Transformation of interfaces in nanoamorphous solids

A model is proposed for the diffusive smearing of interfaces of different geometries in metal nanoglasses. It is shown that the smearing of planar interfaces occurs faster than that of triple junctions, which, in turn, takes place faster than that of the nodes. This accounts for the variation of the mean positron lifetime in metal nanoglasses induced by thermal treatment. Fiz. Tverd. Tela (St. Petersburg) 41, 1627–1629 (September 1999)     

Configuration of normal vibrations in an unbounded medium containing dislocations with allowance for energy dissipation

On the basis of the calibration model of plastically incompressible medium with dissipation we consider the structure of vibrations in an unbounded medium with dislocations. Inclusion of dissipation of weakly damped low-frequency vibrations of volume contraction and high-frequency longitudinal sound vibrations is shown to cause a considerable change in the structure of normal vibrations. For a certain dissipation weakly damped low-frequency vibrations of volume contraction and high-frequency longitudinal sound vibrations are described by one branch of the dispersion relation. The structure of the vibrations changes from liquid-like vibrations to longitudinal sound vibrations in the neighborhood of , which together with the boundary frequency ₀ of optical vibrations is characteristic. The calibration model including energy dissipation does not describe the Cosser liquid.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 40–44, March, 1995.     

Molecular dynamics study of interfaces in solids

The interfaces of heterostructures were studied in the framework of molecular dynamics to reveal a periodic interface stress distribution on the micro scale. The calculations were performed for copper and silver and for two different types of heterostructures: a heterostructure consisting of perfect Cu and Ag crystals and a heterostructure formed by Ag vapor deposition on a perfect Cu substrate. The calculation results allowed several conclusions about the origin of a chessboard interface structure and about certain mechanisms of its modification depending on the nanostructure size, boundary conditions, and method of heterostructure formation.     

Elastic wave propagation and scattering in solids with uniaxially aligned cracks

In this article, elastic wave propagation and scattering in a solid medium permeated by uniaxially aligned penny-shaped microcracks are studied. The crack alignment refers to the case in which the unit normals of all cracks are randomly oriented within a plane of isotropy. The analysis is restricted to the limit of the noninteraction approximation among individual cracks. Explicit expressions for attenuations and wave speeds of the shear horizontal, quasilongitudinal, and quasishear vertical waves are obtained using stochastic wave theory in a generalized dyadic approach. The ensemble average elastic wave response is governed by the Dyson equation, which is solved in terms of the anisotropic elastic Green's dyadic. The analysis of expressions is limited to frequencies below the geometric optics limit. The resulting attenuations are investigated in terms of the directional, frequency, and damage dependence. In particular, the attenuations are simplified considerably within the low frequency Rayleigh regime. Finally, numerical results are presented and discussed in terms of the relevant dependent parameters.     

Interaction of second sound with acoustic waves in solids

An expression has been derived for the collision operator for phonons in a solid, which is valid at very low temperatures. The set of coupled equations for the elastic deformation and the phonon density or second sound has been reduced to a simple tractable form and the dispersion equation for the coupled waves consisting of the acoustic modes and second sound has been derived. It is shown that only the longitudinal mode interacts with the second sound. It is also shown that as a result of the interaction with the second sound, the longitudinal velocity along the principal axis acquires a correction term that is proportional to bothγ ² andT ⁴. The author felicitates Prof. D S Kothari on his eightieth birthday and dedicates this paper to him on this occasion.     

Frictional dissipation and interfacial glass transition of polymeric solids

We present single contact friction experiments between a glassy polymer and smooth silica substrates grafted with alkylsilane layers of different coverage densities and morphologies. This allows us to adjust the polymer-substrate interaction strength. We find that, when going from weak to strong interaction, the response of the interfacial junction where shear localizes evolves from that of a highly viscous threshold fluid to that of a plastically deformed glassy solid. This we analyze as resulting from an interaction-induced "interfacial glass transition" helped by pressure.     

Evolution of microscopic cracks and pores in solids under loading

Experimental data on the development and partial healing of microscopic cracks and pores in loaded crystalline materials are considered. An analysis of the data indicates that fracture development has a number of specific features depending on the state of the materials and the testing conditions and is a kinetic thermal fluctuation process occurring virtually throughout the entire time of loading.     

Three-dimensional vibrations of cylindrical elastic solids with V-notches and sharp radial cracks

This paper provides free vibration data for cylindrical elastic solids, specifically thick circular plates and cylinders with V-notches and sharp radial cracks, for which no extensive previously published database is known to exist. Bending moment and shear force singularities are known to exist at the sharp reentrant corner of a thick V-notched plate under transverse vibratory motion, and three-dimensional (3-D) normal and transverse shear stresses are known to exist at the sharp reentrant terminus edge of a V-notched cylindrical elastic solid under 3-D free vibration. A theoretical analysis is done in this work utilizing a variational Ritz procedure including these essential singularity effects. The procedure incorporates a complete set of admissible algebraic-trigonometric polynomials in conjunction with an admissible set of “edge functions” that explicitly model the 3-D stress singularities which exist along a reentrant terminus edge (i.e., α>180°) of the V-notch. The first set of polynomials guarantees convergence to exact frequencies, as sufficient terms are retained. The second set of edge functions—in addition to representing the corner stress singularities—substantially accelerates the convergence of frequency solutions. This is demonstrated through extensive convergence studies that have been carried out by the investigators. Numerical analysis has been carried out and the results have been given for cylindrical elastic solids with various V-notch angles and depths. The relative depth of the V-notch is defined as (1−c/a), and the notch angle is defined as (360°−α). For a very small notch angle (1° or less), the notch may be regarded as a “sharp radial crack.” Accurate (four significant figure) frequencies are presented for a wide spectrum of notch angles (360°−α), depths (1−c/a), and thickness ratios (a/h for plates and h/a for cylinders). An extended database of frequencies for completely free thick sectorial, semi-circular, and segmented plates and cylinders are also reported herein as interesting special cases. A generalization of the elasticity-based Ritz analysis and findings applicable here is an arbitrarily shaped V-notched cylindrical solid, being a surface traced out by a family of generatrix, which pass through the circumference of an arbitrarily shaped V-notched directrix curve, r(θ), several of which are described for future investigations and close extensions of this work.     

Effects of the orientational distribution of cracks in solids

We derive a theory for the elastic characterization of multicracked solids based on a homogenization technique. We consider a material containing a two-dimensional arbitrary distribution of parallel slit cracks which is elastically equivalent to a crystal with orthorhombic symmetry. We obtain explicit expressions for the macroscopic elastic stiffness tensor which is found to depend upon both the density of cracks and their angular distribution, here described by a suitable order parameter. For the isotropic case, we find that the degradation depends exponentially on the crack density. In addition, we show an unusual elastic behavior of a multicracked medium in the plane strain condition: for a negative Poisson ratio, we obtain an effective Young modulus greater than the actual value of the host matrix.     

Inversion of acoustic diffraction fields in anisotropic solids

We show that the fast Fourier transform (FFT) technique provides a computationally efficient method of calculating 2D amplitude and phase images of complex wave fields generated and measured in elastically anisotropic solids by phase sensitive acoustic microscopy. Further, we discuss how this technique can be used to treat inverse problems such as source reconstruction, image quality assessment, and the determination of elastic constants.     

声束在固体中的聚焦及其在声成像中的应用

本文主要研究表面换能器阵列在固体中产生的聚焦及轴上焦场的情形,讨论各级次焦点处场的值与条宽阵元的关系.用数值计算求出某一频率时不同的阵元宽度的焦场分布,并从实用角度出发,讨论将这种聚焦用于无损检测和成像时,横向和纵向分辨率等问题,为了进一步说明理论,做了一些探索性的实验。     

Determining the structure of energy in heterostructures with diffuse interfaces

A standard way of determining the type of energy structure in heterostructures, based on analyzing the spectral shift of the band of steady-state photoluminescence (PL) as a function of the excitation power density, is developed with allowance for the effect of the fluctuation tails of the density of energy states. The technique is validated for InAs/AlAs, GaAs/AlAs, and AlSb _y As_1–y /AlAs heterostructures with quantum wells.     

Bonding and energy dissipation in a nanohook assembly

Combining total energy and molecular dynamics calculations, we explore the suitability of nanotube-based hooks for bonding. Our results indicate that a large force of 3.0 nN is required to disengage two hooks, which are formed by the insertion of pentagon-heptagon pairs in a (7,0) carbon nanotube. Nanohooks based on various nanotubes are resilient and keep their structural integrity during the opening process. Arrays of hooks, which are permanently anchored in solid surfaces, are a nanoscale counterpart of velcro fasteners, forming tough bonds with a capability of self-repair.