超声兰姆波的时频分析

声-超声方法在被测板件厚度较小时,板中传播的超声波模式主要是多模式的兰姆波。由于兰姆波的复杂性,声-超声方法回避了对信号进行进一步深入的分析,而采用所谓“应力波因子”对材料中能量耗散进行相对度量。当多模式与频散同时存在时,信号的识别异常困难,本文采用时-频分析的方法对声-超声信号进行了分离,只需一次测量就可满足分析要求。通过对兰姆波信号的伪Wigner-Ville分布进行时频重排和图像特征提取,能够精确地获得兰姆波的局部群速度频散曲线。     

采用单个压电传感器的单模式兰姆波激发频率的选择

考虑了压电传感器(PZT)与板之间的耦合作用,从理论上研究了单个压电传感器激发时产生单模式兰姆波的频率调节方法,实验给出了模式选择在兰姆波结构健康监测中的应用. 在板材、板厚以及PZT尺寸一定的情况下,从理论上能够预测到作为频率函数的各兰姆波模式的幅值变化. 根据某特定兰姆波模式的幅值最大而其他模式幅值相对最小时所对应的频率,即可识别出该兰姆波模式优化的激发频率. 数值仿真验证了理论的有效性以及单模式兰姆波选择的可能性. 在不同的激发频率下,分别激发了优化的A0 模式,优化的S0模式以及共存的A0和S0模 关键词： 兰姆波 压电传感器 激发频率 结构健康监测     

基于频散补偿和分数阶微分的多模式兰姆波分离

由于兰姆波的多模和频散特性,实际检测时在同一激发频率下存在多种模式的混合信号,而各模式信号有不同的频散特性,使得在时频混叠的情况下兰姆波的检测变得十分复杂.本文在频散补偿的基础上,通过时延函数建模,依靠不同模式频散趋势的差异性,将时频混叠信号的分离问题转化为部分模式混叠信号的分离问题.基于分数阶微分的理论,用信号幅值谱分数阶微分极大值和对应频率分别与微分阶次拟合多项式实现特征参数的提取并依靠特征参数重建幅值谱.结合相位谱重构时域信号以实现部分混叠信号中频散补偿后的模式的分离.最后恢复频散获得分离后的兰姆波信号.仿真和实验结果表明,本文方法不仅可以实现时频混叠多模式兰姆波信号的分离,更能保证分离精度,有助于复杂多模式频散信号的分离与处理的进一步研究.     

基于模式分离的兰姆波逆时偏移成像

应用频率域逆时偏移方法实现各向同性和各向异性板中缺陷的兰姆波成像.由于缺陷引起的多模态散射信号会在重建图像中形成伪像,根据基本导波模式振动对称性的差别进行了模式分离预处理.基于多元阵列超声技术,开展了铝板和复合板内缺陷频率域逆时偏移超声成像方法的数值仿真研究.首先,建立有限元模型,采用环形传感器数值采集由缺陷引起的兰姆波散射信号,然后,将采集到的多模式散射信号进行模式分离处理,再将模式分离后的兰姆波散射信号经时间反转后并在相应的接收器处重新激励,在频域中运用格林函数反向传播兰姆波散射信号,获取监测区域的声场信息,与正向传播声场进行互相关,重建缺陷图像.首先对铝板中单缺陷以及复合材料板中相邻的两个相同缺陷进行数值仿真,对比有无模式分离处理的缺陷逆时偏移成像效果,体现出模式分离的重要性.在此基础上,采用逆时偏移方法对复合板材内位置邻近、深度不同的双缺陷进行识别.数值结果表明,模式分离预处理后的缺陷重建图像能够有效去除多模式干扰产生的伪像.文中提出的成像方法对各向同性板和各向异性板内缺陷的检测和成像具有很好的发展潜力,可以准确地探测多个缺陷的形状、尺寸和深度.     

兰姆波非线性效应的实验观察(Ⅱ)

基于Ritec-SNAP系统对固体板中传播的兰姆波的非线性效应进行了实验观察。根据导波的模式展开分析方法和兰姆波的频散曲线,简述了兰姆波的积累二次谐波发生条件。采用一定倾角的斜劈换能器在固体板表面激发和接收兰姆波的基波和二次谐波时域信号,阐述了兰姆波的基波、二次谐波时域脉冲包络的积分振幅的物理意义,在固体板表面分别测量了不同传播距离的兰姆波的基波、二次谐波的幅频曲线。在兰姆波具有非线性效应的频率值附近,分析了兰姆波的二次谐波振幅随传播距离的变化关系。实验结果进一步证明了兰姆波在一定条件下具有强烈的非线性效应,其二次谐波表现出随传播距离积累增长的性质。     

一种定征复合板材粘接层性质的非线性超声兰姆波方法

借助于兰姆波频散曲线及导波激发的模式展开分析方法,对基频兰姆波时域信号及二次谐波时域信号的发生过程进行了直观的论述。结合Ritec-SNAP系统的测量功能,详细分析了二次谐波时域脉冲包络积分表达式的物理意义;该积分表达式可表征基频兰姆波时域脉冲传播过程中的二次谐波发生效率,以及基频与二倍频兰姆波模式之间的频散程度。在基频与二倍频兰姆波相速度相等(或近似相等)的频率附近,实验观察到显著的且无模式混叠的二次谐波信号,显示出在兰姆波的传播过程中的确可存在强烈的非线性效应。对于三种不同粘接情形的复合板材,实验结果表明,采用本文引入的非线性兰姆波应力波因子,结合二次谐波幅频曲线峰值所对应的频率值,可有效地对板材粘接层性质进行表征。     

用分数阶微分实现时频重叠多模式兰姆波的模式分离

为了分离时频重叠多模式超声兰姆波,提出了一种用分数阶微分理论实现多模式兰姆波模式分离的方法。以时频重叠的S1和A1模式混合信号为例,用赛利斯分布作为幅值谱的模型,首先对信号做频散补偿,由补偿后信号幅值谱分数阶微分推导了幅值谱特征参数的计算式并根据信号模型重建幅值谱,然后结合兰姆波的频散特性推导出对应的相位谱,并通过幅值谱和相位谱重构各模式的信号,实现模式分离。对1 mm钢板中仿真时频重叠的S₁和A₁模式信号分离结果显示分离出信号的幅值和带宽相对误差小于6%,中心频率相对误差小于0.25%。实验结果也证实了方法的可行性。因此当混合信号幅值谱不完全交叠时,本方法可以实现多模式兰姆波信号的分离,有助于多模式兰姆波频散信号的分析和识别。      

卷积噪声环境下语音信号鲁棒特征提取

提出了一种基于独立分量分析(ICA)的语音信号鲁棒特征提取算法,用以解决在卷积噪声环境下语音信号的训练与识别特征不匹配的问题。该算法通过短时傅里叶变换将带噪语音信号从时域转换到频域后,采用复值ICA方法从带噪语音的短时谱中分离出语音信号的短时谱,然后根据所得到的语音信号短时谱计算美尔倒谱系数(MFCC)及其一阶差分作为特征参数。在仿真与真实环境下汉语数字语音识别实验中,所提算法相比较传统的MFCC其识别正确率分别提升了34.8%和32.6%。实验结果表明基于ICA方法的语音特征在卷积噪声环境下具有良好的鲁棒性。      

基于声源方位信息和非线性时频掩蔽的语音盲提取算法

针对欠定卷积混合的语音信号模型,提出一种基于声源方位信息和非线性时频掩蔽的语音盲提取算法。首先对低频段混合语音信号进行时频分析估计瞬时相对时延(ITD)并采用势函数聚类分析方法估计出声源个数及其ITD,接着锁定目标提取准确的目标语音方位信息,最后利用独立语音在时频域上的近似W一分离正交性,采用非线性时频掩蔽的方法提取目标语音。仿真实验表明,该方法能锁定任意感兴趣目标方位,能有效提取目标语音,文中实验条件下信噪比增益平均达9.5 dB。      

台阶板中兰姆波A0模式传播特性的光弹研究

为了研究变壁厚结构中兰姆波的传播特性，以K9玻璃制成的台阶板为波导，运用动态光弹法实现了兰姆波在阶梯板中传播过程的可视化。使用MATLAB对兰姆波的理论应力分布进行了仿真，将仿真结果与光弹图像相对比以进行模式识别。此外，根据兰姆波的动态光弹图像计算了特定模式的相速度，并与相速度频散曲线相对比，以支持模式识别的结果。对4mm-3mm和4mm-2mm厚度差台阶板进行了对比分析。研究发现在激励目标模式的兰姆波时，可能会出现非目标模式，并且兰姆波的模态转换与频散程度与台阶处的厚度差有关。本文的结果可对具有相同性质类似结构材料的无损检测提供参考。     

两侧有固体层负载时板中Lamb波的传播

本文研究了薄板二面有固体导负载时板中Ｌａｍｂ波的传播，从弹性波理论出发并结合应的边界条件，导出板中Ｌａｍｂ波的色散方程，数值计算表示，不管作为自由状态时板中Ｌａｍｂ波相速（板厚取定时）是大于或小于外层固体的声表面波波速，板中对称及反对称模式的Ｌａｍｂ波相速都随着外层固体层厚度增加而变化并且渐近于外层固体的声表面波波速，数值计算又表明，对很薄的板，板中对称及反对称模式的相速均随负载板的厚度呈线性变化     

Transient Lamb waves: comparison between theory and experiment

This paper presents an experimental investigation of the generation of transient Lamb waves inside an immersed plate by a time limited signal. The transient Lamb waves are the solutions of the characteristic equation of the plate for complex frequency and real slowness, in contrast to the leaky Lamb waves, for which the slowness is complex and the frequency is real. By using a large transducer in agreement with the real slowness assumption, the excitation of these transient Lamb waves is observed experimentally both in reflection and transmission for a tone burst excitation. Particular attention is paid to the A0 mode for which the amplitude increases exponentially in time. As a result, the total reflected waveform contains in this case a transient contribution, coming from the generation of the A0 mode, that precedes the forced regime. It is shown that there is a very good agreement between the theoretical and the experimental time exponential amplitude variations of the transient Lamb waves. For the A0 mode, the transient signal is qualitatively explained by a group velocity higher than the phase velocity.     

不锈钢折弯板裂纹Lamb波检测技术*

金属弯板广泛应用于车辆、船舶、飞机等大型装备结构件中，折弯处易形成应力集中产生裂纹，直接影响构件的使用安全和寿命。本文开展了不锈钢弯板裂纹缺陷Lamb波检测技术研究，计算了钢板的频散曲线，优选三种不同频率的S0模态：0.25MHz、0.5MHz、1MHz。利用COMSOL软件建立频域仿真模型，模拟了Lamb波在3mm厚“L”形弯板内部的传播情况，开展折弯板裂纹缺陷检测实验，并使用小波包变换的方法对实验信号进行分析。结果表明，经小波变换后，0.25MHz、0.5MHz、1 MHz的S0模态Lamb波均可用于裂纹缺陷的识别检测.     

Time-frequency analysis of the dispersion of Lamb modes.

Accurate knowledge of the velocity dispersion of Lamb modes is important for ultrasonic nondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo-Wigner-Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyze acoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocity dispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the A0, A1, S0, and S2 Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.     

Propagation of time-reversed Lamb waves in acrylic cylindrical tubes as cortical-bone-mimicking phantoms

The present study aims to investigate the propagation of time-reversed Lamb waves in acrylic cylindrical tubes as cortical-bone-mimicking phantoms. Time-reversed Lamb waves could be successfully launched in 6 acrylic tubes with wall thicknesses from 2 to 12 mm by using a modified time reversal method. The group velocities of the time-reversed Lamb waves in the acrylic tubes were measured by using the axial transmission technique. They decreased very slightly with increasing wall thickness, showing good agreement with the theoretical group velocity of the A0 Lamb wave in the acrylic plate. These results suggest that the time-reversed Lamb waves in the acrylic tubes would essentially behave as the A0 Lamb wave, consistent with the behavior of the slow guided wave in long cortical bones. It is expected that the application of the time-reversed Lamb waves in long bones would enhance clinical potential of ultrasonic technologies for the diagnosis of osteoporosis.     

Lamb waves beam deviation due to small inclination of the test structure in air-coupled ultrasonic NDT

In Lamb waves inspection, an air-coupled transmitter transducer is oriented at a specific angle such that it generates a pure Lamb mode which propagates along the structure and interacts with any existing defects. For this inspection system, amplitude losses appears when small inclinations of the tested structure occurs. An important factor which affects directly these losses has been observed, it consists of the Lamb waves beam (LWB) deviation due to this bad alignment. In this work, a simple expression of LWB deviation has been deduced. This expression includes the test structure angle, phase velocity of generated Lamb mode, and the phase velocity of waves propagating in the coupled medium. A(0) Lamb mode is generated and detected in 1 mm thick aluminium plate sample using 1 MHz PZ27 piezoelectric transducers of 20 mm of diameter. Experimental LWB deviation angles are measured for different inclination angles of the test sample. A comparative study is released with theoretical results. For 1 degree of misalignment in the aluminium plate inclination, and transducers separation distance of 35 mm, LWB deviation angle is around 7 degrees and the amplitude is reduced by around 11%. Then, for a large separation distance, we must move the receiver transducer to detect the deviated LWB. It is shown that, for both theoretical and experimental studies, the LWB deviation and its measured amplitude are very sensitive to the alignment of the tested structure with respect to the transmitter-receiver transducers plane. In metal plates it is most satisfactory to use A(0) mode compared with S(0) mode since it is easy to excite and has a large amplitude and small deviation beam angles.     

The propagation of Lamb waves in a plate bordered with layers of a liquid.

The influence of liquid layers on the propagation of Lamb waves in a plate of finite thickness is studied theoretically. The dispersion equations of Lamb waves in a plate bordered with layers of liquids are derived. Numerical solutions of the equations show that the phase velocity of Lamb waves changes with the thickness of the liquid layers. For the lowest antisymmetrical mode of very thin plates, the numerical results calculated from the dispersion equations are compared with those derived from the bending wave acoustic impedance approach. The limitation of the latter is discussed. Applications of Lamb waves pertinent to biosensing are also presented.     

小样本字典学习的兰姆波模态识别方法*

针对兰姆波多模态识别问题,提出了基于小样本字典学习的模态识别方法。将多层复合板的频散特性看作一个线性时不变系统,首先,根据频散知识模拟各个模态传播特定距离后的信号,提取走时和能量特征创建字典;其次,获取待测信号的走时特征,通过查询字典来识别兰姆波模态;最后,根据能量参数估计结果,实现待测信号中各模态信号的分离和重构。通过对三层粘接的AAA板(铝板-亚克力板-铝板,每层厚度为2mm)中传播距离为0.3m、0.5m的直达波和反射波的实验验证,结果显示该方法对A0、S0模态的有效识别和各个波包信号的准确重建。     

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。