Comparison between the dispersion curves calculated in complex frequency and the minima of the reflection coefficients for an embedded layer

Analytical solutions of Lamb functions for symmetric and antisymmetric elastodynamic modes propagating within a solid layer embedded in an infinite medium are presented. Alternative theoretical analyses of such modes are performed, first in terms of the usual approach of harmonic heterogeneous plane waves (real frequency and complex slowness) and then in terms of transient homogeneous plane waves (complex frequency and real slowness). An example structure of a 0.1-mm-thick "alpha case" (an oxygen-rich phase of titanium that is relatively stiff) plate embedded in titanium is used for the study. A large difference between the usual dispersion curves calculated in real frequency and complex slowness and those calculated in complex frequency and real slowness is shown. Thus the choice between a spatial and a temporal parameter to describe the imaginary part of the guided waves is shown to be significant. The minima and the zeros of the longitudinal and shear plane-wave reflection coefficients are calculated and are compared with the dispersion curves. It is found that they do not match with the dispersion curves for complex slowness, but they do agree quite well with the dispersion curves for complex frequency. This implies that the complex frequency approach is better suited for the comparison of the modal properties with near-field reflection measurements.     

Anisotropy of sound velocity for a new PbB4O7 crystal as determined by the laser ultrasonic technique

4 O₇ crystal is a new nonlinear optical crystal. For the first time the anisotropy of the velocities of its longitudinal and surface acoustic waves (SAWs) are determined by laser ultrasonic technique. The velocities of surface waves for X-, Y-, and Z-cut crystals are also calculated. The theoretical calculations of slowness curves are in good agreement with experimental results. The SAW slowness curve is elliptical for Y- or Z-cut crystal wafers, and circular for an X-cut wafer. Received: 27 January 1997/Accepted: 30 July 1997     

Acoustic properties of wood in tridimensional representation of slowness surfaces

Mechanical behaviour of wood considered as an orthotropic solid can be determined with ultrasonic technique. The propagation phenomena in wood are complex and theoretically are regulated by Christoffel's equation. Three type of waves can propagate in wood. During the propagation phenomena three slowness sheets are observed, corresponding to a fast longitudinal wave (inner sheet) and two shear waves, one fast and one slow (outer sheet). These waves are submitted continuously to mode conversion phenomena. The polarization angle changes when the propagation direction is out of the principal directions of symmetry of the material. In this article an analysis of the propagation phenomena in tridimensional representation is performed for different wood species. This approach contributes to the understanding of dynamic aspects of particle displacement associated with the wave fronts propagation. Globally, the anisotropy of each species, expressed by their acoustical behaviour is well represented.     

A method for analyzing the microwave absorption properties of magnetic materials

Based on transmission line theory, a complex thickness is introduced to analyze the electromagnetic wave absorption properties of materials with high magnetic loss and low dielectric loss. At each frequency the real part of the complex thickness denotes the thickness of the absorber to reach maximum absorption, while the absolute value of the imaginary part of the normalized complex thickness represents the degree of impedance mismatch between the material and free space. Using this method, the intensity of the reflection loss and the corresponding material thickness can be well predicted and understood. This method describes well the microwave absorption behaviors of PANI/α-Fe and PANI/CIP/Fe₃O₄ composites. It is found that the electromagnetic energy loss in the material, as well as the interference cancellation of the reflected waves at the air-material interface, contributes to the global minimum of reflection loss for the two composites.     

Laser picosecond acoustics in isotropic and anisotropic materials

We present experimental results concerning the laser generation of picosecond acoustic pulses and their propagation in isotropic and anisotropic materials. We make use of a conventional reflectance detection technique as well as interferometric detection to probe the real and imaginary changes in reflectance. We also demonstrate the detection of transverse acoustic waves by mode conversion at an interface between an isotropic polycrystalline film and an anisotropic substrate.     

Surface electromagnetic waves on magnetic media

We assume complex magnetic and dielectric properties of two adjacent, isotropic, semi-infinite media and examine surface electromagnetic waves (SEW) at their plane interface. The dispersion curves and non-radiative conditions are derived and compared with other results. Comments are made about which materials will allow surface electromagnetic waves. The importances of the dielectric constants of the sample and the overlayer materials are emphasized with an example, Si and NiO.     

Modeling the propagation of ultrasonic waves in the interface region between two bonded elements

An important task in nondestructive materials evaluation is the development of techniques to characterize the bond quality of adherent joints. Binding forces are nonlinear and cause a nonlinear modulation of transmitted and reflected ultrasonic waves. As a consequence, the higher harmonics generated by an insonified monochromatic wave give information about the adhesive bonds. The local binding forces in thin bonded interfaces can be obtained by the amplitudes of the ultrasonic waves of the insonified frequency and its higher harmonics as transmitted through the interface. Additional phase measurements may enable one to obtain the evaluation of the full hysteretic cycle of the interaction force. In order to gain a deeper understanding of the interface region and to improve the technique, numerical simulations of the ultrasonic wave propagation through specimens of two bonded elements can be used. A simple model based on the local interaction simulation approach (LISA) is described in this contribution, and a comparison between the results of the simulations and the experimental data is presented. Besides its intrinsic relevance for NDE, the problem considered in this paper may be very useful to analyze and test models for the simulation of ultrasonic wave propagation in nonclassical nonlinear mesoscopic elastic materials.     

Attenuation of electromagnetic waves in a semiconductor superlattice in a magnetic field

Magnetoplasma waves and inhomogeneous (complex) waves in an infinite semiconductor superlattice in a magnetic field are studied, and dispersion curves are obtained. It is shown analytically and numerically that, in periodic structures, among inhomogeneous waves, there are many complex waves for which the imaginary part of the wave vector is greater than the real part. The effect of dissipation in a medium on the dispersion curves of magnetoplasma waves is examined. The dependence of the minimum phase velocity on the collision frequency and the magnetic field strength is studied.     

Modeling of the electric-field distribution of plasma photonic crystals having inhomogeneity in the materials

We study the electric-field distributions in binary 1D-PPCs in the presence of inhomogeneity in the materials of unit cells. Here we consider four 1D-PPC structures: the first two structures have linear and exponential graded dielectric materials with a homogeneous plasma, and the other two structures have homogeneous dielectric material with linear and exponential density profiles of the plasma. Matching the electric fields and their derivatives on the dielectric-plasma interface, we obtain the required field-distribution relations. We observe that the amplitude of electric field changes in the presence of inhomogeneity in the materials, and the angle of incidence of electromagnetic waves highly affects the electric-field distributions.     

A theoretical study of ultrasonic wave transmission through a fluid-solid interface

This article develops a model for the study of the transient ultrasonic waves radiated by a transducer in a liquid and transmitted into a solid through a plane interface. The method is an extension to the transient case, of the angular spectrum method previously developed for the monochromatic case. It is based on the decomposition of the ultrasonic field, in impulse plane waves. The radiated waveform is calculated at any point in the field by a simple summation of these impulse plane waves, where the propagation delay and the refraction have been taken into account. These plane waves are, first of all, delayed by an amount of time corresponding to the travel time up to the considered field point. The transmission through the plane interface is taken into account by using Snell refraction laws and transmission coefficients. In the obtained results all the waves previously described by other authors are highlighted: direct wave, edge waves, head waves as well as subsurface waves with a clear resolution between compression and shear waves.     

Generalized laws of reflection and refraction from real valued boundary conditions

The generalized Snell's laws are usually derived from phase-matching condition by using harmonic inhomogeneous plane waves. The inhomogeneity makes it difficult to trace curves of energy flow. Here we show that, at a lossy interface, real valued boundary conditions are valid universally. Thus a time-dependent way is developed to directly derive generalized laws of reflection and refraction from real valued boundary conditions by using harmonic homogeneous plane waves. Finally, several novel properties of transmitted wave associated with energy losses are predicted numerically, which may be applied to experimentally test our theoretical analysis.     

Subsonic leaky Rayleigh waves at liquid-solid interfaces

The paper is devoted to the study of leaky Rayleigh waves at liquid-solid interfaces close to the border of the existence domain of these modes. The real and complex roots of the secular equation are computed for interface waves at the boundary between water and a binary isotropic alloy of gold and silver with continuously variable composition. The change of composition of the alloy allows one to cross a critical velocity for the existence of leaky waves. It is shown that, contrary to popular opinion, the critical velocity does not coincide with the phase velocity of bulk waves in liquid. The true threshold velocity is found to be smaller, the correction being of about 1.45%. Attention is also drawn to the fact that using the real part of the complex phase velocity as a velocity of leaky waves gives only approximate value. The most interesting feature of the waves under consideration is the presence of energy leakage in the subsonic range of the phase velocities where, at first glance, any radiation by harmonic waves is not permitted. A simple physical explanation of this radiation with due regard for inhomogeneity of radiated and radiating waves is given. The controversial question of the existence of leaky Rayleigh waves at a water/ice interface is reexamined. It is shown that the solution considered previously as a leaky wave is in fact the solution of the bulk-wave-reflection problem for inhomogeneous waves.     

Enhanced anisotropy in Paratellurite for inhomogeneous waves and its possible importance in the future development of acousto-optic devices

The anisotropic feature of most crystals, involves a direction dependent wave velocity for each of the possible modes. Paratellurite (Tellurium dioxide) is extraordinary because, for one of the propagation modes, i.e. the quasi shear horizontal (QSH) mode, the anisotropy is exceptional. This results, on the one hand in a very strong directional dependent sound velocity and on the other hand, in a low wave velocity in certain directions, resulting in a high figure of merit for the acousto-optical interaction. In the case of inhomogeneous waves, the slowness surfaces change their shape and magnitude, for all crystals. However, for paratellurite, this effect is again extraordinary. As soon as a relatively small inhomogeneity is considered, the sound velocity for the QSH mode becomes really exceptionally anisotropic, resulting in a slowness surface that is almost spherical, covered by pins. The velocity corresponding to those 'pins', is much lower than in the case of homogeneous plane waves, which is very promising for the future development of acousto-optic cells involving an even higher figure of merit.     

Spin-wave spectroscopy and application of its methods to heterostructures of silicon dioxide with Co nanoparticles on a GaAs substrate

A method has been developed for determining magnetic and electrical characteristics of film nanostructures containing magnetic nanoparticles from dispersion curves of surface spin waves propagating in these nanostructures. The dispersion curves of spin waves are determined by the dynamics of the spin component described by the generalized Landau-Lifshitz equations and an alternating electromagnetic field induced by a spin wave. Since spin waves are very sensitive to inhomogeneity of magnetic parameters, spin disorder, and conductivity of an object near or inside which these waves propagate, they can be used for determining magnetic and electrical characteristics of the objects under investigation. The developed calculation method, which can be employed both in spin-wave spectroscopy and in analysis of dispersion curves obtained by other methods, has been used for determining parameters of heterostructures consisting of a SiO₂ film with Co nanoparticles on a GaAs substrate. It has been found from the shape of dispersion curves of the surface spin waves that, in the film near the interface, spins of the nanoparticles are close to a ferromagnetic ordering, whereas near the free surface, the spin orientation of nanoparticles is more chaotic. It has been revealed that a conducting layer is formed in GaAs, and the SiO₂(Co) film near the interface has an increased conductivity.     

Determination of transport parameters in air-saturated porous materials via reflected ultrasonic waves

An ultrasonic reflectivity method of evaluating the acoustic parameters of porous materials saturated by air (or any other gas) is discussed. The method is based on experimental detection of waves reflected at normal incidence by the first and second interface of the material. This method is based on a temporal model of direct and inverse scattering problems for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material with a rigid frame [Fellah et al., J. Acoust. Soc. Am. 113, 61-73 (2003)]. Generally, the conventional ultrasonic approach can be used to determine tortuosity, and viscous and thermal characteristic lengths via transmitted waves. Porosity cannot be estimated in transmitted mode because of its very weak sensitivity. First interface use of the reflected wave at oblique incidence leads to the determination of porosity and tortuosity [Fellah et al., J. Acoust. Soc. Am. 113, 2424-2433 (2003)] but this is not possible at normal incidence. Using experimental data of reflected waves by the first and second interface at normal incidence simultaneously leads to the determination of porosity, tortuosity, viscous and thermal characteristic lengths. As with the classic ultrasonic approach for characterizing porous material saturated with one gas, both characteristic lengths are estimated individually by assuming a given ratio between them. Tests are performed using weakly resistive industrial plastic foams. Experimental and numerical results, and prospects are discussed.     

The reflection of bounded inhomogeneous waves on a liquid/solid interface

Fourier analysis and normal mode theory are used to describe the reflection of bounded inhomogeneous waves on a liquid/solid interface. Nonspecular reflection phenomena in the Rayleigh angle are studied in detail. In this way, an explanation is given for the Rayleigh dip phenomenon for positive inhomogeneity factors and the related result of a reflection coefficient larger than unity when the sign of the inhomogeneity factor is reversed. In the limit of large beamwidths, the reflection coefficient predicted by the infinite plane inhomogeneous wave theory is obtained. These results are entirely consistent with the experimental work published by Deschamps [J. Acoust. Soc. Am. 96, 2841-2848 (1994)]. The energy efficiency of Rayleigh wave excitation is investigated as well. It is shown that for large beamwidths, the energy efficiency for bounded inhomogeneous waves is considerably higher in comparison with Gaussian and square-profiled beams.     

水平连铸圆钢超声检测结果的信号处理

水平连铸圆钢为多晶种的不均匀粗晶材料，当采用超声波对其进行检测时，将产生较强的组织散射噪声，从而使得对缺陷回波的判断与评价无法进行本文采用分离频谱的方法对原始检测信号进行处理，再用截止地对缺陷回波进行恢复运算。实验结果本文所采用的方法能较为有效地消除原始检测信号中的组织噪声，从而提高超声检测的信噪比。     

表面粗糙度对固体内部超声背散射的影响

超声背散射法可通过多晶体金属内部的空间方差信号,实现微观结构参数的无损评价,但表面粗糙度对评价模型的精度及实用性存在显著影响.基于高斯声束理论推导垂直入射粗糙界面的纵波声场,以此研究声能的Wigner分布规律;在超声的波长远大于粗糙度的前提下,构造表面粗糙度修正系数,并建立粗糙界面的单次散射响应模型,揭示粗糙度对超声波背向散射的影响规律.用304不锈钢制备轮廓均方根值为0.159μm的光滑试块和25.722μm的粗糙试块开展超声背散射实验,结果表明模型在粗糙度修正前后均可实现光滑试块的晶粒尺寸有效评价,但未经修正的传统模型对粗糙试块的晶粒尺寸评价结果与金相法结果的相对误差高达-21.35%,而本模型的评价结果与金相法结果符合得很好,相对误差仅为1.35%.可见,本模型能有效补偿粗糙度引起的超声背散射信号衰减,从而提高晶粒尺寸无损评价的精度.     

Stiffness evaluation of contacting surfaces by bulk and interface waves

This paper describes ultrasonic measurements of normal and tangential stiffnesses of the contacting interface between polished aluminum blocks subjected to nominal contact pressures up to about 3.8 MPa. These stiffnesses were evaluated by ultrasonic spectroscopy methods for the bulk (longitudinal and transverse) wave reflection coefficients and the anti-symmetric mode interface wave velocity. The measurements revealed the interfacial stiffnesses as functions of the frequency as well as the applied contact pressure. The ratio of the tangential and normal stiffnesses is discussed in the light of foregoing theoretical and experimental findings. Furthermore, possible explanations for the frequency dependence of the measured stiffnesses are reviewed, invoking the spatial inhomogeneity of the interfacial stiffness as well as its lossy nature.     

国产的多极子阵列声波测井仪(MPAL)在油田开发生产中的应用

本文对国产的多极子阵列声波测井(MPAL)资料进行了分析,验证了仪器有较好的稳定性,测试结果表明:硬地层的纵波、横波、斯通利波波形清楚,幅度变化正常;通过其在岩性识别、裂缝识别、气层识别、岩石机械特性分析及各向异性的应用进行研究,能够较为准确地求取地层的纵波、横波、斯通利波的声波时差及幅度和衰减曲线,计算岩石力学参数,并进行地层各向异性评价。