基于时间反转加权分布的复合材料Lamb波损伤成像

超声Lamb波是检测板状结构损伤的常用方法,然而碳纤维增强聚合物基复合材料（Carbon Fiber ReinforcedPlastics,CFRP）本身的各向异性会对Lamb波的损伤成像和定位造成很大的影响。且大多数检测方法均采用健康结构的检测信号作为参考信号,用差信号的方法来实现损伤成像,该过程容易受到待测结构和实验环境变化等外界因素的影响。针对该问题,采用时间反转和加权分布成像相结合的方法,将其应用在复合材料板状结构的Lamb波损伤检测和成像中。仿真结果表明,该方法能够有效地实现板中单源脱层损伤和多源脱层损伤的二维成像与定位,且具有较高的精度和准确性。     

C/C热防护结构弹性波仿真分析方法及损伤对弹性波的影响

热防护结构（Thermal protection structures,TPS）作为可重复使用航天飞行器的关键结构,其安全性和可维护性至关重要。已有的针对可重复使用航天飞行器TPS的结构健康监测的研究不多,特别是C/C、C/SiC等复合材料制造的TPS。针对可重复使用航天飞行器C/C TPS,研究其基于主动弹性波的结构健康监测方法。提出了一种TPS弹性波数值仿真方法,在验证方法有效性的基础上,开展了面向C/C TPS损伤监测的弹性波传播特性研究,从信号时域特征、弹性波传播波场、损伤散射信号、损伤散射信号波场及损伤因子5个方面全面分析了弹性波在C/C TPS中的传播特性及损伤带来的影响。结果表明,当分层损伤边长大于25 mm后,损伤因子剧烈变化,因此采用基于弹性波的结构健康监测方法可以很好地监测C/C TPS内部分层损伤及损伤的扩展情况。      

基于超声导波的复合材料结构疲劳损伤监测实验研究

为快速评估复合材料结构的疲劳损伤状况,采用超声导波和时-频分析相结合的方法,对疲劳状态下的复合材料结构进行在线连续监测。对玻璃纤维增强复合材料试件进行拉伸疲劳试验,使用激光引伸计获取试件纵向刚度的变化,探究试件内部疲劳损伤的累积情况。利用小波变换对由压电传感元件激励和接收的超声导波信号在时频域进行分析,提取与疲劳损伤有关的信号特征;最后,通过多元偏值分析引入马氏平方距离,融合多个信号特征,确定复合材料试件中疲劳损伤的存在性以及表征其演变过程。实验结果表明所提出方法在复合材料结构疲劳损伤连续监测方面的有效性。     

基于Bayesian理论的无参考信号主动Lamb波损伤定位方法

目前主动Lamb波损伤检测方法大都基于健康状态下的参考信号以获取损伤散射信号,并采用确定性的方法进行损伤定位,在适用性方面受到较大影响。基于Lamb波的时间反转聚焦原理,考虑损伤定位过程中的不确定性因素,发展了一种基于Bayesian理论的无参考信号主动Lamb波损伤定位方法。根据时反聚焦信号中主、旁瓣波包峰值的相对时刻,提取各测量通道损伤散射信号传播时间与直达波传播的时间之差作为样本,并基于Bayesian理论,获得损伤位置、波速等未知参数的联合后验概率分布,再利用马尔科夫链蒙特卡洛(MCMC)方法在后验分布中对未知参数进行采样估计,得到马尔科夫链的极限分布即未知参数的后验分布。通过对一铝质矩形薄板的数值仿真和模型实验研究结果表明,该方法能够较为准确地识别出损伤位置、波传播速度及其不确定性。     

温变工况下超定独立成分分析导波监测方法

基于超声导波的结构健康监测技术在实际工程应用中受到变化的环境工况条件的影响,由于独立成分分析方法对于处理时变工况条件下的大量监测导波信号存在局限性,以及对不同程度的损伤表征研究存在不足,提出一种基于超定独立成分分析的导波监测方法并改进了基于k均值聚类的损伤指标。以广泛存在的环境温度作为环境变量,通过主成分分析从大量导波信号组成的观测矩阵中确定独立分量个数,使用独立成分分析将处理后的导波信号分解为独立分量,能够有效地将损伤与环境工况的影响分离到不同的独立分量中。对长期经受环境温度变化的铝板进行了导波监测实验,结果表明该方法能够有效减少独立分量数目并从大量导波信号中排除环境温度的干扰识别出损伤,并且对处于温变条件下的完整和不同损伤程度的铝板进行了损伤识别实验,进一步研究了该方法在排除环境温度干扰的同时表征损伤程度的效果。     

基于损伤存在概率成像方法的复合材料结构损伤识别

基于小波分析理论和概率统计原理,提出一种损伤存在概率成像方法,对复合材料结构进行在线的健康监测。首先对结构损伤前后Lamb信号进行比较,提取信号的能量特征差异系数,作为损伤指标;然后用概率统计方法判断该损伤指标是由损伤引起的还是因环境变化造成的;最后用成像算法给出存在概率图像识别损伤。对复合材料板结构进行实验验证该方法的可行性,具有一定的实际工程应用价值。     

基于Shannon 复数小波的复合材料结构时间反转聚焦多损伤成像方法

研究了一种基于压电传感器阵列和主动Lamb 波的结构损伤成像方法,有助于克服Lamb 波在板结构中、特别是在复合材料板结构中存在的频散、多种模式及模式转换的现象给结构健康监测带来的困难。分析了结构多损伤散射信号的时间反转聚焦原理,在此基础上提出了一种基于Shannon 复数小波和时间反转聚焦的信号合成成像方法。该方法中,确定Lamb 波响应信号的到达时刻是信号能够准确聚焦的关键因素之一。提出了利用Shannon 复数小波变换计算Lamb 波响应信号到达时刻的方法。在碳纤维复合材料板结构上对整套信号合成成像方法进行了验证。研究结果表明,该方法能够有效地对同一个监测区域中的多个损伤进行成像定位。相对于30 cm ×30 cm 的监测区域,定位误差不超过2 cm。该方法有助于结构健康监测技术的工程应用。      

基于超声散射回波的组织损伤程度监测方法

高强度聚焦超声(HIFU)治疗肿瘤过程中,根据组织损伤程度调整HIFU治疗剂量非常重要.本文从信号处理的角度出发,提出基于超声散射回波能量和声衰减系数并结合BP神经网络监测生物组织损伤程度的方法.将高强度聚焦超声打击新鲜猪肉组织前后获得的散射回波通过A/D转换输入到计算机,对接收到的组织散射回波信号进行预处理,再从中提取出超声散射回波能量和声衰减系数参量信息,比较不同参量对损伤程度的辨识效果.实验结果表明:综合超声散射回波能量和声衰减系数特征并输入BP神经网络,相对于仅使用某种特征参数而言能更好地监测HIFU治疗中生物组织的损伤程度,尤其在组织损伤严重时优势更加明显.     

基于小波变换方法的高层建筑模态参数辨识

针对高层建筑抗震、抗风、健康监测及损伤诊断等研究中结构模态参数辨识问题,利用随机减量技术从环境激励下的高层建筑振动响应数据中提取自由衰减信号,根据自由衰减信号小波变换结果可以有效地辨识高层建筑固有频率和粘性阻尼系数。通过仿真结果与理论值比较,证实了该方法的有效性和精确性。该方法成功地运用于大连市某高层楼房的固有频率和粘性阻尼系数的辨识,为该高层楼房健康监测提供了重要的参考依据。     

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。     

Research on the Signal Reconstruction of the Phased Array Structural Health Monitoring Based Using the Basis Pursuit Algorithm

The signal processing problem has become increasingly complex and demand high acquisition system, this paper proposes a new method to reconstruct the structure phased array structural health monitoring signal. The method is derived from the compressive sensing theory and the signal is reconstructed by using the basis pursuit algorithm to process the ultrasonic phased array signals. According to the principles of the compressive sensing and signal processing method, non-sparse ultrasonic signals are converted to sparse signals by using sparse transform. The sparse coefficients are obtained by sparse decomposition of the original signal, and then the observation matrix is constructed according to the corresponding sparse coefficients. Finally, the original signal is reconstructed by using basis pursuit algorithm, and error analysis is carried on. Experimental research analysis shows that the signal reconstruction method can reduce the signal complexity and required the space efficiently.     

基于多重信号分类算法的复合材料冲击定位

为了提高复合材料结构冲击定位的精度和实时性, 将阵列信号处理技术引入到结构健康监测领域, 提出了利用小波变换和多重信号分类算法实现复合材料结构冲击定位的新方法: 通过小波变换提取冲击响应信号某一窄带频率成分, 运用多重信号分类(MUSIC) 算法实现冲击源到达方向的估计; 根据Lamb 波传播特性, 用小波变换求出某一中心频率下的对称模式和反对称模式的Lamb 波到达同一传感器的时间差, 结合对称模式和反对称模式Lamb 的速度差就可以估计出冲击源到达传感器的距离, 实现冲击定位。对玻璃纤维/环氧树脂复合材料层合板和碳纤维/双马树脂基复合材料层合板2 种试件的实验均表明该方法能快速、精确地识别出冲击源位置。      

Temperature Independent Damage Detection in Plates Using Redundant Signal Measurements

This paper describes a method to detect notch like damages in plates using piezoelectric transducers. The method does not use prior baseline data for damage detection. A single pair of piezoelectric wafer transducers made of Lead Zirconate Titanate (PZT) is attached back to back on the opposite sides of a plate and are used for simultaneous actuation and sensing. A notch, which is a sudden change in thickness of the plate, leads to mode conversion of Lamb waves. The mode converted wave component in the measured signal is then separated from the other Lamb wave mode components using polarization characteristics of the piezoelectric wafer transducers. The damage index is a function of the amplitude of this mode converted component of the signal. In real world situations, the damage index will not be exactly zero due to inaccuracy in transducer collocation and non-uniformity in their bonding conditions. Therefore, a (non-zero) threshold for the damage index needs to be established to avoid false alarms. True to the spirit of baseline-free damage detection, this threshold is computed from the signals acquired only from the current state of the structure. This is achieved by using redundancy in signal measurements. Since the method detects damages without having to rely on baseline data, environmental variations like temperature change do not affect its performance. Results from numerical simulations as well as experiments on aluminum specimens are provided to demonstrate the effectiveness of the method described above.     

Single Mode Tuning Effects on Lamb Wave Time Reversal with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Lamb wave time reversal method is a new and tempting baseline-free damage detection technique for structural health monitoring. With this method, certain types of damage can be detected without baseline data. However, the application of this method using piezoelectric wafer active sensors (PWAS) is complicated by the existence of at least two Lamb wave modes at any given frequency, and by the dispersion nature of the Lamb wave modes existing in thin-wall structures. The theory of PWAS-related Lamb wave time reversal has not yet been fully studied. This paper addresses this problem by developing a theoretical model for the analysis of PWAS-related Lamb wave time reversal based on the exact solutions of the Rayleigh-Lamb wave equation. The theoretical model is first used to predict the existence of single-mode Lamb waves. Then the time reversal behavior of single-mode and two-mode Lamb waves is studied numerically. The advantages of single-mode tuning in the application of time reversal damage detection are highlighted. The validity of the proposed theoretical model is verified through experimental studies. In addition, a similarity metric for judging time invariance of Lamb wave time reversal is presented. It is shown that, under certain condition, the use of PWAS-tuned single-mode Lamb waves can greatly improve the effectiveness of the time-reversal damage detection procedure.     

Design of an Orthotropic Piezoelectric Composite Material Phased Array Transducer for Damage Detection in a Concrete Structure

ABSTRACT

In order to verify the feasibility and effectiveness of the sensor used in complex concrete materials, an innovative application of ultrasonic phased array detection for a concrete structure was investigated. Comparing with the traditional piezoelectric composites, orthotropic piezoelectric composite material (OPCM) can be used as transducers in damage detection show clear advantages because of their high sensitivity and directivity along the polarization direction. A low frequency ultrasonic phased array transducer consisting of 16 OPCM elements is studied. The optimal array parameters, such as the phased array element interval, the array element width, and number of elements, are obtained by studying the total displacement changes as various parameters change at the focus point in the concrete structure. This configuration allowed the variation and control of the wave field directivity in the concrete structure during the measurements. The measurements were taken on concrete specimens using a precise time-delay device. The experimental measurements were compared to theoretical calculations to investigate the influence of different array element parameters. The results show that an OPCM phased array transducer can be used to detect damage in a concrete block.