Electromechanical Impedance Technique Research Articles

Preliminary acousto-ultrasonic investigation for multi-parameter transducer self-diagnostic system in composites

Commercial aerospace vehicles are demanded to withstand harsh conditions with a low weight impact. Composites are massively used to achieve the optimal trade-off about such performances. However, actually the effect of barely visible flaws on the damage tolerance approach reduces strongly the benefits introduced. Structural Health Monitoring is the most promising technology aimed to compensate such shortcoming aspect. Among several methods, guided waves are efficiently adopted to detect hidden flaws using small integrated transducers, whose reliability is affect by hazard operating conditions. The self-sensing of such devices became indeed crucial to reduce false alarms in damage detection. Although the electromechanical impedance technique is proved to be effective for sensor fault due to different degradation levels, several failures may induce different effects warning a possible compensation. The present work discusses such effects with a comprehensive acousto-ultrasonic experimental campaign. The combined wave propagation and electromechanical impedance investigation mostly assesses the necessity and the feasibility of a multi-parametric self-sensing system. This approach is verified performing several measurements using Laser Doppler Velocimetry (LDV) as well as electromechanical signature measurements on typical aeronautical composites.

STRUCTURAL HEALTH MONITORING USING PZT A REVIEW

Due to the involution of damage occurring in the structure it is difficult to monitor structural health. Electromechanical Impedance technique (EMI) is used for the damage detection in the structural member experimental results are obtained by the applying piezoceramic patches surface bonded and embedded PZT (Lead Zirconate Titanate) patchs are used for the detection of structure failure. This papers reviews online strength gain monitoring of early age concrete is detected with the help of EMI sensing technique. By using this methods detection s well as location, severity estimation is also possible. The piezoceramic materials performs the both works sensor as well as actuator. The proposed method of damage detection are quit effective and sensitive to concrete.

Evaluation the compressive strength of the cement paste blended with supplementary cementitious materials using a piezoelectric-based sensor

This paper aims to investigate the feasibility of using piezoelectric-based sensors to characterize the compressive strength gain process of cement paste blended with supplementary cementitious materials. The electromechanical impedance technique was used for in-situ monitoring of the strength gain of cement pastes. Two different indices of root mean square deviation (RMSD) and correlation coefficient (CC) have been used to establish a quantitative correlation between the conductance signature obtained by lead zirconate titanate (PZT) sensors and the compressive strength of cement paste. Both indices exhibited a reasonable qualitative trend which was compatible with the trend of strength gain of cement pastes. However, the RMSD was found to be more efficient than CC index in estimating the compressive strength of cement paste over time. The experimental results indicate that EMI can be used as a nondestructive testing (NDT) method to enable in-situ measurement of strength gain process of cement paste with supplementary cementitious materials.

Prognosis of low-strain fatigue induced damage in reinforced concrete structures using embedded piezo-transducers

Fatigue induced damage under low-strain conditions is a commonly encountered phenomenon in reinforced concrete (RC) structures subjected to continuous vibrations, such as bridges under vehicular loads, buildings under wind and structures directly supporting or in the vicinity of vibration emitting machinery. This type of fatigue tends to weaken the structure silently, and quite often, its timely cognizance is missed out in the life-cycle management of the structure. Among the various structural health monitoring (SHM) frontiers, very scarce research has been devoted to this kind of damage in RC structures. This paper covers a long-term experimental study related to low-strain fatigue damage monitoring encompassing a real-life sized RC structure. The experimental specimen was subjected to over eight million cycles of flexural loading with the maximum bending strain values restricted to 50 µm/m. The structure was instrumented with piezo-based composite concrete vibration sensors (CVS), which were embedded inside the beam near the surface. The CVS operated in dual mode, acting as sensor for the global vibration technique as well as the local electro-mechanical impedance (EMI) technique. As EMI sensors, they facilitated the determination of the equivalent stiffness parameter (ESP), and were found to be very expedient for damage detection as well as localization during the incipient stages of fatigue damage, coinciding with the appearance of first few cracks. The ESP served to represent the diminishing trend to actual residual stiffness fairly well during the initial stages. Acting in the global mode, determination of the overall stiffness of the structure by the same CVS provided an alternate damage measure, in terms of a realistic estimate of the overall residual flexural stiffness. This proved useful parameter during moderate to severe damage conditions, and in particular near the final failure of the structure. The monitoring paradigms presented in the paper pave way for effective prognosis of fatigue induced damage in real-life RC structures.

Investigating the performance of “Smart Probe” based indirect EMI technique for strength development monitoring of cementitious materials – Modelling and parametric study

Electromechanical impedance (EMI) technique has been proven effective in strength development monitoring of early age cementitious materials with the aid of statistical tools. The critical limitation of this technique lies in that no parametric evaluation of the cementitious material can be provided. This is partially due to the inconsistency of the baseline of the measured impedance signatures. The recently developed Smart Probe based indirect EMI technique overcomes the abovementioned drawbacks of this technique, enabling parametric estimation of the cementitious material being monitored. The influence of various physical parameters of the Smart Probe on its performance in curing monitoring of cementitious material has not been investigated. In this study, the analytical model of the Smart Probe previously proposed by the authors is improved. In the improved model, the distributed stiffness and mass of the embedded segment of the Smart Probe are explicitly formulated in the governing equations of motion, allowing more realistic predictions of the actual dynamic features of the Smart Probe-cementitious material system. A 3-D coupled field finite element (FE) model is also established to predict the admittance spectrum of the Smart Probe embedded in cementitious material. It is found that multiple resonance peaks in the conductance spectrum computed by FE simulation are in good agreement with the experimental data in terms of resonance frequency. The resonance peaks predicted by both the FE and the analytical models exhibit the same trend of movement as the experiment throughout the curing process, which verifies the effectiveness of the proposed models in monitoring the strength development process of cementitious materials. Parametric study conducted on the Smart Probe shows that the effect of the Poisson’s ratio of the cementitious material on the resonance frequency is limited when compared with that of the dynamic modulus. It is also found that the dimensions and position of the PZT patch influence the magnitude and the sharpness of the resonance peaks in the admittance spectrum. The improved analytical model can be a useful tool for optimizing the design of Smart Probes for monitoring purpose.

Investigations on effectiveness of embedded PZT patches at varying orientations for monitoring concrete hydration using EMI technique

The electro-mechanical impedance (EMI) technique has already been established to be an effective technique for detecting incipient damages in engineering structures including reinforced concrete (RC) using smart piezo-electric sensors. Its effectiveness in monitoring the hydration of concrete, using surface bonded and embedded lead zirconium titanate (PZT) patches has also been investigated well. However, the influence of patch orientation while embedding them in concrete is still unexplored. The present study investigates the effectiveness of the EMI technique using PZT patches embedded at different orientations (here, 0°, 45° and 90°) with the axis of the structure, while monitoring the hydration of a real-life prototype RC beam. Further, the effectiveness of the PZT patches in monitoring the strength gain in concrete is investigated. It is observed that the PZT patch placed in the inclined orientation (45°) least efficient in capturing the progressive changes of hydration in the concrete in comparison to the other two orientations. The EMI technique showed best results when PZT patch was placed in horizontal position (0°) with the longitudinal axis of the beam.

A New Approach For Dressing Operation Monitoring Using Voltage Signals Via Impedance-Based Structural Health Monitoring

Among the methods used in structural health monitoring (SHM), the electromechanical impedance technique (EMI), which uses piezoelectric transducers of lead zirconate titanate (PZT), stands out for its low cost. This paper presents a new approach for monitoring of the dressing operation based on structural health monitoring from the digital processing of voltage signals based on the time-domain response of a PZT transducer by EMI method. Experimental tests of the dressing process were performed by using a single-point dresser equipped with a natural diamond. The voltage signals in the time-domain were collected in different damage levels using a measurements EMI System. By using damage metrics, it was possible to qualify different damage levels that the diamond suffered during the dressing operation, observing variations from the magnitude of the signals. The dressing operation is of utmost importance for the grinding process and the dresser wear negatively affects the result of the process, which owns high added value. In this way, this work contributes with a new monitoring tool which aims ensuring a consistent dressing operation.Keywords: Manufacturing process, automation, electromechanical impedance, dressing operation.

Monitoring of pin connection loosening using eletromechanical impedance: Numerical simulation with experimental verification

Pin connection, as an important structural connection mechanism, is widely used in various structures, especially spatial structures. In this article, numerical and experimental investigations are performed for monitoring the loosening of pin-connected structures using the electromechanical impedance technique. For this purpose, a finite element model for a pin-connected structure considering the contact interfaces between the pin and the support base is proposed to study the effect of pin connection loosening on the electromechanical impedance signatures. A multi-physics analysis is conducted to simulate the electromechanical behavior of lead zirconate titanate transducers bonded on the pin head and the steel base. The relationship between the force applied on the pin connection and the variation in electromechanical impedance signatures is established. Experiments are carried out to verify the accuracy of the proposed finite element model. The results show that the changes in the electromechanical impedance signatures consist of frequency shifts and peak splitting. The contact condition of the pin connection can be assessed by observing the changes in the electromechanical impedance signatures. The location of the piezoelectric patch has significant effect on the sensitivity of the electromechanical impedance signatures. The numerical study in this research helps to optimize the design of sensor placement and improve the detection accuracy of the electromechanical impedance method in practical applications of detecting the loosening of pin-connected structures.

Use of Savitzky-Golay Filter for Performances Improvement of SHM Systems Based on Neural Networks and Distributed PZT Sensors.

A considerable amount of research has focused on monitoring structural damage using Structural Health Monitoring (SHM) technologies, which has had recent advances. However, it is important to note the challenges and unresolved problems that disqualify currently developed monitoring systems. One of the frontline SHM technologies, the Electromechanical Impedance (EMI) technique, has shown its potential to overcome remaining problems and challenges. Unfortunately, the recently developed neural network algorithms have not shown significant improvements in the accuracy of rate and the required processing time. In order to fill this gap in advanced neural networks used with EMI techniques, this paper proposes an enhanced and reliable strategy for improving the structural damage detection via: (1) Savitzky–Golay (SG) filter, using both first and second derivatives; (2) Probabilistic Neural Network (PNN); and, (3) Simplified Fuzzy ARTMAP Network (SFAN). Those three methods were employed to analyze the EMI data experimentally obtained from an aluminum plate containing three attached PZT (Lead Zirconate Titanate) patches. In this present study, the damage scenarios were simulated by attaching a small metallic nut at three different positions in the aluminum plate. We found that the proposed method achieves a hit rate of more than 83%, which is significantly higher than current state-of-the-art approaches. Furthermore, this approach results in an improvement of 93% when considering the best case scenario.

Real-Time Strength Monitoring for Concrete Structures Using EMI Technique Incorporating with Fuzzy Logic

This study estimates the strength of a special mixture of high-strength concrete (HSC) with admixtures for use in a nuclear power plant (NPP). Nuclear power plant structures need a HSC with some additional qualities to operate the safe options. For this purpose, the experimented concrete was specially designed to fulfill the required qualities of NPP. For gaining these desirable qualities, it needs to monitor the concrete strength development process. Here, the PZT materials were used as sensors to acquire data by measuring the electromechanical impedance (EMI), and then cross correlation (CC) was calculated to look at changes according to strength development. Data were measured for 28 days, and over this period concrete can gain up to 96% of its design strength. This technique is based on a single sensor. After casting concrete, the PZT material starts vibrating as an actuator to produce vibrations. At the same time, it also works as a sensor to measure the dynamic response of the structure to the vibrations. With strength development, the resonant frequencies of the EMI start changing. To estimate the strength development, a fuzzy logic tool was used to analyze the parameters, allowing for us to estimate and predict the concrete strength. For cross-checking, the estimated strength was compared with the actual strength of concrete; this was determined by examining cuboid cores taken from specimens during experiments at the 1st, 3rd, 7th, 14th, and 28th days. According to the results, this approach of strength estimation and monitoring the strength development is useful for forecasting the stability of structures.

Comparative study of deterioration of composite due to ice formation using strain, electromechanical impedance and guided waves

Glass fibre reinforced plastic (GFRP) composites are finding increasing application in aerospace structures. The monitoring of these structures is not only necessary but also mandatory by the safety codes. The present state of the art allows isolation of damage and the quantification of damage is the next challenge. The quantification may allow for better maintenance scheduling and as a result lower downtime for airplanes, which makes it significant in the aerospace discipline. It is known that the moisture absorption in the composites affects its mechanical properties. The moisture can interact with the composite sample in the gaseous, liquid or solid phase. The mechanism for deterioration of the composite due to the moisture depends on the phase in which the interaction occurs. The paper thus investigates the interaction of water absorbed by the sample in the solid phase. The GFRP sample is exposed to low temperatures after the ingress of the moisture. The sample is studied using three distinct damage detection methods namely, Electromechanical Impedance technique (EMI), Guided waves method (GW) and NA based technique based on measured strains. The aim of the research is to compare the performance of the three methods for the assessment of the deterioration of the composite samples due to ice. The EMI, GW and strain based method have been shown to be sensitive to moisture induced deterioration. In addition to the detection of deterioration, the use of measured strains provides an intuitive way for the quantification of the moisture induced deterioration in the sample.

Time-domain Analysis Based on the Electromechanical Impedance Method for Monitoring of the Dressing Operation

Among the methods used in structural health monitoring (SHM), the electromechanical impedance technique (EMI), which uses piezoelectric transducers of lead zirconate titanate (PZT), stands out for its low cost. Then, this paper presents a new approach for monitoring of the dressing operation from the digital processing of voltage signals based on the time-domain response of a PZT transducer by EMI method. Experimental tests of the dressing process were performed by using a single-point dresser equipped with a natural diamond. The voltage signals in the time-domain were collected in different damage levels. By using temporal statistics, it was possible to qualify different damage levels that the diamond suffered during the dressing operation, observing variations from the magnitude of the signals. The dressing operation is of utmost importance for the grinding process and the dresser wear negatively affects the result of the process, which owns high added value. In this way, this work contributes with a new monitoring tool which aims ensuring a consistent dressing operation.

2-D Statistical Damage Detection of Concrete Structures Combining Smart Piezoelectric Materials and Scanning Laser Doppler Vibrometry

In the present study a new structural health monitoring (SHM) technique is proposed as well as a new damage index based on 2-D error statistics. The proposed technique combines the electromechanical impedance technique (EMI) which is based on the use of piezoelectric Lead Zirconate Titanate (PZT) patches and Scanning Laser Doppler Vibrometry (SLDV) for damage detection purposes of concrete structures and early age monitoring. Typically the EMI technique utilizes the direct and inverse piezoelectric effect of a PZT patch attached to a host structure via an impedance analyzer that is used for both the actuation and sensing the response of the PZT-Host structure system. In the proposed technique the attached PZTs are actuated via a function generator and the PZT-Host structure response is obtained by a Scanning Laser Doppler Vibrometer. Spectrums of oscillation velocity of the surface of the attached PZTs vertical to the laser beam versus frequency are obtained and are evaluated for SHM purposes. This damage detection approach also includes the use of a damage index denoted as ECAR (Ellipse to Circle Area Ratio) based on 2-D error statistics and is compared to the Root Mean Square Deviation (RMSD) damage index commonly used in SHM applications. Experimental results include ascending uniaxial compressive load of concrete cubic specimens, ascending three point bending of reinforced concrete beam specimens and early age monitoring of concrete. Results illustrate the efficiency of the proposed technique in damage detection as well as early age monitoring as, in the first case, both severity and location of damage can be determined by examining the values of damage indices for each damaged state and in the early age monitoring case damage indices follow the strength gain curve.

Influence of piezoelectric parameters on admittance diagnostic signals for structural health monitoring: a numerical study

The electromechanical impedance (EMI) technique for structural health monitoring (SHM) relies on the analysis of the electrical admittance response, known as the admittance signature, of a piezoelectric ceramic (PZT) chip bonded on or embedded within a structure. Damage in the structure alters its mechanical impedance. Owing to the dynamic coupling that exists between the mechanical impedances of the structure and the PZT chip, this is in turn reflected in the changed admittance signature. The changes in the admittance signature due to deterioration of the PZT characteristics need to be distinguished from those due to structural damage for effective SHM. This paper presents the results of a numerical study carried out to understand the influence of PZT material parameters and geometric characteristics on the admittance signature, which may be useful for effective analysis of diagnostic signals for SHM purposes.

Design and Implementation of an Accurate, Portable, and Time-Efficient Impedance-Based Transceiver for Structural Health Monitoring

Reducing maintenance costs while increasing the safety and reliability, especially in moving structures, needs a reliable nondestructive analyzer. The aim of this research is to provide a practical solution for this problem based on high-frequency excitation of stationary and moving structures by propagating standing Lamb waves in the range of 1 to 1000 kHz. The proposed solution comprises of a controlled frequency swept signal source, a number of piezoelectric sensors, a portable analyzer, and a rotary mechanism. Measuring the accurate electromechanical impedance (EMI) is made possible by monitoring the applied voltages, currents, and phase differences accurately. In this study, design and implementation of a low-cost, compact, and portable transceiver is explored for periodic structural health monitoring of a proposed rotary structure using EMI technique. The compactness of the proposed system is an essential requirement for rotary structures as compared with bulky, heavy, and expensive impedance analyzers. Challenges in design and development of such a system are discussed in this paper, together with mitigations to make the system functional and practical. An experimental study is carried out in frequency domain to measure the real and imaginary parts of impedance spectrum of piezo-transducers. The results show that the portable transceiver has the capability to detect structural incipient damages before any catastrophic failure, thus avoiding undesirable shut down during the operation.

Equivalent Circuit of Piezoelectric Diaphragms for Impedance-Based Structural Health Monitoring Applications

Piezoelectric diaphragms are low-cost acoustic components commonly used in a wide variety of electronic devices, where the typical function is to generate an audible alarm. However, scientific study concerning the use of these components in advanced applications has increased in recent years because of their low cost and widespread availability in several standard sizes. Based on a growing interest in these components, we propose a basic equivalent electromechanical circuit considering a 1-D vibration assumption of piezoelectric diaphragms for structural health monitoring applications based on the electromechanical impedance (EMI) technique, which was used in this paper to assess the sensitivity of piezoelectric diaphragms to structural damage. The proposed circuit is a three-port type and is suitable for small metal structures. The sensitivities of three diaphragms of different sizes to structural damage were assessed using the proposed equivalent circuit and compared with the sensitivity of a conventional transducer commonly used in the EMI method. In addition, tests were performed on aluminum bars to experimentally validate the equivalent circuit, and the theoretical and experimental results correspond well.

Crack detection in pipelines using multiple electromechanical impedance sensors

An extensive network of pipeline systems is used to transport and distribute national energy resources that heavily influence a nation’s economy. Therefore, the structural integrity of these pipeline systems must be monitored and maintained. However, structural damage detection remains a challenge in pipeline engineering. To this end, this study developed a modified electromechanical impedance (EMI) technique for crack detection that involves fusing information from multiple sensors. We derived a new damage-sensitive feature factor based on a pipeline EMI model that considers the influence of the bonding layer between the EMI sensors and pipeline. We experimentally validated the effectiveness of the proposed method. Finally, we used a damage index—root mean square deviation—to examine the degree and position of crack damage in a pipeline.

A novel electromechanical impedance–based model for strength development monitoring of cementitious materials

Strength monitoring of early age concrete improves the efficiency of construction as it provides information on the optimum time for shoring removal and pre-stress transferring. Electromechanical impedance technique has been proven to be a useful tool for strength monitoring of cementitious materials. One of the key limitations of this technique is the lack of physical models, which resulted in strong reliance on statistical analysis tools to quantify the strength of structure being monitored. In this proof-of-concept study, a novel electromechanical impedance–based model with the potential of monitoring the strength of cementitious materials using the concept of Smart Probe is proposed. Instead of directly bonding a lead zirconate titanate patch on the host structure, a lead zirconate titanate was first surface-bonded on a pre-fabricated aluminum beam, which is termed Smart Probe. The Smart Probe was then partially embedded into cementitious materials for strength monitoring. The structural resonant frequencies of the Smart Probe can be identified from the conductance signatures measured from the lead zirconate titanate patch throughout the curing process and serve as strength indicator. By modeling the cementitious material as an elastic foundation supporting the Smart Probe, an analytical model was developed to predict the dynamic modulus of elasticity of cementitious materials based on the resonance frequency of the Smart Probe. Experimental study was carried out on a mortar slab specimen to verify the model and to investigate the performance of the Smart Probe. It was found that the dynamic modulus of elasticity of the host structure could be predicted from the conductance signatures using the proposed model. Compressive strength assessment was achieved by establishing an empirical relation with the dynamic modulus. The proposed electromechanical impedance–based model with Smart Probe is physically parametric in nature and shows high repeatability, which renders its superiority over the conventional statistical method–based electromechanical impedance technique for strength monitoring of cementitious materials.

Impedance-based damage detection under noise and vibration effects

The electro-mechanical impedance technique has been extensively studied in recent decades as a non-destructive method for detecting structural damage in structural health monitoring applications using low-cost piezoelectric transducers. Although many studies have reported the effectiveness of this detection method, numerous practical problems, such as the effects of noise and vibration, need to be addressed to enable this method’s effective use in real applications. Therefore, this article presents an experimental analysis of noise and vibration effects on structural damage detection in impedance-based structural health monitoring systems. The experiments were performed on an aluminum bar using two piezoelectric diaphragms, where one diaphragm was used to measure the electrical impedance signatures and the other diaphragm was used as an actuator to generate noise and controlled vibration. The effects of noise and vibration on impedance signatures were evaluated by computing the coherence function and basic damage indices. The results indicate that vibration and noise significantly affect the threshold of the lowest detectable damage, which can be compensated by increasing the excitation signal of the piezoelectric transducer.

Study of electromechanical impedance changes caused by modifications of CFRP adhesive bonds

AbstractNumerous techniques of nondestructive testing and structural health monitoring of CFRP structural parts are studied. In this research, we focus on electromechanical impedance (EMI) technique. This technique is based on a piezoelectric sensor that is surface mounted on or embedded in the inspected structure. Because of direct and converse piezoelectric effects, the electrical response of the sensor is related to mechanical characteristic of the structure. In the reported research, adhesively bonded CFRP samples were investigated. The EMI characteristics of samples with modified bonds were compared with properly bonded referential samples. The following modifications were considered: prebond thermal treatment, prebond contamination with de‐icing fluid, and precuring of the adhesive. The EMI spectra were investigated searching for anomalies and changes caused by modification of the adhesive bonds. Numerical indexes were used for the comparison of EMI characteristics. The sensitivity of the EMI method to modified bonds was observed.