Plastic deformation at high strain rates

In the present paper a new method, an original testing device and techniques for the study of plastic deformation in materials at high strain rates up to 2×10⁴ s^–1 achieved with the use of very short stress pulses (length within 10–20 s) are described. The suggested method of yieldpoint determination respects the effects of delayed plastic deformation in the yield point at loading by very short intensive stress pulses. The investigations were carried out in polycrystalline ARMCO-iron and low-alloyed steel, and besides on the specimens irradiated by neutrons with integral dose 1·35×10¹⁹ ncm^–2. The experimental results obtained are interpreted from the point of view of the present knowledge of the process of plastic deformation in bcc metals.     

Diabatic instability of a solid deformed at high strain rates

Summary The model of a thermo-inelastic solid undergoing a very fast deformation is considered beyond the usual linear approximation. It is shown, via a stability analysis of small fluctuations, that a universal stability diagram in control parameter space exists exhibiting a complex topology but only hard-mode (acousticlike) instabilities, due to the hyperbolic nature of the governing equations. It turns out that an adiabatic homogeneous expansion against cohesion forces is no longer possible if a critical strain rate is exceeded, leading to spatial symmetry breaking towards fracture. The validity of this last result is thought to hold qualitatively irrespectively of the nature of the dissipation laws. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Relaxation processes at high excitation levels and Bose-Einstein condensation of excitons in ZnTe crystals

The singularities of low-temperature emission spectra obtained for ZnTe crystals excited resonantly by a strongly monochromatic source are investigated. The possibility of attaining the coherent luminescent emission due to the exciton-exciton interaction under these conditions is proved. The appearance of the non-polarized needle-like peak and the processes of inelastic interaction of polaritons of the upper polariton branch (UPB) as well as the disappearance of the Raman component are interpreted as the spectral attributes of Bose-Einstein condensation of UPB polaritons.     

Deformation at very high strain rates of Al and ERGAL 7075

Summary Comparative examinations have been carried out on Al and ERGAL specimens deformed by compression either fast or slow. The high-strain-rate compressions were obtained by explosive. The dislocation structures have been examined by transmission electron microscopy and by Fourier analysis of the X-ray diffraction lines. Crystalline textures were also determined. After fast deformation smaller interaction energies appear among the dislocations with comparable total densities because of the accumulation in grain boundaries. The results have been interpreted by assuming that the loads which are reached in the shock and reflected wave fronts give rise to generations or multiplications of the dislocations on levels more differentiated than by slow compression.

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Riassunto Sono stati condotti esami comparativi su campioni di Al e di ERGAL deformati mediante compressione lenta o veloce. Le compressioni ad elevate velocità sono state ottenute per mezzo di esplosive. Le strutture delle dislocazioni sono state osservate con microscopia elettronica in trasmissione e analisi di Fourier delle righe di diffrazione dei raggi X. Sono state inoltre determinate le tessiture cristalline. Con densità totali di dislocazioni comparabili, dopo deformazione rapida appaiono minori energie d'interazione tra le dislocazioni stesse a causa di fenomeni di accumulo nei bordi di grano. I risultati sono stati interpretati ammettendo che i carichi raggiunti in corrispondenza delle onde d'urto, diretta e riflesse, diano luogo a generazione e moltiplicazione di dislocazioni su livelli piú differenziati che con compressione lenta.

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Резюме Проводится сравнительный анализ образцов Al и ERGAL, подвергнутых деформации сжатия либо быстро, либо, медленно. Высокая скорость деформации сжатия получается в результате взрыва. Исследуется структура дислокаций с помощью трансмисионной электронной микроскопии и с помощью Фурье-анализа линий дифракции рентгеновских лучей. Определяются кристаллические структуры. После быстрой деформации возникают небольшие энергии взаимодействия между дислокациями с соизмеримыми полными плотностями, вследствие явления аккумуляции на границах зерен. Проводится интерпретация полученных результатов, предполагая, что нагрузки приводят к образованию или размножению дислокаций на уровнях более дифференцированных, чем в случае медленного сжатия.

     

高导无氧铜的高压与高应变率本构模型研究

基于Y/G及G/B为常数的假设,构建了高导无氧铜的七种高压与高应变率本构模型.对于高导无氧铜进行了平面冲击波试验,采用纵向与横向锰铜应力计记录了试件中的纵向与横向应力,从而得到了屈服应力历史.用所构建的七种本构模型进行了数值模拟,并与高导无氧铜的平面冲击波试验结果进行比较.结果表明,平面冲击波载荷下高导无氧铜的屈服强度对于压力、密度、温度以及塑性应变的依赖性是本构描述的关键.而由Hopkinson试验取得的高导无氧铜高应变率本构模型,并不适合描述平面冲击波载荷下的本构特性. 关键词： 本构模型 高导无氧铜 平面冲击波试验 锰铜应力计     

Multiscale simulations of damage of perfect crystal Cu at high strain rates

We use the molecular dynamics code, large-scale atomic/molecular massively parallel simulator (LAMMPS), to simulate high strain rate triaxial deformation of crystal copper to understand void nucleation and growth (NAG) within the framework of an experimentally fitted macroscopic NAG model for polycrystals (also known as DFRACT model). It is seen that void NAG at the atomistic scales for crystal copper (Cu) has the same qualitative behaviour as the DFRACT model, albeit with a different set of parameters. The effect of material temperature on the nucleation and growth of voids is studied. As the temperature increases, there is a steady decrease in the void NAG thresholds and close to the melting point of Cu, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows disappearance of the long-range order due to the creation of stacking faults and the system no longer has a face centred cubic (fcc) structure. Molecular dynamics simulation of shock in crystal Cu at strain rates high enough to cause spallation of crystal Cu are then carried out to validate the void NAG parameters. We show that the pre-history of the material affects the void nucleation threshold of the material. We also simulate high-strain-rate triaxial deformation of crystal Cu with defects and obtain void NAG parameters. The parameters are then used in a macroscale hydrodynamic simulation to obtain spallation threshold of realistic crystal Cu. It is seen that our results match experimental results within the limit of 20% error.     

Strength of liquid tin at extremely high strain rates under a femtosecond laser action

The destruction of liquid tin at the extremely high strain rate caused by femtosecond laser pulses has been studied. The ablation and cavitation thresholds, as well as tensile stresses responsible for the destruction of liquid tin at a strain rate of ～10⁹ s^–1, have been experimentally determined.     

Slip at high shear rates

There are contradictory published data on the behavior of fluid slip at high shear rates. Using three methodologies (molecular dynamics simulations, an analytical theory of slip, and a Navier-Stokes-based calculation) covering a range of fluids (bead-spring liquids, polymer solutions, and ideal gas flows) we show that as shear rate increases, the amount of slip, as measured by the slip length, asymptotes to a constant value. The results clarify the molecular mechanics of how slip occurs. Furthermore, they indicate that in this limit, molecular dynamics simulations must accurately account for heat transfer to the solid.     

Relaxation processes in mesoscopic superconductors

We investigate the possibility of a novel kind of optical pump probe spectroscopy where the two laser pulses are focused on different areas of the sample. The response to the destruction of the superconducting state in a large part of a mesoscopic ring is studied numerically. We use the time dependent Ginzburg-Landau equations with periodic boundary conditions and external magnetic field. We evaluate the relaxation rates of the superconducting order parameter as well as the voltage induced by the charge imbalance. Computer simulations confirm that the perturbation of superconductivity on one part of the ring induces a voltage which decelerates the superconducting electrons on the other part of the ring. This deceleration results in the decrease of the superconducting current and the superfluid density. The relaxation times are of the order of the picosecond, the induced voltage of few millivolts and the variation of the superconducting gap of 10% which we believe to be suitable for time resolved femtosecond optical spectroscopy.     

Relaxation processes in non-Debye dielectrics

The specific features of the relaxation processes in non-Debye dielectrics have been investigated. The nature of the difference between the relaxation frequencies of the dielectric constant and dielectric loss (conductivity) has been explained. It has been shown that the average relaxation frequency of the conductivity is considerably (in some cases, by several orders of magnitude) higher than the relaxation frequency of the dielectric constant owing to an increase in the conductivity spectra of the statistical weight of the relaxation processes with short relaxation times.     

Relaxation times in cyclotron resonance in metals

The relation between an effective relaxation time τ in cyclotron resonance, mass anisotropy and magnetic field tipping is studied and a true relaxation time τ₀ is extracted by comparison with measurements. It is found that for copper τ has a maximum value in a slightly tipped magnetic field and that in potassium τ decreases monotonically with increasing tip angle. This is explained in terms of the Fermi surfaces of these metals.     

Elastic phase transitions in metals at high pressures

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-?to fourth-order elastic constants. The second-,third-?and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P?≈?69?GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.     

Void coalescence in metals at high irradiation doses

Mechanism of void coalescence in a metal irradiated by fast particles : p to high doses in dpa is theoretically investigated. It is shown that the decrease of void concentration with the dose increase due to the coalescence of immobile but growing voids must obey the dependence N ₀ ～ exp {-α(δ V/V)(t)}, where δV/V(t) is the void swelling at a time moment t, and α is a constant, the value of which is in the interval 1 < α < 8 and seems to be close to α ≈ 6.

It is shown that the void concentration as a function of irradiation dose : hould have a maximum when the void swelling δ V/V reaches some definite value between 2% and 17% depending upon the void size distribution form.     

Peripherical processes in QED at high energies

In this review we consider the high energy processes in QED frames in peripherical kinematics. The key feature of this kinematics is that processes have a large cross section which do not decrease with the increasing of the initial center of mass energy. Two purposes to study peripheral processes are: the background processes with large total cross sections and the structure of jets in the fragmentation region.We describe the various QED peripherical processes in terms of Impact Factors and give the explicit expressions for the differential distributions and spin correlation effects, as well as estimates of the total cross section of peripheral processes in photon-photon, photon-lepton and lepton-lepton collisions.A special attention is paid to the small-angle Bhabha scattering process which is relevant for beam monitoring at LEP I, LEP II.Based on analytical properties of the amplitudes some relations (QED sum rules) between the high energy asymptotic of the cross sections of inelastic processes in e ⁺ e ^? collisions and higher order perturbative contributions to the electron Dirac and Pauli form-factors are derived.For practical using we present some loop momentum integrals.     

Relaxation round vacancies in the alkali metals

The long-range ionic displacements round a vacancy are shown to come from both the elastic, long-wavelength, limit and from the Kohn anomaly. The effect of the topology of the Fermi surface is stressed.Attention is then focussed on the alkali metals with almost spherical Fermi surfaces, where it can reasonably be assumed that the elastic displacements dominate. Complete relaxation around a vacancy relates formation energy E_v, bulk modulus B and atomic volume Ω through E_v = αBΩ where α is a constant. The formation volume follows as a balance between a negative harmonic contribution and a positive and larger anharmonic term.     

Volume dilatation in a polycarbonate blend at varying strain rates

Impact loaded polymers show a variety of strain-rate dependent mechanical properties in their elastic, plastic and failure behaviour. In contrast to purely crystalline materials, the volume of polymeric materials can significantly change under irreversible deformations. In this paper, uni-axial tensile tests were performed in order to measure the dilatation in the Polycarbonate-Acrylnitril-Butadien-Styrol (PC-ABS) Bayblend T65. The accumulation of dilatation was measured at deformation speeds of 0.1 and 500 [mm/s]. Instrumented with a pair of two high-speed cameras, volume segments in the samples were observed. The change in volume was quantified as relation between the deformed and initial volumes of the segments. It was observed that the measured dilatations are of great significance for the constitutive models. This is specifically demonstrated through comparisons of stress-strain relations derived from the two camera-perspectives with isochoric relations based on single-surface observations of the same experiments.