Warped basis pursuit for damage detection using lamb waves

This paper presents a novel time-frequency procedure based on the warped frequency transform (WFT) to process multi-mode and dispersive Lamb waves for structural health monitoring (SHM) applications. The proposed signal processing technique is applied to time waveforms recorded at an array of scan points after waveguide excitation. The WFT is combined with a basis pursuit algorithm to extract the distance traveled by the ultrasonic waves even in the case of multi-modal dispersive propagation associated with broadband excitation of the waveguide. This is obtained through a decomposition of the acquired signals using dictionaries composed by optimized atomic functions which are designed to match the spectro-temporal structure of the various propagating modes. The warped basis pursuit (W-BP) analysis of several acquired waveforms results in distance signals that can be combined through classical beamforming techniques for acoustical source imaging purposes. A masking procedure is also proposed to suppress imaging noise. This approach is tested on experimental data obtained by broadband guided wave excitation in a 1-mm-thick aluminum plate with an artificially introduced through crack and tiny holes, followed by multiple waveguide displacement recording through a scanning laser Doppler vibrometer. Dispersion compensation, high-resolution source, and defect imaging are demonstrated even in domain regions that are not directly accessible for measurement.     

基于弯折频率变换的Lamb波高分辨率损伤成像

Lamb波在结构健康监测中受到广泛关注,但其在传播过程中存在着多模和频散特性,不利于损伤定位和高分辨率成像。弯折频率变换（Warped Frequency Transform, WFT）通过构建合理的弯折映射可实现对频率轴的弯折。基于Lamb波群速度频散曲线设计弯折频率变换,则可用于Lamb波信号的处理。本文从直接补偿角度出发,利用WFT对传感信号进行频散抑制。提出了基于WFT的高分辨率损伤成像方法,利用有限元软件ABAQUS进行了带损伤铝板中Lamb波传播的仿真。仿真结果表明WFT能有效压缩频散的波包,通过本文所提成像方法可实现高分辨率损伤成像。     

Ultrasonic guided-waves characterization with warped frequency transforms

Guided wave (GW) dispersion curves can be extracted from a time-transient measurement by means of timefrequency representations (TFRs). Unfortunately, any TFR is subject to the time-frequency uncertainty principle. This, in general, limits the capability of TFRs to characterize closely spaced guided modes over a wide frequency range. To overcome this limitation, we implemented a new warped frequency transform that presents enhanced mode extraction capabilities because of a more flexible tiling of the time-frequency domain. The tiling is designed to match the dispersive spectro-temporal structure of a GW by selecting an appropriate map of the time-frequency plane. The proposed transformation is fast, invertible, and covariant to group delay shifts. An application to Lamb waves propagating in an aluminum plate is presented. Time-transient GWs propagation events obtained both numerically and experimentally are considered. The results show that the proposed warped frequency transform limits the interference patterns which appear with other TFRs and produces a sparse representation of the Lamb wave pattern that can be suitable for identification and characterization purposes.     

Self-reconstruction of wave packets due to spatio-temporal couplings

Abstract

The spatio-temporal self-reconstruction of the wave function fulfilling the wave equation is examined. The effect is based on the sampling of the spatial spectrum of the wave function in the cylindrical coordinates and on its coupling to the temporal frequency spectrum. The predetermined spatio-temporal profile of the wave function is approximately reconstructed at the periodic spatial intervals along the propagation direction for the continuous temporal spectrum. The periodicity appearing in both the spatial and temporal evolutions are examined for wavefields with the discrete temporal spectrum. The reconstructed field then simultaneously exhibits propagation properties known as the self-imaging and the mode-locking.     

基于振动模态分析的钢轨中超声导波传播特性数值计算方法

针对钢轨等复杂异型波导结构中超声导波频散特性难于求解的问题,本文通过振动模态分析提取出钢轨中超声导波传播特性。基于振动特征频率和波动解互相转化的原理,将振动模态分析获得的特征频率解,转换成波动解,进而提取出钢轨的频散特性。同时,还可以由结构振动分析中获得的变形信息,计算出超声导波的波结构。通过对计算出的钢轨中低阶超声导波模态频散特性和波结构分析,选择出适合钢轨无损检测的超声导波模态类型和频率范围,为后续专用超声导波传感器设计和检测实验提供理论支持。     

基于导波的焊接方管损伤检测的仿真与实验分析

基于导波的损伤检测方法是结构健康监测领域的研究热点,并在工程应用上表现出很大的潜力。通过有限元仿真和实验分析的方法对某型列车底盘转向架局部焊接方管结构上的损伤检测问题进行研究。仿真中建立结构的三维模型以模拟导波在其中的传播过程,实验中采用主动式换能器网络激发和接受在结构中传播的导波。优选激励频率以减少导波固有的多模式现象对后续信号分析的影响。借助小波变换和希尔伯特变换等方法对所有采集到的信号进行处理,并利用“导波三角定位法”进行损伤定位。实验结果验证了该方法的有效性。     

Structural health monitoring of cylindrical structures using guided ultrasonic waves

The research field of structural health monitoring (SHM) in the realm of civil engineering has emerged rapidly. SHM concepts are based on integrated sensors and actuators to evaluate the structural state. Beside common structural response methods and other nondestructive testing techniques, wave-based ultrasonic techniques are widely used especially because of their flexibility. Monitoring cable structures such as overhead transmission lines or stay cables in suspension bridges is one objective of those wave-based methods. These structures are subject to aging, corrosion and other static and dynamic loads (e.g., wind, temperature). The cylindrical structures act as waveguides whereby monitoring of large distances with a single ultrasonic transducer is possible. However, the wave propagation is multimodal and dispersive, which complicates analysis of the wave motion and development of monitoring applications. This work addresses several aspects of the propagation of guided waves in cylinders, especially the analysis of reflection and transmission at discontinuities using finite element and boundary element methods.     

A curvelet transform approach for the fusion of MR and CT images

There are several medical imaging techniques such as the magnetic resonance (MR) and the computed tomography (CT) techniques. Both techniques give sophisticated characteristics of the region to be imaged. This paper proposes a curvelet based approach for fusing MR and CT images to obtain images with as much detail as possible, for the sake of medical diagnosis. This approach is based on the application of the additive wavelet transform (AWT) on both images and the segmentation of their detail planes into small overlapping tiles. The ridgelet transform is then applied on each of these tiles, and the fusion process is performed on the ridgelet transforms of the tiles. Simulation results show the superiority of the proposed curvelet fusion approach to the traditional fusion techniques like the multiresolution discrete wavelet transform (DWT) technique and the principal component analysis (PCA) technique. The fusion of MR and CT images in the presence of noise is also studied and the results reveal that unlike the DWT fusion technique, the proposed curvelet fusion approach doesn't require denoising.     

Three-dimensional analytical solution for transient guided wave propagation in liquid-filled pipe systems

The objective of this study is to investigate the three-dimensional (3-D) analytical solution for transient guided wave propagation in liquid-filled pipe systems using the eigenfunction expansion method (EEM). The eigenfunctions corresponding to finite liquid-filled pipe systems with a traction-free lateral boundary and rigid smooth end boundaries are obtained. Additionally, the orthogonality of the eigenfunctions is proved in detail. Subsequently, the exact 3-D analytical transient response of finite liquid-filled pipe systems to external body forces is constructed using the EEM, based on which, the approximate 3-D analytical transient response of the systems to external surface forces is derived. Furthermore, the analytical solution for transient guided wave propagation in finite liquid-filled pipe systems is extended explicitly and concisely to infinite liquid-filled pipe systems. Several numerical examples are given to illustrate the analysis of the spatial and frequency distributions of the radial and axial displacement amplitudes of various guided wave modes; the numerical examples also simulate the transient displacement of the pipe wall and the transient pressure of the internal liquid from the present solution. The present solution can provide some theoretical guidelines for the guided wave nondestructive evaluation of liquid-filled pipes and the guided wave technique for downhole data transfer.     

Models of ultrasonic wave propagation in epoxy materials

This paper is concerned with modeling ultrasonic wave propagation in epoxy materials to better understand NDE procedures and to provide reliable input to more complex models of guided wave propagation in layered structures. Different physical models are considered in the context of how well they simulate the (known) linear relationship between bulk wave attenuation coefficients and frequency. The identified models are then extended to simulate wave propagation in materials with mechanical properties, which vary gradually in the spatial dimension. This is achieved using electric circuit transmission line analogs to the viscoelastic mechanical system. Verifying experimental results are included.     

Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface

A movie of the dispersive and dissipative propagation of surface plasmon polariton (SPP) wave packets at a silver/vacuum interface is recorded by the interferometric time-resolved photoemission electron microscopy with 60 nm spatial resolution and 330 as frame interval. The evolution of SPP wave packets is imaged through a two-path interference created by a pair of 10 fs phase correlated pump-probe light pulses at 400 nm. The wave packet evolution is simulated using the complex dielectric function of silver.     

铝薄板裂纹电磁超声导波B扫描检测实验研究

采用Disperse软件,计算了3 mm厚度铝合金薄板的水平剪切(Shear horizontal, SH)导波频散曲线,设计制作了中心频率为0.22 MHz的SH导波电磁超声换能器(Electromagnetic acoustic transducer, EMAT),对铝薄板试样中直裂纹进行了B扫成像检测实验分析,采用同步挤压小波变换(Synchrosqueezedwavelettransform,SWT)对来自铝薄板试样中直裂纹SH导波信号进行了去噪处理。实验结果表明,SWT可以去除原始导波信号中的噪声,并实现导波模态分离,有助于改善B扫图像的质量。所设计的SH导波EMAT能够有效地检出3 mm厚铝薄板试样中10 mm长、1 mm宽、2 mm深的人工直裂纹。     

基于空间拓展法的管道高阶螺旋导波波包追踪方法研究

螺旋导波是管道短距离高精度层析成像技术采用的主要导波模态,可以有效地提高成像分辨率,弥补目前管道导波长距离大范围检测精度低的缺点,在管道腐蚀检测中具有重要意义。然而由于导波的频散及螺旋导波的多路径传播特性,换能器接收到的检测信号波包众多,且经常发生重叠。为了便于分析这些波包的来源,需要有效的方法来计算各阶螺旋传播路径的长度及波包到达时间,进行波包追踪。推导了基于空间拓展法的管道高阶螺旋导波传播路径的计算模型,并针对各向均匀辐射的柱面波前兰姆波形成的管道螺旋导波,进行了数值计算和实验验证,证明了模型的正确性及其在波包追踪中的有效性。对利用管道螺旋导波进行检测及层析成像具有理论指导作用。     

悬索桥缆索钢丝损伤超声导波检测数值模拟

为了实现对大跨悬索桥缆索钢丝损伤的有效检测,采用理论分析和数值模拟相结合的方法,对超声导波无损检测技术进行研究。通过理论求解钢丝中导波的频散曲线,分析频散特性和波结构,选取中心频率为200 kHz的L(0,1)模态进行钢丝断丝损伤检测;利用有限元软件,研究了钢丝中导波的频散特性和波结构,通过二维傅里叶变换技术对钢丝中的低阶导波模态进行识别,进一步分析了缺陷尺寸和角度对L(0,1)模态缺陷反射系数的影响;最后,对L(0,1)模态在两根钢丝和七根钢丝的断丝处的缺陷回波进行了数值模拟。数值模拟与理论分析结果相吻合,说明低频L(0,1)模态可以有效地对缆索钢丝断丝损伤进行远距离检测。     

The Potential of Guided Waves for Monitoring Large Areas of Metallic Aircraft Fuselage Structure

The potential for long-range propagation of ultrasonic guided waves through metallic aircraft fuselage structure has been investigated using dispersion analysis and numerical modelling, validated by experiment. In order to satisfy the pressing need for integrated structural health monitoring of ageing metallic aircraft, it is likely that an active guided wave system based on current technology must feature efficient propagation over distances of at least 1m with an attenuation of not more than about 40dB/m. Propagation was examined across free skin, tapering skin, skin loaded with sealant and paint, double skin jointed with either sealant or adhesive, and lap and stringer joints, which together adequately characterise metallic monocoque fuselage construction. Whilst the simple and tapering skins allow long range propagation of non-dispersive modes with little reflection at the transition to tapering skin, the attenuation caused by application of a sealant layer generally leaves no viable modes. Guided wave propagation through double skin features the inevitable generation of twin modes with similar phase velocity, which interact with each other during propagation. This interaction crucially determines the efficiency of propagation across narrow joints and effectively precludes propagation across a succession of joints. This work leads to the conclusion that an active aircraft system that relies on guided wave propagation of more than 1m is not feasible, whereas localised guided wave monitoring of structurally significant areas is a more practical approach.     

广义兰姆波的积累二次谐波发生研究

借助色散导波的部分波分析方法、二阳微扰理论及同的声非线性反射处理技术,本文对声波导中广义兰姆波的积累二次谐波发生进行了理论研究。结果表明,在一定条件下,源于声波导介质的体非线性、由文义兰姆波的部分波非线性相互作用所发生的二次谐波具有随传播距离积累增长的性质,文中给出了广义兰姆波的积累二次谐波发生所要满足的条件,并求解出相应的积累二次谐波之解析式。     

火炮身管超声导波频散特性及传感器配置

超声导波检测技术具有检测距离远、效率高的优点,适用于检测火炮身管等圆管类结构,具有突出的军事应用价值和前景。首先对圆管超声导波理论进行推导,并对身管损伤的几种常见类型进行分析研究。采用数值计算的方法绘制频散曲线,通过分析其频散特性对导波检测频率进行优选,初步得到了最佳的检测频率范围;其次着重对传感器种类选择、数量及分布方式与导波传播特性的关系进行研究。实验表明,导波检测的最佳频率范围为0～300 kHz,且导波弯曲模态一般不宜作为检测模态;传感器数量的增加不仅增强了激励信号的强度,还有效抑制了身管中导波的频散。     

Simulation of acoustic wave propagation in dispersive media with relaxation losses by using FDTD method with PML absorbing boundary condition

We present a method to incorporate the relaxation dominated attenuation into the finite-difference time-domain (FDTD) simulation of acoustic wave propagation in complex media. A dispersive perfectly matched layer (DPML) boundary condition, which is suitable for boundary matching to such a dispersive media whole space, is also proposed to truncate the FDTD simulation domain. The numerical simulation of a Ricker wavelet propagating in a dispersive medium, described by second-order Debye model, shows that the Ricker wavelet is attenuated in amplitude and expanded in time in its course of propagation, as required by Kramers-Kronig relations. The numerical results also are compared to exact solution showing that the dispersive FDTD method is accurate and that the DPML boundary condition effectively dampens reflective waves. The method presented here is applicable to the simulation of ultrasonic instrumentation for medical imaging and other nondestructive testing problems with frequency dependent, attenuating media.     

基于曲波域与核主成分分析的人脸识别

针对小波变换不能充分描述人脸曲线特征的缺点,本文提出一种基于曲波域与核主成分分析(KPCA)的人脸识别算法.采用多尺度、多方向的曲波(Curvelet)变换提取图像特征,不仅具有更高的逼近精度和更好的稀疏表达能力,而且其变换系数能有效表示沿曲线的奇异性.进一步使用核主成分分析(KPCA)将曲波特征系数投影到更具表达力的...