Effect of surface stresses on surface waves in elastic solids

This paper deals with the propagation of surface waves in homogeneous, elastic solid media whose free surfaces or interfaces of separation are capable of supporting their own stress fields. The general theory for the propagation of surface waves in a medium which supports surface stresses is first deduced, and then this theory is employed to investigate the particular cases of surface waves, viz. (a) Rayleigh waves, (b) Love waves and (c) Stoneley waves. It is seen that the Rayleigh waves become dispersive in nature; and, in case of low frequency with residual surface tension, a critical wavelength exists, below which the propagation of Rayleigh waves is not possible. This critical wave length is directly proportional to the surface tension. Some numerical calculations have been made in the case of Love waves and conclusions have been drawn.     

真空中陶瓷绝缘子的沿面闪络现象及其研究进展

陶瓷绝缘子沿面闪络是制约电真空器件绝缘性能、影响设备正常运行的关键因素之一。本文介绍了关于真空中陶瓷绝缘子沿面闪络物理机制的各种假说，着重阐述了二次电子发射雪崩理论，分析了影响陶瓷绝缘子闪络电压的各种因素，提出了从陶瓷材料角度提高闪络电压的方向，总结了绝缘子沿面闪络现象的研究现状，展望了今后的研究方向与发展趋势。     

Boundary integral equations for the scattering of elastic waves by elastic inclusions with thin interface layers

Elastic waves are scattered by an elastic inclusion. The interface between the inclusion and the surrounding material is imperfect: the displacement and traction vectors on one side of the interface are assumed to be linearly related to both the displacement vector and the traction vector on the other side of the interface. The literature on such inclusion problems is reviewed, with special emphasis on the development of interface conditions modeling different types of interface layer. Inclusion problems are formulated mathematically, and uniqueness theorems are proved. Finally, various systems of boundary integral equations over the interface are derived.     

Contributions of higher order terms to nonlinear waves in fluid-filled elastic tubes: strongly dispersive case

In the present work, employing the nonlinear equations of an incompressible, isotropic and elastic thin tube and the approximate equations of an incompressible inviscid fluid, and then utilizing the modified reductive perturbation technique presented by us [15] the amplitude modulation of weakly nonlinear waves is examined. It is shown that the first order term in the perturbation expansion is governed by a nonlinear Schrödinger equation and the second order term is governed by the linearized Schrödinger equation with a nonhomogeneous term. In the longwave limit a travelling wave type of solution to these equations are also given.     

Propagation of low-frequency elastic waves in double-porositymedia containing viscoelastic component in the pores

A self-consistent scheme named the effective field method (EFM) is applied for the calculation of the velocities and quality factors of elastic waves propagating in double-porosity media. A double-porosity medium is considered to be a heterogeneous material composed of a matrix with primary pores and inclusions that are represent by flat (crack-like) secondary pores. The prediction of the effective viscoelastic moduli consists of two steps. First, we calculate the effective viscoelastic properties of the matrix with the primary small-scale pores (matrix homogenization). Then, the porous matrix is treated as a homogeneous isotropic host where the large-scale secondary pores are embedded. Spatial distribution of inclusions in the medium is taken into account via a special two-point correlation function. The results of the calculation of the viscoelastic properties of double-porosity media containing isotropic fields of crack-like inclusions and double-porosity media with some non-isotropic spatial distributions of crack-like inclusions are presented.     

Dynamical model of total and partial pressures in a vacuum system due to intermittent desorption

The dynamical characteristics of a vacuum system in respect of pressure are critical for studying the dynamical desorption processes such as those produced by mechanically stimulated desorption in vacuum. In the present work, a mathematical model of the instant pressure in a vacuum system has been developed assuming an intermittent square-wave desorbed gas flow. In this case, the duration of the pressure transient is proposed to be used as a generalized criterion of the rate of the desorbed gas flow and, consequently, of the instant pressure behaviour. The dependences of the transient duration on the pumping speed, system volume, molar mass of the desorbed gas as well as on the period and the on-off ratio of the desorption process have been revealed. The developed model made no assumptions on the physical nature of the gas source, i.e. desorption, leaking, etc., and can be used for different applications.     

The scattering of harmonic elastic anti-plane shear waves by a Griffith crack in a piezoelectric material plane by using the non-local theory

In this paper, the dynamic behavior of a Griffith crack in a piezoelectric material plane under anti-plane shear waves is investigated by using the non-local theory for impermeable crack face conditions. For overcoming the mathematical difficulties, a one-dimensional non-local kernel is used instead of a two-dimensional one for the anti-plane dynamic problem to obtain the stress and the electric displacement near the crack tips. By using the Fourier transform, the problem can be solved with the help of two pairs of dual integral equations. These equations are solved using the Schmidt method. Contrary to the classical elasticity solution, it is found that no stress and electric displacement singularity is present near the crack tip. The non-local dynamic elastic solutions yield a finite hoop stress near the crack tip, thus allowing for a fracture criterion based on the maximum dynamic stress hypothesis. The finite hoop stress at the crack tip depends on the crack length, the circular frequency of incident wave and the lattice parameter. For comparison results between the non-local theory and the local theory for this problem, the same problem in the piezoelectric materials is also solved by using local theory.     

Study on the surface topology of vacuum-fired stainless steel by scanning tunnelling microscopy

The surface of 304L stainless steel, after normal bakeout and vacuum firing, has been imaged in the scanning tunnelling microscope (STM). Probe tips with sharp apexes and well-known reproducible geometry which were previously characterized in the field ion microscope (FIM) were used. After bakeout the STM surface profiles show on chemical polished surfaces an average roughness of 3-5 nm and over 300-500 nm variations in height of 30-50 nm. After vacuum firing a significant change of the surface structure and topology is observed. Crystallites with almost (1 1 1) orientation show wide (1 1 1) terraces with monoatomic steps. On tilted crystallites again wide (1 1 1) terraces, intersected with bunched steps and facets forming nearly regularly pattern and corresponding in orientation almost to the (1 0 0) and (1 1 0) planes can be observed. From the general appearance of the surface after vacuum firing it can be concluded that the surface became less active with respect to gas surface interaction which supports the present understanding of outgassing of this material.     

Plasmon mediated Brillouin scattering of surface acoustic waves (in different directions to polymer line structures on a silver coated glass substrate)

In this paper measurement on the propagation of Rayleigh waves on the surface of polymer line structures on top of a silver layer were examined by using plasmon mediated Brillouin scattering. The line structures grating constants were 8, 4 and 1 μm. Spectra were recorded with different scattering angles and propagation directions with respect to the line direction. Three Rayleigh modes have been observed in samples with grating constants 8 and 4 μm. In the parallel direction, one mode is unshifted relative to a pure silver surface. In the perpendicular direction there is one unshifted and one shifted mode. The observed shift of 6 μm^− 1 is independent of the grating constant. The 1 μm sample shows a somewhat contrasting behavior. Two different scattering mechanisms, involving surface plasmons at each interface, are proposed to explain the observed effects.     

Diffraction of elastic waves by four rigid strips embedded in an infinite orthotropic medium

In the present paper, the elastodynamic response of four coplanar rigid strips embedded in an infinite orthotropic medium due to elastic waves incident normally on the strips is analyzed. The resulting mixed boundary-value problem is solved by an integral-equation method. The normal stress and the vertical displacement are derived in closed analytic form. Numerical values of stress-intensity factors at the edges of the strips and vertical displacements at point in the plane of the strips for several orthotropic materials are calculated and plotted graphically to show the effect of material orthotropy.     

Waves generated by a source below a free surface in water of finite depth

The free-surface flow due to a submerged source in water of finite depth is considered. The fluid is assumed to be inviscid and incompressible. The problem is solved numerically by using a boundary integral equation formulation due to Hocking and Forbes [6]. The numerical results show that there is a train of waves on the free surface in accordance with the results of Mekias and Vanden-Broeck [5]. For small values of the Froude number, the amplitude of the waves is so small that the free surface is essentially flat in the far field. These waveless profiles agree with the calculations of Hocking and Forbes [6].     

Determining elastic properties of skin by measuring surface waves from an impulse mechanical stimulus using phase-sensitive optical coherence tomography

The mechanical properties of skin are important tissue parameters that are useful for understanding skin patho-physiology, which can aid disease diagnosis and treatment. This paper presents an innovative method that employs phase-sensitive spectral-domain optical coherence tomography (PhS-OCT) to characterize the biomechanical properties of skin by measuring surface waves induced by short impulses from a home-made shaker. Experiments are carried out on single and double-layer agar–agar phantoms, of different concentrations and thickness, and on in vivo human skin, at the forearm and the palm. For each experiment, the surface wave phase-velocity dispersion curves were calculated, from which the elasticity of each layer of the sample was determined. It is demonstrated that the experimental results agree well with previous work. This study provides a novel combination of PhS-OCT technology with a simple and an inexpensive mechanical impulse surface wave stimulation that can be used to non-invasively evaluate the mechanical properties of skin in vivo, and may offer potential use in clinical situations.     

Analysis of partial discharge activity by a conducting particle in liquid nitrogen under AC voltages adopting UHF technique

Liquid nitrogen (LN₂) is used as an insulant as well as coolant in high temperature superconducting power equipments. Particle contamination in liquid nitrogen is one of the major cause for formation of partial discharges during operation. An attempt has been made in the present study to understand the feasibility of using Ultra High Frequency (UHF) sensors for identification of partial discharge (PD) formed due to particle movement in liquid nitrogen under AC voltages. It is observed that the partial discharge formed in LN₂ radiates UHF signal. The results of the study indicate that the conventional partial discharge measurement and UHF peak amplitude measurement have direct correlation. The Phase Resolved Partial Discharge (PRPD) analysis indicates that the partial discharge formed due to particle movement occurs in the entire phase windows of the AC voltage. The PD magnitude increases with increase in applied voltage. The frequency content of UHF signal generated due to particle movement in liquid nitrogen under AC voltages lies in the range of 0.5-1.5 GHz. The UHF sensor output signal analyzed using spectrum analyzer by operating it in zero-span mode, indicates that burst type PD occurs due to particle movement.     

Influence of barrier on partial discharge activity by a conducting particle in liquid nitrogen under AC voltages adopting UHF technique

The UHF signals are generated due to PD formed by particle movement in liquid nitrogen under AC voltages. The levitation voltage of a particle in liquid nitrogen measured through UHF technique and by conventional PD measurement technique is the same, confirming the sensitivity of UHF technique for identification of PD activity. The frequency content of UHF signal generated due to particle movement in liquid nitrogen, under AC voltages, lies in the range 0.5–1.5 GHz. The characteristics of UHF signal generated due to particle movement between the barrier and high voltage/ground electrode is much similar to the signal generated by particle movement in clean electrode gap. Pseudo resonance phenomena can occur in liquid nitrogen due to particle movement. It is also observed that the partial discharge magnitude, in general, be high when the particle moves between the barrier and high voltage electrode when compared to the barrier and the ground electrode. Percentage of clay in epoxy nanocomposites has not altered the levitation voltage of the particle in the electrode gap. Zero span analysis clearly indicates that pseudo resonance occurs when particle moves (in a short gap) between the barrier and high voltage/ground electrode.     

Characterization of stiffness degradation caused by fatigue damage in textile composites using circumferential plate acoustic waves

The aim of this paper is to show the possibility of fatigue damage characterization in GFRP-tube-like components by using circumferential plate waves. For that purpose, fatigue tests with different loading directions have been conducted and the stiffness degradation has been monitored. After a preset number of loading cycles, non-destructive ultrasonic tests using circumferential plate waves were performed. The correlation between damage induced amplitude changes of the plate waves and stiffness degradation in non-crimped fabric composites is discussed. The results indicate that the technique is applicable to fatigue damage assessment in complex-shaped components of composite materials and is relevant to a wide field of applications.     

Analysis of accuracy in prism coupling methods and a proposal of two simple ways for coupling to guided waves and/or for exciting surface plasmon resonance

The stationarity of the coupling spot in relation with different shapes of couplers is discussed. The accuracy in measuring the effective index by various couplers is analyzed. Two simple methods are proposed to couple light into guided waves and/or to excite surface plasmon resonance (SPR). One uses an optical block as a coupler and the incident beam falls onto it in two perpendicular directions. This method is particularly useful for exciting SPR at an interface between metal and anisotropic dielectric media, but also can be used to couple light into guided waves. Another method does not need any coupler. The incident beam is directly launched into the sample from the sustrate side. When a thin metal film is deposited on the substrate and covered either by air or another dielectric layer, SPR can be excited at the interface of metal/air of metal/dielectric. This method can be viewed as an alternative of Kretschmann configuration.     

Study of strontium- and magnesium-doped lanthanum gallate solid electrolyte surface by X-ray photoelectron spectroscopy

The chemical states of the surface of the oxygen ion conducting solid electrolyte La_0.9Sr_0.1Ga_0.85Mg_0.15O_3−δ (LSGM 1015) as prepared by solid-state synthesis was analyzed by X-ray photoelectron spectroscopy. It was found that adventitious carbon did not interact with any of the constituent elements of LSGM 1015. Ga and La were found to exist in trivalent states. But, due to ionic bombardment presence of Mg could not be detected in the electrolyte surface.     

Comments on “surface waves in fibre-reinforced anisotropic elastic media” by Sengupta and Nath [Sādhanā 26: 363–370 (2001)]

In the paper under discussion, the problem of surface waves in fibrereinforced anisotropic elastic media has been studied. The authors express the plane strain displacement components in terms of two scalar potentials to decouple the plane motion into P and SV waves. In the present note, we show that, for wave propagation in fibre-reinforced anisotropic media, this decoupling cannot be achieved by the introduction of the displacement potentials. In fact, the expressions for the displacement potentials used by the authors do not satisfy one of the equations of motion. Consequently, most of the equations and results of the subject paper are either irrelevant or incorrect     

Diffraction of elastic waves by defects in plates: Calculated arrival strengths for point force and thermoelastic sources of ultrasound

This paper applies geometrical ray theory to the calculation of the surface displacements generated by point force and thermoelastic sources of ultrasound in plates containing planar defects. The calculation includes direct wave arrivals, waves undergoing back-wall reflection with or without mode conversion, and waves diffracted by the crack-tip. Ultrasonic B-scan data are also simulated so that comparison can be made with experimental data. It is shown that the thermoelastic source, which can be generated by a pulsed laser, is particularly well suited to defect detection by the ultrasonic time-of-flight technique.     

Condition assessment of concrete in hydraulic structures by surface wave non-destructive testing

Maintenance and rehabilitation of concrete structures affected by alkali-aggregate reaction (AAR) require conducting detailed assessment of the concrete conditions, mainly close to the surface where the damage is more severe. This paper presents in situ investigations by surface wave testing of near-surface AAR damage in two hydraulic structures. The survey was carried out using a non-intrusive multi-sensor method that involves frequency–wavenumber analysis of surface waves. The method allows solving Rayleigh surface wave propagation modes required for the determination of the shear wave velocity in terms of depth. The variation of Young’s modulus with concrete depth can be estimated from the obtained shear wave velocity profile. Two different cases of surface wave propagation, typical of concrete structures, are discussed in this paper. The tests were conducted from the concrete surface only and the subsurface quality was mapped up to a depth of 1.50 m. The applications show that the proposed surface wave method is a potential non-destructive evaluation method that can be used to detect and locate near surface damage in concrete structures.

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