轨道板中超声兰姆波传播的有限元计算和实验*

该文针对我国高速铁路轨道板缺陷的非接触动态检测问题,研究了空气耦合超声兰姆波在轨道板中的传播规律。首先,给出了轨道板中超声兰姆波的相速度和群速度频散曲线,结果表明:随着频厚积的增加,频散现象越明显,并且A0相速度收敛于Rayleigh波的波速。然后,建立轨道板中波传播的有限元模型,计算得到兰姆波传播的群速度为2220 m/s,且二维傅里叶变换系数的较大值沿Rayleigh波的频散曲线分布。最后,在沪杭高铁嘉兴南站进行了现场测试,以8.8°倾斜角向轨道板激励产生超声兰姆波,激发产生的兰姆波模态群速度为2325 m/s,且二维傅里叶变换分析其系数的较大值沿Rayleigh波的频散曲线分布。有限元计算结果和实验结果均与理论计算结果一致。该研究为后续轨道板缺陷的非接触动态检测提供了理论依据和实验方法。     

Effect of anisotropy on acoustoelastic birefringence in wood

The ultrasonic velocity of shear waves propagating through radial direction of a wood plate specimen, transversely to the loading direction, was measured. By rotating an ultrasonic sensor, the oscillation direction of the shear waves was varied with respect to the wood plate axis and loading direction. The relationship between shear wave velocity and oscillation direction was examined to discuss the effect of anisotropy on the acoustoelastic birefringence in wood. The results obtained were summarized as follows. When the oscillation direction of the shear wave corresponded to the tangential direction of the wood specimen regardless of the stress direction, shear wave velocity decreased markedly and the relationship between shear wave velocity and rotation angle tended to become discontinuous. That is, when the shear waves oscillated in the anisotropic axis of the wood, the shear wave velocity peaked unlike in the case of oscillation in the stress direction. In an isotropic material (acrylic, aluminum 5052), on the contrary, when the shear waves oscillated in the stress direction of the specimen, the shear wave velocity peaked regardless of the main-axis direction of the specimen. On the basis of the discussion of these results, the ultrasonic shear wave propagating in wood under stress is confirmed to be polarized in the anisotropic axis of the wood.     

Non-contact transportation using near-field acoustic levitation

Near-field acoustic levitation, where planar objects 10 kg in weight can levitate stably near the vibrating plate, is successfully applied both to non-contact transportation of objects and to a non-contact ultrasonic motor. Transporting apparatuses and an ultrasonic motor have been fabricated and their characteristics measured. The theory of near-field acoustic levitation both for a piston-like sound source and a flexural vibration source is also briefly described.     

Numerical simulation of liquid velocity distribution in a sonochemical reactor

Ultrasonically induced flow is an important phenomenon observed in a sonochemical reactor. It controls the mass transport of sonochemical reaction and enhances the reaction performance. In the present paper, the liquid velocity distribution of ultrasonically induced flow in the sonochemical reactor with a transducer at frequency of 490 kHz has been numerically simulated. From the comparison of simulation results and experimental data, the ultrasonic absorption coefficient in the sonochemical reactor has been evaluated. To simulate the liquid velocity near the liquid surface above the transducer, which is the main sonochemical reaction area, it is necessary to include the acoustic fountain shape into the computational domain. The simulation results indicate that the liquid velocity increases with acoustic power. The variation of liquid height also influences the behavior of liquid velocity distribution and the mean velocity above the transducer centre becomes a maximum when the liquid height is 0.4 m. The liquid velocity decreases with increasing the transducer plate radius at the same ultrasonic power.     

Improved biofilm removal using cavitation from a dental ultrasonic scaler vibrating in carbonated water

The use of cavitation for improving biofilm cleaning is of great interest. There is no system at present that removes the biofilm from medical implants effectively and specifically from dental implants. Cavitation generated by a vibrating dental ultrasonic scaler tip can clean biomaterials such as dental implants. However, the cleaning process must be significantly accelerated for clinical applications. In this study we investigated whether the cavitation could be increased, by operating the scaler in carbonated water with different CO₂ concentrations. The cavitation around an ultrasonic scaler tip was recorded with high speed imaging. Image analysis was used to calculate the area of cavitation. Bacterial biofilm was grown on surfaces and its removal was imaged with a high speed camera using the ultrasonic scaler in still and carbonated water. Cavitation increases significantly with increasing carbonation. Cavitation also started earlier around the tips when they were in carbonated water compared to non-carbonated water. Significantly more biofilm was removed when the scaler was operated in carbonated water. Our results suggest that using carbonated water could significantly increase and accelerate cavitation around ultrasonic scalers in a clinical situation and thus improve biofilm removal from dental implants and other biomaterials.     

Horizontal continuous casting of metals with mould excited by ultrasonic waves

The thermal and mechanical conditions which exist during continuous casting of metals at the interface between bar and mould determine the cooling of the bar as well as the friction between bar and mould.Ultrasonic excitation of a horizontally-operating, continuous casting mould increased considerably the maximum throughput. The average value of the inner wall temperature of the mould rose during ultrasonic excitation. Periodic temperature fluctuations decrease during exposure to ultrasonic waves and a more even cooling of the bars is achieved. The force of removal was also reduced. In particular, peaks in the force when the bars are stuck together are reduced.The ultrasonic treatment also improved the surface finish. There is a relation between the temperature fluctuations of the inner side of the mould and the peak-to-peak roughness height. Grain refinement occurs as well as a more even grain distribution in the casting direction.     

非线性兰姆波在阶梯板中传播的有限元仿真

在非线性兰姆波检测中,累积二次谐波是重要的检测信号.该文利用有限元法仿真了非线性兰姆波在阶梯板中的产生和传播过程,通过二维离散傅里叶变换分析了兰姆波的传播模态和各个模态通过波导横截面的能流大小,给出了各个模态在阶梯板中的反射、透射系数,总结了其随阶梯板厚度的变化规律.研究发现,随着台阶增高,散射效应使得兰姆波更难入射波...     

Shear wave caustics and the shadows of surface breaking cracks

A refinement to the detection of defects in rods and pipes by means of their ultrasonic shadows is presented. It has been previously shown that in standard non-destructive test configurations ultrasonic waves inside cylindrical test objects form caustics, surfaces of high sound intensity. Here it is demonstrated that when a crack edge crosses a caustic the diffraction of sound around the defect is enhanced, which allows the point at which this intersection occurs to be detected. Because the caustic is a well-defined geometric entity the position of the crack edge is now known. This effect presents a novel method of sizing defects that extend in from the surface. Experiments were performed in water immersion using 5 MHz sound. The test specimens were 76 mm diameter aluminium cylinders into which slots were cut to act as artificial defects.     

Resonance attenuation of ultrasonic waves in a superconductor with a moving vortex structure

Equations describing the interaction of ultrasonic waves with a moving vortex structure are derived. The addition to attenuation and the relative change in the velocity of longitudinal ultrasonic waves due to this interaction are calculated. It is found that when a longitudinal ultrasonic wave propagates along the direction of motion of the vortex structure and the velocity V of the structure is equal to half the velocity of the wave, then anomalous acoustic attenuation occurs and the contribution from the ultrasound-vortex interaction to the velocity of the ultrasonic wave vanishes. It is shown that if the vortex structure moves at a sufficiently high velocity, then (in contrast to the case of the structure at rest) a weakly damping collective mode propagating with velocity 2V arises in the structure. It is this mode that is responsible for anomalous attenuation of longitudinal ultrasonic waves.     

A pinhole-plate ultrasonic atomizer

A new pinhole-plate ultrasonic atomizer has been proposed and developed. A pinhole-plate having one or more pinholes is joined to a cavity filled with liquid and is vibrated by a circular ring piezoelectric element bonded onto it. The vibrating plate generates ultrasonic waves only in the vicinity of the pinholes to jet and atomize the liquid from them. The atomizer is very simple and compact, and extremely low in power consumption.     

多通道超声兰姆波检测板状结构中的裂纹

超声兰姆(Lamb)波在结构缺陷检测方面愈来愈受到重视,但目前Lamb波的应用局限于单信号源激励,单通道接收的方法,容易受到噪声干扰,其后续的信号分析处理也比较复杂。本文旨在采用多通道Lamb波对板状结构中的裂纹进行定量分析与诊断。在铝板的表面凿刻出不同深度的凹槽作为裂纹,通过线阵换能器采集在一定传播距离内的多通道Lamb波信号,并采用二维傅里叶变换分析在不同深度的裂纹下,Lamb波模式能量的变化规律。结果表明,相对于完好铝板中的Lamb波信号,裂纹的存在会使Lamb波发生模式转换现象,并且转换模式能量百分比随裂纹深度的增加而线性增加。其结果为Lamb波评价板状结构中的裂纹状况提供了一种可能的方法。     

Measurements of the longitudinal wave speed in thin materials using a wideband PVDF transducer

A flat transducer was constructed, using a 9-microm-thick PVDF (polyvinylidene fluoride) film for generation and detection of high-frequency ultrasonic waves, and used for measurements of the phase velocity of longitudinal waves traveling along the thickness direction in a very thin material. The transducer has a useful wideband frequency characteristic extending from 10 MHz to over 150 MHz. Measurements of the phase velocity of the longitudinal waves are carried out using a 0.212-mm-thick glass slide and a 0.102-mm-thick stainless-steel shim, using water as a coupling medium. The thickness limit for this measurement appears to be approximately 20 microm. The phase velocity of the longitudinal mode is obtained as a function of frequency in the frequency domain by using a modified sampled continuous wave (cw) technique. It can also be measured in the time domain by using a broadband pulse of short duration.     

Ultrasonic resonator with electrical discharge cell for ozone generation

The effect of ultrasound waves in an ultrasonic resonator cell on ozone generation by a hollow needle to plate electrical discharge enhanced by the airflow through the needle is experimentally investigated. It was found that the application of ultrasound waves increases ozone generation for discharge when the needle is negatively biased, and has no effect on ozone generation for the discharge when the needle is biased positively. We try to explain the influence of ultrasound on the discharge mechanisms.     

Three-dimensional hybrid model for predicting air-coupled generation of guided waves in composite material plates

For contact-less, non-destructive testing (NDT) purposes using air-coupled ultrasonic transducers, it is often required to numerically simulate the propagation of ultrasonic waves in solid media, and their coupling through air with specific transducers. At that point, one could simulate the propagation in the air and then in the solid component, using a Finite Element (FE) model. However, when three-dimensional (3D) modeling becomes necessary, such a solution reveals to be extremely demanding in terms of number of degrees of freedom and computational time. In this paper, to avoid such difficulties, the propagation in air from an ultrasonic transmitter to a tested solid plate is modeled in 3D using a closed-form solution. The knowledge of the transducer characteristics (diameter, frequency bandwidth, efficiency in Pa/V) allows the spatial distribution and actual pressure (in Pa) of the acoustic field produced in the air to be predicted, for a given input voltage. This pressure field is applied in turn as a boundary condition in a 3D FE model, to predict the plate response (displacement and stress guided beams) for a given distance between the transmitter and the plate, and for a given angle of orientation of the transmitter with respect to the plate. The FE model is so restricted to modeling of the solid structure only, thus reducing very significantly the number of degrees of freedom and computational time. The material constituting the plate is considered to be an anisotropic and viscoelastic medium. To validate the whole modeling process, an air-coupled ultrasonic transducer is used and oriented at a specific angle chosen for generating one specific Lamb mode guided along a composite plate sample, and a laser probe measures the normal velocity at different locations on the surface of the plate. In the field of NDT, it is generally suitable to excite a pure Lamb mode in order to ease the interpretation of received signals that would represent waves scattered by defects. After a validation step, the numerical model is then used to investigate the effect of the material anisotropy on the purity of the incident guided mode.     

激光超声检测带过渡圆角平板表面缺陷的数值研究

为了研究激光激发出的超声波在带过渡圆角的金属平板上的传播规律和检测表面缺陷的方法,采用有限元法模拟了该类平板中的激光超声现象,分析了表面波在圆角区域的传播规律和与表面缺陷的作用过程。数值结果表明:激光激发出纵波、横波和Rayleigh波等,其中Rayleigh波主要存在于表面mm量级,并且在过渡圆角处发生模式转换生成了直达波R′和模式转换波R_R等多种表面波;经过过渡区域后的声波在表面缺陷处发生了反射和透射现象,通过B扫图可以检测缺陷的位置。随着缺陷深度的增加,表面波的透射系数不断减小,且透射波R_t和R_st存在0.5 μs左右的到达时间差,该时间差与缺陷深度近似成线性正相关。数值结果为激光超声检测带过渡圆角的平板表面缺陷提供了有价值的参考。     

液体材料超声处理过程中声场和流场的分布规律研究

针对合金熔体等液体材料的超声处理过程,选取水作为透明模型材料,采用数值模拟计算和示踪粒子实验方法,研究了20和490 kHz两种频率超声作用下水中的声场和流场分布.结果表明,增大变幅杆半径能够提高水中声压水平,扩大空化效应的发生区域.当超声频率为20 kHz时,水中声压最大值出现在超声变幅杆下端面处,且声压沿传播距离的增大而显著减小.如果超声频率增加至490 kHz,水中的声压级相比于20 kHz时明显提高,且声压沿着超声传播方向呈现出周期性振荡特征.两种频率超声作用下水中的流场呈现相似的分布特征,且平均流速均随着变幅杆半径增大表现出先升高后降低的趋势.变幅杆半径相同时,20 kHz频率超声作用下水中的平均流速高于490 kHz频率超声.采用示踪粒子图像测速技术实时观察和测定了水中的流速分布,发现其与计算结果基本一致.     

Transverse and torsional complex vibration systems for ultrasonic seam welding of metal plates

Transverse and torsional complex vibration systems for ultrasonic seam welding of metal plate specimens, using a 27 kHz complex vibration disk welding tip vibrating in transverse and torsional vibration modes, were studied. Using a complex vibration welding system with a welding tip vibrating in elliptical or circular locus, thick plate specimens can be welded with a more uniform and larger area compared to a conventional ultrasonic welding system. The disk welding tip vibrates in an elliptical or circular locus. The complex vibration system can continuously weld multiple parts of metal plate specimens such as heat sinks with a large number of fins.     

Analytic approximation to nonlinear hydroelastic waves traveling in a thin elastic plate floating on a fluid

An analytic approximation method known as the homotopy analysis method (HAM) is applied to study the nonlinear hydroelastic progressive waves traveling in an infinite elastic plate such as an ice sheet or a very large floating structure (VLFS) on the surface of deep water. A convergent analytical series solution for the plate deflection is derived by choosing the optimal convergencecontrol parameter. Based on the analytical solution the effects of different parameters are considered. We find that the plate deflection becomes lower with an increasing Young’s modulus of the plate. The displacement tends to be flattened at the crest and be sharpened at the trough as the thickness of the plate increases, and the larger density of the plate also causes analogous results. Furthermore, it is shown that the hydroelastic response of the plate is greatly affected by the high-amplitude incident wave. The results obtained can help enrich our understanding of the nonlinear hydroelastic response of an ice sheet or a VLFS on the water surface.     

Plate-shaped non-contact ultrasonic transporter using flexural vibration

We developed a plate-shaped non-contact transporter based on ultrasonic vibration, exploiting a phenomenon that a plate can be statically levitated at the place where its gravity and the acoustic radiation force are balanced. In the experiment, four piezoelectric zirconate titanate elements were attached to aluminum plates, on which lattice flexural vibration was excited at 22.3 kHz. The vibrating plates were connected to a loading plate via flexible posts that can minimize the influence of the flexure induced by heavy loads. The distribution of the vibration displacement on the plate was predicted through finite-element analysis to find the appropriate positions of the posts. The maximum levitation height of this transporter was 256 μm with no load. When two vibrating plates were connected to a loading plate, the maximum transportable load was 4.0 kgf.     

Ultrasonic diffraction grating spectroscopy and characterization of fluids and slurries

The ultrasonic diffraction grating is formed by machining triangular grooves, 300 microns apart, on a stainless steel surface. The grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m=1 beam in the liquid. The angle of this beam in the liquid increases with decreasing frequency and the critical frequency FCR occurs when the angle is 90 degrees. At frequencies below FCR, this m=1 wave does not exist and its energy is shared with other types of waves. The signal of the reflected m=0 wave is observed and an increase is observed at FCR. This information yields the velocity of sound in the liquid and particle size.