A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.     

Time-domain analysis of second-harmonic generation of primary Lamb wave propagation in an elastic plate

Within the second-order perturbation approximation,this paper investigates the physical process of generation of the time-domain second harmonic by a primary Lamb wave waveform in an elastic plate.The present work is performed based on the preconditions that the phase velocity matching is satisfied and that the transfer of energy from the primary Lamb wave to the double frequency Lamb wave is not zero.It investigates the influences of the difference between the group velocities of the primary Lamb wave and the double frequency Lamb wave,the propagation distance and the duration of the primary Lamb wave waveform on the envelope shape of the time-domain second harmonic.It finds that the maximum magnitude of the envelope of the second-harmonic waveform can grow within some propagation distance even if the condition of group velocity matching is not satisfied.Our analyses also indicate that the maximum magnitude of the envelope of the second-harmonic waveform is kept constant beyond a specific propagation distance.Furthermore,it concludes that the integration amplitude of the time-domain second-harmonic waveform always grows with propagation distance within the second-order perturbation.The present research yields new physical insight not previously available into the effect of generation of the time-domain second harmonic by propagation of a primary Lamb wave waveform.     

Impulse propagation in the nocturnal boundary layer: analysis of the geometric component

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind jet lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal propagating in such a duct is received at long ranges from the source as an extended wave train consisting of a series of weakly dispersed distinct arrivals followed by a strongly dispersed low-frequency tail. The leading distinct arrivals have been previously shown to be well modeled by geometric acoustics. In this paper, the geometric acoustics approximation for the leading arrivals is investigated. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the distinct arrivals have been determined. The time delay between and relative amplitudes of the direct-refracted and single ground reflection arrivals have been investigated as parameters for an inversion scheme. A two parameter quadratic approximation to the effective sound speed profile has been fit and found to be in strong agreement with meteorological measurements from the time of propagation.     

黏弹细棒中直达波提取及其动态力学参数的宽频测试

黏弹性材料的动态力学参数可通过对材料样品中纵波测试并结合波速法计算得到。由于测试样品尺寸的限制,除了高损耗材料在高频范围外,不管是在样品端头或侧面都难以获得准确的直达波信号,导致参数测量不准确。针对这个问题,对一端固定一端自由的黏弹性细棒中的纵波传播过程进行了分析,提出了黏弹性细棒直达波提取的方法,并实现了黏弹性材料动态力学参数的宽频测试。采用宽带脉冲作为测试激励信号,利用两台激光测振仪对细棒的固定端和自由端的振动速度进行测试,计算得到细棒自由端处的直达波。然后,利用提取出的直达波信号进行波速法计算,得到了黏弹性细棒宽频范围内的储能模量和损耗因子,宽频脉冲测试结果与黏弹仪测试结果相吻合,验证了该方法的有效性。基于直达波提取的宽频测试方法不仅效率高,而且可以使波速法应用于更低的频段。      

Parameter measurement of the cylindrically curved thin layer using low-frequency circumferential Lamb waves

The characteristic parameters of a cylindrically curved thin layer include its elastic constants, thickness and curved radius. A layer is considered thin if the echoes from the front and back surfaces of the layer cannot be separated in the time domain, and/or that the wave arrivals corresponding to longitudinal and shear wave part cannot be identified in the time or space domain. This paper describes a low-frequency circumferential Lamb wave method to characterize those parameters of a cylindrically curved thin layer. The technique is based on the measurement of circumferential Lamb wave phase velocity and the unknown parameter is estimated through minimizing the mean square error obtained by comparing theoretical and experimental phase velocities. The sensitivity and accuracy of the proposed technique to different parameters are analyzed. Using the present technique, a cylindrically curved thin layer with thickness down to ten percent of the longitudinal wavelength can be successfully measured with an average relative error less than two-percent in our experiment.     

Observations of acoustic surface waves in outdoor sound propagation

Acoustic surface waves have been detected propagating outdoors under natural conditions. Two critical experimental conditions were employed to ensure the conclusive detection of these waves. First, acoustic pulses rather than a continuous wave source allowed an examination of the waveform shape and avoided the masking of wave arrivals. Second, a snow cover provided favorable ground impedance conditions for surface waves to exist. The acoustic pulses were generated by blank pistol shots fired 1 m above the snow. The resultant waveforms were measured using a vertical array of six microphones located 60 m away from the source at heights between 0.1 and 4.75 m. A strong, low frequency "tail" following the initial arrival was recorded near the snow surface. This tail, and its exponential decay with height (z) above the surface (approximately e(-alpha z)), are diagnostic features of surface waves. The measured attenuation coefficient alpha was 0.28 m(-1). The identification of the surface wave is confirmed by comparing the measured waveforms with waveforms predicted by the theoretical evaluation of the explicit surface wave pole term using residue theory.     

脉动流化床内球柱形颗粒流动行为研究

本文结合离散单元模型和计算流体力学方法对脉动流化床内气固流动特性进行数值模拟.结果表明,脉动气流的波形和振幅对球柱形颗粒的流动行为有明显的影响.方波脉动气流作用下的时均颗粒总平动和旋转动能最大,其次是正弦波脉动和三角波脉动,恒定气流下的最小.功率谱分析结果可以发现方波气流下主频幅度最大,其次是正弦波、三角波和恒定气流....     

Effect of spatial dispersion on transient acoustic wave propagation in 3D

Spatial dispersion is the variation of wave speed with wavelength. It sets in when the acoustic wavelength approaches the natural scale of length of the medium, which could, for example, be the lattice constant of a crystal, the repeat distance in a superlattice, or the grain size in a granular material. In centrosymmetric media, the first onset of dispersion is accommodated by the introduction of fourth order spatial derivatives into the wave equation. These lead to a correction to the phase velocity which is quadratic in the spatial frequency. This paper treats the effect of spatial dispersion on the point force elastodynamic Green's functions of solids. The effects of dispersion are shown to be most pronounced in the vicinity of wave arrivals. These lose their singular form, and are transformed into wave trains known as quasi-arrivals. The step and ramp function wave arrivals are treated, and it is shown that their unfolded quasi-arrival forms can be expressed in terms of integrals involving the Airy function.     

Measuring scanning acoustic microscope with a harmonic sounding signal

A concept of a scanning acoustic microscope with a harmonic sounding signal for measuring the parameters of local homogeneous regions of flat samples is proposed. The distinctive feature of the device is the utilization of the Doppler effect that occurs in the sounding wave reflected from the sample surface when the sample is uniformly moved relative to the focusing ultrasonic transducer of the microscope. It is theoretically demonstrated that the spectrum of the received signal is determined by the product of the reflection coefficient and the transfer function of the transducer. The errors of the measurement technique are considered, and the sources of signal distortions are analyzed. High sensitivity of the measurement results to the errors of the scanning system is demonstrated. The developed measuring microscope is described, in which an acoustic interferometer is used to provide the necessary precision of the scanning coordinate measurement. The microscope transfer function is measured for the frequency of the sounding signal 65 MHz, and the values of density and bulk wave velocities are determined for a homogeneous sample by the measured phase of the reflection coefficient using the technique of nonlinear estimation of parameters. With fused quartz used as an example, it is shown that the measurement error is 7.2% for density and 2.3 and 0.7% for the velocities of longitudinal and transverse waves, respectively. In addition, the velocity of a leaky surface wave (SAW) is determined by two methods. One method is based on measuring the position of the inflexion point for the experimental phase of the reflection coefficient, and the other is based on calculating the SAW velocity corresponding to the measured values of density and bulk wave velocities. The errors of these methods are found to be equal to 0.42 and 0.17%, respectively.     

基于单孔洞近似的高纯铝部分层裂实验的数值模拟研究

基于单孔洞近似,对不同撞击速度下高纯铝的部分层裂实验进行了数值模拟研究,讨论了微孔洞长大对波传播的影响及其在自由面速度波剖面上的表现. 通过分析微孔洞周围的应力场变化,认识到实测自由面速度波剖面出现"回跳"特征并不能说明材料发生完全层裂,其直接原因是样品内部微孔洞长大所引起的局部卸载效应. 将计算得到的自由面速度波剖面和微孔洞相对体积与实验结果进行了对比分析,发现两者均符合很好,表明采用单孔洞增长来近似描述部分层裂样品中随机损伤发展及其对波传播的影响是可行的. 关键词： 层裂 孔洞增长 自由面速度波剖面 微孔洞相对体积     

On driving a transducer to produce pulses shorter than the natural period of the transducer

When an acoustical transducer is given a short electrical impulse, the pressure wave produced is usually some sort of damped sine wave whose exact shape is determined by the physical properties of the transducer and the medium to which it is coupled. In order to produce pressure pulses shorter than this, it is necessary to drive the transducer with a more complex waveform. This paper describes how to calculate and produce the voltage waveform for any desired pressure waveform and shows some examples of the voltage waveforms necessary to drive a loudspeaker to produce pulses shorter than the natural resonant period of the loudspeaker.     

六面顶压机高温高压弹性波速测量装置样品室温度梯度的校正

 大腔体六面顶压机波速测量装置的样品室内存在着较大的温度梯度。根据实验观测的样品室内的温度分布数据和在一定温度条件下波速随温度升高呈线性变化的特征，运用理论分析和计算相结合的方法对样品的弹性波速进行了校正。以斜长角闪岩为例，对其实验测量的纵波速度（v_P）进行了校正，并与原来的数据作了对比：波速校正量绝对值随着温度的升高逐渐增大；在2.0 GPa、530 ℃时，样品X和样品Y校正后的v_P比未校正的v_P分别减小了0.270 km/s（3.3%）和0.118 km/s（1.5%）。这种方法明显地提高了波速测量结果的精度，有助于更准确地进行地球物理资料的解释。     

Three-dimensional elastic wave scattering by a layer containing vertical periodic fractures

Elastic wave scattering off a layer containing a single set of vertical periodic fractures is examined using a numerical technique based on the work of Hennion et al. [J. Acoust. Soc. Am. 87, 1861-1870 (1990)]. This technique combines the finite element method and plane wave method to simulate three-dimensional scattering off a two-dimensional fractured layer structure. Each fracture is modeled explicitly, so that the model can simulate both discrete arrivals of scattered waves from individual fractures and multiply scattered waves between the fractures. Using this technique, we examine changes in scattering characteristics of plane elastic waves as a function of wave frequency, angle of incidence, and fracture properties such as fracture stiffness, height, and regular and irregular spacing.     

A system for measuring elastic wave velocity under high pressure and high temperature using a combination of ultrasonic measurement and the multi‐anvil apparatus at SPring‐8

An experimental system to measure the elastic wave velocities of hot‐pressed polycrystalline samples at high pressure and high temperature has been installed at SPring‐8. It uses a combination of the ultrasonic pulse‐echo‐overlap method and Kawai‐type multi‐anvil apparatus (SPEED‐1500). X‐ray radiographic imaging enables the sample length to be determined at high pressure and high temperature, which is indispensable for precise determination of elastic wave velocity. Precise measurements of the elastic wave velocities of various minerals have been determined at pressures up to 19 GPa and temperatures up to 1673 K. The experimental technique provides the precise elastic wave velocities of various materials corresponding to those in the Earth's mantle.     

Diffraction of picosecond bulk longitudinal and shear waves in micron thick films. Application to their nondestructive evaluation

In this paper, acute focusing of the laser pump beam ( approximately 0.5 microm) on the sample surface allows picosecond acoustic diffraction in thin metallic films. The resulting wavefronts propagate at a group velocity which differs from phase velocities in anisotropic films. Waveforms have been experimentally recorded in a gold layer (2.1 microm thick) for several distances between pump and probe on the sample surface. A specified signal processing based on a Synthetic Focalization Technique allows analyzing the space repartition of the acoustic wave vectors for both longitudinal and shear waves. Stiffness coefficients of the gold layer are then identified from wave arrival times.     

A reflected energy prediction model for long-range hydroacoustic reflection in the oceans

Acoustic energy from underwater earthquakes and explosions can propagate over long distances with very little attenuation in the deep ocean. When this sound encounters a seamount, island, or continental margin, it can scatter and again propagate over long distances. Hydrophones in the deep sound channel can detect these reflections tens of minutes or hours after arrivals from the direct source-to-receiver path. This paper presents the Reflected Energy Prediction (REP) model, a model for predicting these reflected arrivals. For a given source and receiver, the REP model uses a detailed knowledge of the underwater environment and components of the Hydroacoustic Coverage Assessment Model, HydroCAM, to predict the impulse response of the ocean. When this impulse response is convolved with a source function, a waveform envelope prediction is made that can be compared with recorded data. In this paper we present the model and a few applications of the model using data recorded from earthquakes and explosions in the Atlantic and Indian Oceans. These examples illustrate the use of the model and initial steps toward model calibration.     

Ultrasonic SH wave velocity in textured aluminium plates

Neutron diffraction has been used to measure the crystallite orientation distribution function for a sample of rolled aluminium plate. Based on this study an expression is given for the ultrasonic velocity of the fundamental horizontally polarized shear (SH) wave propagating in the plane of the plate at an angle θ to the rolling direction. This expression is correct to second order in the leading texture component and is compared with the exact numerical result and with measurements of the SH wave velocity for this plate. In textured materials the group and phase velocities are not, in general, parallel and an expression for the angle between them is derived. Inclusion of this effect leads to better agreement between experiment and theory. The results have application to the texture-independent determination of stress with ultrasonics recently proposed by MacDonald and Thompson et al.     

用三角波电流励磁改进交流磁滞回线实验简

改进行了交流磁滞回线实验的励磁方法。研制了三角波电流交流恒流源驱动励磁线剖励磁．使软磁环形样品被交流磁化双向交替达到饱和,磁化场H的波形仍为三角波。维持三角波电流的周期和幅值不变．可使H变化的波形和变化的速率不变。可在H匀速变化的条件下观测样舳的交流磁滞回线;能在磁化场相同的条件下观测不同样品的交流磁滞回线;还可直接观测样品的微分磁导率曲线。     

Theoretical examination of ultrasonic pole figures via comparison with the results analyzed by finite element polycrystal model

The ultrasonic wave velocities in a polycrystalline aggregate are sensitively influenced by texture development due to plastic deformation. According to Sayer's model, it is possible to construct ultrasonic pole figures via the crystallite orientation distribution function (CODF), which can be calculated by using ultrasonic wave velocities. In the previous papers, the theoretical modeling to simulate ultrasonic wave velocities propagating in solid materials under plastic deformation has been proposed by the authors and proved to be a good agreement with experimental results. Generally, wave velocities are dependent upon the propagating wave frequency; hence to evaluate texture development via ultrasonic pole figures it is necessary to examine an influence of frequency dependence on the ultrasonic wave velocities. In the present paper, the proposed theoretical modeling is applied to the texture characterization in polycrystalline aggregates of FCC metals under various plastic strain histories via ultrasonic pole figures, and also the frequency dependence is examined by using Granato-Lücke's dislocation strings model. Then the simulated ultrasonic pole figures are compared with the pole figures analyzed by the finite element polycrystal model (FEPM). The good qualitative agreement between both results suggests the sufficient accuracy of our proposed theoretical modeling.     

Continuous recording of ultrasonic velocity by laser Bragg diffraction

A method for improving the measuring accuracy of ultrasonic wave velocities in a liquid sample is given. This utilizes the Bragg diffraction angle. Continuous recording of the ultrasonic velocity for a continuously varying sample temperature is also described. An analogue of the Czerny-Turner mount in the grating monochromater is used for the determination of the angle of diffraction. A frequency modulated ultrasonic wave produces an oscillating photo-electric signal from a diffracted light component incident on a photo-multiplier tube. Using this signal, the peak point in the intensity distribution of the component is determined with extremely high accuracy. Applying the phase-sensitively detected output of this signal to a servo motor which drives the angle of a plane mirror (replacing the grating in the Czerny-Turner mount), gives automatic tracking of the angle of diffraction. This in turn allows the possibility of continuous recording of the velocity dispersion as a function of sample temperature.