Assessment of material degradation due to corrosion-fatigue using a backscattered Rayleigh surface wave

Material degradation due to corrosion-fatigue was evaluated nondestructively using backscattered Rayleigh surface wave. A corrosion-fatigue test was carried out for the specimens made of thermo-mechanically controlled process steel in 3.5 wt.% NaCl solution at 25 degrees C. The Backscattering profile, which is the amplitude variation of backscattered ultrasound according to the incident angle, of the specimens were measured in water at room temperature after the corrosion-fatigue test. The velocity of Rayleigh surface wave, determined from the incident angle at which the profile of the backscattered ultrasound became maximum, decreased for the specimen that had the large number of cycles to failure in the corrosion-fatigue test. This fact implies that the corrosion degradation occurred at specimen surface in this specific test is dominantly dependant on the time exposed to corrosion environment. The result observed in the present work demonstrates the high potential of backscattered Rayleigh surface wave as a tool for nondestructive evaluation of corrosion degradation of aged materials.     

Acoustic mapping of the ocean bottom and requirements of the initial data

The amount and degree of detail of the initial information are analyzed as applied to the new method of ocean bottom mapping on the basis of the fine structure of the bottom-reflected sound field at normal incidence. For the mapping data obtained with a planar multielement receiving array, the distortions caused by insufficiently detailed information derived from discrete measurements of the bottom-reflected amplitude are considered. The relations between the number of receivers, the receiver spacing, the array aperture, and the horizontal correlation length of the sound field are determined. The problem is solved by computer simulation with the use of generalized quantitative data obtained in deep-water ocean experiments.     

Anisotropic effects in surface acoustic wave propagation from a point source in a crystal

Strong anisotropic effects in the propagation of surface acoustic waves (SAWs) from a point-like source are studied experimentally and theoretically. Nanosecond SAW pulses are generated by focused laser pulses and detected with a cw probe laser beam at a large distance from the source compared to the SAW wavelength, which allows us to resolve fine intricate features in SAW wavefronts. In our theoretical model, we represent the laser excitation by a localized impulsive force acting on the sample surface and calculate the far-field surface response of an elastically anisotropic solid to such a force. The model simulates the measured SAW waveforms very well and accounts for all experimentally observed features. Using the data obtained for the (111) and (001) surfaces of GaAs, we describe a variety of effects encountered in the SAW propagation from a point source in crystals. The most interesting phenomenon is the existence of cuspidal structures in SAW wavefronts resulting in multiple SAW arrivals for certain ranges of the observation angle. Cuspidal edges correspond to the phonon focusing directions yielding sharp peaks in the SAW amplitude. A finite SAW wavelength results in internal diffraction whereby the SAW wavefront spreads beyond the group velocity cusps. Degeneration of a SAW into a transverse bulk wave is another strong effect influencing the anisotropy of the SAW amplitude and making whole sections of the SAW wavefront including some phonon focusing directions unobservable in the experiment. The propagation of a leaky SAW mode (pseudo-SAW) is affected by a specific additional effect i.e. anisotropic attenuation. We also demonstrate that many of the discussed features are reproduced in powder patterns, a simple technique developed by us earlier for visualization of SAW amplitude anisotropy.Received: 17 June 2003, Published online: 15 October 2003PACS: 43.35.+d Ultrasonics, quantum acoustics, and physical effects of sound - 68.35.Gy Mechanical properties; surface strains - 62.65.+k Acoustical properties of solidsA.M. Lomonosov: On leave from the General Physics Institute, 117942 Moscow, Russia     

Calculation of surface wave motions due to a subsurface point force: an application of elastodynamic reciprocity

The elastodynamic reciprocity theorem for two time-harmonic elastodynamic states of the same body is used to determine the surface wave motions generated by a subsurface point load of arbitrary direction in an isotropic, homogeneous, elastic half-space. The actual surface wave motions due to the point load expressed in a suitably general form represent one of the states, while the other one is an appropriately selected auxiliary solution consisting of incoming and outgoing surface waves. A direct application of the reciprocity theorem yields the desired information on the generated surface wave motion.     

Note on the surface wave due to the prescribed elevation

In the present paper, a study of the deep-sea water wave caused by an oscillatory wind stress due to the atmospheric depression, resulting in spiral cyclonic wind pressure on the surface of the sea is made. It has been observed that the motion of the water wave in the case of wind stress exhibits a greater elevation on the sea surface asg the acceleration due to gravity decreases and maintains the oscillatory nature with the increase of time. For the case of spiral cyclonic motion for which the sea surface experiences the elliptical pressure on the surface, the motion diminishes asg diminishes and oscillates with the variation of time. The motion also diminishes asymptotically as the radius vector of the elliptical pressure approaches unity.     

Rayleigh wave attenuation due to the scattering by two-dimensional irregularities of the walls of an empty borehole

Attenuation of the Rayleigh waves propagating along an irregular surface of an empty borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular boundary of a half-space. The technique used to evaluate the attenuation coefficient is based on the perturbation method and the mean field method. As a result, an expression is obtained that relates the partial attenuation coefficients of the surface Rayleigh wave to the scattering by the irregular surface of an empty borehole into the bulk longitudinal and transverse waves (the R → P and R → S processes) and into the surface Rayleigh waves (the R → R processes). The frequency-dependent behavior of the partial attenuation coefficients is analyzed for different correlation functions of irregularities.     

Field due to a point source in a layer over an inhomogenous medium

Summary A point source at an interface between a homogeneous layer of finite depth and an inhomogeneous half-space is considered. The transmitted wave and the dispersion relation for the Love waves is calculated analytically. The authors of this paper have agreed to not receive the proofs for correction.     

Methods for measuring bistatic characteristics of sound scattering by the ocean bottom and surface

It is very interesting to measure the bistatic characteristics of sound scattering by the ocean bottom and surface for the development of present-day hydrolocation net-centric schemes. Possible methods for measuring scattering bistatic characteristics are evaluated. The angular characteristics of reverberation related to illumination of a water area by active linearly frequency-modulated signals are studied using linear passive arrays to test a measuring scheme in a shallow water area with a depth of ~20 m in the 1–3 kHz frequency range. The bistatic sound-scattering characteristics in the water area are calculated based on the measurements. The obtained characteristics are compared with the known data on the backscattering of sound.     

Feed-out of rear surface perturbation due to rarefaction wave in laser-irradiated targets

We report experimental results on hydrodynamic perturbation transfer from the rear to the front of laser-irradiated targets. Flat polystyrene foils with rear-surface perturbations were irradiated by partially coherent light. We observed phase inversion of the rear surface after the shock breakout at the rear surface. Perturbations on the laser-irradiated surface arose due to the rippled rarefaction wave. Experimental results were well reproduced by a simple model with unperturbed hydrodynamic quantities calculated from the one-dimensional simulation.     

海面动态波动引起的船舶辐射噪声线谱伴随调制特性

针对海上实验发现的船舶辐射噪声载波线谱两侧对称出现伴随线谱现象,建立了基于抛物方程近似理论的动态起伏海面条件下连续波信号传播预报模型,揭示了海面风速、收发距离、声源深度等因素对伴随调制线谱频率间隔和强度的影响规律。数值仿真结果表明,伴随调制线谱与其载波线谱的频率间隔由具有稳定频率的海面涌浪决定;伴随调制线谱强度随海面风速增大而增大;不同收发距离和声源深度等条件下伴随调制线谱强度随距离的变化趋势与其载波线谱强度基本一致,近水面（平均深度3 m以内）声源上移和下移伴随调制线谱能量大致相当,比载波线谱能量低约10 dB;除了载波传播损失大的深度外,非近水面声源上移和下移伴随调制线谱强度能量相差较大,比载波线谱能量整体上低约20 dB以上。对海上实测水面船辐射噪声数据进行长时间窗时频分析表明,上移和下移频率伴随调制线谱与载波线谱的间隔为0.1 Hz左右,伴随调制线谱强度与载波线谱强度相差约10 dB,与仿真分析结果一致。海面动态波动引起的船舶辐射噪声线谱伴随调制特性对水中目标特征识别等具有重要价值。     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

A note on the sound field due to a point source inside an absorbent-lined enclosure

It is shown that the field distribution due to a point source of sound within an absorbent-lined enclosure can be simply calculated using a geometrical ray treatment and the appropriate plane-wave pressure-reflection coefficient. A practical application of this result is in predicting the performance of free-field test chambers of the type commonly used for industrial measurement of the sound power of noise sources.     

Radiation transfer due to a point source in an isotropically scattering,inhomogeneous solid sphere

A method of analysis is presented for solving radiation transfer due to a point source in an absorbing, isotropically scattering, inhomogeneous sphere, which is subjected to a diffusely reflecting boundary. The space-dependent single-scattering albedo is represented in the form ω(r) = Σ^K_k=0 D_K r^k, where r is the optical variable and D_k are the known expansion coefficients. The effects of various functional forms of albedo on the incident radiation, angular distribution of radiation intensity and emissivity are examined.     

The contribution of a lateral wave in simulating low-frequency sound fields in an irregular waveguide with a liquid bottom

A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations.     

Solution of a diffusion equation of smoke molecules due to constant point source in gravitational field

From both the Langevin equation, including a gravitational term, the Fokker-Planck equation based on the dynamical behavior of Brownian particles, and equation of smoke molecular diffusion due to a constant point source is introduced and is solved by applying the Laplace transformation with the convolution theorem.The solution is expressed by the complementary error function with a mean pathway of smoke molecules affected by gravity and is proved to be reduced to conventional forms by a certain restriction neglecting gravity without any forced term.     

Scattering of a narrow wave beam in the pulsed irradiation of a randomly irregular surface in the atmosphere

We consider the problem of wave beam scattering in the pulsed irradiation of a randomly irregular surface in the atmosphere in the case of significant shading of some surface elements by other surface elements. Using the Kirchhoff method, we obtain an analytical expression for the average power that is registered by a receiver. The influence of shading on the shape of the received pulse in the turbulent atmosphere is studied. Ekotsentr-VNIRO (All-Russia Research Institute of Sea Fish Economy and Oceanography), Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 6, pp. 713–721, June, 1997.     

Study of horizontal multipaths and ray chaos due to ocean mesoscale structure

Long-range ocean acoustic propagation in the presence of idealized mesoscale structure is studied by first deriving a two-dimensional horizontal-plane parabolic wave equation that follows from the adiabatic mode approximation. In the geometric limit, a nonautonomous Hamiltonian dynamical system having one degree of freedom is derived. A stochastic formalism is developed to analyze this nonintegrable dynamical system. The main result is that on average two rays that are initially separated by an infinitesimal amount diverge exponentially at a rate given by the Lyapunov exponent that has been calculated theoretically and compared to numerical experiments with agreement to two decimal places. The practical implication of this result is that tomographic inversions based on assumed pointwise accurate ray predictions might not be possible beyond the "predictability horizon" of many thousands of kilometers, due to horizontal-plane multipaths induced by naturally occurring mesoscale activity.     

Simulation of the vector wave field of a low-frequency sound source in the ocean: Some important features

An approach to the simulation of low frequency vector wave fields in stratified media (mainly in the ocean) is considered. The approach is characterized by an improved stability with respect to dividing the medium into many layers of arbitrary thickness. The model for the sound field of a point source is based on an integral representation of two-dimensional, cylindrically symmetric vector wave fields in inhomogeneous media, so that the contributions of all types of waves are included automatically. The model medium is subdivided into N horizontally homogeneous layers for which 4(N?1) equations are formulated to satisfy the boundary conditions between adjacent layers. The method of the generalized Schmidt matrix is used to obtain the coefficients of the equations; these coefficients are substituted into the expressions (of the Fourier-Bessel integral type) for the local parameters of the field. The latter are calculated according to the numerical procedure, and the results are used to model the distributions of the acoustic pressure and the horizontal and vertical components of the particle velocity in liquid and elastic media. The instability of the calculation procedure may result in a disagreement between the model and the exact solution. However, the disagreement is shown to occur mainly in models containing excessively thick layers. A way for improving the stability of the numerical model is suggested. The simulation results are compared with the exact analytical solution for the simplest example and with the results obtained according to the commonly used generalized matrix procedure (the benchmark problem). The examples of the practical application of the model for investigating more complex seismoacoustic wave fields in the ocean are presented.