Mechanical impedance-based technique for steel structural corrosion damage detection

The electromechanical impedance (EMI) technique has gained acceptance for structural health monitoring, due to its merits of model-free, high frequency detection and fast response features. This paper presents an innovative mechanical impedance-based technique to monitor the development of corrosion damage on steel structure, which is different from the traditional admittance (inverse of impedance)-based EMI technique. Firstly, structural mechanical impedance (SMI) which directly manifests the structural properties is theoretically deprived. Secondly, by measuring the raw admittance signature, the sensitivity of SMI is investigated in the experiment of steel structural corrosion damage conducted within 117 days. Finally, the corrosion damage quantification is also discussed by calculating the root mean square deviation (RMSD) index. The results indicate that structural mechanical impedance is sensitive to corrosion damage but the detecting range is limited. The real part of SMI can be adopted for an effective indicator of steel structural corrosion damage. The proposed technique is found to be effective in steel structural corrosion damage detection.     

Non-linear feature identifications based on self-sensing impedance measurements for structural health assessment

This paper presents the application of a non-linear feature identification technique for structural damage detection. This method is coupled with the impedance-based structural health monitoring (SHM) method, which utilises electromechanical coupling properties of piezoelectric materials. The non-linear feature examined in this study is in the form of autoregressive coefficients in the frequency domain autoregressive model with exogenous (ARX) inputs, which explicitly considers non-linear system input/output relationships. The applicability of this non-linear feature for damage identification is investigated in various frequency ranges using impedance signals measured from a laboratory-test structure. The performance of the non-linear feature is also compared with those of linear features typically used in impedance methods. This paper reinforces the utility of non-linear features in SHM and suggests that their sensitivity in different frequency ranges may be leveraged for certain applications.     

载荷影响下的机电阻抗协整结构损伤识别方法

高频机电阻抗(electromechanical impedance, 简称EMI)方法利用粘贴在结构表面的压电传感器(piezoelectric transducer, 简称PZT)进行主动激励，通过连续监测和分析PZT机电导纳信号的变化评估结构的健康状态；然而EMI方法容易受到环境工况变化的影响，导致结构损伤的误报。针对此问题，采用时间序列协整方法处理及消除结构工作载荷对阻抗谱特征信号的影响。该方法是基于结构动荷载作用下PZT阻抗谱导纳信号的非平稳特征，将动荷载影响下的阻抗谱非平稳时间序列经线性组合变换成平稳时间序列，根据得到的协整余量序列有效判断结构的健康状态。为验证该方法的有效性，开展了动应力影响下铝梁结构的螺栓松动损伤识别实验。结果表明，协整消除了动态应力对EMI方法的影响，当铝梁内存在持续变化的应力时，仍可以准确识别螺栓松动。机电阻抗协整方法能够消除结构健康监测中荷载作用的影响，及时准确地进行结构损伤识别。     

Frequency response function shape-based methods for structural damage localisation

The ultilisation of structural shape signals for damage localisation has shown some promise, especially in the applications where an accurate finite element model of the structure is not available. For this purpose, traditional shape signals, like mode shapes, flexibility matrices, uniform load surface (ULS) and operational deflection shapes (ODS) have been widely used. Using frequency response function (FRF) shapes for structural damage localisation is however, a relatively new but promising technique. Unlike mode shapes, ULS and ODS, FRF shapes are defined on broadband data and so have potential to reveal damage location more clearly. Another advantage of using FRF shapes is that the test data can be directly used without the necessity of conducting modal identification. Nevertheless, some problems associated with this approach still remain to be solved. No solid foundation or deduction about the use of FRF shapes for damage localisation has been given in any literature so far. In addition, it has been observed that this method only works for a low-frequency range. This limitation of FRF shapes has not been explained or well treated so far. In this study, a scheme of using FRF shapes for structural damage localisation is proposed. Methods within this scheme include some important modifications like using the imaginary parts of FRF shapes and normalising FRF shapes before comparison. The theoretical explanation of using FRF shapes for damage localisation is presented and the limitations of the previous FRF shape methods have been overcome. The proposed methods have shown great potential in structural damage localisation.     

回传射线矩阵法含裂纹框架的电阻抗模型

基于回传射线矩阵法(RMM)提出了修正的电阻抗理论模型,使得电阻抗信号能与含裂纹结构的各物理参数定量联系起来.分别用Timoshenko梁理论和经典杆理论描述门式框架各结构元件的弯曲振动和轴向振动,结构裂纹模拟为具有一定刚度的转动弹簧.对于粘贴有压电片的结构元件,把它作为压电片-粘结剂-主体结构这一耦合结构系统加以考察,最后推导出包含了结构裂纹信息的电阻抗的解析表达式.基于该模型,从压电片中提取的电阻抗信号可以有效识别门式框架中的裂纹.     

Computer-aided hybrid time reversal process for structural health monitoring

For structural health monitoring (SHM) applications, this paper proposes a new baseline-free time reversal process (TRP)-based SHM technique. The newly proposed technique is characterized by two key features: the hybrid TRP (HTRP) and the diagnostic digital imaging method. In the conventional TRP (CTRP), the forward and backward processes are all conducted by the measurement using piezoelectric actuators and sensors. However, in the proposed HTRP, the backward process is conducted by the computer-aided computational approach while the forward process is conducted by the same measurement approach as used for CTRP. As a benefit of the computer-aided backward process, some Lamb wave modes which do not carry any damage information can be removed to obtain the refined reconstructed signals which are finally used for the damage diagnosis: this can make the remaining signal processing for the damage diagnosis much efficient. In contrast to the previous CTRP-based damage diagnosis methods where the pattern comparison methods have been widely used, the digital imaging method is used in the proposed SHM technique to detect damages by using the time-of-arrivals of the wave packets generated by the damages.     

基于Lamb波的压电陶瓷/环氧树脂复合材料传感器制备及应用

风力机叶片、飞机机翼等结构均为大面积曲面板类结构,基于Lamb波的结构健康检测技术对细微损伤比较敏感,是目前最具应用前景的技术之一。传统的Lamb波传感器大都采用PZT陶瓷制成,质地脆且硬度大,不能适应于曲面结构检测。将PZT陶瓷粉末与环氧树脂复合制备了一种新型柔性0-3型压电复合材料,研究了质量比、极化电场、极化温度和极化时间等因素对压电复合材料性能参数的影响,开展了多因素正交实验来确定材料制备的最优极化工艺参数。实验研究了该压电复合材料制成的传感器对Lamb波的传感响应特性,与现有的MFC、PVDF和传统压电片等传感元件的响应特性进行了对比分析。将制备的传感器应用于翼型曲面板,利用椭圆定位方法进行损伤检测。研究结果表明,PZT陶瓷/环氧树脂压电复合材料传感器具有良好的传感响应特性,能够很好地贴合于曲面板表面,与只能部分耦合于曲面板的传统压电片相比,采集的Lamb波信号更加准确,从而为曲面板类结构健康监测提供一种新型的柔性压电传感技术。     

基于压电阻抗频率变化的螺栓松动检测技术

针对螺栓松动问题,提出基于压电阻抗频率变化的螺栓松动监测方法。在螺栓头部安装压电材料,采用材料试验机拉伸螺栓来精确模拟螺栓所受预紧力;研究了洁净环境和油污环境中,压电材料导纳谱中的峰值频率随螺栓预紧力的变化。结果表明：随着螺栓预紧力的增大,压电导纳谱中的峰值频率降低,且两者间具有较好的线性关系;油污对测试影响较小。     

Modeling of EMI response of damaged Mindlin–Herrmann rod

To ensure high sensitivity to incipient damages, electromechanical impedance (EMI) is measured at high frequencies for damage detection. In the working high-frequency range, very high vibration modes, of the order of the thousandth mode or higher, of a structural member are likely to be activated. This imposes a great difficulty on the accurate modeling of EMI response of a structure. In this paper, the reverberation matrix method (RMM) is adopted to study the dynamics of a Mindlin–Herrmann rod with surface piezoelectric patches. The rod is inhomogeneous along the axial direction so that damage-induced reduction of cross-section or Young's modulus could be easily incorporated in the model. A piecewise-homogeneous rod model is subsequently introduced to approximate the inhomogeneous rod, along with a shear lag model of interfacial bonding between the lead zirconate titanate (PZT) patches and the host rod. An analytical expression for the electric impedance (or admittance) of the coupled model of PZT patch-bonding layer-host rod system is derived. Comparison with other established results is presented. Parametric investigations are also performed to show the dynamic properties of the coupled smart structural system. The analysis in this paper provides necessary theoretical basis for damage detection of rod via the EMI signatures.     

激励电压对压电阻抗法检测灵敏度的影响

研究了锆钛酸铅压电陶瓷(lead zirconate titanate piezoelectric ceramic,简称PZT)传感器的激励电压与压电阻抗法(electro-mechanical impedance,简称EMI)结构健康监测技术灵敏度之间的关系.借助WK 6500B精密阻抗分析仪及自行搭建的高激励电压电阻抗测试系统(high excitation voltage electrical impedance measurement system,简称HEVEIMS),在0.01～20 V之间8种不同激励电压下,对尺寸为1250 mm×100 mm×3 mm铝梁结构上距离PZT传感器1 000 mm的直径为1.2,2.5和3.5 mm的通孔损伤进行了检测.研究结果表明,激励电压与检测灵敏度之间的关系是非单调的.在1～10 V范围内,提高激励电压能够显著增大EMI方法的检测灵敏度.高激励电压下PZT电阻抗信号中非线性成分增多是导致EMI方法检测灵敏度升高的主要原因.研究结果不但为EMI检测中激励电压的选取提供了参考,也为明确激励电压与检测灵敏度之间的相关关系提供了数据支持.     

基于阻抗技术的压电元件自损伤检测-

为了保证结构健康监测系统的有效运行,基于压电阻抗技术原理,提出了一种压电传感、驱动元件自损伤检测方法.通过研究压电元件等效电容值(压电元件导纳虚部数值)的变化,判别压电元件是否破损及其与主体结构之间是否剥离,并在悬臂梁结构上进行压电元件自损伤检测试验.结果表明,相比压电元件完好的情况,各频率下压电元件电纳值随脱粘部分面积的增加而上升,随破裂部分面积的增加而下降,并且损伤程度和等效电容变化量成比例关系,与理论分析结果吻合较好.     

Damage detection by the distribution of predicted constraint forces

Damage causes the deterioration of dynamic and static performance of intact structures. Regarding the measured static displacement or modal displacement data as constraints for describing damaged responses, this study derives analytical equations to estimate constraint forces in the satisfaction of constraints. The constraint forces are forces required for describing the flexural shape of the damaged beam under static and dynamic loadings. Based on the concept that the occurrence of damage causes the change of force mechanism, this work proposes an analytical method to detect damage from the distribution of constraint forces. When compared to the displacement curvature and the frequency response function (FRF) curvature methods using 2% noise, the results have shown that the proposed method is less sensitive to noise and is more effective in detecting multiple damaged areas in the beam of short span length and their intensity at low levels of damage.     

Vibration based damage detection using large array sensors and spatial filters

In this paper, a novel approach for vibration based damage detection is proposed. The approach relies on the use of a large network of sensors (possibly hundreds of them) to which a programmable linear combiner is attached. The linear combiner is programmed to work as a modal filter. The frequency content of the output of the modal filter is proposed as feature for damage detection. It is shown that if a local damage is present, spurious peaks appear in the FRF of the modal filter whereas if temperature changes are considered, the FRF of the modal filter is shifted but its shape remains unchanged. The approach is interesting because of the ability to differentiate between local damage and global environmental changes to a structure. Issues about the practical implementation of the method are discussed.     

Locating damage in waveguides from the phase of point frequency response measurements

A method for detecting damage in uniform waveguide structures from two or more point frequency response functions (FRFs) is described. Attention is focussed on bending waves in beams although the method can in principle be applied to any waveguide structure. The input FRF is the superposition of directly injected waves and waves reflected from the damage and from other scattering regions in the structure. The phase of this FRF modulates with wavenumber, with the period of modulation in wavenumber space being related to the distances between the excitation point and scattering locations. The phase spectrum of the input FRF is found: the phase is determined, dispersive effects are removed by transforming from the frequency domain to the wavenumber domain, and the inverse Fourier transform from the wavenumber to the space domain found. Peaks in this phase spectrum indicate the distance to the scatterer. Two (or more) input FRFs can be used to determine the location of the scatterer unambiguously. Signal processing issues are discussed. Numerical results for a uniform beam with a breathing crack are given and experimental results for beams with a slot cut into them are presented. The approach lies in the middle ground between low frequency, modal methods and high frequency, ultrasonic methods. It allows one to interrogate a region of a structure rather than the complete structure and there is no requirement for a validated model of the structure in order to locate damage, apart from an estimate of the dispersion relation.     

A new damping modelling approach and its application in thin wall machining

In this paper, a new approach to modelling the damping parameters and its application in thin wall machining is presented. The approach to predicting the damping parameters proposed in this paper eliminates the need for experiments otherwise used to acquire these parameters. The damping model proposed was compared with available damping models and experimental results. A finite element analysis and Fourier transform approach has been used to obtain frequency response function (FRF) needed for stability lobes prediction. Several predicted stable regions using both experimental and numerical FRF’s for various examples gave a good comparison.     

频响函数在结构损伤检测中应用的试验研究

结构动力测试中,频响函数可直接测得,不仅避免了在模态提取时的误差,而且在相同频段上可提供更多结构损伤信息,因此可方便地应用于结构损伤的检测中。对于结构基础的随机激励,试验结果表明频响函数法可有效地检测结构的损伤。同时说明传感器布置在结构的不同位置都可检测结构的损伤。并且随机振动的大小不影响结构损伤检测的效果。为频响函数法在现场实测结构的损伤提供了依据。     

一种新的频响函数无偏估计方法

在对几种典型的频响函数估计进行分析的基础上，利用不相关信号的互功率谱多次平均趋于零的性质，提出了一种新的频响函数无偏估计方法。该估计方法不需要测试第3个信号即可达到无偏估计的目的，避免了测试系统过于复杂，减小了测试工作量和因过多环节的引入而降低估计精度的可能性。测试n组数据，可以进行n(n—1)／2次平均，平均次数是其他方法的(n—1)／2倍，提高了频响函数估计的精度。     

Self-powered structural health monitoring with nonlinear energy harvesting system

The present paper describes the application of the fully self-powered structural health monitoring (SHM). Based on the nonlinear process of microgenerators that directly convert ambient mechanical energy into electrical energy, using the synchronized switch harvesting (SSH) method developed in our laboratory, the nonwired SHM system is equipped. The system is separated into two parts. One is an autonomous wireless transmitter (AWT), its mass is 28.9 g, and it generates a radio frequency (RF) signal and a Lamb waveform as a damage index signal. Another part is these receivers, called autonomous wireless receiver (AWR), and its weight is of 67.6 g. A preliminary design of the device using shelf electronics and surface mounted piezoelectric patches is presented. The energy balance shows that more than enough energy to operate these processes can be obtained within 10 s (when around 50 Hz and more than 2 MPa of the stress level). Some different damage index measurements of SHM are finally discussed.     

Model updating of damped structures using FRF data

Due to the important contribution of damping on structural vibration, model updating of damped structures becomes significant and remains an issue in most model updating methods developed to date. In this paper, the frequency response function(FRF) method, which is one of the most frequently referenced model updating methods, has been further developed to identify damping matrices of structural systems, as well as mass and stiffness matrices. In order to overcome the problem of complexity of measured FRF and modal data, complex updating formulations using FRF data to identify damping coefficients have been established for the cases of proportional damping and general non-proportional damping. To demonstrate the effectiveness of the proposed complex FRF updating method, numerical simulations based on the GARTEUR structure with structural damping have been presented. The updated results have shown that the complex FRF updating method can be used to derive accurate updated mass and stiffness modelling errors and system damping matrices.     

Self-powered structural health monitoring with nonlinear energy harvesting system

The present paper describes the application of the fully self-powered structural health monitoring (SHM). Based on the nonlinear process of microgenerators that directly convert ambient mechanical energy into electrical energy, using the synchronized switch harvesting (SSH) method developed in our laboratory, the nonwired SHM system is equipped. The system is separated into two parts. One is an autonomous wireless transmitter (AWT), its mass is 28.9 g, and it generates a radio frequency (RF) signal and a Lamb waveform as a damage index signal. Another part is these receivers, called autonomous wireless receiver (AWR), and its weight is of 67.6 g. A preliminary design of the device using shelf electronics and surface mounted piezoelectric patches is presented. The energy balance shows that more than enough energy to operate these processes can be obtained within 10 s (when around 50 Hz and more than 2 MPa of the stress level). Some different damage index measurements of SHM are finally discussed.