Characteristics of the effect of low-current gas discharge on a strong shock wave

The effect of nonequilibrium plasma medium created by low-current gas discharge on a shock wave (Mach number M = 6.8) has been investigated. The shock wave is generated in a shock tube with a circular cross section and then passes through the area of the effect in the working chamber having a square cross section. The gas discharge is ignited in the area of the effect before the arrival of the shock wave by applying voltage to special pin electrodes mounted in the upper and lower walls of the working chamber. In the experiment, the phenomenon of broadening of the shock wave and an increase of its propagation velocity when passing through the region of nonequilibrium gas-discharge plasma was discovered.     

Propagation of the front of a strong shock wave in an inhomogeneous atmosphere

An equation describing the propagation of the front of a point explosion in an inhomogeneous atmosphere is derived here with the aid of dimensional analysis. The solution for an exponential atmosphere agrees with already known results.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 6, pp. 987–991, June, 1972.     

Mechanism responsible for the cooling of air heated by a strong shock wave

The problem of the cooling of a spherical volume of air after departure of a shock wave in air of normal density is studied numerically. Based on the results obtained, a conclusion is made that no radiative cooling wave exists in a real atmosphere.Academic Scientific Complex A. V. Luikov Heat and Mass Transfer Institute of the Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 2, pp. 155–158, February, 1994     

Deformation of polycrystalline Ti2AlC under compression

Deformation of polycrystalline Ti₂AlC under room and high temperature compression was investigated. The results demonstrated that Ti₂AlC was damage tolerant at room temperature and the samples were shear fractured upon failure. At high temperatures Ti₂AlC deform plastically. The brittle-to-ductile-transition temperature (BDTT) of Ti₂AlC was between 1000 °C and 1050 °C. The microstructure and fracture surfaces were examined using scanning electron microscopy. Due to insufficient number of dislocation systems, the room-temperature deformation was constitute of kinking and delaminating of laminated Ti₂AlC grains, basal plane dislocation slip, formation of voids and cavities in the vicinity of main crack. At high temperatures below BDTT, the deformation was a combination of cavities formation and intergranular sliding. At temperatures above BDTT, the deformation was mainly plastic flow. Received: 15 January 2001 / Reviewed and accepted: 12 April 2001     

A uniformly valid solution for the three-dimensional inviscid supersonic flow past blunt bodies with strong compression in the shock wave

Summary An analytical uniformly valid approximate solution is developed for the steady threedimensional supersonic flow past blunt bodies. The inverse problem is investigated, i.e. the shock shape is prescribed. Viscosity and heat conduction are neglected. The approximation is based on two main assumptions: i) the density ratio across the shock is very small, ii) the pressure does not change its order of magnitude along a normal to the shock surface. The pattern of the streamlines projected on to the shock surface is calculated from an ordinary differential equation of second order by taking into consideration the pressure gradients. By evaluating two integrals the flow quantities and the streamlines are determined in the shock layer together with the body shape. The solution is also valid for sharp nosed bodies.The method is applied to paraboloidal or hyperboloidal shock shapes of various cross sections. Results are presented for the streamline projections in the entire flow field. The flow quantities and the streamline pattern in the shock layer are calculated in the symmetry plane of the flow field. The streamlines differ from the geodesics, i.e. the solution according to the Newtonian model. The flow quantities on the body and the body shape show good agreement with numerical results of the direct problem by Rusanov.With 10 Figures     

Similarity solutions of a strong shock wave propagation in a mixture of a gas and dusty particles

The similarity solutions of a strong shock wave propagation in a mixture of a gas and small solid particles have been investigated. Similarity solution exists only when the shock is very strong and the surrounding medium is of a constant density and at rest and with negligible counterpressure. The non-dimensional fundamental equations are derived and studied. The results depend on three non-dimensional parameters; i.e. (i) the ratio of the specific heats of the gas γ, (ii) the mass concentration of the solid particles k_p in the mixture and (iii) the ratio of the density of the solid to that of initial density of the gas G. Numerical solutions for various values of γ k_p and G are presented and discussed. The speeds of the shock wave front and its location with various energy releases are given.     

单轴压缩载荷下闭孔泡沫铝的变形机制

基于X射线计算机断层扫描技术,重构了能够反映闭孔泡沫铝真实细观结构的三维有限元模型。采用数值模拟与试验测试相结合的方法,研究了泡沫铝在准静态单轴压缩载荷作用下的力学响应及其变形机制,重点关注了平台阶段及致密化阶段的变形模式。结果表明:试件中变形带的出现是压缩过程进入平台阶段的一个标志,此时棱杆和孔壁的变形以塑性弯曲为主;平台阶段,棱杆及孔壁的变形逐渐向塑性起皱与塑性屈曲转变;伴随致密化阶段的发生,变形带内部的胞孔严重坍塌,呈‘双凹圆盘’状。闭孔泡沫铝细观结构变形模式的数值模拟与试验结果相符,验证了该模型的有效性,为进一步研究各相关物理量(相对密度、加载速率等)及变形机制对其宏观吸能性能的影响奠定了基础。     

Measurements of the viscosity of water under shock compression

The results of measurements of the shear viscosity of shock-loaded water, obtained by the method of decay of perturbations on a corrugated shock front suggested by Sakharov et al. [1], are treated in the light of new data on the phase diagram of water. The experimental data on viscosity in the pressure range from 4 to 25 GPa are compared to the results of measurements by other methods. The high values of viscosity at high pressures are attributed to the formation a water-ice VII mixture under shock compression of water.Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 1, 2005, pp. 136–145. Original Russian Text Copyright © 2005 by V. N. Mineev and A. I. Funtikov.     

Deformation band and texture of a cast Mg-RE alloy under uniaxial hot compression

Microstructural evolution and texture of a cast Mg-9Gd-4Y-0.6Zr ingot under hot compression were studied in this paper. Post-deforming microstructures were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, while crystallographic orientation information was obtained from X-Ray macro-texture measurement and EBSD micro-texture analysis. Dynamic recrystallization (DRX) initiated from the deformation bands (DB) forming on original grain boundaries; the DB became widen with continuously conversion of low-angle-boundary grains into high-angle-boundary grains. The tendency of strain localization increased with Z parameter. The macro-texture analysis indicates that uniaxial compression yielded out the randomized basal texture component. This texture component was found to be strengthened with increasing Z parameter. The micro-texture analysis shows that the deviation from the ideal basal texture arose from orientated growth within DBs. Moreover, the localization deformation promoted dynamic precipitation within DBs, which inhibited the development of DRX.     

Computer simulation of copper and silver under shock compression conditions

A correction to the embedded atom method potentials for copper and silver is proposed which ensures good agreement between molecular dynamics simulation results and experimental data on the thermodynamic properties of Cu and Ag under shock compression conditions (at pressures of up to 1473 GPa in the case of copper and up to 478 GPa in the case of silver). The potentials are represented in analytical form. The calculations take into account the contributions of the valence electrons to the energy and pressure of the metals. This reduces the simulated temperature on the Hugoniot adiabat (by 30% in the case of copper and by a factor of 1.5–2 in the case of silver) relative to estimates in the Grüneisen model.     

Measurements of the viscosity of iron and uranium under shock compression

Interest in studying the viscosity of iron and uranium is primarily associated with studying the stability of motion of envelopes made of these materials towards the center in spherically symmetric systems during the acceleration of the envelopes by the products of explosion and by shock waves. The experimental measurements of viscosity in the pressure range from 30 to 250 GPa involve the use of the method of evolution of harmonic oscillation preassigned at the front of shock wave propagating in iron and uranium. The resultant data are considered along with the estimates of the thermodynamic state of matter under shock compression.     

Axisymmetric deformations of straight cylindrical tubes under shock wave loading

The tube strains caused by shock waves passing through the tube are investigated. The tubes are straight and cylindrical and the deformations axisymmetric. Two models are developed that allow the computation of the strains for differently fixed tubes. The models are simple and lead to equation of motions that can easily be solved. The characteristics of the tube deformations caused by shock waves can be studied and described by these models. It is shown that the chosen approach to compute the tube strains is suited to reproduce experimental results. The comparison between computation and experiment also indicates that the structural damping activated by these high speed waves may be small.     

Acoustic wave propagation in disordered microscale granular media under compression

We investigate the effect of dynamic and uniaxial static loading on the wave speeds and rise times of laser generated acoustic waves traveling through a disordered, multilayer aggregate of 2 \(\mu {\mathrm {m}}\) diameter silica microspheres, where the excited dynamic amplitudes are estimated to approach the level of the static overlap between the particles caused by adhesion and externally applied loads. Two cases are studied: a case where the as-fabricated particle network is retained, and a case where the static load has been increased to the point where the aggregate collapses and a rearrangement of the particle network occurs. We observe increases in wave speeds with static loading significantly lower than, and in approximate agreement with, predictions from models based on Hertzian contact mechanics for the pre- and post-collapse states, respectively. The measured rise time of the leading pulse is found to decrease with increasing static load in both cases, which we attribute to decreased scattering and stiffening of the contact network. Finally, we observe an increase in wave speed with increased excitation amplitude that depends on static loading, and whether the system is in the pre- or post-collapse state. The wave speed dependence on amplitude and static load is found to be in qualitative agreement with a one-dimensional discrete model of adhesive spheres, although the observed difference between pre- and post-collapse states is not captured. This investigation, and the approach presented herein, may find use in future studies of the contact mechanics and dynamics of adhesive microgranular systems.     

载荷作用下EPS混凝土中弹性波传播特性研究

应用EPS混凝土来模拟含缺陷的岩石材料。对EPS粒径分别为1、2和3mm的三种EPS混凝土试样进行了载荷作用下不同频率的弹性波传播实验研究。采用单一频率脉冲叠合的方法来精确确定材料的波速,结果表明：EPS混凝土的p波波速随载荷增加在试件的开始压密实阶段有较明显的增大趋势,当试件相对密实,波速增加不是很明显;s波波速随载荷增加有一定程度增加,但幅度比p波波速增加得小得多。应用一种相对波速的方法,即将波速与当前载荷下材料的声波速度进行对比,可以较好地分析波速与载荷和频率的关系。最后对波速与载荷和频率的关系进行了理论模拟分析。此研究对于应用弹性波进行材料和结构的无损检测等技术方面有很好的参考意义。     

Generation of a flat-top laser beam for gravitational wave detectors by means of a nonspherical Fabry-Perot resonator

We have tested a new kind of Fabry-Perot long-baseline optical resonator proposed to reduce the thermal noise sensitivity of gravitational wave interferometric detectors--the "mesa beam" cavity--whose flat top beam shape is achieved by means of an aspherical end mirror. We present the fundamental mode intensity pattern for this cavity and its distortion due to surface imperfections and tilt misalignments, and contrast the higher order mode patterns to the Gauss-Laguerre modes of a spherical mirror cavity. We discuss the effects of mirror tilts on cavity alignment and locking and present measurements of the mesa beam tilt sensitivity.     

Deformation of copper under the action of strong magnetic fields and cryogenic temperatures

Results are presented for an experimental study of the effect of a strong constant and pulsed magnetic field on the mechanical characteristics and deformation features of copper M1 at 293–4.2 K. Dependences have been obtained for the increase in specimen strain on the intensity of a pulsed magnetic field and flow stress, and possibilities have been studied for increasing the effectiveness of pulsedmagnetic deformation of metals. A calculation relationship is suggested for determining the critical stress level under strong skin-effect conditions of a magnetic field.Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 12, pp. 32–36, December, 1989.     

Deformation twinning to dislocation slip transition in single-crystal tantalum under dynamic compression

Journal of Materials Science - The deformation twinning to dislocation slip transition is observed in large-scale molecular dynamics simulations of single-crystal tantalum under dynamic loading,...