Electro-mechanical Impedance Signatures Research Articles

Health Monitoring of Steel Structures Using Sub-frequency Electromechanical Impedance Technique

The electro-mechanical impedance (EMI) technique using piezoelectric lead zirconate titanate (PZT) transducers has been shown promising for health monitoring of steel structures. For damage assessment, statistical techniques such as root mean square deviation (RMSD) have been employed to compare the electro-mechanical (EM) admittance signatures acquired before and after damage. However, using the conventional RMSD approach, it is difficult to specify the damage severity and location. To overcome this limitation, a sub-frequency interval approach has been proposed by the authors for the concrete structures. In the present study, this approach is extended and applied to the steel structures. Considering low attenuation of wave and large sensing region of PZTs in steel structures, multiple PZTs are utilized to acquire the EMI signatures of an I-section steel beam. Artificial damage points are induced in the steel beam and the corresponding RMSD values for sub-frequency ranges (RMSD-S) are calculated. To qualitatively study the damage severity and location, a method combining the RMSD-S of multiple PZTs and the logarithmic attenuation of ultrasonic waves is proposed. The results demonstrate that the proposed method is robust and sensitive for damage assessment of steel structures.

Application of Electro-Mechanical Impedance Sensing Technique for Online Aging Monitoring of Rubber

Abstract Electro-mechanical impedance (EMI) sensing technique based on smart piezoelectric materials has emerged as a potential tool for the implementation of an online monitoring system for structural health monitoring. Many nondestructive methods have limited capabilities in monitoring of the rubber aging in a continuous manner. In this study, the feasibility of the EMI sensing technique for the online aging monitoring of rubber is investigated. The finite element method (FEM) is employed to analyze the impedance of piezoelectric patch which is bonded on the aging rubber. The results show that the EMI signatures not only can detect the aging of rubber but also can evaluate the aging extent, and the EMI signatures are more sensitive to the early aging rubber with increasing excites frequency.

Finite Element Model for Damage Detection in Three-Dimensional Cube Structures

Based on three-dimensional finite element method (FEM), an accurate electro-mechanical impedance (EMI) model for a damaged cube structure is established in the paper. The damages are simulated by the reduction in Young’s modulus in the certain area of the cube structure. A coupled structural system consisting of PZT patch, bond layer and host structure is taken into account. Both the effects of the damage severity and damage propagation on EMI signatures are then investigated. The numerical computation indicates that the present EMI model can be employed to detect the damages in the structures.

Hybrid damage monitoring of steel plate-girder bridge under train-induced excitation by parallel acceleration-impedance approach

A hybrid damage monitoring scheme using parallel acceleration-impedance approaches is proposed to detect girder damage and support damage in steel plate-girder bridges which are under ambient train-induced excitations. The hybrid scheme consists of three phases: global and local damage monitoring in parallel manner, damage occurrence alarming and local damage identification, and detailed damage estimation. In the first phase, damage occurrence in a structure is globally monitored by changes in vibration features and, at the same moment, damage occurrence in local critical members is monitored by changes in impedance features. In the second phase, the occurrence of damage is alarmed and the type of damage is locally identified by recognizing patterns of vibration and impedance features. In the final phase, the location and severity of the locally identified damage are estimated by using modal strain energy-based damage index methods. The feasibility of the proposed scheme is evaluated on a steel plate-girder bridge model which was experimentally tested under model train-induced excitations. Acceleration responses and electro-mechanical impedance signatures were measured for several damage scenarios of girder damage and support damage.

Finite Element Model for Cracked Beams with Bonded Piezoelectric Transducers

Based on the finite element software ANSYS, an electromechanical impedance (EMI) model for a cracked beam with imperfectly bonded piezoelectric patches is established in the paper. The property of bonding layer between the PZT sensor/actuators and the host beam is taken into account and thus the three-dimensional (3D) model of piezoelectric patch-adhesive-cracked beam coupled system is developed. Comparison with existing numerical results validates the effectiveness and accuracy of the present analysis. Then, parameter study is conducted by considering effects of the vibration mode of the host beam, the mass density of the adhesive and crack depth etc. on EMI signatures. The numerical results indicate that the present EMI model can be used to detect the cracks in the structures.

잡음 환경 하에서의 전기-역학적 임피던스 기반 조류발전 구조물의 장기 건전성 모니터링

In structural health monitoring (SHM) using electro-mechanical impedance signatures, it is a critical issue for extremely large structures to extract the best damage diagnosis results, while minimizing unknown environmental effects, including temperature, humidity, and acoustic vibration. If the impedance signatures fluctuate because of these factors, these fluctuations should be eliminated because they might hide the characteristics of the host structural damages. This paper presents a long-term SHM technique under an unknown noisy environment for tidal current power plant structures. The obtained impedance signatures contained significant variations during the measurements, especially in the audio frequency range. To eliminate these variations, a continuous principal component analysis was applied, and the results were compared with the conventional approach using the RMSD (Root Mean Square Deviation) and CC (Cross-correlation Coefficient) damage indices. Finally, it was found that this approach could be effectively used for long-term SHM in noisy environments.

Electromechanical impedance response of a cracked functionally graded beam with imperfectly bonded piezoelectric wafers

An analytical model of a cracked functionally graded beam with attached Lead Zirconate Titanate (PZT) actuator/sensors is proposed in the paper for structural health monitoring. In this model, the dynamic behavior of the piezoelectric patches is considered and a viscoelastic law is adopted to describe the bonding imperfection between the piezoelectric patches and the beam. A piecewisely homogeneous beam model is then employed to approximate the original inhomogeneous beam based on the Timoshenko beam theory. The crack in the beam is treated as a massless rotational spring. In order to develop the recursive formulations to reduce the dimension of the final equations in the method of reverberation-ray matrix (MRRM), new local scattering relations are established for this smart beam using a matrix reduction technique. An analytical expression of the electromechanical impedance (EMI) is derived based on the improved MRRM via the recursive formulations. Comparison with existent experimental results and those predicted by other methods, such as the conventional MRRM, the transfer matrix method (TMM), and the finite element method (FEM), is made to validate the proposed analysis. Furthermore, the effects of various parameters including the crack depth on the EMI signatures are highlighted.

Delamination assessment of a laminated composite beam using distributed piezoelectricsensor/actuator

This paper focuses on delamination detection of laminated beams using electromechanicalimpedance (EMI) signatures, which are directly related to the mechanical impedance of thestructure. An accurate model of the laminated beam including a delamination introducedinside as well as installed PZT transducers is established. In the model, we consider thedynamic behavior of the piezoelectric patches as well as the presence of bondingimperfection using a Kelvin-type viscoelastic interface. In order to develop the recursiveformulations to reduce the size of the final equations in the method of reverberation-raymatrix (MRRM), new local scattering relations are established for this smartbeam with distributed piezoelectric patches and delaminated members. Then,an analytical expression of the impedance, which involves information on thesize and position of the delamination in the smart beam, is derived based onthe improved MRRM via the recursive formulations. Comparison with existingexperimental and numerical results is made to validate the proposed analysis. Thenumerical simulations allow us to study more closely the influence of some physicalparameters, such as transducer size and position, on the EMI signatures. Moreover, thecovariance, a kind of non-parametric damage index, integrated with distributedsensor/actuators is also employed to identify the delamination in the laminate.

Three-Dimensional Electromechanical Impedance Model for Damaged Beams with Imperfectly Bonded Piezoelectric Patches

Dynamic analysis is conducted for a cracked beam with imperfectly bonded piezoelectric patches using the finite element method in the paper. The property of adhesive between the PZT patches and the host beam is taken into account based on the peel stress model as well as the shear lag model and thus the three-dimensional (3D) model of piezoelectric patch-adhesive-host beam coupled system is developed. Based on the established three-dimensional EMI model, the effect of some physical parameters such as vibration mode of main structure, the mass of adhesive layer and crack depth etc. on electromechanical impedance signatures is investigated. Finally, the root-mean-square deviation (RMSD), a kind of non-parametric damage index, is also employed to identify the damage severity of the cracked beam.

Non-destructive evaluation of concrete quality using PZT transducers

This paper presents a new concept of using PZT (lead zircornate titanate) transducers as a non-destructive testing (NDT) tool for evaluating quality of concrete. Detection of defects in concrete is very important in order to check the integrity of concrete structures. The electro-mechanical impedance (EMI) response of PZT transducers bonded onto a concrete specimen can be used for evaluating local condition of the specimen. Measurements are carried out by electrically exciting the bonded PZT transducers at high frequency range and taking response measurements of the transducers. In this study, the compression test results showed that concrete specimens without sufficient compaction are likely to fall below the desired strength. In addition, the strength of concrete was greatly reduced as the voids in concrete were increased. It was found that the root mean square deviation (RMSD) values yielded between the EMI signatures for concrete specimens in dry and saturated states showed good agreement with the specimens' compressive strength and permeable voids. A quality metric was introduced for predicting the quality of concrete based on the dry-saturated state of concrete specimens. The simplicity of the method and the current development towards low cost and portable impedance measuring system, offer an advantage over other NDE methods for evaluating concrete quality.

Practical implementation of piezo-impedance sensors in monitoring of excavation support structures

SUMMARY In the last decade, electromechanical impedance (EMI)-based monitoring technique using piezoceramic (PZT) sensors have been successfully implemented in health monitoring of lab-sized engineering structures. However, its implementation in real life application, such as monitoring underground support structures, has not been done before. In general, the EMI technique utilizes the unique EMI signature where any changes in the signature during the period of monitoring indicate possible damage in the host structure. This paper presents a part of monitoring results of the soil excavation carried out for the construction of new mass road transport (MRT) station in the southern part of Singapore using a PZT-based EMI technique. The MRT site consists of typical clayed soil of varying properties along the depth of excavation. To prevent the soil collapse during excavation, temporary support structures were laid with suitable monitoring systems. The paper presents the results obtained from the PZT sensors and the comparisons with conventional measurement devices. However, there were no damages reported in the structure, and hence the PZT sensors, which were initially aimed to capture possible damages, were used later to capture load variations on the struts due to the surrounding soil. Copyright © 2010 John Wiley & Sons, Ltd.

An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.

Monitoring interfacial defects in a composite beam using impedance signatures

A simply supported laminated beam bonded or embedded with a piezoelectric layer, which acts as sensor/actuator, is investigated using the state-space method. Linear spring-like constitutive relations are employed to model the behavior of interfacial bonding defects. The imperfect bonding between the host laminate and the piezoelectric transducer is also taken into account. Numerical results show that the electro-mechanical impedance (EMI) signatures extracted from the piezoelectric layer are very sensitive to the bonding imperfection of composite laminates. The covariance of the simulated data, which is a non-parametric damage index, is also employed to identify the severity of bonding imperfection quantitatively.

Dynamic Behavior of Structure-mounted Disk-shape Piezoelectric Sensors Including the Adhesive Layer

Piezoelectric (PZT) transducers are commonly used for numerous diagnostic techniques for nondestructive evaluation (NDE) of structures. Researchers in the past have shown that the presence of an adhesive layer between the PZT patch and the structure could have a significant impact both on the amplitude of the wave propagated into the structure and on its Electromechanical impedance (EMI) signature. To account for losses, models have been developed to describe the effects of the adhesive layer. Nonetheless, the models found in the literature are strictly valid for either 1D or rectangular PZT patches only. The aim of this article is to develop a dynamic model for a disk, PZT patch connected to a structure by means of a thin adhesive layer. Unlike previous works, it was found that for disk-shape sensors the effect of the adhesive layer is frequency dependent and that the losses due to the shear deformation of the adhesive layer for the case near the first radial resonant mode were considerably lower than for the quasi-static case. In order to validate the model developed in this article, experimental data are compared to the predicted EMI signatures of disk-shape sensors mounted on a structure for various configurations.

Effect of External Vibration on PZT Impedance Signature.

Piezoelectric ceramic Lead Zirconate Titanate (PZT) transducers, working on the principle of electromechanical impedance (EMI), are increasingly applied for structural health monitoring (SHM) in aerospace, civil and mechanical engineering. The PZT transducers are usually surface bonded to or embedded in a structure and subjected to actuation so as to interrogate the structure at the desired frequency range. The interrogation results in the electromechanical admittance (inverse of EMI) signatures which can be used to estimate the structural health or integrity according to the changes of the signatures. In the existing EMI method, the monitored structure is only excited by the PZT transducers for the interrogating of EMI signature, while the vibration of the structure caused by the external excitations other than the PZT actuation is not considered. However, many structures work under vibrations in practice. To monitor such structures, issues related to the effects of vibration on the EMI signature need to be addressed because these effects may lead to misinterpretation of the structural health. This paper develops an EMI model for beam structures, which takes into account the effect of beam vibration caused by the external excitations. An experimental study is carried out to verify the theoretical model. A lab size specimen with different external excitations is tested and the effect of vibration on EMI signature is discussed.

Piezoelectric sensor based nondestructive active monitoring of strength gain inconcrete

The recent advent of smart materials, such as piezoelectric materials, shape-memory alloys,and optical fibers, has added a new dimension to present structural health monitoringtechniques. In particular, the electro-mechanical impedance (EMI) sensing techniqueutilizing piezoelectric materials has emerged as a potential tool for the implementation of abuilt-in monitoring system for damage detection of civil structures. However, there is littleeffort to apply this technique for concrete monitoring. In this study, an effort to extend theapplicability of the EMI sensing technique is made for strength gain monitoringof early age concrete. PZT (piezoelectric lead zirconate titanate) patches areemployed to sense the EMI signature of curing concrete. A series of experiments wasconducted on concrete specimens to verify the applicability of the EMI sensingtechnique. The results show the excellent potential of the EMI sensing techniqueas a practical and reliable nondestructive method for strength gain monitoring.

Monitoring Vibrating Structures Using PZT Impedance Transducers

Electromechanical impedance (EMI) technique using lead zirconate titanate (PZT) transducers has been increasingly applied to structural health monitoring (SHM) of aerospace, civil and mechanical structures. The PZT transducers are usually surface bonded to or embedded in a structure and subjected to actuation so as to interrogate the structure at the desired frequency range. The interrogation results in the electromechanical admittance (inverse of EMI) signatures which can be used to estimate the structural health or integrity according to the changes of the signatures. In the existing EMI method, the monitored structure is only excited by the PZT transducers for the interrogating of EMI signature, while the vibration of the structure caused by the external excitations other than the PZT actuation is not considered. However, in real situation many structures work under vibrations. To monitor such structures, issues related to the effects of vibration on the EMI signature need to be addressed because these effects may lead to misinterpretation of the structural health. This paper develops an EMI model for beam structures, which takes into account the effect of beam vibration caused by the external excitations. An experimental study is carried out to verify the theoretical model. A Lab sized specimen with external excitation is tested and the effect of excitation on EMI signature is discussed.

Application of electro-mechanical impedance sensing technique for online monitoring of strength development in concrete using smart PZT patches

Electro-mechanical impedance (EMI) sensing technique utilizing smart piezoelectric materials has emerged as a potential tool for the implementation of an online monitoring system for structural health monitoring. Many nondestructive methods for strength gain monitoring seem to have limited capabilities in monitoring of the strength gain in a continuous manner. In this study, the feasibility of the EMI sensing technique for the online strength gain monitoring of early-age concrete is investigated. The experimental study is conducted on the piezoelectric patch instrumented concrete specimen. It is found that the EMI signature is very sensitive to the strength gain in early age concrete.

Sensor Self-diagnosis Using a Modified Impedance Model for Active Sensing-based Structural Health Monitoring

The active sensing methods using piezoelectric materials have been extensively investigated for the efficient use in structural health monitoring (SHM) applications. Relying on high frequency structural excitations, the methods showed the extreme sensitivity to minor defects in a structure. Recently, a sensor self-diagnostic procedure that performs in situ monitoring of the operational status of piezoelectric (PZT) active sensors and actuators in SHM applications has been proposed. In this investigation, previously developed impedance models were revisited in order to investigate the effects of sensor and/or bonding defects on the admittance measurement. New parameters for sensor quality assessment of a PZT and coupling degradation effects between a PZT and bonding layer were incorporated into the traditional electromechanical impedance model for better estimation of the electromechanical impedance signatures and sensor diagnostics. The feasibility of the modified impedance model for sensor self-diagnosis using the admittance measurements was demonstrated by a series of parametric studies using a simple example of PZT-driven single degree of freedom spring-mass-damper system. This paper summarizes the description of the proposed modified electromechanical impedance model, parametric studies for impedance-based sensor diagnostics, and several issues that can be used as a guideline for future investigation.

Application of EMI Technique for Crack Detection in Continuous Beams Adhesively Bonded with Multiple Piezoelectric Patches

Electro-mechanical impedance (EMI) is very effective for detecting the local incipient damages including small cracks in structures and has been validated by many experimental investigations. Meanwhile, some analytical models have been proposed to deal with dynamics of structures for health monitoring. However, there is little analytical work done to relate EMI signatures with crack parameters at ultrasonic frequencies. In this paper, a modified model combining EMI technique and reverberation matrix method (RMM) is proposed to quantitatively correlate crack parameters in continuous beams with high-frequency signatures for structural health monitoring. The model is based on Timoshenko beam theory with the crack treated as a massless rotational spring. The bonded PZT wafers are restricted to one-dimensional axial vibration. A shear lag model is adopted to simulate the interfacial bonding between PZT patches and the host beam. For the first time, an analytical expression of impedance (or admittance) involving information of cracks in this coupled smart structure system is derived. Comparison with existent numerical and experimental results is presented to validate the analysis. Based on this model, EMI signatures extracted from the PZT wafers can be used to identify cracks in a continuous beam.