Electromechanical Impedance Response Research Articles

Assessing cement paste strength evolution under curing: An experimental and numerical investigation through equivalent stiffness parameter identified by embedded piezo sensors

Equivalent stiffness is one of the most important mechanical parameters of any concrete structure which allows engineers to predict the behaviour of the structure. This paper identifies the equivalent stiffness parameter through an embedded piezo sensor (EPS) in the strength development of cement paste using the electro-mechanical impedance (EMI) technique through experimental and numerical investigation. The experiments were conducted on the cement paste specimens; and the compressive strength (destructively) and EMI response were obtained during curing process. Further, a numerical model of cement paste specimen with EPS is developed to validate the EMI response. In addition, an equivalent stiffness parameter is identified from the experimental and simulation EMI data. It is found that the variation in conductance signature during curing process and strength development is effectively captured by the EPS. The identified equivalent stiffness either calculated from experimental or simulation data followed a similar behaviour of the compressive strength.

Integrating the Capsule-like Smart Aggregate-Based EMI Technique with Deep Learning for Stress Assessment in Concrete.

This study presents a concrete stress monitoring method utilizing 1D CNN deep learning of raw electromechanical impedance (EMI) signals measured with a capsule-like smart aggregate (CSA) sensor. Firstly, the CSA-based EMI measurement technique is presented by depicting a prototype of the CSA sensor and a 2 degrees of freedom (2 DOFs) EMI model for the CSA sensor embedded in a concrete cylinder. Secondly, the 1D CNN deep regression model is designed to adapt raw EMI responses from the CSA sensor for estimating concrete stresses. Thirdly, a CSA-embedded cylindrical concrete structure is experimented with to acquire EMI responses under various compressive loading levels. Finally, the feasibility and robustness of the 1D CNN model are evaluated for noise-contaminated EMI data and untrained stress EMI cases.

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.

Enhancement of PZT-based damage detection in real-scale post-tensioned anchorage under ambient conditions

Varying ambient temperature significantly affects electromechanical (EM) impedance responses of PZT sensors, leading to inaccurate damage detection results. In this study, we investigate temperature influences on the PZT's EM impedance response and introduce a filtering method to enhance PZT-based damage assessment of a real-scale post-tensioned anchorage. Firstly, the principles of the PZT-based damage identification technique are outlined. Next, the sensitivity of the PZT's EM impedance under simultaneous damage and temperature effects are extensively analyzed and quantified using finite element modelling. Thirdly, a 10-day long-term impedance monitoring of a real-scale anchorage specimen is conducted under ambient temperature conditions and different strand-looseness scenarios. The issues on damage detection performance of the PZT under varying ambient temperature are discussed. Finally, a temperature filtering technique based on principal component analysis is adopted to enhance the accuracy of strand looseness detection in the test anchorage. The numerical and experimental outcomes show that the impact of ambient temperature can overshadow the effect of strand looseness, leading to false damage diagnosis. By removing the influence of temperature on the EM impedance response, the strand looseness in the test anchorage was successfully diagnosed.

The Health Monitoring of Bonded Composite Joints Using Both Thickness and Radial Impedance Resonance Responses.

With the advent of bonded composites in today's aircraft, there is a need to verify the structural integrity of the bonded joints that comprise their structure. To produce adequate joint integrity, strict process control is required during bonding operations. The latest non-destructive joint inspection techniques cannot quantify the strength of the bond and only indicate the presence of disbonds or delaminations. Expensive and timely proof-load testing of the joints is required to demonstrate structural performance. This work focuses on experimentally evaluating joint-health monitoring with piezoelectric sensors exposed to repeated loadings until failure. Single-lap-shear composite joints are structurally tested while using sensor electromechanical impedance response as a health indicator. Based on these experiments, validation of this novel method is achieved through detailed evaluation of the sensor impedance response characteristics during loading, which enable initial and prognostic joint health assessments. The experimental results indicate that the embedded piezoelectric sensors are able to measure the sensor impedance radial and thickness resonance response changes prior to joint failure, without sacrificing the joints' structural performance.

Determination of the Dynamic Modulus of Elasticity of Pine Based on the PZT Transducer

A new method for the determination of the dynamic modulus of elasticity (Ed) of pine wood, based on the transverse vibration excitation and electromechanical impedance (EMI) response of the lead zirconate titanate (PZT) transducer is proposed. The influence of the length to thickness ratio of the pine specimen on the measurement accuracy was studied through modal simulation analysis. Based on the results of the modal simulation, the size of the pine specimen was optimized, and the scanning frequency range of the EMI response was determined. On this basis, the EMI simulation and test of the pine specimen coupled with a PZT patch were carried out to verify the effectiveness of the novel method. The impedance simulation results of three kinds of pine specimens show that a unique and significant formant appears in the real part of each EMI response curve, and the maximum relative errors of the rectangular PZT patch and circular PZT patch are 1.34% and 1.81%, respectively. The impedance test results of three kinds of pine specimens indicate that the maximum relative errors of the rectangular PZT patch and circular PZT patch are 1.41% and 1.68%, respectively, compared with the corresponding results obtained by the traditional transverse vibration method. Simulation and experimental results verify the validity of the proposed method for the elastic modulus determination of pine wood.

Exploring chloride-induced corrosion in reinforced concrete structures through embedded piezo sensor technology: an experimental and numerical study

Corrosion of steel in concrete is one of the major problems with respect to the durability of reinforced concrete (RC) structures. Thus, monitoring the corrosion in real-time is essential to prevent structural damage. However, one of the main challenges is to simulate the real-time development of corrosion in the RC structure. In recent years, smart aggregates, also called embedded piezo sensors (EPS), have become increasingly popular for monitoring localized and corrosion damage in RC structures using electro-mechanical impedance (EMI). This paper presents the experimental and numerical investigation of corrosion in RC structures subjected to the chloride-laden environment using EPS via the EMI technique. To fulfil this objective, the study has been carried out in two stages such as; in the first stage, the experiments are conducted on the RC specimen, and the EMI response was obtained both in a pristine state and when accelerated corrosion progressed. In the second step, a numerical model of the RC specimen has been developed based on the experimental data in the COMSOL software, and the effect of corrosion in the form of varying mass loss percentages has been simulated. Based on the results, it is concluded that the experimental and numerical conductance signatures before and after corrosion are matched. The deterioration in terms of stiffness loss in the RC specimen was 18.20% at 30% mass loss.

Experimental and numerical investigation of the bonding conditions of piezoelectric sensors under high compressive strains on structures

In the past two decades, thin lead zirconate titanate (PZT) sensors have been widely used in the electro-mechanical impedance (EMI) technique for sensing applications, particularly for monitoring civil structures. They are typically surface bonded using an industrial adhesive to the monitored structure. The bond between a PZT sensor and structure must be sufficiently strong to transmit the response of the structure to the sensor. In this study, acrylic cubes bonded with PZT patches are subjected to high compressive strains above 2000 με to develop a better understanding of bonding conditions when structures undergo such high strains. Acrylic can undergo such high strains without developing fissures or cracks. Thus, the recorded EMI response only reflects changes in the bonding condition due to the development of strains. The experiments are also numerically supplemented by simulating various debonding conditions. At higher strains, it was observed that the admittance signatures tend to behave similarly to a freely vibrating PZT patch, indicating debonding around the periphery. Even after the complete unloading of the structure, the signatures did not return to their initial state, indicating a permanent partial debonding. The strains developed on a loaded structure are not uniform and can be localized due to structural imperfections, resulting in higher strains in the region where a sensor is bonded. The insights from this study will aid in expanding the scope of the application of PZT sensors for monitoring civil structures through better comprehension of the PZT-structure bond under high compressive strains.

On the strength of the piezoceramic transducer in the system of structural health monitoring

The aim of the presented paper is the more detailed investigation and improvement of pre-stressing technology for the piezoelectrical transducer (PET) protection from degradation at action of extreme overload and longtime variable load in operation. Earlier developed 1D model of embedded PET is verified by the finite element analysis. Using this model, the parametrical assessment of stress state and strength of conventional transducer is done. It is shown that there is specific class of PET for which satisfying solution cannot always be obtained by optimization of parameters of the conventional piezoelectrical transducer (CPET). Therefore, further analysis is focused on the stress state and strength of the pre-stressed piezoelectrical transducer (PPET) it comparison with CPET. The electromechanical impedance (EMI) and its integral indicis are used for assessment of PET operability. Numerical simulation of EMI response to possible damages of PET and the outcome of static and fatigue tests are shown the significantly higher resistance of PPET against operational degradation in comparison with CPET.

Evaluation of boundary and material influences on the dynamic electromechanical impedance response of the embedded PZT sensor in concrete

Evaluation of boundary and material influences on the dynamic electromechanical impedance response of the embedded PZT sensor in concrete

Crack Detection in Bearing Plate of Prestressed Anchorage Using Electromechanical Impedance Technique: A Numerical Investigation

The bearing plate is an important part of a tendon–anchorage subsystem; however, its function and safety can be compromised by factors such as fatigue and corrosion. This paper explores the feasibility of the electromechanical impedance (EMI) technique for fatigue crack detection in the bearing plate of a prestressed anchorage. Firstly, the theory of the EMI technique is presented. Next, a well-established prestressed anchorage in the literature is selected as the target structure. Thirdly, a 3D finite element model of the PZT transducer–target anchorage subsystem is simulated, consisting of a concrete segment, a steel anchor head, and a steel bearing plate instrumented with a PZT transducer. The prestress load is applied to the anchorage via the anchor head. The EMI response of the target structure is numerically obtained under different simulated fatigue cracks in the bearing plate using the linear impedance analysis in the frequency domain. Finally, the resulting EMI response was quantified using two damage metrics: root-mean-square deviation and correlation coefficient deviation. These metrics are then compared with a threshold to identify the presence of cracks in the bearing plate. The results show that the simulated cracks in the bearing plate are successfully detected by tracking the shifts in the damage metrics. The numerical investigation demonstrates the potential of the EMI technique as a non-destructive testing method for assessing the structural integrity of prestressed structures.

Insights into the piezoceramic electromechanical impedance response for monitoring cement mortars during water saturation curing

Lead Zirconate Titanate (PZT) based electromechanical impedance (EMI) sensors were used to monitor the mechanical properties development of different water to cement ratios (w/c) cementitious mortar mixes, during the first 28 days of curing under water.Through using the analytical procedure proposed in this study to analyse the EMI data, the different mixes mechanical properties development through the curing period were detected, and the EMI response was able to provide a more detailed interpretation regarding the difference between the surface and the bulk material mechanical properties development.Both the peaks from the impedance signature (Z) and the first difference of the impedance signature (dZ) showed shifts to higher frequency ranges as the age of the samples increased, indicating an increase in the material stiffness. Furthermore, the compressive and the flexural stresses showed an R2 > 0.8 and > 0.9, respectively in relation to the frequency shifts.The relationship between the PZT-EMI response through the curing period and the sample’s mechanical properties was shown to be frequency-dependent; hence a numerical analysis using ANSYS Workbench 18.1 was undertaken to understand this frequency-dependence phenomenon. From the numerical model, the impedance signature response at higher frequency ranges was shown to be dominated by the response from the surface of the hosting material, whereas the response from the specimen's interior dominated the lower frequencies EMI response.The analytical approach proposed in this study is expected to assist in differentiating between internal cementitious materials processes, such as internal curing, and those originating at the surface, such as aggressive chemical agents penetration.

Electrochemical impedance based interfacial monitoring for concrete beams strengthened with CFRP subjected to wetting–drying cycling and sustained loading

Environmental exposure and mechanical loads would cause interfacial degradation between the concrete and carbon fiber reinforced polymers (CFRP) and thus premature failure for concrete structures strengthened with externally bonded CFRP, while reliable monitoring techniques are still in demand. This paper presents an investigation into electromechanical impedance (EMI) based monitoring for concrete beams strengthened with CFRP subjected to wetting–drying (WD) cycling and sustained loading. Concrete beams were fabricated and strengthened in flexure with externally bonded CFRP plates. Strengthened beams were subjected to aging tests (WD cycling and sustained loading) and four-point bending tests, whilst the EMI method was applied to monitor the interfacial performance between CFRP and concrete. WD cycling and sustained loading degrade the interfacial performance and reduce the transferrable force via the CFRP-concrete interface. The EMI signals can reflect the changes in the interfacial performance and show good sensitivity to debonding of CFRP plates under sustained loading and monotonic bending. The main characteristics for estimating whether interfacial damage occurs are proposed based on the EMI response. More importantly, the EMI technique is practical and promising for monitoring the existing strengthened structures even when no EMI history data are available.

Specific resonance mode enhancement and suppression using non-uniform polarization of piezoelectric wafers: Theory and experiments

We present experimental demonstration of specific resonance mode enhancement and suppression in circular and rectangular piezoelectric wafers with engineered non-uniform polarization profiles. The polarization profiles are designed based on the electromechanical impedance response of non-uniformly polarized wafers as obtained from the theory. The circular wafers are fabricated with non-uniform polarization profiles that involve a central polarized region surrounded by an unpolarized region. The radius of the polarization zone is designed based on the condition for specific mode enhancement obtained from the electromechanical impedance response of non-uniformly polarized wafers. We actually show how the condition can not only be used to enhance but also to suppress electromechanical resonances. Two kinds of wafers are designed and fabricated to specifically suppress second and third radial modes respectively. Similarly, rectangular wafers are designed with two different kinds of non-uniform polarization profiles — the first of which enhances the second in-plane extensional mode in the impedance spectrum and the second polarization profile suppresses all the electromechanical resonances pertaining to the in-plane extensional modes and selectively excites only the in-plane bending modes. The proposed approach of using non-uniformly polarized wafers finds application in designing multi-frequency sensors/transducers, frequency-tuned receivers, acoustic beamforming, and other non-traditional applications such as information storage.

Modeling and experimental validation of a quantitative bar-type corrosion measuring probe using piezoelectric stack and electromechanical impedance technique

Corrosion coupon method has been widely used to estimate the corrosion rate in multiple industries. However, in this method, the weights of the coupons are measured periodically, which limit the application for on-line monitoring. In this paper, a novel type of quantitative bar-type corrosion measuring probe using piezoelectric stack and electromechanical impedance (EMI) technique was proposed. The probe consists of a piezoelectric stack and a metal bar. The multilayer models were used to derive the solution of the probe in longitudinal vibration mode. Five probe prototypes with designated probe length were fabricated to simulate uniform corrosion induced mass loss and investigate the EMI response with probe length. The relationship between the corrosion induced probe length loss and the first and second resonant and anti-resonant frequencies were analyzed. The measured results agreed well with the theoretical predictions. In addition, the accelerated corrosion tests were also performed to induce corrosion to the probe in a realistic setup and further validate the efficacy of the proposed method. The present study proved the feasibility of using the proposed bar-type corrosion measuring probe to quantitatively assess the corrosion amount by introducing the longitudinal vibration with piezoelectric stack and EMI.

New refined analytical models for various bonding conditions of an adhesively bonded smart PZT transducer using the EMI technique

The electro-mechanical impedance (EMI) technique has emerged as a cost-effective and non-destructive technique to detect the possible damages in the structure using a piezoelectric transducer, especially, lead zirconate titanate (PZT). The adhesive bond layer plays an important role in the PZT patch-host structure interaction for monitoring structural damage. Two bonding conditions are investigated in this research paper. Primarily, the debonding phenomenon of the adhesive bond layer may misinterpret the EMI response on the damage caused in structure. Subsequently, the investigation included the protective layer at the top of the PZT transducer to avoid sensor degradation. However, the analytical models developed so far have not considered a protective layer at the top of the PZT transducer. This paper presents the novel two-dimensional (2D) analytical model for incorporating debonding concepts and the new refined 2D analytical model to include a protective layer in the study of surface-bonded PZT transducers. The proposed analytical models are verified with the experimental studies. The experimental and analytical results show good agreement, which confirms the effectiveness of the new models. This paper also incorporated the effect of each bonding condition for monitoring structural damage by implementing the EMI technique. For the simulation, the numerical investigations on the PZT transducer bonded on the metallic (aluminum and steel) and concrete blocks are performed using coupled field analysis through finite element (FE) modeling. It is found that each bonding condition has influenced the resulting signatures. The signatures obtained from developed theoretical models and numerical simulations using three-dimensional FE models for each bonding condition are compared to highlight the influence on structural damage detection. The trend of signatures is found to be matching satisfactory. Several parametric studies have been conducted to show the efficacy of the new refined model with a protective layer. It considers the different input properties of an adhesive layer, host structure, and temperature conditions. The influence of debonding of the protective layer is also studied, and the obtained results support the need for a protective layer in the models.

A Low-Cost Prestress Monitoring Method for Post-Tensioned RC Beam Using Piezoelectric-Based Smart Strand

This study proposes a cost-effective prestress monitoring method for post-tensioned reinforced concrete (RC) beams using a smart strand. Firstly, the concept of a piezoelectric-based smart strand and its implementation for prestress force monitoring are developed. The smart strand is prepared by embedding inexpensive and high-sensitivity electromechanical impedance (EMI) sensors in a steel strand. Next, the feasibility of the proposed method is experimentally verified for prestress force monitoring of a simple supported post-tensioned RC beam. A smart strand prototype is fabricated and embedded into a 6.4 m RC beam which is then prestressed with different levels. For each prestress level, the EMI responses of the smart tendon are measured and the EMI features are extracted for prestress force monitoring. The results showed that the EMI signals of the smart strand showed strong resonant peaks that varied sensitively to the prestress level of the beam. The prestress change in the prestressed RC beam was successfully estimated by using linear regression models of the EMI features.

Understanding Impedance Response Characteristics of a Piezoelectric-Based Smart Interface Subjected to Functional Degradations

The functionality of piezoelectric devices is of significant importance in the electromechanical impedance (EMI)-based structural health monitoring (SHM) and damage detection. Despite the previous work, the EMI response characteristics of a degraded piezoelectric-based smart interface have not been sufficiently investigated due to the difficulty in making realistic functional defects via the experiment. To overcome this issue, we present a predictive simulation strategy to comprehensively investigate the EMI response characteristics of a smart interface subjected typical functional degradations. For that, a bolted steel girder connection is selected as a host structure to experimentally conduct EMI response measurement via the smart interface. Then, a finite element (FE) model corresponding to the experimental model is established and updated to reproduce the measured EMI response. By using the updated FE model, four common degradation types, including shear lag effect, transducer debonding, transducer breakage, and interface detaching are simulated and their effects on the EMI response are comprehensively analyzed. It is found that the interface detaching defect has significant impacts on the primary resonances of the EMI response and generates additional peaks with complex modal shapes. Also, the functional defects can result in distinctive EMI response characteristics, which are promising for assessing the functional condition of the smart interface.

Deep learning-based functional assessment of piezoelectric-based smart interface under various degradations

The piezoelectric-based smart interface technique has shown promising prospects for electro-mechanical impedance (EMI)-based damage detection with various successful applications. During the process of EMI monitoring and damage identification, the operational functionality of the smart interface device is a major concern. In this study, common functional degradations that occurred in the smart interface are diagnosed using a deep learning-based method. Firstly, the effect of functional degradations on the EMI responses is analytically discussed. Secondly, a critical structural joint is selected as the test structure from which EM measurement using the smart interface is conducted. Thirdly, a numerical model corresponding to the experimental model is established and updated to reproduce the measured EMI responses. By using the updated numerical model, the EMI responses of the smart interface under the common functional degradations, such as the shear lag effect, the adhesive debonding, the sensor breakage, and the interface detaching, are simulated; then, the functional degradation-induced EMI changes are characterized. Finally, a convolutional neural network (CNN)-based functional assessment method is newly proposed for the smart interface. The CNN can automatically extract and directly learn optimal features from the raw EMI signals without preprocessing. The CNN is trained and tested using the datasets obtained from the updated numerical model. The obtained results show that the proposed method was successful to classify four types of common defects in the smart interface, even under the effect of noises.

A simple PZT transducer design for electromechanical impedance (EMI)-based multi-sensing interrogation

A simple sensor design to achieve different electromechanical impedance (EMI) characteristics is required for the serial/parallel multi-sensing interrogation. Aiming at the shortcomings of traditional trial-and-error method for collocating serial/parallel PZT transducer with different EMI characteristics, this paper proposes a simple but effective method to design and fabricate PZT transducers with distinguished EMI characteristics by bonding PZT patch on permanent magnet disks with different thickness. The thickness of the magnetic disk could change the impedance resonant frequency of the transducer almost linearly, which greatly simplifies the transducer design to ensure each one has distinguished peak frequency. The EMI responses of proposed smart modulation transducer (SMT) under different thickness of magnetic disk were obtained by finite element method (FEM) simulations, as well as by experiments. The simulation outcomes are in good agreement with experimental results. And it is concluded that the EMI resonant peak frequency of SMT increases linearly due to the increase of magnet thickness, which ensures each SMT has very different EMI characteristics. Both experiments and simulations have confirmed that the proposed method is effective. To obtain the insight of the performance of the SMT to structural damage detection using multi-sensing EMI method, the four fabricated SMTs with different thickness of magnetic disk were series-connected for multi-bolt loosening monitoring. Results validate that the response signatures with distinguished EMI characteristics can be obtained, and both the severity and location of bolt looseness can be identified via the modified MAPD (mean absolute percentage deviation) damage index. The proposed PZT design method has great guiding role for optimizing the design of sensors for EMI-based multi-sensing interrogation, promoting the practical application of EMI technology in metal structure health monitoring.