摘要:

Shock wave profiles in vitreousGeO2 (6.56&hthinsp;Mg/m3)under planar loading were measured using stress gauges to 14 GPa. New and previous data yield Hugoniot:D=0.974&hthinsp;(km/s)+1.711&hthinsp;ufor shocks of 6 to 40 GPa. We show that the phase change from 4- to 6-fold coordination ofGe+4withO−2in vitreousGeO2occurs from 4 to 15 GPa. Hugoniots of vitreousGeO2andSiO2are found to approximately coincide if the pressure inSiO2is scaled by the ratio ofSiO2toGeO2initial density. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303472

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The B1/B2 phase transformation in potassium chloride (KCl) under shock loading conditions has been investigated using constantan strain gauges. These have been mounted in the samples in such an orientation so that they are sensitive to longitudinal strain. The samples themselves are made from pressed, polycrystalline potassium chloride. At the phase transformation stress ofca.2 GPa, measured strains ofca.−0.1 are slightly higher than values calculated from knowledge of the particle and shock velocities at that stress level. Residual porosity in the microstructure has been suggested as a possible cause. Total strains over the stress range 2.5 GPa to 4.0 GPa show reasonable agreement with the work of others at lower stresses, but some disagreement at higher stress levels. We suggest that this might be due to the material being in a non-equilibrium state at the time of measurement. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303473

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Many experimental studies have demonstrated the existence of shock-induced chemical reactions in organic materials. Some of this work was motivated by the possible cosmochemical significance of shock-induced synthesis resulting from meteor bombardment of planetary atmospheres. In most cases, fundamental reaction mechanisms and kinetics, and threshold conditions for reaction initiation, remain poorly understood. Using a recently developed reactive empirical bond order (REBO) potential for hydrocarbons, we have carried out molecular dynamics simulations of shocked condensed-phase acetylene to investigate the resulting shock-induced chemistry at an atomistic level. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303474

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The shock-induced&agr;→&egr;phase transition in iron has been numerically investigated at the grain level by use of a two dimensional, discrete element model with focus on the metastability and dynamics of the transition. It was found (1) that the transition relaxes to the static metastable surface and (2) that the relaxation time of the transition increases as the wave propagates, and decreases for stronger shock pressure. Also, the calculation showed that the second order kinetics yields the global transition pressure closer to the experimental value than the first order kinetics. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303475

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Results of an acoustical study of mercury at sub- and supercritical states are reviewed. Sound velocity, attenuation and reflectivity against a buffer-rod/mercury interface have been measured at steady state conditions at temperatures up to 2100 K and pressures up to 190 MPa with molybdenum and niobium cells. The obtained phase diagram contains a first order prewetting phase transition curve, which tangentially meets the coexistence curve at a wetting transition point located below 1250 K. These two curves are well separated, and the prewetting critical point lies well above the bulk liquid-gas critical point. The phase diagram allows one to suggest an explanation for mysterious peculiarities seen both in the vapor phase and in the supercritical region of mercury, including thermopower anomalies first noticed in early seventies. The importance of the prewetting phenomena for safety of nuclear reactors with metallic coolants is discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303476

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Using large-scale molecular-dynamics simulations, we have studied the propagation of elastic and plastic waves in three-dimensional crystals oriented along low-index directions. We have observed elastic precursors, followed by steady plastic waves (depending on propagation direction), which are the result of both simple, and quite complex, modes of plastic deformation. We present a review of earlier results, methods of computation and visualization, and prospects for further areas of study. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303477

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A theory is presented for the mechanical response of porous media to high-strain-rate deformations. The model is “consistent” because each feature is incorporated in a manner that is mathematically compatible with all the other features. Unlike simplep-&agr;models, the onset of pore collapse depends (via a user-adjustable yield function) on the amount of shear present. The elastic part of the strain rate is linearly related to the stress rate, except for nonlinear contributions due to the change in the elastic moduli upon pore collapse. The inelastic part of the strain rate includes parts from plastic deformation of the matrix material, pore nucleation, and phase transformations in the matrix material. The plastic strain rate is taken normal to the yield surface. Consequently, if phase transformation and/or nucleation are simultaneously occurring, the inelastic strain rate will be non-normal to the yield surface. The matrix yield stress is permitted to harden linearly. Plastic volume changes of the matrix material are assumed negligible in comparison to macroscopic volume changes associated with pore collapse. Rate dependence is allowed via an overstress model. The theory has been exercised under a rigorous array of canonical loading paths with special care to ensure sensible response upon unloading and reloading. Results show good progress toward modeling a particular 10&percent; porous ferroelectric ceramic. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303478

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Understanding the fragmentation process in solids and liquids, and predicting the fragment size distributions, are important for a number of applications that include meteorite impacts on space structures, armor/antiarmor applications, and debris generation in pulse power facilities. The fragmentation process is one in which microscopic cracks, voids, or shear bands are nucleated at discrete sites and thereafter grow to coalescence. Mesomechanical models of this process are fairly mature for failure via shear banding or cleavage cracking, but not for voids in liquids or plastically deforming solids (1). Even when mesomechanical models are available, they are typically challenging to install in ever-evolving hydrocodes, and simpler engineering models that predict average fragment size as a function of a few material properties and the strain rate have come into wide use (2). We assess the expected accuracies of such engineering models by comparing the predictions of two “equally plausible” simple models for the fragmentation of liquids and plastically deforming solids. The two models agree closely for liquids, but differ by about a factor of ten for plastically deforming solids. We conclude that such engineering models are suitable for scoping problems and predicting trends, but mesomechanical models will be required for more accurate predictions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303479

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Pore collapse plays an important role in shock response of porous materials, which is usually predicted with a Carroll-Holt model based on continuum mechanics. In this paper, single pore collapse processes at various loading speeds have been investigated numerically with Discrete Meso-Element Dynamic Method (DM2). Results show that the pore collapse process at various loading speeds is not isotropic and has different patterns at meso-scale level. The strong localization effects during collapse cause the formation of hot spots and mass mixing, which might be the principal mechanisms for initiation of chemical reaction in porous mixtures. An improved model for pore collapse involving meso-scale information is needed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303480

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Uniaxial strain impact experiments have been performed to obtain shock compression and release response of a 0.22 g/cm3polyurethane foam in a configuration where the foam impacts a thin target witness plate. Wave profiles from a suite of ten experiments have been obtained, where shock amplitudes range from 40 to 600 MPa. A traditional P-&agr; porous material model generally captures the material response. A fully three-dimensional explicit representation of the heterogeneous foam structure modeled with numerical simulations recovers some of the high frequency aspects of the particle velocity records. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303481

出版商:AIP    

年代:1900

数据来源: AIP