Electromechanical Admittance Signatures Research Articles

An Electromechanical Impedance-Based Application of Realtime Monitoring for the Load-Induced Flexural Stress and Damage in Fiber-Reinforced Concrete

Effective real-time structural health monitoring in concrete structures is paramount to evaluating safety conditions and the timely maintenance of concrete structures. Especially, the presence of discrete fibers in fiber-reinforced concrete restrains crack propagation into small and thin cracks, which increases the difficulty in detecting damage. In this study, an array of piezoelectric lead zirconate titanate (PZT) transducers was applied to study the effects of external load-induced flexural stress and damage in fiber-reinforced concrete beams using the electromechanical impedance (EMI) or electromechanical admittance (EMA) methods. Beams were subjected to a four-point bending test under repeatable loading, while PZTs evaluated corresponding flexural stress and induced damage simultaneously. Due to the influence of the medium’s stress fields in the different types of wave propagation in structural elements, PZT transducers measurements are accordingly affected under variable stress fields, in addition to the effect of the higher level of damage that occurred in the medium. According to the results of the tests, variation in EMA signatures, following flexural stress and gradual damage changes, provided convincing evidence for predicting stress and damage development.

Restoration of electromechanical admittance signature via solving constrained optimization problems for concrete structural damage identification

Effective restoration of lossy data is prerequisite to guarantee the reliability of early warning information for damage diagnosis, considering large amounts of sensor data to be collected, transmitted and stored in long-term structural health monitoring (SHM) process. In this study, a compressive sampling matching pursuit (CoSaMP) algorithm was developed for solving the restoration problem of electromechanical admittance (EMA, inverse of impedance) signatures-based damage identification. Observed vectors through a random matrix projection were collected as a substitute of the EMA signatures used in conventional method, and EMA signature restorations from incomplete observed vectors were transformed into solving constrained optimization problems. Then the CoSaMP algorithm was utilized to recover EMA signatures with loss in observed vectors. Validation of the approach was conducted by a series of tests on artificial crack identification for a standardized concrete cube under laboratory conditions, which was compared with that using iterative shrinkage-thresholding algorithm (ISTA)-based algorithm. Additionally, impacts of different loss ratios on restoration accuracy were discussed. The proposed approach showed promising availability for damage identification using EMA technique encountered with lossy data.

Numerical investigation on EMI signatures in pipes with varied damage levels

Piezoelectric transducers have become very effective in damage detection of many important structures. Smart pipes surface bonded with piezoelectric sensors are efficient in damage detection in the form of corrosion. Damaged pipes exhibit changes in its stiffness and damping characteristics which ultimately gives changes in its Electro-Mechanical Admittance signatures. In the present study smart pipe with varied corrosion lengths along the circumference are considered. The corrosion width and depth are varied for all the peripheral damage lengths and the effect on EMI signatures is studied. Based on the conductance signatures of the healthy and damaged pipes damage indices (Root Mean Square Deviation-RMSD) are computed. It is concluded that greater RMSD values indicate the larger extent of the damage. In the present study circumferential damage of length, 0.03π showed more than 4% RMSD values in the case of 0.004 mm depth of corrosion when compared to the other cases considered.

Temperature effect on electromechanical admittance–based concrete structural health monitoring

Concrete structures in service are often subjected to environmental/operational temperature effects, which change their inherent properties and also inflict a challenge to their extrinsic monitoring systems. Recently, piezoelectric lead zirconate titanate (PZT)-based electromechanical admittance technique has been increasingly growing into an effective tool for concrete structural health monitoring; however, uncertainty in the changes of monitoring signals induced by temperature impact on concrete/PZT sensor would inevitably cause interference to structural damage detection, which adversely hinder its application from laboratory to engineering practice. This article, aiming at exploring the temperature effect on the electromechanical admittance–based concrete damage evaluation, primarily covered a series of theoretical/numerical analysis with rigorously experimental verifications. Three aspects of comparative studies were performed in theoretical/numerical analysis: (1) thermal-dependent parameters were inclusively evaluated in contribution to the electromechanical admittance characteristics via PZT-structure interaction models; (2) three-dimensional finite element analysis in multi-physics coupled field was employed to qualitatively assess the singular temperature effect on the electromechanical admittance behaviors of free-vibrated PZT, surface-bonded PZT/inside-embedded PZT coupled healthy concrete cubes; and (3) depending on the modeling of surface-bonded PZT-/inside-embedded PZT-cracked concrete cube, thermal effect on damage evaluation was addressed via quantification on the electromechanical admittance variations. In the experimental study, rigorous validation tests were carried out on a group of lab-scale concrete cubes, where surface-bonded PZT/inside-embedded PZT transducers were simultaneously employed for electromechanical admittance monitoring in view of thermal difference between concrete surface and its inner part. Correlation coefficient deviation value-based effective frequency shifts algorithm was also employed to compensate the temperature effect. Moreover, temperature effect was further testified on the monitoring of a full-scale shield-tunnel segment structure. Experimental results indicated that temperature triggered different behaviors of electromechanical admittance signatures for surface-bonded PZT/inside-embedded PZT transducers and contaminated the electromechanical admittance responses for damage detection. Structural damage severity level can be disadvantageously amplified by temperature increment even if under the same damage scenarios.

Characterization of the Electromechanical Admittance Signatures of Piezo-Impedance Transducers based on its Location

Abstract The Electromechanical Impedance method (EMI), using piezoelectric ceramics transducers such as Lead-Zirconate-Titanate (PZT), has applications in the fields of non-destructive evaluation (NDE) and structural health monitoring (SHM). The piezo-ceramic transducer chip (PZT) serves as an actuator supplying vibrational energy when it is bonded to the structure and subjected to harmonic voltage at ultrasonic frequency ranges. The electrical admittance of the PZT is known to be dependent on the dynamic coupling of the mechanical impedances of the PZT and the host structure. The admittance signature obtained over a frequency range serves as a diagnostic signal to monitor structural health. Any defect or damage changes the structure’s mechanical impedance which gets reflected in the admittance signature. This paper examines the changes in the admittance signatures of the same PZT transducer when it is bonded at different locations of a thin plate with cantilever type end conditions. The study is purely numerical using the coupled-field FE simulations in ANSYSTM, yet it gives better insight into admittance signature analysis, which is essential to assessing the structural health.

Electromechanical Admittance Signature Analysis of Piezo-ceramic Transducers for NDE

Of the multiple applications of the piezoelectric ceramics transducers such as Lead-Zirconate-Titanate (PZT), one prominent one is in the field of non-destructive evaluation (NDE) using the Electromechanical Impedance method (EMI). In this method a piezo-ceramic transducer (PZT) is bonded on to the structure and actuated using a harmonic voltage at ultrasonic frequency ranges. The dynamics of the PZT-structure interaction is captured in the frequency response of the electrical admittance of the PZT, popularly called as the admittance signature. A mechanical damage in the structure alters the mass, stiffness and damping properties locally, which in turn alters the admittance signature. Many analytical and semi-analytical models of the EMI method that have been used are limited in application to simple systems. The coupled-field finite element modeling has been demonstrated to quite accurately predict the admittance signatures as obtained by the experimental EMI technique. This paper presents the results of identification and quantification studies of damage using the analysis of admittance signatures obtained by coupled-field FE model of a simple cantilever type plate in ANSYSTM.

Crack monitoring using multiple smart materials; fiber-optic sensors & piezo sensors

ABSTRACTThis article focuses on health monitoring of structures using multiple smart materials. In this research, two fiber-optic sensors, namely fiber Bragg grating (FBG) and fiber-optic polarimetric sensor (FOPS), are investigated for damage detection in the beam specimen. FBG is used for local strain measurement while FOPS is used for global strain measurement. Both FBG and FOPS show significant changes in the strain due to damages in the specimen. Also, at the center of the specimen, piezoelectric wafer active sensor (PWAS) is attached. The electromechanical admittance (EMA) signature of the specimen beam is recorded by PWAS. The changes in the amplitudes of the peaks obtained at various frequencies in this EMA signature are analyzed, and it is shown that the peak amplitudes respond differently to damages and to change in loading. Thus, multiple smart materials (FBG, FOPS, and PWAS) are used to get improved information on the health of the beam.

Embedded 3D electromechanical impedance model for strength monitoring of concrete using a PZT transducer

The electromechanical (EM) impedance approach in which piezoelectric ceramics (PZT) simultaneously act as both a sensor and an actuator due to their direct and inverse piezoelectric effects has emerged as a powerful tool for structural health monitoring in recent years. This paper formulates a new 3D electromechanical impedance model that characterizes the interaction between an embedded square PZT transducer and the host structure based on the effective impedance. The proposed formulations can be conveniently used to extract the mechanical impedance of the host structure from the electromechanical admittance measurements of an embedded PZT patch. The proposed model is verified by experimental and numerical results from a smart concrete cube in which a square PZT transducer is embedded. Subsequently, this paper also presents a new methodology to monitor the compressive strength of concrete based on the effective mechanical impedance. By extracting the effective mechanical impedances from the electromechanical admittance signatures, measuring the compressive strength of the concrete cubes at different ages and combining these measurements with the index of the correlation coefficient (CC), a linear correlation between the concrete strength gain and the CC of the real mechanical admittances was found. The proposed approach is found to be feasible to monitor the compressive strength of concrete by age.

Piezo-impedance transducers for residual fatigue life assessment of bolted steel joints

This article presents a new approach for fatigue life assessment of bolted steel joints using the equivalent stiffness determined by surface-bonded piezo-impedance transducers. The piezo transducers acquire electromechanical admittance signatures comprising the conductance (real part) and the susceptance (imaginary part), from which the equivalent system parameters of the joint are identified. The equivalent stiffness, one of the identified parameters, is experimentally found to be related to the residual stiffness of the joint and is correlated with the remaining life of the component in terms of the cycles of loading that can be further sustained by the joint. Through tests on three prototype steel joints, empirical equations are derived to relate the residual fatigue life to the loss of equivalent identified stiffness. The most promising feature of the proposed approach is that it employs the admittance signature of the surface-bonded piezo transducers directly, thereby circumventing the determination of the in situ stiffness of the joint.

Dual use of PZT patches as sensors in global dynamic and local electromechanical impedance techniques for structural health monitoring

In this article, a new approach is proposed to effectively detect the initiation and progression of structural damage by combining the global dynamic and the local electromechanical impedance (EMI) techniques, using the same set of surface-bonded piezoelectric ceramic (PZT) patches as sensors. The PZT patches are used to determine the natural frequencies and the strain mode shapes of the structure (for use in the global dynamic technique) as well as to acquire the electromechanical admittance signature (for use in the EMI technique) to facilitate an improved damage assessment. Occurrence and location of the incipient damage are determined using the EMI technique, whereas for moderate to severe damages, the location and the severity are arrived at through the global dynamic technique. Finally, damage severity is determined in terms of the original stiffness of structure using the strain mode shapes directly determined using the PZT patches. The proposed technique is illustrated using two specimens—a 4-m long steel beam and a mild steel plate 1260 mm × 630 mm × 6.5 mm in size. The integrated approach provides greater information about damage, is simple to apply, does not involve any numerical/analytical modeling a priori, and is at the same time very cost effective.

Sub-Frequency Interval Approach in Electromechanical Impedance Technique for Concrete Structure Health Monitoring

The electromechanical (EM) impedance technique using piezoelectric lead zirconate titanate (PZT) transducers for structural health monitoring (SHM) has attracted considerable attention in various engineering fields. In the conventional EM impedance technique, the EM admittance of a PZT transducer is used as a damage indicator. Statistical analysis methods such as root mean square deviation (RMSD) have been employed to associate the damage level with the changes in the EM admittance signatures, but it is difficult to determine the location of damage using such methods. This paper proposes a new approach by dividing the large frequency (30–400 kHz) range into sub-frequency intervals and calculating their respective RMSD values. The RMSD of the sub-frequency intervals (RMSD-S) will be used to study the severity and location of damage. An experiment is carried out on a real size concrete structure subjected to artificial damage. It is observed that damage close to the PZT changes the high frequency range RMSD-S significantly, while the damage far away from the PZT changes the RMSD-S in the low frequency range significantly. The relationship between the frequency range and the PZT sensing region is also presented. Finally, a damage identification scheme is proposed to estimate the location and severity of damage in concrete structures.

Parallel and Individual Interrogations of Piezo-Impedance Transducers for Damage Detection

Lead zirconate titanate (PZT)-based piezo-impedance transducers have been successfully used in structural health monitoring (SHM) of various engineering structures in the recent years. To effectively monitor structural health, identification of damage location is of great importance. Although a single PZT is sensitive enough to detect damage, it is insufficient to indicate the location of damage. Therefore, in this study, nine PZT transducers are surface bonded on an aluminum plate to investigate the feasibility of locating the damages induced in the plate. At each damage state, electromechanical admittance signatures of all PZT sensors are recorded and analyzed using the root mean square deviation (RMSD) index. Results show that the damage location can be estimated by the RMSD patterns obtained from both parallel and individual interrogations of PZTs.

Simplified Impedance Model for Adhesively Bonded Piezo-Impedance Transducers

The electromechanical impedance technique employs surface-bonded lead zirconate titanate piezoelectric ceramic patches as impedance transducers for structural health monitoring and nondestructive evaluation. The patches are bonded to the monitored structures using finitely thick adhesive bond layer, which introduces shear lag effect, thus invariably influencing the electromechanical admittance signatures. This paper presents a new simplified impedance model to incorporate shear lag effect into electromechanical admittance formulations, both one-dimensional and two-dimensional. This provides a closed-form analytical solution of the inverse problem, i.e. to derive the true structural impedance from the measured conductance and susceptance signatures, thus an improvement over the existing models. The influence of various parameters (associated with the bond layer) on admittance signatures is investigated using the proposed model and the results compared with existing models. The results show that the new mod...

Monitoring Beam-Column Joint in Concrete Structures Using Piezo-Impedance Sensors

Despite that piezoelectric ceramic lead zirconate titanate (PZT) has been used for structural health monitoring (SHM) in various engineering systems, limited work has been conducted on real size concrete structures. Beam-column connections are critical regions in reinforced concrete (RC) moment-resisting frame structures. The vulnerability of RC beam-column joints has been identified from structural damage investigations over the past decades, especially in the area of earthquake engineering. In the context of a terrorist bomb attack, the beam-column joints are very vulnerable, especially when the perimeter columns lose their load carrying capacity due to damage and the beam-column joints become one of the crucial load transfer mechanism of the structural frame. To avoid catastrophic failures, it is important to monitor beam-column joints under existing gravitational loads. In this paper, an experiment is carried out on four real size concrete frame structures with different detailing subjected to gradually increased loads. A number of PZT sensors are bonded to the frame structure to acquire PZT electro-mechanical (EM) admittance signature. The structural mechanical impedance (SMI) is extracted from the PZT EM admittance signature and its sensitivity is compared with that of the EM admittance. The relations between the damage index and the loading step and tip deflection of the concrete structure are obtained. Finally the sensitivity of the PZT sensors in detection of the critical loading level is discussed. The results show that the PZT sensors are capable of monitoring the integrity and behavior of the real size concrete structures.

Sensitivity of PZT Impedance Sensors for Damage Detection of Concrete Structures.

Piezoelectric ceramic Lead Zirconate Titanate (PZT) based electro-mechanicalimpedance (EMI) technique for structural health monitoring (SHM) has been successfullyapplied to various engineering systems. However, fundamental research work on thesensitivity of the PZT impedance sensors for damage detection is still in need. In thetraditional EMI method, the PZT electro-mechanical (EM) admittance (inverse of theimpedance) is used as damage indicator, which is difficult to specify the effect of damage onstructural properties. This paper uses the structural mechanical impedance (SMI) extractedfrom the PZT EM admittance signature as the damage indicator. A comparison study on thesensitivity of the EM admittance and the structural mechanical impedance to the damages ina concrete structure is conducted. Results show that the SMI is more sensitive to the damagethan the EM admittance thus a better indicator for damage detection. Furthermore, this paperproposes a dynamic system consisting of a number of single-degree-of-freedom elementswith mass, spring and damper components to model the SMI. A genetic algorithm isemployed to search for the optimal value of the unknown parameters in the dynamic system.An experiment is carried out on a two-storey concrete frame subjected to base vibrations thatsimulate earthquake. A number of PZT sensors are regularly arrayed and bonded to the framestructure to acquire PZT EM admittance signatures. The relationship between the damageindex and the distance of the PZT sensor from the damage is studied. Consequently, thesensitivity of the PZT sensors is discussed and their sensing region in concrete is derived.

Three-Dimensional Electromechanical Impedance Model for Multiple Piezoceramic Transducers—Structure Interaction

In the last few decades, piezoceramic (PZT) transducers have been extensively used either as actuators or as sensors in the vibration and noise control of aero, civil, and mechanical (ACM) systems. Only in the last decade, PZT transducers have been used in the electromechanical impedance (EMI) models as both sensor and actuator for the structural health monitoring of ACM systems. In the EMI models, the PZT transducers are generally surface bonded to the host structure and are then subjected to one-dimensional (1D) voltage to interrogate the structure for the desired frequency range. The interrogation results in the prediction of electromechanical admittance signatures. These signatures serve as indicators of the health/integrity of the structure. However, the existing single PZT–structure interaction models consider the PZT transducer to be negligible in mass and thus ignored. Moreover, they impose restrictions on the PZT shape, size, and isotropy. This paper presents a novel semianalytical multiple three-dimensional PZT–structure interaction model which considers the “mass” of the PZT transducers, and the transducers are subjected to parallel (1D) sinusoidal voltage. Further, the model does not impose restriction on the shape (square or rectangular), size (thin or thick), and electrical properties (isotropic or anisotropic) of PZT. The derived model is also experimentally verified using lab-sized aluminum plate. As it is generic, the model is expected to be applicable for the nondestructive evaluation of most ACM systems.

Influence of loading on the electromechanical admittance of piezoceramic transducers

Damage detection using electromechanical (EM) impedance in structural health monitoring(SHM) of engineering structures is rapidly emerging as a useful technique. In the EMimpedance method, piezoceramic (PZT) transducers are either surface bonded to orembedded inside the host structure and are subjected to electric actuation. The EMadmittance signatures of the PZT transducers, which consist of real and imaginary parts,serve as indicators to predict the health/integrity of the host structure. However, in reallife, structural components such as slabs, beams and columns are constantly subjectedto some form of external loading. The EM admittance signature obtained forsuch a constantly loaded structure is different from that obtained when damagesare present in the structure. This paper presents an experimental and statisticalinvestigation to show the influence of loading on EM admittance signatures. It is alsoobserved that the susceptance signature is a better indicator than the conductancesignature for detecting in situ stress in the host structure. This observation isfurther supported by a statistical analysis. This paper is expected to be useful forthe non-destructive evaluation of engineering structures with external loading.

Development of an electromechanical admittance approach for application in the vibrationcontrol of intelligent structures

The present work considers the possibility of vibration control of a distributed dynamicalsystem, such as flexible plates using local piezoelectric (PZT) actuators/sensors and theelectromechanical admittance concept. When PZT actuators bonded on structures are usedin active vibration and acoustic control, the desired deformation field in the structure isobtained through the application of localized line forces and moments generated byapplying an appropriate electrical field on the outer surfaces of the PZT patches. Theelectromechanical admittance generated at the electrical terminals of a PZT-driven smartstructure is then extracted to synthesize a desired damping performance. This is achievedby a FEM-based minimization of the difference between the computed and thedesired electromechanical admittance signature for investigated frequency ranges.

Structural identification and damage diagnosis using self-sensing piezo-impedancetransducers

The use of smart materials, such as lead zirconate titanate (PZT), has accelerateddevelopments in the fields of structural identification and automated structuralhealth monitoring (SHM). One such technique that has made much progress isthe electro-mechanical impedance (EMI) technique, which employs self-sensingpiezo-impedance transducers. In this technique, a PZT patch is surface bonded to thestructure to be monitored and its corresponding electro-mechanical admittancesignature is used for damage detection. This paper introduces a new methodfor identifying structures from the measured admittance signatures in terms ofequivalent structural parameters, whereby the identified parameters are used fordamage characterization. The new method has been applied to a truss, a beamand a concrete cube, and found to be able to successfully perform structuralidentification and damage diagnosis. In addition, several advantages have beenascertained in comparison with the conventional, non-parametric statistical methods.

Damage severity and propagation characterization with admittance signatures of piezotransducers

The electromechanical (EM) impedance method is emerging as an effective tool forstructural damage detection. Damage is detected by changes in the EM impedancesignatures of the smart piezoelectric transducer bonded on the structure. The damagequantification has so far been restricted to using non-parametric statistical indices tomeasure changes in the signatures. Such measures, although simplistic, fail tocorrelate the changes in the signatures to physical parameters of the structures.Thus, although effective in detecting the presence of damage, the method fails togive further information about the location and severity of the damage. In thispaper, the EM impedance method integrated with a finite element (FE) modelis presented as a means for characterizing damage growth. Damage growth ischaracterized by quantifying the changes in the natural frequency shifts of the structureextracted from the EM admittance signatures. A new damage characterization indexis derived, which is experimentally validated to be capable of distinguishing alocalized increase in severity from damage propagation through the structure.