Electromechanical Impedance Method Research Articles

On damage location techniques and future prospects for industrial applications utilizing the electromechanical impedance method: a systematic review

On damage location techniques and future prospects for industrial applications utilizing the electromechanical impedance method: a systematic review

Evaluation of Damage Severity and Strength of Structural Member Using Macro‐Molecular PZT Sensor

AbstractThe structural health of various engineering systems has been easily monitored using macromolecular piezoelectric ceramic lead zirconate titanate (PZT) based electromechanical impedance (EMI) technique. It is in need to study the evaluation of various levels of damage and the present strength of structural systems to enhance the life of structures. Once damages occur, due to continuing usage they usually propagate in certain directions. Moreover, the increase in the severity of damages may lead to failure of the structural components or even the whole structure. Interaction between structure and PZT, which is found effective in EMI technique, is to identify effective damage detection. The macro‐molecular PZT's self‐actuating and sensing properties are utilized by the electromechanical impedance (EMI) method. The higher modes of vibration are locally activated by the local application of an AC source on the PZT transducers attached to the host structure. In this paper, the EMI technique using surface‐bonded PZT transducers is employed to obtain the structural health signature. The experiment is carried out on a real‐size concrete frame structure subjected to artificial damage to identify and locate the damages using frequency variations, and the severity is checked using extracted equivalent parameters; the damage index. This method also can identify hair‐like cracks at an early stage, which explains how it can shield the building from serious failures.

Polymeric Macro‐Molecular Piezoelectric Sensors for Evaluation of Structural Damage

AbstractFinding damages and their prevalence in structures is a very challenging problem. In order to identify potential structural damage, non‐destructive methods are not very helpful. One of the most popular piezoelectric ceramic materials used worldwide is macromolecular lead zirconate titanate (PZT), also known as Pb[Zr(x)Ti(1‐x)]O3. PZT transducers are proving to be a successful alternative for assessing the structure's health. The Macro‐molecular PZT's self‐actuating and sensing properties are utilized by the electromechanical impedance (EMI) method. In this capacity, the macro‐molecular PZT patches serve as co‐located actuators and sensors and make use of ultrasonic vibrations to produce a distinctive admittance “signature” of the structure. PZT patches perform exceptionally well in terms of damage sensitivity. The higher modes of vibration are locally activated by the local application of an AC source on the PZT transducers attached to the host structure. The alteration of the admittance response is a sign of injury to the transducer's vicinity. In the current study, the damage percentage and its location in a 150 × 150 × 150 mm concrete cube are being examined. Regression analysis is also used to determine the concrete cube's strength. This method also can identify hair‐like cracks at an early stage, which explains how it can shield the building from serious failures.

Monitoring and quantification of ultrasonic fatigue damage in copper based on internal friction measurement

Fatigue damage in metals and alloys would reduce their strength and should be identified as early as possible. However, fatigue damage is usually difficult to detect even using advanced nondestructive testing method. Here we developed an automated damping monitoring system for ultrasonic fatigue testing based on a quantitative electromechanical impedance method. During fatigue testing, the damping and resonance frequency of the Piezo actuator/horn/specimen system is measured regularly using an impedance analyzer. When the system damping suddenly increases, it implies that fatigue damage may be generated in the specimen. Separate impedance measurements on the copper specimen in a vacuum chamber show that the internal frictions of damaged specimens are much larger than the fresh specimens. Subsequent X-Ray micro-CT scanning shows that microcracks up to 0.8 mm in length were generated in those fatigued specimens with increased internal frictions. Further tensile testing shows that compared to the fresh specimen, specimens with fatigue damage exhibits reduced tensile strength and elongation at break. Furthermore, the larger the internal friction, the smaller the remaining yield strength and a nearly linear relationship between them holds. Finally, a damage variable DQ is defined based on internal friction which can act as a measure of fatigue damage in metals.

Structural Condition Monitoring Using Deep Learning on a Metallic Part Fabricated by Additive Manufacturing

Additive manufacturing (AM) was originally developed to manufacture polymer prototypes. Today, it has been used for the manufacturing of many critical machine components. Most of the structural health monitoring (SHM) methods were developed for monitoring the condition of large and thin plates on airplane fuselages. Additively manufactured parts are generally small, thick, and have complex geometries. SHM methods have been improved to sense load, detect defects, and identify loose bolts with the help of a permanently installed sensor. In this study, the adaptability of SHM methods was researched with additively manufactured metal parts with complex geometry. Magnets were used to apply pressure to 9 different locations on the surface of a stainless steel additively manufactured thick plate with deep groves. SHM was used to estimate the magnets’ location. Many SHM (Lamb wave) methods cannot work on smaller parts since their dimensions are shorter or very close to the wavelength of the created oscillations. Surface response to excitation (SuRE) method which has similar characteristics to electromechanical impedance methods was used for data collection. To obtain descriptive features of the time domain data, fast Fourier transformation (FFT), short-time Fourier transformation (STFT), continuous wavelet transformation (CWT), and synchrosqueezing transform (SST) were applied to CWT. 1D and 2D convolutional neural networks (CNN) were used to classify the cases. When CNN was optimized for the analysis of our data, 100% location estimation accuracy was obtained by using 50% of 320 scalograms for training. The scalograms were obtained by enhancing the CWT results with SST. STFT-CNN combination was the second best. It obtained 95% accuracy with the same number of spectrograms and training allocation.

Damage detection and localization based on different types of actuators using the electromechanical impedance method in 3D-printed material

Electromechanical impedance (EMI) measurement, using piezoelectric transducers (PZTs) in the high-frequency range is a potential method for assessing the health of lightweight structures. The major objective of this work is to comprehend how different actuators react to damage in additively manufactured (AM) polymer structures. A novel frequency-range selection technique was suggested based on the maxima of the standard deviation of the impedance frequency spectra gathered for the referential and damage cases. A 3D-printed acrylonitrile butadiene styrene (ABS) plate was used for the investigation, where two PZT and one macro fiber composite (MFC) actuator were glued to the surface. Small magnets were used to simulate damage and were positioned at increasing distances from each transducer as EMI measurements were made using the MFC and 1 PZT. This served both in studying the transducers’ sensitivity to damage and selecting the proper frequency range for damage detection utilizing the standard-deviation approach. The EMI-acquired data from the MFC actuator displays damage-sensitive peaks in a low-frequency band (0–58 ), while the PZT shows a good sensitivity in a higher frequency range (94–304 ). In order to evaluate the PZT and MFC actuators’ sensitivity to damage in the 3D-printed ABS plate, impact damage is also generated in the plate’s center. The impedance-based damage indices obtained from different types of PZTs (2 PZTs and 1 MFC) were projected to the same base level and then fused—for the first time—for impact-damage localization and further added magnetic mass damage localization. The obtained damage index values of impedance are encouraging for the evaluation of AM polymer structures with a 4.48 mm positional error from a real location by fusing data in the different frequency ranges for PZTs and MFC. The damage localization error increases significantly to the new location beyond the damage sensitivity range of the PZT2 and MFC for the added magnetic mass on the 3D printed structure.

Piezoelectric Wafer Active Sensor Transducers for Acoustic Emission Applications.

Piezoelectric materials are defined by their ability to display a charge across their surface in response to mechanical strain, making them great for use in sensing applications. Such applications include pressure sensors, medical devices, energy harvesting and structural health monitoring (SHM). SHM describes the process of using a systematic approach to identify damage in engineering infrastructure. A method of SHM that uses piezoelectric wafers connected directly to the structure has become increasingly popular. An investigation of a novel pitch-catch method of determining instrumentation quality of piezoelectric wafer active sensors (PWASs) used in SHM was conducted as well as an investigation into the effects of defects in piezoelectric sensors and sensor bonding on the sensor response. This pitch-catch method was able to verify defect-less instrumentation quality of pristinely bonded PWASs. Additionally, the pitch-catch method was compared with the electromechanical impedance method in determining defects in piezoelectric sensor instrumentation. Using the pitch-catch method, it was found that defective instrumentation resulted in decreasing amplitude of received and transmitted signals as well as changes in the frequency spectrums of the signals, such as the elimination of high frequency peaks in those with defects in the bonding layer and an increased amplitude of around 600 kHz for a broken PWAS. The electromechanical impedance method concluded that bonding layer defects increase the primary frequency peak's amplitude and cause a downward frequency shift in both the primary and secondary frequency peaks in the impedance spectrum, while a broken sensor has the primary peak amplitude reduced while shifting upward and nearly eliminating the secondary peak.

Monitoring of the curing process and thermal failure of adhesive-bonded structures based on damping measurement using quantitative electromechanical impedance

Adhesives are essential materials in engineering and their strength directly affects the safety of structures. The curing and environmental deterioration are critical areas of focus in adhesive researches. However, conventional studies on adhesive curing emphasize only on the characteristics of the adhesive itself, while studies on deterioration of adhesive-bonded structure tend to concentrate on detecting disbonds. In this work, we proposed to monitor the curing process and thermal failure of adhesive-bonded-structures based on damping measurement using a quantitative electromechanical impedance (Q-EMI) method. The performances of two adhesives, i.e. epoxy and cyanoacrylate, were studied via bonding a piezoelectric transducer onto one end of an alumina bar and measuring the admittance spectrum of the transducer-adhesive-alumina system. Results show that the damping steadily decreases during the curing process and eventually stabilizes for both adhesive-bonded structures. A significantly higher curing rate is observed at 60 °C compared to that at room temperature. At high temperatures, the damping will increase dramatically, or no peaks can be observed in the admittance spectrum, indicating adhesive failure. The results indicate that the epoxy adhesive can function normally at temperatures up to 120 °C and can endure heating-cooling cycles up to 180 °C, while the cyanoacrylate adhesive can function normally at 140 °C but fails upon cooling. The proposed Q-EMI method is quite powerful in monitoring the performances of bonded structures.

Research on bolt looseness monitoring based on electromechanical impedance transmissibility technology

Centralized damage, such as bolt looseness, is one of the most common types of damage in bridge structures. Thus, early detection of such damage is essential for bridge maintenance. Detection methods based on transmittance functions (TFs) have been widely studied. These functions use a T matrix to calculate damage indicators and reflect changes in dynamic parameters, such as natural structural frequencies. However, existing research has shown that the excitation position significantly impacts the T matrix. Therefore, this study proposes a new method based on electromechanical impedance (EMI) for local damage characterization, namely, EMI-TF. A series of comparative tests shows that the EMI-TF process is more sensitive and accurate than the traditional TFs. In addition, the sensitivity of the EMI-TF and EMI methods is compared. Results show that using EMI-TF technology can achieve the localization of minor damage at lower frequencies, which, to some extent, overcomes the limitations of the traditional EMI method that can only detect minor damage at high frequencies. The repeatability of EMI-TF is also studied separately in experiments, with ten repeated experiments conducted. Results show that the experimental results of EMI-TF have high repeatability.

A few-shot electromechanical impedance monitoring method based on a modified prototype network

Bolt loosening monitoring is of great significance to warrant the reliability and safety of bolted structures. The electromechanical impedance (EMI)-based evaluation is effective to perceive bolt loosening. However, EMI signals are highly prone to contamination by temperature fluctuation. Deep learning (DL) based EMI is a promising technique for accurate damage detection in the temperature variation environment. However, DL needs a lot of data to train, which is usually very difficult to collect sufficient structural damage data in real word scenarios. This paper proposed a few-shot EMI monitoring method based on a modified prototype network for bolt looseness detection under temperature varying environment. The approach features a conversion method of the impedance signal to image based on the Hank matrix. A modified prototype network is then developed. An experimental study was carried out on a bolted joint. EMI signals under different bolt loosening conditions were measured in a temperature variation environment. An impedance analyzer and a self-made small lightweight monitoring device were both used to measure the EMI signals to test the cross domain scenario. The proposed method was compared with the transfer learning methods and other typical few-shot learning methods. The experiment results show that the proposed few-shot EMI method can obviously improve the monitoring accuracy of bolt loosening with few samples.

Experimental assessment of impedance-based structural health monitoring in radioactive environment

This work presents an experimental study of the influence of radiation on structural health monitoring systems based on the electromechanical impedance method using low-cost piezoelectric diaphragms. For application of the method, the baseline was obtained without radiation and then compared by damage indices after application of radiation. Considering applications in nuclear power plants, the irradiation and calculation of damage indices were performed in the range of 10 kGy–60 kGy at 10 kGy intervals. Impedance measurements were performed in seven frequency bands between 0 and 65 kHz. The results show that, due to the change in the impedance signatures, the damage indices values increase according to the total applied radiation dose, which may indicate a false indication of damage. Besides, results indicate that there are frequency bands less sensitive to radiation and a threshold can be defined to distinguish radiation from structural damage.

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

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

Damage monitoring of the planting balcony in vertical greenery buildings using the EMI method

Vertical greenery buildings generally have high ecological, aesthetics, and economic benefits. This paper focuses on damage monitoring of a reinforced concrete (RC) planting balcony in a high-rise vertical greenery building using the electro-mechanical impedance (EMI) method. Damage evaluation of the concrete using the EMI method was first carried out through theoretical analysis and experimental investigations. They both indicate that the conductance resonant frequency (CRF) of the piezoelectric transducer has a good linear correlation with damage of the concrete. According to the experimental results, damage evaluation criteria of concrete are proposed based on the CRF. On the basis of the above work, damage monitoring of a RC planting balcony employing a total of 13 piezoelectric transducers was conducted for about 7 months. The results show the planting balcony was slightly damaged which mainly happened before planting during the whole monitoring process.

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.

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.

Monitoring the wear of turning tools with the electromechanical impedance technique

In this study, Inconel 718 and Ti6Al4V alloys were machined using ceramic (WG 300), and carbide (G20M Grade) inserts, respectively. The focused application was grooving. The tool wear was observed with traditional methods and with a new approach, the electromechanical impedance (EMI) technique. First, different machining durations were determined to obtain a different level of worn tools, and the wear amounts of tools were measured using a specialized optical microscope. Then, the electromechanical impedance signature of the new and worn tools was measured with two different piezoelectric sensors. It was understood that the wear level of the tools could also be determined with the EMI method. In order to evaluate the damage quantitatively, RMSD and CCD Damage Indexes (DI) were used. With this proposed method, it was thought that the tool wear level and the remaining machining time from the tool could be predicted with calculated DI. Additionally, a second study was conducted on irregularly worn carbide tools; it was understood that sensors location has a significant role in determining the wear zone; for the current setup, the flank wear could be detected more effectively with the EMI method than the wear on the rake side of the cutting tool.

Damage detection in noisy environments based on EMI and Lamb waves: A comparative study

Structural health monitoring (SHM) systems have shown the potential to detect damage in various types of structures, thus increasing safety and reducing maintenance costs. Among the damage detection methods are those that use small piezoelectric transducers attached to the monitored structure, such as the electromechanical impedance (EMI) and Lamb waves methods. Although both methods have shown to be effective in detecting structural damage, external disturbances such as noise can alter the signals from piezoelectric sensors in real applications and prejudice the correct diagnosis of the monitored structure. Therefore, this paper presents a comparative analysis of the performance of EMI and Lamb waves methods in noisy environments, which is a condition commonly found in real structures. The obtained results add value to the SHM field by allowing to know the tradeoff in choosing the appropriate method regarding the detection, quantification, and location of structural damage under noise conditions. Experimental tests were performed on aluminum structures to assess the effectiveness of both techniques to detect, quantify, and locate structural damage under noise of different intensities. The experimental results show that each technique has different characteristics in terms of effectiveness in detecting, quantifying, and locating damage under noise effects.

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.

Fracture characteristics and stress transfer length of CFRP-concrete bond under field conditioning

To estimate the actual durability of CFRP-concrete bond, an investigation into bond behavior of the interface between CFRP and concrete under field conditioning (FC) in a subtropical area is presented. Single lap-shear tests were conducted on CFRP-concrete specimens subjected to FC for up to 360 days. Bond slip law and fracture characteristics were evaluated using a generalized approach based on the experimental load-displacement curves. The results show that the local bond strength decreased by 30% in the initial 90 days and a 10% recovery appeared after additional 270 days, while the interfacial fracture energy showed a 30% increase after 360 days. Analytical modeling incorporating the evaluated bond slip law was performed and the effect of FC on the stress transfer was revealed. The effective bond length increased by 50% throughout FC. Besides, it can be found that an electromechanical impedance method (EMI) via the use of PZTs was able to monitor the CFRP-concrete interface during single lap-shear tests, and the monitoring results can indicate the bond deterioration.

Bolt looseness monitoring based on damping measurement by using a quantitative electro-mechanical impedance method

In-situ bolt looseness monitoring is very important for the stability and safety of engineering structures. The electro-mechanical impedance (EMI) method, which had been intensively studied in structure health monitoring, has offered a feasible solution. However, in traditional EMI method, the lack of modal pre-selection leads to dense resonant peaks which brings difficulty to signal analysis. The commonly used statistical damage indices typically do not have physics meaning and may fail to distinguish the defects from the environmental changes. In this work, a quantitative EMI (Q-EMI) method was proposed to measure the damping of structures for bolt looseness monitoring. The in-plane excitation method based on distributed thickness-shear-mode piezoelectric transducers was used for selective excitation of vibration modes. The modal-related sensitivity variation was theoretically analyzed and then validated by experiments. Three different specimens were tested and the results showed that during bolt loosening, the modal damping increased by 100%–300%, while the resonance frequency of the structure only decreased by several percent. Moreover, it was found that the loosened bolt may be located using the variation of modal-related sensitivity. Due to its good performance and easy mounting, the proposed Q-EMI method is very promising for bolt looseness monitoring and is expected to be widely used in near future.