Quasidislocation stoneley wave and Eshelby dislocation Scholte wave

A quasidislocation (a dislocationlike entity described here for the first time) moves at the speed of a Stoneley surface wave that travels at the interface between two different elastic solids. An Eshelby glide edge dislocation moves at the speed of a Scholte surface wave that travels at the interface between a solid and an ideal liquid. The quasidislocation and the glide edge dislocation (that moves at the Eshelby velocity) are the Green's functions of their waves. Scholte waves are planar distributions of transonic moving glide edge dislocations. They are not Stoneley waves, although often called by that name, because Stoneley waves are planar distributions of subsonic moving quasidislocations.     

Ultrasonic pulse spectroscopy of a solid inclusion in an elastic solid

Experiments were conducted to measure the arrival time and the power density spectra of wide-band ultrasonic pulses as scattered by a circular, cylindrical, solid inclusion in a matrix of aluminium. Cavities of 3.18 and 6.35 mm diameter were alternately filled with four different solids possessing a wide range of acoustic properties. Results show that the time history and the spectral analysis of the scattered pulses can be used to determine the size of the cavity or the wave velocity in the solid inclusion.     

Numerical analysis of ultrasonic transmission and absorption of oblique plane waves through the human skull.

Ultrasonic transmission and absorption of oblique plane waves through the human skull are analyzed numerically for frequencies ranging from 1/2 to 1 MHz. These frequencies are optimum for noninvasive ultrasound therapy of brain disorders where numerical predictions of skull transmission are used to set the phase and amplitude of source elements in the phased array focusing system. The idealized model of the skull is a three-layer solid with ivory outer and inner layers and a middle marrow layer. Each layer is modeled as a flat, homogeneous, isotropic, linear solid with effective complex wave speeds to account for focused energy losses due to material damping and scattering. The model is used to predict the amplitude and phase of the transmitted wave and volumetric absorption. Results are reported for three different skull thicknesses: 3 mm, 6 mm, and 9 mm. Thickness resonances are observed in the transmitted wave for 3 mm skulls at all frequencies and for the 6 mm skulls below 0.75 MHz. Otherwise, the transmission is dominated by the direct wave. Skull phase errors due to shear waves are shown to minimally degrade the power at the focus for angles of incidence up to 20 degrees from normal even for low material damping. The location of the peak volumetric absorption occurs either in the outer ivory or middle marrow layer and shown to vary due to wave interference.     

Scholte Wave at Air-Metal Interface Generated by a Pulsed Disc-Like Source

The normal displacement and pressure of Scholte and Leaky Rayleigh waves at air-metal interface generated by a pulsed disc-like source are simulated theoretically by the Cagniard-de Hoop method and studied by laser ultrasound technique experimentally. It is found that the Scholte wave detected by a photorefractive interferometer is mainly contributed by the surface pressure and the Leaky Rayleigh wave is dominated by the surface displacement. It is also proven that the pulse width of these interface waves is mainly determined by the acoustic time delay on the generating source size under our experimental conditions.     

Scholte波识别水池缩比实验

该文基于简正波分离识别Scholte波的方法,分析了之前Scholte波识别水池缩比实验结果中出现较大误差的原因,提出从声场中优先提取其他阶简正波的方法来改善实验结果、降低实验误差,并证实增加合成孔径垂直阵采样点数至119点（一次采样期间首末阵元信号保持良好相关性）,有助于改善水池Scholte波识别误差,从而使水池Scholte波识别缩比使实验结果与理论值更为接近。水池缩比实验的结果证明该方法改善了Scholte波识别实验的结果,降低了声源深度较浅、Scholte波激发较弱时的实验误差,使水池缩比实验简正波分离识别Scholte波的方法更加可靠。     

Conventional electromagnetic acoustic transducer development for optimum Lamb wave modes

Lamb waves are normally utilized for inspecting thin metal sheets. Wheel type probes using piezoelectric oscillators have generally been used as the sensors for Lamb waves. Recently, the electromagnetic acoustic transducer (EMAT) has been developed and is beginning to be used as a Lamb wave detector. We have developed a useful type of transducer for Lamb waves. The new EMAT consists of a meander coil with a narrow distance of 2.5 mm and has a symmetrical structure in the vertical direction for both surface sides. The new EMAT can generate Lamb waves with variable wavelengths corresponding to the frequency range from approximately 300 kHz to 2.5 MHz and multiple modes, and can also generate selected symmetrical and anti-symmetrical mode Lamb waves. It is demonstrated that the optimum Lamb wave mode could be produced by the appropriate positioning of the EMATs and controlling the phase (same or inversed) of the electrical signal driving the device. The described EMAT can be used to examine steel (or other material) sheets of different thickness. It is also shown that the S0 (0.3 MHz) mode Lamb wave is the most effective for the deepest (up to 6 mm) penetration.     

Study of the sorption processes in a piezoelectric-molecularly imprinted polymer film structure using Rayleigh surface acoustic waves

The sorption processes in a piezoelectric-molecularly imprinted polymer film structure, where the polymer is synthesized from the monomers of bisphenol-A glycerolate diacrylate using imprinted morpholine molecules as template molecules, are experimentally studied with Rayleigh surface acoustic waves at a frequency of 120 MHz. The desorption processes for morpholine are found to be anomalously slow as compared to other analytes under study. The possibility of the application of the results obtained for creating selective chemical sensors based on surface acoustic waves is discussed.     

Scholte波与含泥沙两相流介质属性关系的分析及仿真验证

基于四种超声悬浮液模型Urick, Urick-Ament, HT, Mcclements分析了Scholte波在两相流体与多孔介质固体界面处的传播特性. 结合各模型的复波数表达式建立含泥沙流体-多孔介质固体界面波特征方程, 分析了Scholte波速与两相流体积含量、粒径等介质属性的关系. 通过仿真实验获得界面波信号, 运用时延估计获得Scholte波速与泥沙含量、粒径的关系, 发现所得的波速与Urick-Ament和HT理论有相对好的一致性. 关键词： Scholte波 两相流体 多孔介质 泥沙含量     

声表面波在圆弧处反射及透射的数值研究*

为了研究直达表面波在圆弧过渡面处传播性能的变化,采用有限元方法模拟了热弹机制下,线性脉冲激光辐照金属铝块表面时激发的表面波在近表面传播过程中,在不同曲率半径的圆弧处发生的反射及透射现象,建立了圆弧半径与反射表面波以及透射表面波时域信号特征之间的联系。计算结果表明:曲率半径与表面波中心波长的数值关系对表面波在圆弧处的传播有显著的影响;同时证明了根据透射表面波信号的到达时间可以反演圆弧半径的大小,为之后利用表面波信号定量检测材料表面圆弧凹痕的深度提供了理论依据。     

Influence of constant quantizing electric field on plasma waves in a two-dimensional superlattice

The effect of a constant electric field on plasma waves in a two-dimensional electronic gas in a system with periodic potential is investigated. A dispersion relation ω(k) is obtained using the high temperature approach. The main and resonant modes of plasma waves are investigated numerically. Regions of one-particle and collective excitations are identified. Calculations are performed on the basis of the plasma wave quantum theory in a random-phase approximation with allowance for the Umklapp processes.     

Calculations of the reflection and transmission of ultrasound by cracks in steel filled with solid sodium

The reflection and transmission coefficients are calculated for 2.25 MHz compression and shear waves incident on idealized cracks (ie, a slot with smooth, flat, parallel faces) in steel filled with solid sodium. The coefficients are calculated as a function of crack width and angle of incidence. Both polarizations (SV and SH) of shear wave are considered, as well as compression wave incidence. It is concluded that a crack of width 0.01 mm or greater, full of solid sodium, will return at least 10% of the incident compression wave amplitude. With a suitable choice of inspection angle, 40% reflection of the incident amplitude can be achieved for either compression or shear (SV) waves. The use of shear waves with the uncommon SH polarization also looks promising.     

On the propagation of acceleration waves in incompressible hyperelastic solids

The conditions for the propagation of acceleration waves (sound waves) in incompressible elastic media undergoing finite deformation are investigated. The incompressible hyperelastic solid media is considered in accordance with the general constitutive theory of materials subject to internal mechanical constraints. The equation of motion of acceleration waves is obtained using the theory of singular surfaces. A general comparison is made between the magnitudes of the propagation speeds of waves in incompressible and unconstrained solid media by the use of Mandel's inequalities. The magnitudes of the speeds of propagation of acceleration waves in the incompressible hyperelastic material classes of neo-Hookean, Mooney-Rivlin, and St. Venant-Kirchhoff solids are determined. Comparisons are made of the specific results concerning the magnitudes of wave propagation speeds making use of the corresponding material parameters.     

Measurement of elastic nonlinearity using remote laser ultrasonics and CHeap Optical Transducers and dual frequency surface acoustic waves

A nonlinear ultrasonic technique for evaluating material elastic nonlinearity has been developed. It measures the phase modulation of a high frequency (82MHz) surface acoustic wave interacting with a low frequency (1MHz) high amplitude stress inducing surface acoustic wave. A new breed of optical transducers has been developed and used for the generation and detection of the high frequency wave. The CHeap Optical Transducer (CHOT) is an ultrasonic transducer system, optically activated and read by a laser. We show that CHOTs offer advantages over alternative transducers. CHOTs and nonlinear ultrasonics have great potential for aerospace applications. Results measuring changes in ultrasonic velocity corresponding to different stress states of the sample are presented on fused silica and aluminium.     

Backscattering enhancements associated with subsonic Rayleigh waves on polymer spheres in water: observation and modeling for acrylic spheres

Unlike most common solids, "plastic" polymer solids typically have shear and Rayleigh wave phase velocities less than the speed of sound in water. Subsonic Rayleigh waves on smooth objects in water are not classified as leakey waves and it is necessary to reexamine backscattering mechanisms. Also the intrinsic material dissipation of the Rayleigh wave can be significant. Backscattering by acrylic or polymethlmethacrylate (PMMA) spheres in water is analyzed and measured in the region ka = 1.5-7 and it is found that prominent low-lying resonance peaks of the form function f exist. The peaks can be modeled with quantitative ray theory as the result of coupling of subsonic Rayleigh waves with sound through acoustic tunneling. The most prominent maximum of f=5.63 occurs at ka = 1.73 and is associated with the quadrupole (or n=2) partial wave. In addition to explaining the scattering, the target strength is found to be sufficiently large that such spheres may be useful for passive low frequency targets.     

研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.     

Two-dimensional analysis of the effect of an electrode layer on surface acoustic waves in a finite anisotropic plate

The effect of a metal layer over an elastic substrate on surface acoustic wave propagating in the structure can be evaluated precisely for semi-infinite solids and infinite plates, but there is no accurate analytical solution if the finite size of the plate has to be considered. By expanding displacements with eigensolutions of surface acoustic waves in a semi-inifite solid, a set of two-dimensional equations similar to the Mindlin plate theory are obtained. Then for a thin electrode layer, the effect is considered through the approximation of displacements in the metal layer with the ones in the substrate, and an integration over the thickness incorporated the properties of the metal layer into equations through the modification of material properties with the decaying indices of surface acoustic waves and the thickness of the metal layer. Using AT-cut quartz crystal as the substrate, we present the effect of silver electrode layers of finite thickness on the phase velocity of propagating surface acoustic waves.     

Estimation of wall thinning in mild steel using laser ultrasound Lamb waves and a non-steady-state photo-emf detector

In this work, a non-steady-state photo-emf receiver has been used to detect the lower frequency fundamental a0 Lamb waves in mild steel. Experimentally, the Lamb waves are laser-generated in the thermoelastic regime using a Q-switched, 20 ns, pulsed Nd:YAG laser and a line source. Typical Lamb waves had centre frequencies of 250 kHz but with frequency components that extended beyond 1 MHz. In mild steel, higher order Lamb wave modes were not considered to be significant below a frequency thickness product of 1.6 MHz mm. Below this level, associated velocity dispersion curve offered phase velocity changes that were sensitive to thickness change. Samples up to 5 mm in thickness were examined without significant interference from higher order modes. A non-steady-state photo-emf detector used as the ultrasonic detector had the advantage of a lower frequency cut-off at 100 kHz compared to a confocal Fabry-Pérot interferometer (CFPI) of about 2 MHz. Both schemes offered greater stand-off distance (>20 cm) than is possible with EMATs, which have a stand-off distance less than 1.0 mm. Progress made in detecting wall thinning in steel plate with thickness up to 5 mm is reported.     

Effect of water deposition on the surface dynamics of mesopores in MCM-41

The surface acoustic waves in empty cylindrical pores in the amorphous silica MCM-41 as well as in the same pores partially filled with water are studied with the use of a continuum model. The model is shown to be adequate to predict dispersion relations, cut-off wave vectors and the Airy phases for the secular surface waves of the lowest azimuthal indices n. Quantitative predictions are presented both in the liquid and in the polycrystalline solid phase of water. Two sagittal surface waves exist when water is in the liquid phase. The phase transition to the solid phase (ice) results in the disappearance of the high-frequency mode. All the effects occur in the Terahertz frequency region.     

Detonation type waves in phase (chemical) transformation processes in condensed matter

Fast self sustained waves (autowaves) associated with chemical or phase transformations are observed in many situations in condensed matter. They are governed neither by diffusion of matter or heat (as in combustion processes) nor by a travelling shock wave (as in gaseous detonation). Instead, they result from a coupling between phase transformation and the stress field, and may be classified as gasless detonation autowaves in solids. We propose a simple model to describe these regimes. The model rests on the classical equations of elastic deformations in a 1-dimensional solid bar, with the extra assumption that the phase (chemical) transformation induces a change of the sound velocity. The transformations are assumed to occur through a chain branched mechanism, which starts when the mechanical stress exceeds a given threshold. Our investigation shows that supersonic autowaves exist in this model. In the absence of diffusion (dissipation factor, losses), a continuum of travelling wave solutions is found. In the presence of diffusion, a steady state supersonic wave solution is found, along with a slower wave controlled by diffusion. Received 15 October 1998     

Detection of moving objects and flows in liquids by ultrasonic phase conjugation

The possibility for the application of the method of parametric phase conjugation of ultrasonic waves in measuring the velocity of moving objects and flows is investigated. Results of experimental measurements of the Doppler frequency shift are presented for a low-frequency wave (1 MHz) generated by phase-conjugate waves (10 MHz and 11 MHz) propagating in opposite directions in the presence of a moving scatterer. The super high sensitivity of the phase of the low-frequency wave to variations in the spatial position of the scatterer is used to measure the velocity of the object. The presence of flows in the region of propagation of phase-conjugate waves returned leads to an uncompensated Doppler shift of the phase of the phase-conjugate wave at the primary radiation source. The implementation of this feature of ultrasonic phase conjugation for the detection and measurement of the flow velocities in a liquid is demonstrated experimentally.