechanisms that can even resemble phase transformations. We have just begun to explore the richness of the shock loading regime in crystalline solids, while much of the behavior upon unloading has heretofore remained in the province of speculation. We discuss some of the recent advances we have made at Los Alamos in simulations of shock waves and related phenomena, including plastic deformation, high-speed interfacial sliding, and fragmentation. As experimental observations become more and more refined, and molecular-dynamics simulations become larger, even approaching the mesoscale, fruitful overlap is achievable in the near future. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303416

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Molecular dynamics simulations have been performed for highly compressed fluid hydrogen in the density and temperature regime of recent shock-compression experiments. Both density functional and tight-binding electronic structure techniques have been used to describe interatomic forces. Two tight-binding models of hydrogen have been developed with a single s-type orbital on each atom that reproduce properties of the dimer, of various crystalline structures, and of the fluid. The simulations indicate that the rapid rise in the electrical conductivity observed in the gas-gun experiments depends critically on the dissociated atoms(monomers). Hugoniots derived from the equations-of-state of these models do not exhibit the large compressions predicted by the recent laser experiments. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303417

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Thermodynamic properties and electrical conductivity measurements of nonideal plasmas of hydrogen were carried out under multiple shock compression up to 1.5 Mbar. An abrupt increase of the electrical conductivity (by five orders of value) was registered starting at the density0.3÷0.5&hthinsp;g/cm3.This electrical conductivity value approaches∼103&hthinsp;&OHgr;−1&hthinsp;cm−1which is near to the liquid metal values. The data obtained are described by the nonideal plasma model taking account of the increase of number of conductivity electrons due to the “pressure ionization.” ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303418

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Simple statistical mechanics models have been assembled into a wide-range equation of state for the hydrogen isotopes. The solid is represented by an Einstein-Gru¨neisen model delimited by a Lindemann melting curve. The fluid is represented by an ideal gas plus a soft-sphere fluid configurational term. Dissociation and ionization are approximated by modifying the ideal gas chemical-equilibrium formulation. TheT=0isotherm and dissociation models have been fitted to new diamond-anvil isotherm and laser-generated shock data. The main limitation of the model is in ionization at high compression. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303419

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We develop a model of chemical equilibrium mixtures containing the elements H, C, F. We show that the shock response of a wide variety of molecular and polymeric fluorocarbons can be modeled as a chemical equilibrium mixture of a small number of dissociation product molecules. HF is known to strongly associate in the supercritical fluid phase. We predict that such an association also occurs under shock conditions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303420

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The phase diagram of carbon is not experimentally well known at high pressure and/or high temperature. We have determined the phase diagram of carbon by means of Monte Carlo and molecular dynamics simulation methods using the Brenner bond order potential. The melting line of diamond was found to have a positive slope (pressure vs. temperature) with the graphite-diamond-liquid triple point in agreement with an estimated value based on experiment. The present calculation predicts a first-order phase transition in liquid carbon terminated by a critical point and by a triple point on the graphite melting line. The low-density liquid is predominantlyspbonded with littlesp3character. The high-density liquid is mostlysp3bonded with littlespcharacter. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303421

出版商:AIP    

年代:1900

数据来源: AIP