板状结构中基于Lamb波单模态的缺陷成像试验研究

Lamb波技术非常适于板类结构的各种缺陷检测。为了得到模态较为单一的Lamb波,使得接收信号更具有解释性,缺陷更容易识别,该文中在板上下表面的相同位置分别布置压电传感器阵列,采用同一位置上下表面的双压电片同相或反相激励,激励出单一S₀或A₀模态。运用这种单模态的Lamb波激励方式对板中缺陷进行检测,结合椭圆成像算法与数据融合方法对压电传感器阵列接收到的多组信号进行缺陷成像。该方法有效的实现了板中缺陷的二维成像定位,检测出人工缺陷,并提高了缺陷的可识别能力和定位精度。     

兰姆波在裂纹处的模态转换及散射特性研究

该文采用散射矩阵表征兰姆波在缺陷处的模态转换和散射特性, 研究了S₀模态兰姆波与亚波长级槽形裂纹的交互作用. 由板中导波模态位移场的特性, 提出了只存在零阶模态时的模态分离方法. 求解了S₀模态在不同尺寸裂纹处的多模态散射矩阵. 结果表明, S₀模态入射到裂纹时, 散射波动场周向分布不均, 与入射角度相关;散射特征的强弱由裂纹长度决定, 长度越短, 散射特征越明显;各散射模态的能量分配由裂纹的深度决定, 随着裂纹深度增加, 散射的S₀和SH₀模态能量增加, 而A₀模态先增加后急剧减小.     

Development of a low-frequency high purity A0 mode transducer for SHM applications

The reliability of structural health monitoring (SHM) systems based on guided ultrasonic waves is improved if pure modes are generated by the transducers. A piezoelectric- based transducer generating high purity A₀ mode guided waves at low frequencies (around 20 kHz) was developed. The through-thickness resonance of a piezoelectric element was lowered to the frequency region of interest by use of backing masses and low-stiffness front layers. Soft front layers also reduced significantly the transmission of in-plane displacements caused by Poisson's ratio effects in the piezoelectric element to the structure. Parametric studies were undertaken by varying the backing mass length, the transducer diameter, and the thickness of the front layer. The thickness of the plates on which the transducers were operated was found to be a critical issue and these effects were evaluated. Results obtained by finite-element analysis were validated by experimental measurements and showed that signals with A₀/S₀ energy ratios substantially above 40 dB can be obtained.     

Sensitivity of Lamb wave sensors in liquid sensing

The differences of the sensitivities of liquid- and solid-sensing of Lamb wave sensors are discussed. It is shown that the sensitivity of S₀ mode in liquid sensing is much smaller than that in solid sensing, and also much smaller than that of the A₀ mode     

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.     

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。     

A flexible piezoelectric transducer design for efficient generation and reception of ultrasonic Lamb waves

This paper describes the development of a flexible piezoelectric transducer for the generation and detection of ultrasonic symmetrical Lamb waves in plate-like structures. This piezoplatelet transducer structure comprises an array of miniature piezoceramic plates embedded within a soft setting polymer filler material, combining the efficiency of the active piezoceramic phase with a degree of flexibility, which is a function of the platelet/polymer dimensions. For many condition-monitoring applications, the generation of ultrasonic Lamb waves is often appropriate, and this was achieved by incorporating interdigital design techniques via the transducer electrode pattern. The performance of the piezoplatelet transducer structure was evaluated using a combination of linear systems and finite-element modeling, substantiated by experimental results. Importantly, the transducer is shown to operate as an ensemble of platelets, each operating in the thickness mode and well decoupled from neighboring piezoelectric elements. Using this transducer configuration, an unimodal s1 Lamb wave, at 1.45 MHz, has been generated and detected in a 3-mm thick steel plate. Furthermore, a propagation distance of almost 1 m was recorded for s0 Lamb wave generation/detection in a fiber-reinforced composite plate.     

Characteristics of ultrasonic Lamb waves in 128° rotated Y-cutlithium niobate

The characteristics of ultrasonic Lamb waves propagating along the X-direction of a 128° rotated Y-cut lithium niobate plate are investigated. The first higher-order antisymmetric mode, the A₁ mode, is found to exhibit an anomalous behavior. The velocity of this mode remains nearly constant for all values of h/λ, where h is the plate thickness and λ is the acoustic wavelength. The particle displacement of the mode tends towards that of a pure shear horizontal (SH) wave as the ratio h/λ tends to zero. The electromechanical coupling coefficient of the wave has a value of k²=0.78×10^-2 at h/λ≅0. The coupling decreases as h/λ increases, becoming negligible for h/λ>1. The velocity and coupling coefficient of the mode have been measured for various values of h/λ, and are found to be in fair agreement with theoretical calculations     

Lamb wave excitation by Hertzian contacts with applications in NDE

Excitation of Lamb waves in solid plates by point-like Hertzian contacts for material characterization and nondestructive testing is investigated. A 2 dimensional model using normal mode theory is used to predict the relative excitation efficiency of the lowest order Lamb waves in anisotropic solid plates. Hertzian contact transducers with PZT-5H piezoelectric material and quartz buffer rods are realized to operate in the 200 to 500 kHz range for experimental verification. Single mode operation with the lowest order antisymmetric Lamb wave (A ₀) mode is achieved in various plates at in agreement with theoretical predictions. The technique is applied for material characterization on single crystal silicon samples and defect detection in composite plates. The phase velocity anisotropy of the A₀ mode is measured with signal-to-noise levels exceeding 65 dB. In (111) cut silicon plates the absolute phase velocity is measured with ±0.05% accuracy. The phase velocity anisotropy and effects of delamination in layered composite plates are calculated using the surface impedance approach. The experiments on graphite/epoxy composite plates agree with these calculations and show the potential of the method for defect detection with high resolution     

RANDOM OPPORTUNITIES FOR REDUCED COST REPLENISHMENTS

In this paper we consider the situation where the typical assumptions underlying the EOQ model are appropriate for an item, in particular a reasonably level demand rate exists. However, opportunities for special replenishments at a reduced unit cost occur at random. The following type of 3-parameter policy is considered. When a non-special (regular) replenishment is made, an order-up-to-level, S₁;, is used. When a special opportunity arises we only take advantage of it if the current inventory is low enough (L₂ or lower) and we order enough to raise the inventory level to S₁. This situation was modelled earlier in the literature, but the solution procedure suggested involved a 3-dimensional search on the three control parameters, S₁,, L₂, and S₂. We develop an approximate solution method which is much easier to use and which performs excellently on a wide range of examples. The performance of an even more simple method, based on always taking advantage of a special opportunity to replenish at reduced unit cost (i.e., setting L% = S₂), is shown to be highly dependent upon the values of the five uncontrollable parameters of the problem.     

Determination of lamb wave dispersion data in lossy anisotropic plates using time domain finite element analysis. Part II: application to 2-2 and 1-3 piezoelectric composite transducer arrays

The use of finite element modeling, combined with optical generation and detection of Lamb waves in plate structures, was extended to encompass periodic ceramic-polymer materials typical of those encountered in 1-3 and 2-2 piezoelectric composite array transducers. The resultant dispersion data was employed to predict the occurrence of Lamb wave-induced cross talk in composite monolithic arrays. The finite element modeling method was then used to simulate the dispersion behavior of two array structures that were subsequently manufactured: a 1-D 45% volume fraction linear array coupon and a 2-D 35% volume fraction array coupon. Excellent agreement between theory and experiment was obtained using impedance measurements and laser scans of the surface displacement profile at selected frequencies. Regions of strong inter-element cross-coupling were identified and these are shown to correlate very well with the dispersion data obtained for the dual-phase plate material. This work is considered to provide a useful basis for the design of wideband monolithic composite arrays and minimization of guided wave propagation along the array substrate.     

软模板法制备高频超声换能器用1-3复合压电材料

医用高频超声成像技术广泛应用于皮肤、眼睛及血管壁等人体组织的精细结构成像。1-3复合压电材料因具有较高的机电耦合系数而成为高频超声换能器的核心材料。传统的机械切割-填充、等离子蚀刻等1-3复合材料制备方法成本高、效率低, 难以实现工业化制备。本研究提出一种新的基于软模板的高频复合材料制备方法, 在获得高机电耦合系数的同时, 实现高性能1-3复合压电材料的低成本制备。研究采用微米孔径的软模板实现PZT粉的浆料填充, 通过热压烧结获得均匀竖立的PZT陶瓷微柱阵列, 进而制备出PZT/环氧1-3复合材料。对复合材料进行系统的机电性能测试, 并利用不同方法对复合材料的微结构及其均匀性进行表征。结果表明, 软模板法可使压电微柱具有完整的相结构和较高的成分均匀性, 能够实现较高的胚体压缩率, 提高陶瓷微柱的致密度, 同时形成了微柱阵列且微柱直径可控制在70 μm。软模板法有利于在提高复合材料超声频率(30~50 MHz)的同时获得64%的高机电耦合系数, 为医用高频超声成像以及超声生物显微镜等应用提供了一种高效的1-3复合压电材料工业化制备方法。     

Dual five-port analyzer using fixed probes

A dual 5-port network analyzer using fixed electric probes is presented. To measure S₁₁ and |S₂₁|, the RF signal is applied to port 1 of the DUT through one 5-port junction. The same RF signal reflected by a short circuit at the end of the line is used for measuring S₂₂. In this way, dividers, switches, and phase shifters as used in the standard dual technique are avoided. But S₂₂ can only be measured for devices with |S₂₁|>0.3. The 5-port junctions are inexpensive and very simple to design and operate. The mathematics involved uses simple linear algorithms, and iterative methods are not needed. Measurements for several loads have been performed in X-band, and the results agree with those obtained using an HP 8510B network analyzer     

Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays

We report on the characterization of 1D arrays of capacitive micromachined ultrasonic transducers (cMUT). A 275×5600 μm 1D CMUT array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm/√Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V. The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements are measured, and two major sources of acoustical crosstalk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close to the speed of sound in the fluid. This wave causes internal reflections, spurious resonance, and radiation from the edges of the silicon substrate. The large lateral component of the particle velocity generated by the membranes at the edge of the cMUT array elements is found to be the source of this interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the crosstalk between elements. These waves are excited at the edges of individual vibrating membranes, where they are anchored to the substrate, and result in a narrowing of the beam profile of the array elements. Several methods, such as trench isolation and wafer thinning, are proposed and implemented to modify the acoustical cross coupling between array elements     

Adaptive beamforming for array imaging of plate structures using lamb waves

Lamb waves are considered a promising tool for the monitoring of plate structures. Large areas of plate structures can be monitored using active arrays employing beamforming techniques. Dispersion and multiple propagating modes are issues that need to be addressed when working with Lamb waves. Previous work has mainly focused on standard delay-and-sum (DAS) beamforming while reducing the effects of multiple modes through frequency selectivity and transducer design. This paper presents a minimum variance distortionless response (MVDR) approach for Lamb waves using a uniform rectangular array (URA) and a single transmitter. Theoretically calculated dispersion curves are used to compensate for dispersion. The combination of the MVDR approach and the two-dimensional array improves the suppression of interfering Lamb modes. The proposed approach is evaluated on simulated and experimental data and compared with the standard DAS beamformer. It is shown that the MVDR algorithm performs better in terms of higher resolution and better side lobe and mode suppression capabilities. Known issues of the MVDR approach, such as signal cancellation in highly correlated environments and poor robustness, are addressed using methods that have proven effective for the purpose in other fields of active imaging.     

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。     

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.     

Development of a PVDF membrane hydrophone for use in air-coupled ultrasonic transducer calibration

This work describes the use of a polyvinylidene fluoride (PVDF) membrane hydrophone for application in air-coupled transducer calibration. A one-dimensional theoretical analysis is used to demonstrate the potential and performance of PVDF as a hydrophone material over the frequency range 100 kHz to 5 MHz included in the evaluation is the influence of deposited metallic electrode layers on the sensitivity of the material. Experimental validation over the restricted range 400 kHz to 1 MHz is provided by a coplanar 0.028 mm thick membrane hydrophone in conjunction with a custom built 1-3 piezocomposite transmitter. Calibration of the membrane hydrophone is performed by employing a standard hydrophone that has been calibrated to a primary standard in a water medium. Justification for such an approach is presented within the theoretical analysis which provides a close correlation with experimental data. The generation of Lamb waves at critical angles in the PVDF and their subsequent influence on the directional response of membrane hydrophones operating in air is also addressed. A method for partial suppression of the Lamb waves, based around perforation of the membrane (either in whole or in part), is evaluated experimentally with reasonable results.     

Measuring Lamb wave dispersion curves of a bi-layered plate and its application on material characterization of coating

This paper investigates the Lamb wave dispersion curves of a bi-layered plate and evaluates the feasibility of using the dispersion data to characterize the coating's material properties. The measurement of dispersion curves is based on a focusing PVDF transducer operating in a pulse/echo mode. An image displaying technique is used to determine the dispersion relation of Lamb waves from the measured data. Multiple dispersion curves of Lamb waves are accurately determined over a wide frequency range (4 to 20 MHz). Lamb wave dispersion curves for thin metal sheets electrodeposited with nickel coatings are measured. The elastic constants of the nickel coating are determined by comparing the experimental dispersion data with the theoretical ones calculated numerically. Potential applications of this measurement method are addressed     

ON THE ABSORPTION SPECTRUM OF C60 IN THE VISIBLE SPECTRAL REGION

A complete (720-380 nm), strong visible absorption spectrum (OD₅₉₈=1.6) for C₆₀ in a decalin/methylcyclohexane inert matrix at 77 K is reported for the first time. The structure of this spectrum allows one to construct an acceptable approximation to the gas-phase spectrum for this substance at a low temperature. The 0-0 component of the S₀→S₁ transition in the gas phase at a low temperature was estimated to be at about 635 nm.