【作者】 苗晓婷；

【导师】 孟光；

【作者基本信息】 上海交通大学 ， 机械设计及理论， 2011， 博士

【摘要】 针对简单结构和复杂结构以及工作环境变化时系统地研究了基于导波的结构健康监测技术。提出了有效的特征提取技术与损伤识别方法，使其更适用于工程实践。首先综述了基于导波的结构健康监测技术的研究概况、结构中导波的各个模式与缺陷相互作用的现象、用于提取导波信号特征的信号处理技术和结构损伤的识别方法几个方面的国内外研究成果和发展趋势。在此基础上提出了本文的主要研究内容、方法、路线以及结构纲要。利用动态有限元方法，研究薄铝板中的低频兰姆波的基础阶对称（S₀）模式和基础阶反对称（A₀）模式与切缝缺陷和孔洞缺陷的相互作用现象。利用圆柱坐标系把节点位移沿缺陷所散射的波信号的传播方向和垂直于该方向进行正交分解。分别采用径向板内位移（Uh）、板外位移（Uz）和垂直于径向的板内位移（Uv）描述缺陷散射的S₀模式、A₀模式和基础阶剪切（SH₀）模式。在缺陷周围的波场内校对缺陷散射的能量在不同角度的感应节点上的分布。结果表明在某些损伤情形下缺陷散射的S₀模式、A₀模式和SH₀模式的能量并不随缺陷尺寸的单调变化而单调变化。因此不能仅从缺陷所散射的能量大小推断缺陷的大小。根据各个模式的传播群速度推断S₀模式与厚度方向通透的切缝缺陷和孔洞缺陷相互作用时，不仅生成散射的S₀模式，而且S₀模式转换后生成SH₀模式。A₀模式与厚度方向通透的切缝缺陷和孔洞缺陷相互作用时仅生成散射的A₀模式。在较小频厚积（小于1MHz·mm）的情况下，S₀模式更适合用于利用主动传感器网络进行损伤识别的研究。对兰姆波与缺陷相互作用现象的研究有助于进一步研究提取代表损伤信息的信号特征技术和相应的损伤识别方法。为实现基于导波的旋转圆柱结构的健康监测，提出了一个圆柱模型和相应的损伤识别方法。当该圆柱模型存在不同缺陷时，基于动态有限元方法定义并校对所感应的纵（L）波的反射系数和残余系数。仿真结果表明残余系数可以作为一个有效的特征参数用于损伤识别。这种损伤识别方法在实验中得以验证。利用一个圆形的薄铝板模拟该圆柱模型的径向切片。两个圆形的压电应变片（piezoelectric transducer，PZT）分别固定在铝板上下表面的圆心位置用于激励和采集导波信号。在无缺陷（基准）状态下和有缺陷（检测）状态下从导波信号中提取第一阶基本模式分量（intrinsic mode function，IMF）作为信号特征，并进行比较以校对S₀模式的残余系数。实验结果和仿真结果的一致性验证了所提出的损伤识别方法的有效性，并且为实现基于导波的旋转圆柱结构的健康监测奠定了基础。为实现在宽带噪声干扰的工作环境下的损伤识别，提出一种合理的特征提取技术。利用分谱处理（split spectrum processing，SSP）算法，通过比较基准波信号（结构无缺陷时所采集的波信号）和检测波信号（结构有缺陷时所采集的波信号）的瞬时幅值变化度（instantaneous amplitudevariation degree，IAVD）来提取代表损伤信息的信号特征。通过评估缺陷散射的S0模式的飞行时间（time-of-flight，ToF），最终实现基于导波的损伤识别。首先在理想的工作环境中（无噪声），对无缺陷的铝板进行检测。然后在不同的工作环境中（无噪声和有噪声）检测带有切缝缺陷的铝板。实验结果表明，当检测波信号的信噪比（signal-to-noise ratio，SNR）较低时，噪声能量严重干扰了检测波信号的能量分布。然而，SSP算法所提取的检测波信号的IAVD几乎不受噪声的干扰。因此利用SSP算法能够有效地抵抗宽带噪声的干扰，精确地评估缺陷散射的S0模式的ToF，并进一步结合三角定位算法成功地定位出铝板中的切缝缺陷。针对复杂结构，为了避免在分析兰姆波信号时由激励和采样的时间非同步性以及基准状态下和检测状态下激励波信号的差异所导致的损伤识别误差提出了一个基于导波能量谱的相关性分析来校对传感路径损伤指数（damage index，DI）的方法。分别依据两种方案校对DI，并把所校对的DI作为代表损伤信息的特征参数。方案I：分析一条传感路径在基准状态下与检测状态下感应波信号能量谱的相关性，并把相关系数作为所校对的该条传感路径的DI。方案II：校对一条传感路径激励波信号与感应波信号能量谱的相关系数，检测状态下相关系数相对于基准状态下相关系数的变化量被作为该条传感路径的DI。实验中分别结合依据方案I和方案II所校对的DI与损伤诊断成像算法定位带有加强筋的碳纤维复合材料板上的锥形孔缺陷。利用加权分布函数，传感器网络中各条传感路径所校对的DI值被映射到检测区域内的各个离散坐标上，构建了缺陷出现在这些离散坐标上的概率图像。结果表明，这种基于导波能量谱相关性分析的方法可以避免信号激励和采样的时间同步性影响；方案II相比于方案I可以更有效地避免由基准状态下和检测状态下激励波信号的差异所导致的识别误差，实现对复杂结构中缺陷的精确识别。为实现不参考基准状态信息的损伤识别技术，并尽量减少传感器网络中传感路径的数量以识别多缺陷，提出一个结合时间逆转的兰姆波与损伤诊断成像算法的损伤识别方法。通过分析在时间逆转过程中所获得的重建波形与原始激励的调幅脉冲之间的波形扭曲来校对基于时间逆转的DI，并把所校对的DI作为代表损伤信息的特征参数。利用动态有限元方法在无缺陷和有不同切缝缺陷（不同长度）的铝板中研究兰姆波的时间可逆性，并评估这个基于时间逆转的DI对损伤识别的有效性。实验中，在环境温度变化的情况下利用所提出的损伤识别方法诊断一个带有双切缝缺陷的铝板。实验结果表明该方法不但不需要参考基准状态信息，而且可以利用较少数量的传感路径精确地识别多缺陷。本文对简单结构和复杂结构以及在变化的工作环境中波信号的特征提取技术与针对参考基准状态信息和不参考基准状态信息的损伤识别方法的研究为基于导波的结构健康监测技术在工程实践中的广泛应用奠定了基础。更多还原

【Abstract】 For simple and complicate structures and variational workingenvironment, guided-wave-based structural health monitoring is researchedsystemically. Effective feature extraction techniques and damageidentification methods are proposed, being more adaptive to the actualengineering practice.Some research conclusions and developing trend about guided-wave-based structural health monitoring, interaction with damage of individualmode of guided waves, signal processing techniques for extracting featuresof guided wave signals and damage identification methods are summarized.The advantages and disadvantages of them are discussed and the mainresearch contents, approaches and framework of this thesis are introduced.The interaction with notch and hole in a thin aluminum plate of thefundamental symmetric （S₀） mode and the fundamental anti-symmetric （A₀） mode which are activated at low frequency is researched using dynamicfinite element method. The displacement of node is decomposed along andvertical to the direction of damage-scattered wave propagation usingcylindrical coordinate system. The radial in-plane displacement （Uh）, theout-plane displacement （Uz） and the in-plane displacement vertical to theradial （Uv） are used to describe the damage-scattered S₀mode, A₀mode andfundamental shear （SH₀） mode, respectively. Their energy distributions atthe sensor nodes with different angles are calibrated in wave field around thedamage. Results indicate that the damage-scattered energy in some damagecases does not vary monotonically with the monotonic variation in thedamage dimension, so the damage size can not be deduced only dependedon the damage-scattered energy. It is concluded that when the S₀modeinteracts with the through-thickness notch or hole, not only the S₀mode isscattered by the damage but also the SH₀mode which is mode-converted bythe S₀mode appears. When the A₀mode interacts with the through-thicknessnotch or hole, only the A₀mode is scattered by the damage. For the littlefrequency-thickness product （less than1MHz·mm）, the S₀mode is moreadaptive for damage identification using active sensor network. Research ofthe interaction with damage of Lamb waves is helpful for the furtherresearch of feature extraction technology and damage identification method. A cylindrical model is proposed and a corresponding damageidentification method is promoted to realize the guided-wave-based healthmonitoring of revolving cylindrical structure. Reflection coefficient andresidual coefficient of the captured longitudinal （L） wave in the proposedcylindrical model are defined and calibrated corresponding to differentdamage cases based on dynamic finite element analysis. Simulation resultsdemonstrate that the residual coefficient can be utilized as feature parameterto interpret different damage cases. The promoted damage identificationmethod is validated experimentally. A circular thin aluminum plate with twocentral circular piezoelectric transducers （PZTs） is used to model a radialslice of the proposed cylindrical model. The first intrinsic mode functions（IMFs）, extracted from the captured wave signals as signal features, arecompared between the benchmark （without damage） and damage case （withdamage） to calibrate the residual coefficient of the S₀mode. Experimentalresults consist with the simulation results, indicating the efficiency of thepromoted damage identification method and building foundation for thefuture research about detecting revolving cylindrical structure.Aiming on extracting effective singal feature for damage identificationin the working environment with broad band noise, a split spectrumprocessing （SSP） algorithm is proposed. The time of flight （ToF） of the damage-scattered S₀mode is estimated by comparing the instantaneousamplitude variation degree （IAVD） of the captured wave signal frombenchmark with that from a damage case, so as to realize theguided-wave-based damage identification. Wave signals captured from anintact aluminum plate （benchmark） are firstly acquired in ideal environment（without noise）. Then a notch is artificially introduced into the aluminumplate and wave signals captured from the notched aluminum plate （damagecase） are acquired in different environments （without and with noise）.Experimental results demonstrate that even though the captured wave signalfrom the damage case has a low signal-to-noise ratio （SNR）, the IAVD ofthe captured wave signal extracted using the SSP algorithm is almost notinfluenced by the noise. As result, the proposed SSP algorithm is capable ofestimating the ToF of the damage-scattered S₀mode with high precision,regardless of the broad band noise disturbance, leading to the successfulidentification of the notch in the aluminum plate using triangulationalgorithm.With the objective of avoiding damage identification error, which iscaused by time unsynchronization of activating and sampling signals and theactivated signal differences between benchmark and damage cases, amethod of calibrating damage index （DI） based on the correlation of signal energy spectrum is promoted for the complex structure. Two schemes aredeveloped to calibrate DI and the calibrated DI is utilized as featureparameter to represent damage information. Scheme I: for a sensing path,analyze the correlation of energy spectrums of captured wave signalsbetween benchmark and damage cases, and the correlation coefficient istaken as the calibrated DI. Scheme II: for a sensing path, calibrate thecorrelation coefficient of energy spectrums of the activated wave signal andcaptured wave signal, and the variation of correlation coefficient for damagecase relative to that for benchmark is taken as the calibrated DI. Inexperiment, integrate the calibrated DI using scheme I and scheme II with adamage diagnostic imaging algorithm, respectively, to estimate theprobability of damage presence, locating a cone hole in a composite platewith stiffener. With a weight distribution function, the calibrated DI valuesfor individual sensing paths are mapped to individual grids in the inspectedarea covered by a senor network, constructing a probability image for thepresence of damage. Results demonstrate that the promoted method iscapable to avoid the influence of the time synchronization and scheme II iscompetent for identifying damage in the complex strucre more precisely,avoiding the error induced by the activated signal differences betweenbenchmark and damage cases. To realize damage identification without referring benchmark andidentify multi-damage with numbered sensing pathes in a sensor network,time-reversal Lamb waves combined with a damage diagnostic imagingalgorithm is developed. The distortion between the original activated toneburst and the reconstructed waveform is analyzed to calibrate atime-reversal-based DI which is utilized as feature parameter to representdamage information. The time-reversibility of Lamb waves and theefficiency for identifying damage of the time-reversal-based DI isinvestigated and evaluated in aluminum plates without and withthrough-thickness notches （with different lengths） based on dynamic finiteelement method. An experimental evaluation is conducted for the developeddamage identification method at the different environmental temperatures.The successful locations of dual notches in an aluminum plate demonstratethat the developed method is not only benchmark-free, but also competentfor identifying multi-damage with numbered sensing pathes.In this thesis, reasearches of feature extraction techniques and damageidentification methods with and without referring benchmark for simple andcomplicate structures and variational working environment build afoundation for abroad application of guided-wave-based structural healthmonitoring in actual engineering practice.更多还原