On reactive shear bands

One mechanism for ignition of propellants and explosives involves shear banding, in which the healing due to visco-plastic effects combines with the reactive heating to reduce the critical size. To study this process, the analysis of Frank-Kamenetskii is generalized herein to account for mechanical effects (shear). By means of a dimensional analysis and numerical integration the general solution is reduced to a graph involving a dimensionless width and a dimensionless heating.     

Widths of conduction bands in metals

The widths of the conduction bands in metals have been calculated by using experimental plasma-loss values. The calculated bandwidths are in fair agreement with those observed experimentally for K, L and M X-ray emission.     

Generation of laser-driven flyer dominated by shock-induced shear bands: A molecular dynamics simulation study

High-power pulsed lasers provide an ingenious method for launching metal foils to generate high-speed flyers for high-pressure loading in material science or aerospace engineering.At high-temperature and high-pressure laser-induced conditions,the dynamic response of the metals and the mechanism of flyer formation remain unclear.In this study,the overall process of the laser-driven aluminum flyer,including laser ablation,rupture of metal foil,and the generation of the flyer was investigated by molecular dynamics combined with the two-temperature model.It was found that under high laser fluence(over 1.3 J/cm;with 200-fs laser pulse duration),the laser induced a shock wave with a peak pressure higher than25 GPa,which led to shear bands expanding from the edge of the laser ablation zone in the foil.Compared with the cases of low laser fluence less than 0.5 J/cm^-1,the shear band induced by high laser fluence promotes the rupture of the foil and results in a high-speed flyer(>1 km/s)with better flatness and integrity.In addition,the shock wavefront was found to be accompanied by aluminum crystal phase transformation from face-centered cubic(FCC)to body-centered cubic structure.The crystal structure reverts with the decrease of pressure,therefore the internal structure of the generated flyer is pure of FCC.The results of this study provide a better understanding of the laser-induced shock effect on the foil rupture and flyer quality and forward the development of the laser-driven flyer.     

Arch generated shear bands in granular systems

We propose an arch based model, on cubic and square lattices, to simulate the internal mobility of grains, in a dense granular system under shear. In this model, the role of the arches in granular transport presents a non-linear dependence on the local values of the stress components that can be modeled geometrically. This non-linearity is very important since a linear dependence on the stress will make the models behave similarly to viscous fluids, which will not reproduce highly interesting properties of the sheared systems such as shear bands. In particular, we study a modified Couette flow and find the appearance of shear bands in accordance with the literature.     

Amorphous shear bands in deformed TiNi alloy

The structure and strain relief of TiNi alloy are examined following combined deformation consisting of quasi-hydro-extrusion followed by uniaxial compression. The shear nature of the amorphous bands resulting from such strain is demonstrated. A connection between the amorphous bands and the strain relief has been found. Fiz. Tverd. Tela (St. Petersburg) 39, 1237–1239 (July 1997)     

One-dimensional model of inhomogeneous shear in sliding

The paper briefly describes a one-dimensional dynamic model of plastic shear deformation in a material surface layer in sliding friction, giving grounds to the reduction of the problem dimension from 3D to 1D. A selection of simulation results is presented to illustrate the peculiarities of plastic deformation under the action of two competitive processes — work hardening and thermal softening due to frictional heating. Presented also are experimental data on which to base the conclusion on the possibility of surface layer flow similar to flow of a viscous fluid. To assess from the Reynolds number whether turbulization of the surface layer is feasible, simulation results are used.     

Turbulent shear layers in confining channels

We present a simple model for the development of shear layers between parallel flows in confining channels. Such flows are important across a wide range of topics from diffusers, nozzles and ducts to urban air flow and geophysical fluid dynamics. The model approximates the flow in the shear layer as a linear profile separating uniform-velocity streams. Both the channel geometry and wall drag affect the development of the flow. The model shows good agreement with both particle image velocimetry experiments and computational turbulence modelling. The simplicity and low computational cost of the model allows it to be used for benchmark predictions and design purposes, which we demonstrate by investigating optimal pressure recovery in diffusers with non-uniform inflow.     

Nonlinear dynamics of an interface between shear bands

We study numerically the nonlinear dynamics of a shear banding interface in two-dimensional planar shear flow, within the nonlocal Johnson-Segalman model. Consistent with a recent linear stability analysis, we find that an initially flat interface is unstable with respect to small undulations for a sufficiently small ratio of the interfacial width l to cell length L(x). The instability saturates in finite amplitude interfacial fluctuations. For decreasing l/L(x) these undergo a nonequilibrium transition from simple traveling interfacial waves with constant average wall stress, to periodically rippling waves with a periodic stress response. When multiple shear bands are present we find erratic interfacial dynamics and a stress response suggesting low dimensional chaos.     

Microdoping of subsurface gallium arsenide layers with hydrogen ions

A method of microdoping subsurface semiconductor layers with hydrogen ions (protons) with the use of a plasma-beam discharge is suggested. The method was tested on gallium arsenide layers and was proven to be more efficient than other well-known methods used for modifying subsurface layers.     

Two-component model for the growth of porous subsurface layers

This paper studies a kinetic model that describes the interaction of two fluctuating densities. The model makes it possible to stably reproduce the growth of dense, porous, and fractal structures near the surface of solids placed in an active medium. The solutions of local and nonlocal equations of the model are studied, and the results are used to comment on the possible scenarios of the evolution of systems whose behavior can be reduced to such a model. Finally, the exponents of the growth of the front width in a steady-state regime are calculated for various values of the parameters. Zh. éksp. Teor. Fiz. 114, 1500–1515 (October 1998)     

Stress in dielectric contact layers on metals

Several sources of stress in dielectric contact layers (e.g. growing oxide films) are discussed and order-of-magnitude estimates are given for the accompanying strains. The strain distributions in isotropic layers are deduced for stresses produced by Coulomb forces, volume changes associated with diffusing defects, orienting molecular force (epitaxial) effects, and electrochemical potential gradients producing diffusion currents and growth of the layer.     

Thin liquid layers in shear: non-Newtonian effects

When shear flow is generated in molecularly thin liquid films of simple liquids confined between two parallel plates, the effective viscosity of the liquid increases by many orders of magnitude compared to its bulk value. Non-Newtonian effects such as shear thinning with a universal power law exponent of are observed in experiments and computer simulations. We present a simple model of these phenomena based on shear melting of solid-like layers induced by the strong coupling with the crystalline walls.     

Internal electron diffraction from atomically ordered subsurface nanostructures in metals

We demonstrate that a part of interface at a subsurface nanocavity in Cu(110) can efficiently induce electron scattering back to the surface even if it is inclined with respect to the surface, if the condition for electron diffraction is fulfilled. This backscattering induces oscillations of electron local density of states at the surface versus electron energy. In agreement with our model calculations, the diffraction is assigned to a specific atomic structure at the interface, and is found to be significantly enhanced by focussing of electron waves for propagation along the [110] direction.     

Sound source in the presence of wall shear layers

Numerical solution techniques for evaluating the acoustic field generated by a single line source located inside or outside a wall shear layer of an infinitely long lined rectangular duct are presented. A formula for calculating wave attenuation due to an acoustic lining is given.     

Molecular dynamics simulation of subsurface deformed layers in AFM-based nanometric cutting process

Three-dimensional molecular dynamics simulations of AFM-based nanometric cutting monocrystalline copper with pin tool radius of 0.713 nm are performed to investigate the effect of uncut chip thicknesses (0.1805 nm, 0.361 nm, 0.5415 nm, 0.722 nm, 0.9025 nm, 1.0875 nm, and 1.268 nm) on the depth of subsurface deformed layers. The EAM potential and Morse potential are utilized respectively to compute the interactions between workpiece atoms, the interactions between workpiece atoms and tool atoms. The single-atom potential energy variations of the workpiece atoms within the subsurface regions during the cutting process are obtained and analyzed through a deformation criterion to determine the deformation behaviors of subsurface atoms. The simulation results reveal that the depth of subsurface deformed layers is affected by the AFM pin tool's rake angle. At each uncut chip thickness, the AFM pin tool presents different negative rake angles, consequently different degrees of deformation in the subsurface take place.     

Features of X-ray diffraction in thick perfect crystals with deformed subsurface layers

Diffraction of X-rays in thick perfect crystals with deformed subsurface layers has been investigated. It is shown that in deformed layers kinematic scattering occurs, while in the bulk of perfect part dynamic scattering takes place. It is revealed that in the beams diffracted on surface defects, an essential role plays the second harmonic of the employed characteristic MoK_α1 radiation.