Electromechanical Impedance Spectra Research Articles

The Influence of Coupling-Specific Parameters and Structural Parameters on the Electromechanical Impedance of Acoustic Emission Sensors

Acoustic emission (AE) sensors are widely used for testing and monitoring purposes, necessitating effective field-testing methods to assess their functionality during use, ideally resulting in a method for self-diagnostic. A novel approach for field-testing is to measure the electromechanical impedance (EMI) of the AE sensor. This method requires minimal additional equipment and eliminates human interaction, which is needed for current field-tests like pencil-lead break tests. However, the EMI includes the mechanical impedances of the sensor, the coupling layer, and the structure. Therefore, the measurements contain information about all three and influences of the structure and the coupling layer need to be investigated to be able to establish a method for self-diagnosis in field. Additionally, the piezoelectric effect is temperature-dependent, which affects the measured EMI spectra and may result in erroneous assessments of the sensor's condition if not accounted for. This work presents experimental studies on the influence of measuring equipment, environmental temperature, coupling-specific parameters and structural parameters on the EMI of AE sensors. Moreover, numerical simulations have been conducted to provide additional support and to extend the experimental studies of the latter two. To ensure reproducible sensor coupling to a structure, a setup was constructed to couple the sensor to the structure at the same position and with the same force. The thickness of the coupling layer is determined by a small amount of silicon carbide particles with a specific diameter. This setup is also used to examine the effect of structural parameters.

Electromechanical Impedance of Acoustic Emission Sensors used for Self-Diagnosis

Acoustic Emission (AE) is a well established NDT method which is used more and more for continuous monitoring, heading towards SHM. Today, AE sensors in the field are tested with a pencil lead break test (Hsu-Nielsen source), which requires manual interaction. Sensors are replaced if the sensitivity is too low. However, since the test is performed on the monitored structure, the structure itself may influence the test. Therefore, a detailed check of the sensor in the lab, according to SE02 and ISO 24543, is done afterwards. An automated solution, that can test AE sensors in the field, without human interaction, that needs minimal additional hardware would benefit AE based monitoring systems. The electromechanical impedance (EMI) spectrum is mostly used for local damage detection with piezoelectric transducers. In the past it has been shown that the EMI spectrum can also be used to enable automated self-diagnostic capabilities in the low frequency spectrum (Park et al., 2004) and in the higher frequency regime (Mueller, 2017) including the eigenfrequencies of the transducers. The usage of EMI for AE sensors can, thorough research in this matter provided, solve the issues mentioned above. This work presents results of EMI spectrum analysis of two sets of 12 and 17 sensors, respectively, with different levels of damage from pressure vessel burst tests. The results are compared to that of a face-2-face inspection, which is an established method of verification in the lab according to ISO 24543. It is shown that the EMI spectrum is capable of similar self-diagnostic capabilities. Thus, combining the advantages of global monitoring with AE sensors, while providing a remote and automatic method of self-diagnostic of AE sensors at any point with minimal additional hardware and software requirements.

Feature Engineering for Physics-informed Evaluation of Electromechanical Impedance Measurements for Sandwich Face Layer Debonding Identification

The present research exploits various physically explainable features for the evaluation of electromechanical impedance (EMI) measurements for sandwich face layer debonding identification using a recently published two-step physics- and machine-learning (ML)-based approach. In the previous work, the ML approach used employs a One-Class Support Vector Machine and a K-Nearest Neighbor model for debonding detection and size estimation, respectively. Feature engineering was used to compensate for the simplifications of the finite element (FE)-based physical model and a simple data calibration step was used to adapt to distribution shifts. This enabled to train both ML models exclusively with FE simulation-based synthetic EMI spectra data. The efficacy of the method was demonstrated on real-world EMI spectra measurements of a circular aluminum sandwich panel by a piezoelectric transducer. The considered face layer debonding damage was idealized and stepwise increased by a milling process. In the present work, we explore opportunities for further optimization of our method with respect to data distillation, where we reduce the computational requirements of both training and inference while at the same time preserving essential information. Furthermore, we investigate the usefulness of a separate approach for debonding detection, formulated as an anomaly detection problem. The data preprocessing and ML models are validated by unseen real-world EMI measurement data and benchmarked with the previously published damage evaluation results, considering both reliability and accuracy. The data and the code used in this study are provided in their entirety to enable reproducibility, enhance comprehensibility, and encourage future research.

Unsupervised autoencoders with features in the electromechanical impedance domain for early damage assessment in FRP-strengthened concrete elements

This paper presents the development of a robust automatic diagnosis technique that uses raw Electro-Mechanical Impedance (EMI) signals and deep autoencoder models to detect damage in fiber-reinforced-polymers (FRP) strengthened reinforced concrete (RC) elements, for which the most common failure modes occur in a sudden and brittle way by debonding. The contribution of this work is threefold: First, for the first time, two autoencoder models, convolutional and fully connected, based on an unsupervised learning framework supplemented by appropriate pre-processing techniques, are proposed for effective tracking of FRP-strengthened RC elements from raw EMI response variations in different locations of the auscultated structure; their implementation is also extensively investigated. The proposed framework consists of two main components, namely, dimensionality reduction and relationship learning. The first component is to reduce the dimensionality of the raw EMI signal while preserving the necessary information required, and the second component is to perform the relationship learning between the features with the reduced dimensionality and the stiffness reduction parameters of the structure. The approach is beneficial as only the EMI spectrum from the healthy structure state is considered for the training of the autoencoders. Second, the superior performance of the proposed framework is demonstrated. The results show that the proposed technique can accurately detect minor damage in its earliest stages for this kind of strengthened structures, while removing the need for manual or signal processing-based damage sensitive feature extraction from EMI signals for damage diagnosis. Finally, research presented in this work can potentially open up new opportunities for successful condition monitoring of this type of strengthened structures.

Influence of temperature and preload force on capacitance and electromechanical impedance of lead zirconate titanate piezoelectric wafer active sensors for structural health monitoring of bolts

This study focuses on the effect of temperature and preload force on capacitance and electromechanical impedance of lead zirconate titanate piezoelectric wafer active sensors for structural health monitoring of bolts. We explain the influence of temperature on the basis of the phenomenological thermodynamic theory of ferroelectricity by Landau, Ginsburg and Devonshire. The article illustrates the effect of damping the radial deformation of piezoelectric sensors on the capacitance and electromechanical impedance spectra in structural health monitoring of bolts. We also explains the similarities between the effects of temperature and preload force on the electromechanical impedance spectra. We establish a clear correlation between the mechanical strain in the region of the sensor (here due to a preload force), the capacitance and the electromechanical impedance spectra and thus show that piezoelectric sensors made of lead–zirconate–titanate can be used excellently in areas of variable mechanical strain. The article enhances the understanding of the measurement method and facilitates the transfer of the measurement method to other problems in structural health monitoring. Furthermore, the acquired knowledge serves as a solid basis for verifying the plausibility of data sets containing electromechanical impedance spectra.

Electromechanical Impedance Instrumented Wearable Piezoelectric Ring-Type Sensor for Bolted Connection Monitoring

Due to the loss of preload, structural failure caused by bolt loosening is common in engineering structures. Effective monitoring of bolted connections is of great importance in providing early warning to the structural maintenance department. In this study, a novel and reusable electromechanical impedance (EMI) instrumented wearable piezoelectric ring (WPR) was proposed to detect bolted connection loosening. In the experimental investigation, the EMI spectra of three identical WPRs during the bolt-loosening process were acquired and analyzed. It was concluded that bolt looseness changes can be qualitatively and quantitatively characterized by conductance spectra. Specifically, the resonance frequency and amplitude at certain frequencies were reduced correspondingly as bolts loosen. This was further validated by finite element model simulation results. Moreover, two statistical metrics including mean absolute percentage deviation (MAPD) and root mean square deviation (RMSD) were adopted to evaluate different degrees of bolted connections quantitatively. Both metrics showed an increasing tendency during the bolt loosening process in three different frequency bands. It was found that both metrics demonstrated a significant linear correlation with bolt torque variations. Both of them serve as reliable indicators to detect loose bolts in the bolted connection. The present work verifies the feasibility and applicability of using EMI-instrumented WPR to monitor bolted connections.

Influence of void damage on the electromechanical impedance spectra of Single Lap Joints

Adhesively bonded joints are susceptible to a variety of damage sources, which may be of difficult detection with current Non-Destructive Testing (NDT) methods. Structural Health Monitoring (SHM) techniques, such as the Electromechanical Impedance Spectroscopy (EMIS), aims to outpace NDT by continuously monitoring structures for defects. However, given the complexity of adhesive joint structures, EMIS capability for damage detection still needs to be thoroughly studied. In this manner, electrical impedance measurements were done to Single Lap Joints (SLJ), with both a modified epoxy adhesive and Pressure Sensitive Adhesive (PSA) for void detection. Void defects could not be detected on adhesive joints with the PSA but, in the case of joints with the modified epoxy adhesive, impedance signatures were significantly altered by the presence of both the artificially created void, and by a second damage source caused during the SLJ manufacture. Experimental measurements from modified epoxy joints were compared with simulation results, yielding good approximation when considering the effect of the second source of damage. An approximation of the mechanical impedance was also computed using numerical results from the pristine SLJ model. In this manner, it is possible to detect damage in the adhesive layer of SLJs with electrical impedance measurements from EMIS.

Sandwich Face Layer Debonding Detection and Size Estimation by Machine-Learning-Based Evaluation of Electromechanical Impedance Measurements

The present research proposes a two-step physics- and machine-learning(ML)-based electromechanical impedance (EMI) measurement data evaluation approach for sandwich face layer debonding detection and size estimation in structural health monitoring (SHM) applications. As a case example, a circular aluminum sandwich panel with idealized face layer debonding was used. Both the sensor and debonding were located at the center of the sandwich. Synthetic EMI spectra were generated by a finite-element(FE)-based parameter study, and were used for feature engineering and ML model training and development. Calibration of the real-world EMI measurement data was shown to overcome the FE model simplifications, enabling their evaluation by the found synthetic data-based features and models. The data preprocessing and ML models were validated by unseen real-world EMI measurement data collected in a laboratory environment. The best detection and size estimation performances were found for a One-Class Support Vector Machine and a K-Nearest Neighbor model, respectively, which clearly showed reliable identification of relevant debonding sizes. Furthermore, the approach was shown to be robust against unknown artificial disturbances, and outperformed a previous method for debonding size estimation. The data and code used in this study are provided in their entirety, to enhance comprehensibility, and to encourage future research.

Extraction and selective promotion of zero-group velocity and cutoff frequency resonances in bi-dimensional waveguides using the electromechanical impedance method

This study showcases the electromechanical impedance (EMI) technique for extracting and promoting zero-group velocity (ZGV) and cutoff frequency resonances in a waveguide structure. We identify the mechanisms of multiple resonances in the EMI spectra via a wave propagation perspective. Both simulation and experiments reveal the fact that sharp resonances in the conductance spectra are associated with either ZGV or cutoff frequency points. Consequently, we design four test configurations to enhance local resonances by aligning induced motions with considered mode shapes. Reasonable agreement between simulation and experiment results is observed. We evaluate the performance of considered configurations in terms of mode enhancement, and configurations that can selectively promote certain mode families are summarized. This study also shines the light on the EMI technique for quantitative non-destructive evaluation (NDE) by potentially supporting the inverse characterization of mechanical properties of host structures.

Monitoring of Aging of Solid‐Rocket‐Motor HTPB Propellant Grains Using an Electromechanical Impedance Method

AbstractIt is of great significance to monitor the aging state and evaluate the useful life of stored solid‐rocket‐motor (SRM) propellant grains. In this work, the aging state of hydroxyl terminated polybutadiene (HTPB) propellant grains was studied based on the electromechanical impedance (EMI) method. The relationship between the complex modulus and the mechanical impedance of viscoelastic material was derived. The finite element analysis of the structure with different moduli was carried out. The results show that the peak value of electromechanical impedance increases with an increase of propellant modulus. This report focuses on experimental research on monitoring the aging of cylindrical SRMs based on the EMI method. It is found that the peak of electromechanical impedance spectrum increases with aging time, while the peak frequency decreases slightly. As an aging damage variable, the root mean squared error (RMSD) of electromechanical impedance increases with aging time, and its growth rate is large at first, then decreases, and finally increases. Finally, a tensile test of aging propellant specimens was carried out, and the results show that the rate of change (ROC) of the modulus can be fit by a linear relationship to the RMSD of the electromechanical impedance.

Electromechanical response of laterally clamped piezoelectric wafers: Absence of in-plane mechanical resonances in the electromechanical impedance spectrum

We discuss the electromechanical response characteristics of laterally clamped piezoelectric wafers. The impedance spectra (both magnitude and phase) obtained from finite element simulations of uniformly polarized free and clamped circular and square wafers are first presented. It is observed that the magnitude and phase of the impedance spectrum for free piezoelectric wafers shows all the in-plane extensional modes as electromechanical resonances, while that for a clamped wafer does not show any of them. However, it is also shown that the spectrum corresponding to the out-of-plane displacement on the surface of the wafer actually shows the in-plane resonances for both free and clamped wafers. In other words, it is found that the clamped wafer shows in-plane mechanical resonances but these do not appear in the electromechanical impedance spectrum. This observation is then generalized and explained using an analytical model for thin piezoelectric wafers of arbitrary shape. In particular, it is shown that the impedance spectrum of laterally clamped piezoelectric wafers of any arbitrary shape do not show in-plane extensional modes as electromechanical resonances. Moreover, the impedance spectrum of a laterally clamped wafer is shown to depend only on its area and thickness but not its shape. The above findings are relevant to measurement techniques that rely explicitly on the characteristics of the impedance spectrum observed for a piezoelectric wafer. In addition, the absence of resonances resulting from lateral clamping allows for broadband operation of piezoelectric wafers as sensors and actuators for applications such as ultrasonic range finding, underwater acoustics and acoustic imaging.

Non‐destructive damage detection on welded threaded bolts based on electromechanical impedance spectra

AbstractWithin a research project carried out at the Department of Civil Engineering and the Department of Mechanical Engineering of the University of Siegen, a new monitoring method for non‐destructive damage detection on bolts was developed and tested according to its applicability on real life structures. The so‐called EMI‐method is based on the measurement of electromechanical impedance spectra, which change with varying structural states and thus allow for a condition assessment.Structures vibrate in a combination of their natural modes, which can provide information about their structural condition. Their modal characteristics are system‐inherent quantities and are governed by geometry, system stiffnesses and mass allocation. If a system parameter changes, e.g. the bending stiffness of a bridge, this is reflected in the vibration analysis and the modal characteristics. To identify the modal characteristics of civil engineering structures, these are excited and the system response is measured. This principle is also used for damage detection by means of electro‐mechanical impedance spectra (EMI). The observed system‐inherent quantity is the frequency‐dependent mechanical impedance Zs(ω) of the structure. The method is based on the idea that the mechanical impedance of a structure or component to be measured changes as a result of damage (or loss of prestress in the case of bolts) and thus these changes can be detected.In the project, investigations were carried out on steel plates with orthogonally welded threaded bolts, which are used, for example, as fasteners for rail supporting points on bridges. Research objectives were on one hand the detection of the prestress stage and on the other hand the detection of fatigue damage in the weld seam. Within the scope of the project, it has been investigated to what extent a change of the prestressing load can be reliably detected by means of the EMI method. Furthermore, the influence of damage to the bolt on the impedance spectra was investigated.

On the in-plane vibrations and electromechanical resonance characteristics of non-uniformly polarized rectangular piezoelectric wafers: Selective mode-type excitation and specific mode enhancement

We investigate the in-plane vibrations and electromechanical resonance characteristics of non-uniformly polarized rectangular piezoelectric wafers. Non-uniform polarization is represented as a non-uniform electromechanical coupling coefficient using a polarization function. Governing equations are derived for the forced in-plane vibrations of a thin wafer under the assumption of generalized plane stress. The effect of non-uniform polarization is explicitly obtained in the forcing terms of the governing equations. These equations are then recast into a variational weak form that is then solved using the finite element method to obtain the displacement fields for different modes. The electromechanical response of a non-uniformly polarized piezoelectric wafer is derived in terms of the out-of-plane displacement profile on the surface of the wafer. Using the derived analytical expression, a necessary and sufficient condition for the presence/absence of a vibrational mode in the electromechanical impedance spectrum is obtained. Based on this condition, criteria for selective mode-type excitation and specific mode enhancement of vibrational modes in the electromechanical impedance spectrum are postulated. Selective mode-type excitation of in-plane extensional and shear modes is demonstrated for a square wafer and that of in-plane bending modes is demonstrated for a rectangular wafer. Specific mode enhancement is demonstrated for both square and rectangular wafers. In addition, it is also demonstrated how the criteria can be used to suppress specific vibrational modes in the electromechanical impedance spectrum. The proposed methodology of using non-uniformly polarized piezoelectric wafers finds application in the design of single element transducers with multi-frequency operation, frequency-tuned receivers/sensors, acoustic holograms, designing acoustic beams of prescribed shape/lobes, and other non-traditional applications such as information storage.

Improving the EMI-based damage detection in composites by calibration of AD5933 chip

In this paper results of calibration of Analog Devices AD5933 chip for damage detection problems based on electromechanical impedance (EMI) are presented. A simple calibration method using mid-frequency point and resistor with known resistance was utilised. Authors investigated the influence of calibration process on the measurement results and its agreement with professional measurement equipment. Results obtained from AD5933 were compared with results obtained by professional impedance analyser HIOKI IM3570. Moreover, the influence of lack and wrong calibration of the AD5933 chip is presented. Calibration was validated for the case of damage detection in the carbon fibre reinforced polymer CFRP panel with delamination. Damage indices in the form of RMSD and CCD were utilised. Influence of changing temperature was also included and compensated. Good agreement of results from AD5933 and HIOKI IM3570 was achieved in the case of EMI spectra as well as calculated damage indices. The EMI spectra of magnitude of impedance, phase angle, resistance, conductance, reactance and susceptance were investigated. The obtained results pave the way for effective employment of the AD5933 chip for damage assessment in composite structures.

Monitoring early-age hydration and setting of portland cement paste by piezoelectric transducers via electromechanical impedance method

Monitoring the early-age hydration and setting process of cement paste is of great significance to help engineers determine the cement solidification status and evaluate the development of the strength and stiffness of cements. As a promising non-destructive testing (NDT) method, the electromechanical impedance (EMI) method has been extensively applied in many fields of structure health monitoring (SHM) using piezoelectric transducers. In this research, the feasibility of this method in monitoring the early-age hydration and setting process of Portland cement paste was explored. Experiments and observations on the conductance signatures obtained from the embedded piezoelectric transducers inside the cement pastes with two different water-to-cement (w/c) ratios have been conducted within 72 h of hydration. Moreover, the trends of EMI spectra in response to two different excitation frequency ranges, 100 Hz–1000 kHz and 100 Hz–3000 kHz, were captured. It is revealed that the variation of the conductance signatures in the higher frequency range well corresponds to each stage of the cement setting process determined by the Vicat needle testing. Both the resonance peak shift and the statistic index-based approaches perform effectively in monitoring the transformation of cement pastes from liquid to solid state controlled by the cement hydration. In addition, the hydration mechanism has also been interpreted through the hydration exothermic test, XRD and SEM measurements on cement specimens at different ages.

Real-time monitoring stiffness degradation of hardened cement paste under uniaxial compression loading through piezoceramic-based electromechanical impedance method

Real-time health monitoring of cementitious materials under external loadings is of great significance to help engineers evaluate structural safety state and ensure timely maintenance. As a promising nondestructive testing method, the electromechanical impedance (EMI) method has been widely applied in the field of structure health monitoring (SHM) in civil engineering. In this study, this method was explored in monitoring the stiffness degradation of hardened cement paste subjected to compression loadings. Based on the theoretical modelling, a simplified one-dimensional analytical model was established based upon the piezo-elasticity theory to interpret the correlation between the PZT transducer and the cement specimen in the presence of uniaxial compression loading. Moreover, by embedding the PZT transducer inside cement specimens, the conductance signatures in two different frequency ranges throughout the entire loading process were captured and analyzed. It is revealed that the trend of the EMI spectra is compatible with the cement deterioration. More importantly, the resonance frequency and the statistic metric correlation coefficient (CC) can serve as good indicators to quantitatively assess the stiffness degradation of cement paste under uniaxial compression loadings. This research well validates the effectiveness and applicability of the EMI method in real-time monitoring of cement stiffness degradation under compression loadings.

Damage detection using electromechanical impedance spectra – development and validation of a non‐destructive structure monitoring system

Steel ConstructionVolume 13, Issue 2 p. 91-91 Editor's Recommendations Damage detection using electromechanical impedance spectra – development and validation of a non-destructive structure monitoring system First published: 15 May 2020 https://doi.org/10.1002/stco.202070204AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume13, Issue2May 2020Pages 91-91 RelatedInformation

Quantitative assessment of compress-type osseointegrated prosthetic implants in human bone using electromechanical impedance spectroscopic methods.

Osseointegrated (OI) prostheses are a promising alternative to traditional socket prostheses. They can enhance the quality of life of amputees by avoiding fit and comfort issues commonly associated with sockets. The main structural element of the OI prosthesis is a biocompatible metallic implant that is surgically implanted into the bone of the residual limb. The implant is designed to provide a conducive surface for the host bone to osseointegrate with. The osseointegration process of the implant is difficult to clinically evaluate, leading to conservative postoperative rehabilitation approaches. Elastic stress waves generated in an OI prosthesis have been previously proposed to interrogate the implant-bone interface for quantitative assessment of the osseointegration process. This paper provides a detailed overview of the various elastic stress wave methods previously explored for in situ characterization of OI implants. Specifically, the paper explores the use of electromechanical impedance spectroscopy (EIS) to assess the OI process in compress-type OI prostheses. The EIS approach measures the electrical impedance spectrum of lead zirconate titanate elements bonded to the free end of the implant. The research utilizes both numerical simulation and experimental verification to establish that the electromechanical impedance spectrum is sensitive (between 400 and 460kHz) to both the degree and location of osseointegration. A baseline-free OI index is proposed to quantify the degree of osseointegration at the implant-bone interface and to assess the stability of the OI implant for clinical decision making.

Effect of Thermal Treatment on CFRP Parts Before and After Adhesive Bonding

Numerous non-destructive techniques are being investigated for assuring quality of the adhesive bonds. The research presented here is focused on non-destructive assessment of carbon fibre reinforced polymer (CFRP) parts. The surface condition directly influences the performance of adhesive bonds. The structural joints should ensure safe usage of a structure. However, some modifications of the surface may lead to weak bond that cannot carry the desired load. This is why there is a search for methods of surface assessment before bonding. Moreover, reliable techniques are required to allow to verify the integrity of the adhesive bond after manufacturing or bonded repair. We focus on the laser induced fluorescence (LIF) method for assessing the surface state. The LIF is a non-contact measurement method. In the context of adhesive bond assessment the electromechanical impedance (EMI) method is studied. The EMI uses surface bonded piezoelectric sensors for excitation and sensing. The investigated samples were made of CFRP layers. The samples were treated at elevated temperatures. The influence of the thermal treatment was studied using LIF. The thermal treatment at 220℃ could be clearly distinguishedrom the rest of the considered samples. The thermally treated plates were bonded to untreated plate and then they were measured with the EMI method to study the influence of the treatment on the adhesive bond. The changes of EMI spectra were significant for the treatment at 280℃ and for some thermally treated samples that were later contaminated with de-icing fluid.

Time-domain spectral element method for modelling of the electromechanical impedance of disbonded composites

The research focuses on the electromechanical impedance method. The electromechanical impedance method can be treated as non-destructive testing or structural health monitoring approach. It is important to have a reliable tool that allows verifying the integrity of the investigated objects. The electromechanical impedance method was applied here to assess the carbon fibre–reinforced polymer samples. The single and adhesively bonded samples were investigated. In the reported research, the electromechanical impedance spectra up to 5 MHz were considered. The investigation comprised of modelling using spectral element method and experimental measurements. Numerical and experimental spectra were analysed. Differences in spectra caused by differences in considered samples were observed.