Reflection of plate acoustic waves produced by a periodic array of mechanical load strips or grooves

The reflection of fundamental acoustic waves propagating in a thin piezoelectric plate by a periodic array of conducting strips of finite thickness or grooves has been theoretically and experimentally investigated. The analysis has shown that electrical shorting and mass loading affect the relationship of neighboring region impedances in a contrary manner. In some cases, these effects are comparable, and there exists a certain strip thickness for each piezoactive fundamental plate mode at which the reflection coefficient can become zero. A high efficiency of grooved reflector for plate acoustic waves was theoretically revealed. Experimental results for mass loading and grooved reflections, which have been obtained for an SH/sub 0/ wave propagating in the Y-X lithium niobate plate, are in a good agreement with the theory. They show a high efficiency of such reflectors and confirm the validity of using a model based on an equivalent circuit for the analysis of their operation. Investigations indicate that nearly 100% reflection of the SH/sub 0/ wave in the lithium niobate plate can be obtained with the use of a mass loading reflector containing 10 silver strips of thickness d/h=0.08 or a grooved reflector containing eight grooves of depth d/h=0.25. Here h is the plate thickness and d is the reflector thickness or depth.     

识别薄板中兰姆波和体波的阶梯板方法

斜探头在某些频率下激励出的兰姆波,其群速度与体波的传播速度相近,所以通过判断传播速度不易区分出兰姆波和体波。通过数值模拟和实验,分别研究了激励频率为2 MHz的纵波和S0模态兰姆波在阶梯板上的反射特性,发现:在阶梯板上入射S0模态兰姆波时,有反射回波;而入射纵波时,无反射回波。基于这种反射特性的差别,提出了一种利用阶梯板区别薄板中兰姆波和体波的方法,该方法可用于确认探头的激励特性。     

Ultrasonic guided wave scattering in a plate overlap

Guided wave scattering in a plate overlap is investigated by numerical calculations and experimental measurements of transmission and reflection factors from the overlap region. In the numerical study, a hybrid boundary element-finite element method is used to calculate the guided wave scattered field from the overlap region. Transmission and reflection factors are calculated for incident A0 and S0 Lamb and n0 shear horizontal waves, including higher modes generated through mode conversion phenomena. In addition, parametric studies of transmission and reflection factors in this problem are performed numerically over various incident modes, frequencies, and overlap lengths. For verification and comparison with numerical results, experiments were conducted to measure the transmission and reflection factors for incident Lamb and shear horizontal waves in steel plates with two different overlap areas. The experimental results agree well with the numerical calculations. The numerical and experimental results show that it is highly feasible to carry out efficient Lamb wave nondestructive evaluation (NDE) in overlapped plates and in multilayer structures with various lap joints by selecting various modes and tuning frequency.     

板材厚度变化对Lamb波透射系数的影响

对Lamb波与厚度变化的板材之间的相互作用进行了分析。针对S0模式Lamb波入射情况,利用混合边界元模型计算了台阶状散射体的透射系数与厚度变化的关系,并进行了实验验证。结果表明S0模式的透射系数能够很好的反映板材的厚度变化,从而为该类材料的定量无损检测提供一定的理论支撑,在实际工业中具有重要的应用意义。     

Hamilton体系下的功能梯度电磁弹性固体内的导波研究

根据Hamilton原理建立了三维压电压磁动力学耦合系统的Hamilton对偶体系，将经典的弹性力学一类变量问题转化为二类变量，建立了Hamilton正则方程组，研究了功能梯度电磁材料（FGMM）板／管内的弹性导波的频散特性及波结构特征。结果表明：（1）压电效应提高了Lamb波的频率和波速，而磁效应则相反，压电效应对波动的影响远大于磁效应；它们而对SH波没有影响（厚度方向极化）。（2）短路及断路电学边界条件对SH波不发生任何影响（厚度方向极化），而短路对Lamb波的频率和波速有不同程度的降低（相同波数下）。（3）在波结构上，对平板而言，所谓的“对称”和“反对称”Lamb波由于材料的梯度特性而变得不再严格的关于中心线对称或反对称。对管而言，由于材料的非均匀分布导致存轴对称栩转波模态中出现了横截面翘曲现象．轴对称纵向波也出现厚度剪切应力。     

Elastodynamic guided wave scattering in infinite plates

The scattering of elastic guided waves by defects in two‐dimensional infinite plates is analysed in the plane and antiplane cases, corresponding, respectively, to Lamb and SH modes. A hybrid boundary element–finite element technique is used, where the defect neighbourhood is discretized with quadratic boundary elements and the radiation condition in the plate is satisfied through a normal mode expansion. A semi‐analytical finite element technique is applied in the infinite plate to calculate its dispersion curves and normal modes. This hybrid technique, which showed excellent performance in the solution of Lamb wave reflection at the edge of semi‐infinite plates, is extended in this paper to a wider range of problems, such as Lamb mode scattering by delaminations and surface defects and SH mode interaction with step discontinuities. Copyright © 2003 John Wiley & Sons, Ltd.     

Propagation of QSH (quasi shear horizontal) acoustic waves in piezoelectric plates

The characteristics of QSH (quasi shear horizontal) acoustic waves propagating in thin plates of Y-cut, X-propagation lithium niobate are investigated theoretically and experimentally. The fractional velocity change (Deltanu/nu) produced by electrical shorting of the surface is calculated as a function of the normalized plate thickness h/lambda (h=plate thickness, lambda=acoustic wavelength). It was found that values of Deltanu/nu as high as 0.18 could be obtained. Experimental measurements show good agreement with theory. The properties of QSH waves propagating in the presence of a perfectly conducting electrode separated from the piezoelectric plate by a small air gap have been studied theoretically and experimentally. It was found that by varying the height of the gap, the phase shift through a 3.2-MHz QSH wave delay line can be varied by more than 230 degrees . We have also theoretically investigated the influence of a thin layer of arbitrary conductivity on the velocity and attenuation of the QSH wave. Calculations show that the variations in these parameters can be as high as 18% and 5 dB per wavelength for a change in layer surface conductance from 10(-7) to 10(-5) S. Results obtained in this paper confirm the attractive properties of QSH waves for a variety of sensing and signal processing applications.     

Influence of damaged area on lamb wave propagation in composite plate

The influence of a specified damage on transient propagation of Lamb wave in a composite laminated plate is studied by finite element analysis. The finite element formulation is developed for the laminated plate with embedded or surface-bonded piezoelectric layers. A higher order laminate model is used to describe the displacement field of both composite laminate and piezoelectric layer. The damaged area is modeled by a localized loss of stiffness and quantified by a degradation coefficient . Piezoelectric materials act as both actuators and sensors for generating and receiving Lamb waves. Numerical results show that the waveform, wave peaks and the arrival time of the transmitted Lamb wave are distinctly correlated with quantified degradation coefficient of the damaged area, which are helpful to damage detection for a composite laminated plate in a new way.Tel (Res).: 86-29-88242204, Tel (Off).: 86-29-88213623-8026     

基于三维弹性理论的碳纤维板中Lamb波建模

基于主动Lamb波的结构健康监测是目前复合材料结构损伤监测技术研究的热点之一,了解Lamb波的传播特性对进行可靠的损伤监测非常重要.本文结合经典三维弹性理论与Lamb波的运动位移方程,对碳纤维复合材料板中传播的Lamb波传播特性进行了建模研究,在此基础上推导了碳纤维板的相速度频散曲线,并讨论了Lamb波传播方向与坐标轴之间的夹角及碳纤维铺层方向对频散曲线的影响,建模结果证明了这种建模方法的正确性.     

A lamb wave source based on the resonant cavity of phononic-crystal plates

In this paper, we propose a Lamb wave source that is based on the resonant cavity of a phononic-crystal plate. The phononic-crystal plate is composed of tungsten cylinders that form square lattices in a silicon plate, and the resonant cavity is created by arranging defects inside the periodic structure. The dispersion, transmission, and displacement of Lamb waves are analyzed by the finite-difference time-domain (FDTD) method. The eigenmodes inside the cavities of the phononic-crystal plate are identified as resonant modes. The fundamental and higher order resonant modes, which vary with the length of cavities, are calculated. By exciting the specific resonant mode in an asymmetric cavity, the 232.40 MHz flexural Lamb wave has a magnified amplitude of 78 times larger than the normal one. Thus, the cavity on the tungsten/ silicon phononic-crystal plate may serve as a source element in a microscale acoustic wave device.     

Generation and reception of ultrasonic guided waves in composite plates using conformable piezoelectric transmitters and optical-fiber detectors

A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s(0)) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s(0) Lamb wave generation using a conventional transducer and angled perspex wedge arrangement.     

Lamb wave tomography and its application in pipe erosion/corrosion monitoring

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers is used to excite and detect theA ₀ mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of theA ₀ mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.     

Surface-bonded and embedded optical fibers as ultrasonic sensors

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.     

The excitation and detection of lamb waves with planar coil electromagnetic acoustic transducers

Planar coil electromagnetic acoustic transducers (EMATs) are investigated for the excitation and detection of Lamb waves in nonferromagnetic metallic wave-guides. Such EMATs are attractive for certain applications due to their omni-directional sensitivity to wave modes with predominantly in-plane surface displacement, such as the So Lamb wave mode. A model is developed that enables the modal content of the radiated Lamb wave field from a transmitting EMAT to be calculated, and the output voltage from a receiving EMAT to be predicted when a Lamb wave mode is incident on it. The predictions from this model are compared with experimental data obtained from 12 different EMATs tested on a 5-mm thick aluminum plate, and good agreement is obtained. The model then is used to analyze the different effects that contribute to the overall Lamb wave modal sensitivity of an EMAT. The relationship between coil geometry and wavelength is examined.     

Leaky lamb waves of a piezoelectric plate subjected to conductive fluid loading: an experimental study

This paper proposes a novel experimental method for measuring the propagating characteristics of leaky Lamb waves in a piezoelectric plate surrounded by a fluid. It is a differential type of measurement and is very sensitive to the velocity change and wave attenuation of leaky Lamb waves induced by fluid-loading effects. Experimental measurements on an X-cut LiNbO3 plate immersed in a dielectric and conductive fluid have been carried out. The velocity change and wave attenuation of the leaky Lamb waves caused by dielectric and conductive loadings of the fluid have been experimentally determined. The measured data have been compared with the theoretical ones that are calculated from a partial wave analysis. For the wave velocity, very good agreements between the experimental and theoretical results are observed. For the wave attenuation, there are some discrepancies, but an important characteristic in the relationship between wave attenuation and fluid conductivity as predicted by the theory have been verified experimentally.     

纵波脉冲回波法和兰姆波法在表征薄层结构中的应用比较

介绍了两种常用的材料薄层超声表征方法——纵波脉冲回波法和兰姆波法。阐述了其基本原理，并结合实例比较了它们的应用情况、优缺点和适用条件等。纵波脉冲回波法简单易行，能够获得薄层厚度、声速和衰减等信息。但实验结果受薄层介质的声衰减和表面粗糙度影响较大，对探头的性能要求较高。兰姆波法主要用于表征薄板类材料，无需较高检测频率，可以测得板中纵波和横波的声速、板厚和板与板之间的结合特性等，检测精度较高。缺点是发射和接收探头之间的距离和倾斜角度需要严格控制，影响因素较多。     

光纤布拉格传感器兰姆波检测的反射谱分析

传统的兰姆波多采用压电陶瓷换能器激发和接收。建立了新的超声兰姆波无损检测系统,其基本思想是采用布拉格光纤传感器作为兰姆波的接收器。光纤光栅传感的基本原理是通过检测光栅反射的中心波长移动实现对外界参量如超声的测量。超声作用下光纤光栅的反射谱发生变化,对超声作用下光纤光栅的反射谱变化进行了数值分析,结果表明,超声对光栅反射谱的影响与超声波长与光栅长度的比值是高度相关的。只有当这个比值相当大时,反射谱的形状才不会变化而中心波长发生偏移,此时光纤传感器可用来探测兰姆波。这个结论为利用新的兰姆波无损检测系统在布拉格光栅长度的设计和兰姆波波长的选择方面提供了有用的工具。     

Achromatic half-wave plate for submillimeter instruments in cosmic microwave background astronomy: modeling and simulation

We adopted an existing formalism and modified it to simulate, with high precision, the transmission, reflection, and absorption of multiple-plate birefringent devices as a function of frequency. To validate the model, we use it to compare the measured properties of an achromatic five-plate device with a broadband antireflection coating to expectations derived from the material optical constants and its geometric configuration. The half-wave plate presented here is observed to perform well with a phase shift variation of < 2 degrees from the ideal 180 degrees over a bandwidth of Deltav/v approximately 1 at millimeter wavelengths. This formalism represents a powerful design tool for birefringent polarization modulators and enables its optical properties to be specified with high accuracy.     

The propagation of Lamb waves in a laminated composite plate with a variable stepped thickness

In this study Lamb waves propagating in a laminated composite plate with stepped thickness variance are characterized through experiments. Understanding the characteristics of Lamb waves is very important for developing a structural health monitoring system, as the number, size, and location of transducers should be determined at the structural design stage. Thin piezoelectric (PZT) transducers bonded to the surface were used for the generation and reception of Lamb waves. The influences of stepped thickness variance were investigated with measurement of group velocities and frequency analyses of the received signals. In addition, a mode analysis was conducted by a mutual transmitting–receiving method making use of the difference between the signals received from a pair of PZTs consisting of transmitter and receiver.     

Guided lamb waves and L-SAFT processing technique for enhanced detection and imaging of corrosion defects in plates with small depth-to-wavelength ratio

The Lamb synthetic aperture focusing technique (L-SAFT) imaging algorithm in the Fourier domain is used to produce Lamb wave imaging in plates while considering the wave dispersive properties. This artificial focusing technique produces easy-to-interpret, modified B-scan type images of Lamb wave inspection results. The high level of sensitivity of Lamb waves combined with the L-SAFT algorithm allows one to detect and to produce images of corrosion defects with small depth-to-wavelength ratio. This paper briefly presents the formulated L-SAFT algorithm used for Lamb waves and, in more details, some experimental results obtained on simulated and real corrosion pits, demonstrating the benefit of combining L-SAFT with pulse-echo Lamb wave inspection. The obtained images of the real corrosion defects showed detection of pits with a depth-to-wavelength ratio of approximately 2/11.