激光超声导波分频段时域滤波及频率波数域逆时损伤成像

构建了一个激光激励和激光多普勒扫描拾振的完全非接触式超声导波损伤检测实验平台,实现对结构中超声导波场的高分辨率拾取,克服了常规接触式导波换能器致使波场检测空间分辨率低等不足。提出分频段时域滤波的方法,将激光超声宽频带信号分解为多个窄带信号以抑制频散效应,进而在无基准信号的前提下实现对损伤信号的提取;在此基础上应用频率波数域逆时损伤成像方法重构入射波场及损伤散射波场,以确定损伤边界,进而综合各窄带信号的损伤成像结果以获得更为精确的成像。仿真和实验结果表明:结合小波分析分频段时域滤波法与频率波数域逆时损伤成像方法能够实现各向同性板状结构中损伤的非接触检测,在工程实践上具有更广的应用价值。     

基于导波相控阵全域波束形成及局部时反的损伤定位和量化研究

为满足结构健康监测在线、实时性的要求,该文提出导波相控阵波束形成全域损伤定位及时反局部损伤量化方法。该方法使用一个混合检测系统,其包括一个用于产生导波的压电陶瓷换能器和用于拾取波场信息的非接触扫描式激光多普勒测振仪(SLDV)。由SLDV逐点扫描组成的相控阵阵列,基于导波相控阵波束的形成和成像算法定位薄板中的损伤位置。然后,将已定位的损伤区域作为目标区域,基于频率-波数(f-k)域时反成像方法对局部区域定量评估损伤。数值仿真和实验结果表明:所提方法能够快速定位损伤区域,并定量评估损伤大小。     

蜂窝夹层板结构中导波的传播特性及其脱粘损伤的检测

利用有限元模型研究了蜂窝夹层板结构中导波的传播特性,并进行了蜂窝夹层板结构中蒙皮与蜂窝芯脱粘损伤检测的实验研究。建立了基于实际蜂窝夹层板结构的有限元模型,利用COMSOL Multiphysics软件模拟了导波在完好结构和含有脱粘损伤结构中的传播规律。结果表明,导波在蜂窝夹层板中传播时具有频散和多模态特性,可通过频散关系确定导波的各阶模态,且A_0模态对脱粘损伤最敏感。采用压电片作为激励源,选取窄带脉冲作为激励信号激发导波,利用Polytec激光测振仪采集蜂窝夹层板中的导波信号。对信号进行小波变换,提取A_0模态的幅值,并在此基础上通过损伤概率算法定位脱粘损伤的位置。结果表明,A_0模态幅值可作为损伤检测的参数,且重构的脱粘损伤与实际的脱粘损伤位置吻合较好。     

基于多维阵列和空间滤波器的损伤无波速成像定位方法

随着复合材料在航空结构中的广泛应用,基于压电传感器（PZTs）阵列和Lamb波的结构健康监测成像方法已经成为复合材料结构健康监测技术的研究热点,但是复合材料的各向异性特点导致依赖于信号传播速度的延迟-累加、相控阵等成像方法难以实现其损伤的准确监测。鉴于此,研究了一种与信号传播速度无关的空间滤波器损伤成像定位方法,该方法利用Lamb波在结构中传播时的空间-波数域特征,通过设置空间权重函数使压电传感器阵列形成波数带宽为[k_min,k_max]的空间-波数域滤波器对特定空间方位的Lamb波进行滤波,得到损伤的角度图像; 然后,利用多维线性压电传感器阵列各自针对损伤得到的角度图像进行融合,得到损伤的坐标图像,从而实现了在不依赖传播速度的情况下对损伤的成像定位。在碳纤维层合板上对该方法进行了实验验证。实验结果表明：基于多维线性压电传感器阵列和空间-波数域滤波算法的无波速成像定位方法可以对复合材料结构损伤进行不依赖信号传播速度的成像定位,定位误差在1 cm以内。     

T型加强筋条对弹性导波在薄板中传播行为的影响

加筋薄壁板是航空航天、船舶等装备结构中经常使用的构件。采用弹性导波方法对此类结构进行损伤检测时，需要深入理解筋条对弹性波在薄壁结构中传播行为的影响。针对飞行器结构中典型的T型加筋壁板结构，采用数值模拟方法研究了超声导波与T型加强筋条的作用机制，分别从时域、频域和波数域等维度，分析了由于筋条存在引起的波传播行为的变化特性。结果表明：筋条存在会在结构中引起明显的传播波模式转换现象；随着激励频率的变化，S0与A0模式导波反射及透射行为变化明显。     

强噪声下碳纤维增强树脂复合材料结构Lamb波层析损伤成像方法

Lamb波因其检测范围广、对缺陷敏感性高等特点在复合材料无损检测中广泛应用。但强噪声环境给有效信号的提取带来难度,影响损伤位置判定精度。针对该问题,提出了一种在强噪声背景下基于计盒维数和Lamb波层析成像技术的损伤定位成像方法。首先通过仿真分析了Lamb波在碳纤维增强树脂(CFRP)复合材料板损伤前后传播的特性。在选定的复合材料板上均匀布置圆形传感器阵列,以粘结质量块改变结构局部刚度的形式模拟真实损伤;其次每个传感器依次作为激励器产生Lamb波,其他传感器采集有无损伤下的响应信号,采用小波变换进行信号去噪。将去噪后的信号添加不同等级的白噪声实现噪声干扰;最后采用计盒维数计算有无损伤的信号差异确定损伤因子,并通过概率成像算法实现损伤的定位成像。实验结果表明,在强噪声环境中单损伤与多损伤成像定位最大和平均误差分别为11.18 mm和6.88 mm,该方法无需信号降噪技术,且避免了多损伤时复杂反射信号的提取过程,在强噪声下复合材料损伤定位识别方面具有较大的潜力。     

基于弹性导波的厚钢梁结构的损伤检测

弹性导波由于其对损伤的敏感性和长距离传播特性,成为近年来结构健康监测领域的一个研究热点.探讨了利用 PZT 换能器在较厚结构中进行基于弹性导波方法的损伤识别的可能性.首先参考 Lamb 波的频散曲线,设置了导波的激励信号参数如激励频率、激励波形周期数等;基于优化的激励波形和 PZT 换能器布局,在结构健康监测实验平台上对试件进行了检测.通过对实验结果的处理分析,计算出导波的群速度,并根据群速度和飞行时间(ToF)得到了损伤的位置信息.结果表明利用导波方法能够针对较厚结构进行损伤定位并能识别出不同大小的损伤.     

基于超声导波的列车转向架结构损伤定位成像研究

转向架构架为列车重要部件,其健康状况直接关系到列车安全运行,而基于超声导波的无损检测技术能够用于结构健康监测。采用导波反演法获取构架材料的体波波速,结合Lamb波频散方程得到构架的Lamb波频散曲线,进而选择激励信号。载荷和构架边界都会使响应导波信号特征变得复杂,使得损伤定位误差增加。为减少时域信号误差的影响,采用完好结构与检测结构的激励信号与相应导波能量谱相关系数进行分析。通过此系数获取检测结构的损伤指数,经权函数对各导波传播路径加权,从而得到损伤概率密度大小,进行损伤定位诊断成像。在构架中分别沿焊接方向以及承载剪切方向引入切槽损伤进行实验测试。结果表明,该方法能够较准确地实现损伤定位。     

流体管道超声导波无损检测

研究工程中流体管道的结构损伤识别技术具有现实的意义。以实现管道损伤定位为目的,探讨超声导波检测技术在流体管道的结构损伤识别中的可行性以及其识别精度。选择材料为20#碳钢的圆管为研究对象,根据频散方程利用数值法求解其各个模态频散曲线。以管中导波的传播原理为基础,从管中导波的模式、频散特征、避开截止频率、传播速度等方面因素选择激励信号,通过Abaqus有限元仿真分析导波在流体管道中的传播机理,利用概率密度函数法实现流体管道损伤定位。最后通过在管道上引入切槽损伤和孔损伤进行实验,对定位方法进行验证,实验结果与仿真分析基本吻合,证明了该方法的工程实用价值。     

复合材料加筋壁板损伤识别的概率成像方法

由于不需要超声导波的波速和传播时间,损伤概率成像方法特别适合于复合材料结构的损伤识别。但是其损伤概率分布函数是一种不精确的分布概率,会造成损伤定位精度的降低,从而会影响其在实际工程结构中的应用。为了提高其损伤定位精度,提出了一种改进的损伤概率成像方法。该方法通过利用损伤因子与损伤距离激励-传感通道直达路径的相对距离的关系,对损伤概率成像方法的损伤概率分布函数进行了改进。通过对复合材料加筋壁板上不同位置损伤的识别,对所提方法的有效性进行了验证。实验结果表明,该方法不仅能对复合材料加筋壁板的单损伤进行准确定位,而且能对两个损伤进行有效识别。     

复合管状结构中超声导波的位移分布

对超声纵向导波在复合管状结构中的传播特性进行了分析。然后分析了系统中的位移分布,以此确定了各模式检测管材的最佳频厚积范围和检测的最佳位置。结果表明,各模式的径向和轴向位移在管内壁上的值较大,而在管壁中间和管外壁上的值较小;当频厚积增大到一特定值后,管壁中间和管外壁上的径向和轴向位移都近似为零,此特定值随模式阶次的提高而增加。选取各模式检测的频厚积时,应尽可能的选径向位移较小而轴向位移较大的频厚积点。     

Comparative Study of Laser Doppler Vibrometer and Capacitive Air‐coupled Transducer for Ultrasonic Propagation Imager and the New Development of an Efficient Ultrasonic Wavenumber Imaging Algorithm

Damage detection techniques using guided waves have been studied for decades with very few successful real‐world applications. The recent development with the full wavefield technique using the Ultrasonic Propagation Imager (UPI) is one of those few exceptions. In this paper, we study two non‐contact sensors: the laser Doppler vibrometer and the capacitive air‐coupled transducer in the context as the sensing modules for the UPI. The aim of this paper is to provide a comprehensive study for optimisation of the two sensors, as well as a comparison between them for use in the UPI. First, the parameters for laser ultrasonic measurement of each sensor were studied: surface treatment, measurement angle and stand‐off distance in the case of the laser Doppler vibrometer and measurement angle, lift‐off distance and bias voltage in the case of the capacitive air‐coupled transducer. Two optimised sensors were then compared in terms of their ability to detect damages using the UPI. Also, in this paper, we presented the ultrasonic wavenumber imaging (UWI) algorithm with the new development towards an efficient implementation. The uniqueness of the UWI algorithm with the capability of damage size estimation makes this algorithm very attractive for the future study with full wavefield signal processing.     

Delamination detection in composites through guided wave field image processing

This study explores the feasibility of using a non-contact guided wave imaging system to detect hidden delamination in multi-layer composites. The study is conducted in two phases. In the first phase, Lamb waves are excited by a lead (Pb) Zirconate Titanate transducer (PZT) mounted on the surface of a composite plate, and the out-of-plane velocity field is measured using a one-dimensional (1D) scanning laser Doppler vibrometer (LDV). From the scanned time signals, wavefield images are constructed and processed to study the interaction of Lamb waves with delamination. The paper presents additional signal and image processing techniques used to highlight the defect in the scanned area. The techniques are demonstrated using experimental data collected from a 1.8 mm thick multi-layer composite. In the second phase, a completely non-contact system is described to excite and measure guided waves. A modulated continuous wave (CW) laser source in conjunction with a photodiode is used to wirelessly excite an attached PZT and the resulting waves are again sensed using the vibrometer. The non-contact system is used to excite and measure elastic waves in a composite channel test article. The elastic wave propagation image sequences are created from the non-contact excitation system.     

50th Anniversary Article: Comparison Studies of Full Wavefield Signal Processing for Crack Detection

The signals acquired by measurements of elastic wave propagation have been used for damage detection since the 1970s. The measurements have been carried out mostly by using piezoelectric transducers of various types. Many different sensor configurations and data processing have been proposed to detect and localise structural defects, both for real‐time and off‐line testing. Nevertheless, in the last decade, significant progress in the measurement techniques such as scanning laser Doppler vibrometry and shearographic interferometry has been made. These techniques enable measurement of a full wavefield of elastic waves. This opens up new possibilities and solutions for the problems of the damage detection in structures. Many researchers successfully applied this type of measurements for the damage detection and localisation in thin‐walled structures. Moreover, advanced signal processing techniques, such as wavenumber filtering, give the possibility of damage size estimation, by filling the gap between damage detection and damage prognosis. The aim of this paper is to give a comprehensive review of methods used for the full wavefield measurement. It also describes and compares selected signal processing algorithms developed for damage detection and visualisation based on these measuring techniques. Criticism aspects, as well as advantages of each algorithm, are denoted based on the authors' expertise in the field.     

A system for magnetostrictive transduction of guided waves in fluid-filled pipes of small diameter

This paper is concerned with the design of magnetostrictive transducers for the excitation and detection of guided waves in metal pipes of small diameter (mm) and their application to the study of wave propagation in pipes filled with water or supercritical CO/sub 2/. Optimized system design is based on a simulation of the overall signal pathway which includes the electric circuit conditions at the transducers, mode excitability, and the wavenumber filtering effect of the spatial distribution of the exciting alternating magnetic field. A prototype system was built, and experimental observations on small diameter pipes indicated good agreement with expected results from simulations. The reassigned spectrogram has been used to compare expectation on the basis of guided wave dispersion curves for fluid-filled pipes with experimental data.     

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。     

A rapid signal processing technique to remove the effect of dispersion from guided wave signals

Guided acoustic and ultrasonic waves have been utilized in various manners for non-destructive evaluation and testing. If a guided wave mode is dispersive, a pulse of energy will spread out in space and time as it propagates. For a long-range guided wave inspection application, this constrains the choice of operating point to regions on the dispersion curves where dispersion effects are small. A signal processing technique is presented that enables this constraint on operating point to be relaxed. The technique makes use of a priori knowledge of the dispersion characteristics of a guided wave mode to map signals from the time to distance domains. In the mapping process, dispersed signals are compressed to their original shape. The theoretical basis of the technique is described and an efficient numerical implementation is presented. The robustness of the technique to inaccuracies in the dispersion data is also addressed. The application of the technique to experimental data is shown and the resulting improvement in spatial resolution is demonstrated. The implications of using dispersion compensation in practical systems are briefly discussed.     

基于虚拟时间反转的高分辨率复合材料板结构损伤成像

针对传统时间反转(TR)方法消除Lamb波传播时间和操作复杂的问题, 提出了虚拟时间反转(VTR)方法。该方法采用换元激励和接收机制以保留时间信息, 并运用信号运算代替物理TR操作。讨论了基于阶跃激励获取损伤监测路径传递函数的VTR实现过程。提出了基于VTR的高分辨率损伤成像方法。为了降低复合材料各向异性的影响, 在成像中进行了速度修正。实验结果表明, VTR方法能对碳纤维复合材料板中的Lamb波损伤散射波包进行部分再压缩和TR聚焦, 通过本文中成像方法也能清晰显示出板中单个和两个近邻损伤。      

Assessment of debonding in sandwich CF/EP composite beams using A0 Lamb wave at low frequency

The aim of this study is to quantitatively assess debonding in sandwich CF/EP composite structures with a honeycomb core using acoustic waves activated and captured by surface-mounted PZT elements. For experimental investigation, debonding was introduced at different locations in sandwich CF/EP composite beams. The fundamental anti-symmetric A₀ Lamb mode was excited at a low frequency. The transmitted and reflected wave signals in both surface panels were captured by PZT elements after interacting with the debonding damage and specimen boundaries. Aided by finite element analysis (FEA), the differences in wave propagation characteristics in sandwich composite beams and composite laminate (e.g. skin panel only) were investigated. The debonding location was assessed using the time-of-flight (ToF) of damage-reflected waves, and the severity of debonding was evaluated using both the magnitude of the reflected wave signal and the delay in the ToF of Lamb wave signals. Good correlation between the experimental and FEA simulation results was observed. The results demonstrate the effectiveness of Lamb waves activated and captured by surface-mounted PZT elements on either surface of sandwich composite structures in assessing debonding.