和土体中线弹性连续屏障的隔振分析

基于Biot波动理论,推导了饱和土中S波入射线弹性固体连续屏障时反射P1波、P2波、S波的反射系数与透射S波、P波的透射系数的解析解。进行算例分析,得出各反射透射系数与线弹性固体连续屏障密度、泊松比及弹性模量的关系。结果表明：线弹性固体隔振结构密度对P1波和P2波反射系数基本无影响;而随着线弹性固体隔振结构密度的增大,S波的反射和透射系数及P波的透射系数均逐渐减小,且渐趋稳定;线弹性固体连续屏障泊松比和弹性模量的变化对P1波P2波反射系数及P波透射系数基本无影响,而对S波的反射系数和透射系数影响较大,S波的反射系数随着线弹性固体连续屏障的泊松比和弹性模量的增大而增高,S波的透射系数随着泊松比的增大而减小,随着弹性模量的增大而增大。     

饱和土体中线弹性连续屏障的隔振分析

基于Biot波动理论,推导了饱和土中S波入射线弹性固体连续屏障时反射P1波、P2波、S波的反射系数与透射S波、P波的透射系数的解析解。进行算例分析,得出各反射透射系数与线弹性固体连续屏障密度、泊松比及弹性模量的关系。结果表明:线弹性固体隔振结构密度对P1波和P2波反射系数基本无影响;而随着线弹性固体隔振结构密度的增大,S波的反射和透射系数及P波的透射系数均逐渐减小,且渐趋稳定;线弹性固体连续屏障泊松比和弹性模量的变化对P1波P2波反射系数及P波透射系数基本无影响,而对S波的反射系数和透射系数影响较大,S波的反射系数随着线弹性固体连续屏障的泊松比和弹性模量的增大而增高,S波的透射系数随着泊松比的增大而减小,随着弹性模量的增大而增大。     

基于Lamb波高阶模态厚度共振的薄片厚度表征

爆破片是气路控制系统阀门中的关键元件,其厚度是影响压力开关阈值的关键参数,对其厚度的精确表征具有重要意义。为此,研究了薄片的Lamb波高阶模态的厚度共振响应与薄片厚度之间的关系。对200 μm和50 μm两种不同厚度的不锈钢薄片的厚度共振模式进行了数值模拟,并通过厚度共振频率和厚度之间关系的理论公式计算其厚度。建立了一个使用超声纵波直探头激发、探测不锈钢薄片共振模式的实验系统,得到两种不锈钢薄片的厚度共振频率,通过理论公式计算厚度并与点探头测厚仪的测量结果对比。利用模拟及实验得到的共振峰频率所计算的厚度与实际的厚度测量结果均吻合较好。利用Lamb波在截止频率处可以产生多模态的厚度共振,这些模态均可用于薄片的高精度厚度表征。     

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。     

基于Lamb波和Hilbert变换的复合材料T型加筋损伤监测

为了监测整体成型复合材料结构的损伤,提出了一种基于Lamb波和Hilbert变换的能量损伤指数。首先,通过应用Hilbert变换提取Lamb波信号的波形包络;然后选取具有最大峰值的波包,将此波包在结构出现损伤后的能量变化值与损伤前的能量之比作为损伤指示;该方法不需要选择特定的Lamb波模式,克服了Lamb波在复合材料结构中存在的频散、多模式及模式转换给信号分析带来的困难;最后在复合材料T型加筋的损伤演化试验中,对该能量损伤指数进行了应用验证研究。研究结果表明：该能量损伤指数可以用于复合材料T型加筋的损伤监测,当能量损伤指数（EDI）值达到0.62时,所研究的复合材料T型加筋确定有损伤产生。     

金属薄板中导波的模态识别和波速测定

按照Lamb波理论,板内导波由多阶对称波和反对称波组成。当波长远大于板厚时,利用经典平板理论对Lamb方程进行简化,此时板内主要有零阶对称波S0和反对称波A0两种模态。认识薄板内声波模态,不仅有助于分析声源性质,提取有效信号,而且可以提高声发射检测中时差定位的准确性。利用PAC公司的Fieldcal标准信号发生器和Nielsen Hsu断铅法作为信号源在0.7 mm薄钢板上进行声波传播实验。通过比较实测波速和理论波速甄别板内的声波模态。实验发现对称波S0实测波速和理论波速比较吻合,而反对称波A0的波速由于频散而较难精确测量。综合两种模态波的波速情况,声发射检测中利用对称波速度进行声源定位计算更加接近薄壁结构声波的传播状况。     

超声导波用于液层负载板损伤检测的数值模拟

液层负载薄板结构的损伤检测是结构无损检测和健康监测中的一个重要问题。为寻求合适的液层负载薄板结构损伤检测信号及分析损伤缺陷对检测信号的响应,求解了双侧液层负载薄板的频散方程,计算得到4 mm双侧水域负载铝板的频散曲线,并利用有限元模拟方法研究了缺陷位置、角度及大小对铝板中检测信号的影响。研究结果表明:中心频率小于100 kHz的S0模式漏Lamb波衰减因子趋近于0,适合长距离损伤检测。此外,当缺陷的存在造成板结构的不对称性时,声信号在缺陷处发生明显的模式转换,且转换出的A0模式信号的透射系数随缺陷位置到板厚中心距离的增加而减小,随缺陷与铝板中间面的角度和缺陷长度的增加先增大后减小,并随缺陷宽度的增加而增大。     

碳纤维增强复合材料层合板Lamb波衰减特性研究

为提取适用于碳纤维增强复合材料层合板声发射故障诊断的模态信号,利用三维弹性理论及传递矩阵法获得Lamb波的频散曲线。以碳纤维增强复合材料层合板为研究对象搭建实验平台,改变断铅激励位置从而获得不同声发射信号。对采集的声发射信号进行小波尺度谱分析,结合频散曲线分离出不同模式的Lamb波,分别研究其不同频率的幅度及能量衰减特性。实验结果表明,较其它信号,低频率S0波幅度信号衰减速度较低,对碳纤维增强复合材料层合板的声发射故障诊断研究具有较大优势。     

弹性波传播到声发射传感器的声压透射系数研究

研究固体中弹性波在媒介分界面的透射规律,以提高声发射检测的精度与效率。基于声学理论推导出了三种典型耦合状态的声压透射系数计算公式,明确了各参数对透射系数的影响;搭建了声发射波透射试验研究平台,分别对上述典型状态下的声压透射理论计算系数进行验证;基于建立的声压透射系数计算方法分析了耦合层的声阻抗、厚度值以及耦合层材料的内外位置等对声压透射系数的影响。研究结果表明,理论计算结果与试验检测结果吻合较好,建立的声压透射系数计算公式有助于指导声发射传感器增强对弹性波的拾取。     

Lamb波在含孔隙功能梯度材料中传播特性的研究

研究了含有孔隙的功能梯度材料(Functionally Gradient Materials,FGMs)中Lamb波的传播特性。利用本征函数展开法和孔隙率与弹性常数之间的关系,讨论了不同孔隙率情况下有限厚度铁基氧化铝功能梯度材料中Lamb波的相速度色散曲线,分析了孔隙率对铁基氧化铝功能梯度材料中Lamb波相速度的影响。研究结果表明Lamb波的相速度色散曲线能够同时反映功能梯度材料的弹性模量、泊松比、密度和孔隙率四个方面的信息,为含有孔隙的功能梯度材料弹性性质的反演提供了理论依据。     

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。     

Shear lag solution for tuning ultrasonic piezoelectric wafer active sensors with applications to Lamb wave array imaging

An analytical investigation of the interaction between piezoelectric wafer active sensor (PWAS), guided Lamb waves, and host structure is presented in this paper, supported with application examples. The analytical investigation assumes a PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lag transfer of tractions and strains is assumed, and an analytical solution using the space-wise Fourier transform is reviewed, closed-form solutions are presented for the case of both ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary) and not ideal bonding (i.e., load transfer mechanism localized close the PWAS boundary). The analytical solutions are used to derive Lamb wave mode tuning curves which indicate that frequencies exist at which the A0 mode or the S0 mode can be either suppressed or enhanced. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar, EUSR) and the sparse array imaging. Extensive experimental tests that verify the tuning mechanism and prediction curves are reported. Examples of correctly tuned EUSR images vs. detuned cases illustrate the paramount importance of Lamb wave mode tuning for the success of PWAS based damage detection.     

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.     

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     

表面裂纹的激光超声可视化兰姆波检测研究

兰姆波在板材的缺陷检测中具有重要的地位,但兰姆波在裂纹检测过程的可视化实验研究较少。通过激光超声可视化技术,观察了激光激励出的宽频兰姆波在0.4 mm深、0.2 mm宽的裂纹上的反射和透射现象;并利用带通滤波技术,研究了不同中心频率的兰姆波在裂纹上的散射特性。结果发现:兰姆波在裂纹上发生了模态转换现象,出现了传播速度较快的兰姆波模态;并随着兰姆波频率的增加,反射兰姆波的能量增强。该研究为板中裂纹的兰姆波检测提供了实验参考。     

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.     

应用兰姆波测定材料弹性参数的反演方法

提出基于兰姆波基础阶对称模式的反演方法来估计材料的弹性常数。兰姆波在存在复杂边界的结构中传播时,时域上边界回波相互混叠,各波包成分难以识别。根据兰姆波的频散特性,选取群速度最快对称模式的低频,用于反演计算。利用相位展开法获取基础阶对称模式的相速度实验值;建立以相速度实验值、弹性常数估计值、频率为变量的误差函数,当误差函数最小时,相速度实验值与基于弹性常数估计值的频散曲线吻合度最好。结合薄铝板、发电机大功率电机的线棒绝缘层、转向架的弹性常数反演过程,并进行误差分析,验证基础阶对称模式反演方法的可行性、通用性。     

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     

The interaction of Lamb waves with defects

The interaction of individual Lamb waves with a variety of defects simulated by notches is investigated using finite-element analysis, and the results are checked experimentally. Excellent agreement is obtained. It is shown that a 2-D Fourier transform method may be used to quantify Lamb wave interactions with defects. The sensitivity of individual Lamb waves to particular notches is dependent on the frequency-thickness product, the mode type and order, and the geometry of the notch. The sensitivity of the Lamb modes a(1), alpha(0), and s(0) to simulated defects in different frequency-thickness regions is predicted as a function of the defect depth to plate thickness ratio and the results indicate that Lamb waves may be used to find notches when the wavelength to notch depth ratio is on the order of 40. Transmission ratios of Lamb waves across defects are highly frequency dependent.     

Frequency Dependence of Second-Harmonic Generation in Lamb Waves

The frequency dependence of the second-harmonic generation in Lamb waves is studied theoretically and numerically in order to examine the role of phase matching for sensitive evaluation of material nonlinearity. Nonlinear Lamb wave propagation in an isotropic plate is analyzed using the perturbation technique and the modal decomposition in the neighborhood of a typical frequency satisfying the phase matching. The results show that the ratio of the amplitude of second-harmonic Lamb mode to the squared amplitude of fundamental Lamb mode grows cumulatively in a certain range of fundamental frequency for a finite propagation distance. It is also shown that the frequency for which this ratio reaches maximum is close but not equal to the phase-matching frequency when the propagation distance is finite. This feature is confirmed numerically using the finite-difference time-domain method incorporating material and geometrical nonlinearities. The fact that the amplitude of second-harmonic mode becomes high in a finite range of fundamental frequency proves robustness of the material evaluation method using second harmonics in Lamb waves.