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Eddy current testing plays an important role in numerous industries, particularly in material coating, nuclear and oil and gas. However, the eddy current testing technique still needs to focus on the details of probe structure and its application. This paper presents an overview of eddy current testing technique and the probe structure design factors that affect the accuracy of crack detection. The first part focuses on the development of different types of eddy current testing probes and their advantages and disadvantages. A review of previous studies that examined testing samples, eddy current testing probe structures and a review of factors contributing to eddy current signals is also presented. The second part mainly comprised an in-depth discussion of the lift-off effect with particular consideration of ensuring that defects are correctly measured, and the eddy current testing probes are optimized. Finally, a comprehensive review of previous studies on the application of intelligent eddy current testing crack detection in non destructive eddy current testing is presented.

Wire rope inspection by nondestructive testing methods, sensors and signal processing techniques are mainly reviewed in this paper. Owing to the difference of physical mechanism and testing principles, magnetic flux leakage, eddy current, acoustic emission and ultrasonic guide wave testing as well as other inspection methods for steel wire rope are summarized. Then, the commonly and frequently used testing sensors of inductive coil, hall element, magnetoresistive sensors and others are compared in the perspective of their corresponding operating principles, development situation, advantages and disadvantages. Furthermore, signal processing techniques including the signal filtering techniques such as the time and frequency analysis methods, quantitative data processing methods such as the machine learning and defect classification are studied. Finally, the challenges and future developing trends of wire rope inspection in practical applications are discussed.

Impurities resulting from the erosion of plasma-facing components (PFCs) in the plasma chamber disrupt the normal operation of tokamaks. Additionally, continuous erosion leads to the premature aging of these components, affecting their long-term performance and reliability. To quantify erosion and redeposition directly within the machine between operational phases, non-destructive testing methods are crucial. This study evaluates the 4-point and eddy current (EC) methods for measuring the erosion of PFCs and the design of dedicated erosion markers composed of tungsten and alumina multilayers. The 4-point method, sensitive to thickness loss, necessitates electrically insulating layer. Conversely, the EC method requires a conductivity difference between materials but eliminates the need for interlayers. These non-destructive techniques can be effectively integrated into the tokamak to monitor erosion without requiring component removal. Both methods demonstrate potential for accurate thickness measurements. Our results demonstrate that tungsten erosion of less than 2 μm can be assessed using both the four-point probe and EC methods.

Applying life estimation approaches to determine in-service life of structures and plan the inspection schedules accordingly are becoming acceptable safety design procedures in aerospace. However, these design systems shall be fed with reliable parameters related to material properties, loading conditions and defect characteristics. In this context, the role of non-destructive (NDT) testing reliability is of high importance in detecting and sizing defects. Eddy current test (ECT) is an electromagnetic NDT method frequently used to inspect tiny surface fatigue cracks in sensitive industries. Owing to the new advances in robotic technologies, there is a trend to integrate the ECT into automated systems to perform NDT inspections more efficiently. In fact, ECT can be effectively automated as to increase the coverage, repeatability and scanning speed. The reliability of ECT scanning, however, should be thoroughly investigated and compared to conventional modes of applications to obtain a better understanding of the advantages and shortcomings related to this technique. In this contribution, a series of manual and automated ECT tests are carried out on a set of samples using a split-D reflection differential surface probe. The study investigates the level of noise recorded in each technique and discuss its dependency on different parameters, such as surface roughness and frequency. Afterwards, a description of the effect of crack orientation on ECT signal amplitude is provided through experimental tests and finite element simulations. Finally, the reliability of each ECT technique is investigated by means of probability of detection (POD) curves. POD parameters are then extracted and compared to examine the effect of scanning index, frequency and automation on detection reliability.

Inspection of components with surface discontinuities is an area that volumetric Non-Destructive Testing (NDT) methods, such as ultrasonic and radiographic, struggle in detection and characterisation. This coupled with the industrial desire to detect surface-breaking defects of components at the point of manufacture and/or maintenance, to increase design lifetime and further embed sustainability in their business models, is driving the increased adoption of Eddy Current Testing (ECT). Moreover, as businesses move toward Industry 4.0, demand for robotic delivery of NDT has grown. In this work, the authors present the novel implementation and use of a flexible robotic cell to deliver an eddy current array to inspect stress corrosion cracking on a nuclear canister made from 1.4404 stainless steel. Three 180-degree scans at different heights on one side of the canister were performed, and the acquired impedance data were vertically stitched together to show the full extent of the cracking. Axial and transversal datasets, corresponding to the transmit/receive coil configurations of the array elements, were simultaneously acquired at transmission frequencies 250, 300, 400, and 450 kHz and allowed for the generation of several impedance C-scan images. The variation in the lift-off of the eddy current array was innovatively minimised through the use of a force–torque sensor, a padded flexible ECT array and a PI control system. Through the use of bespoke software, the impedance data were logged in real-time (≤7 ms), displayed to the user, saved to a binary file, and flexibly post-processed via phase-rotation and mixing of the impedance data of different frequency and coil configuration channels. Phase rotation alone demonstrated an average increase in Signal to Noise Ratio (SNR) of 4.53 decibels across all datasets acquired, while a selective sum and average mixing technique was shown to increase the SNR by an average of 1.19 decibels. The results show how robotic delivery of eddy current arrays, and innovative post-processing, can allow for repeatable and flexible surface inspection, suitable for the challenges faced in many quality-focused industries.

This paper employs finite element simulation techniques to devise an innovative array-type petal-shaped pulsed eddy current testing probe and investigates its distinctions related to conventional cylindrical probes. Within the simulation framework, a thorough comparative analysis of the detection capabilities of the two probe types is conducted, encompassing an examination of the eddy current regions formed, the impact of lift-off effects and sensitivity to circular corrosion defects. The simulation outcomes reveal that the probe designed in this study exhibits a more extensive detection range and superior detection performance in comparison to its counterparts. This observation holds significance for defect detection in coated materials and the refinement of pulsed eddy current testing probe designs.

This paper presents the results of experiments using the eddy current system designated for nondestructive inspection of carbon fiber-reinforced composites. For this purpose, the eddy current testing system with a differential transducer with two pairs of excitation coils oriented perpendicularly and a central pick-up coil was utilized. The transducer measures the magnetic flux difference flowing through the pick-up coil. The transducer of this design has already been successfully utilized to inspect isotropic metal structures. However, the anisotropy of the composites and their lower conductivity compared to metal components made the transducer parameters adjustment essential. Thus, various excitation frequencies were considered and investigated. The system was evaluated using a sample made of orthogonally woven carbon fiber-reinforced composites with two artificial flaws (the notches with a maximum relative depth of 30% and 70%, respectively, thickness of 0.4 mm, and a length of 5 mm). The main goal was to find a configuration suitable for detecting hidden flaws in such materials.

Nowadays, electromagnetic pollution originated from artificial intelligence devices, fifth-generation (5G) cellular networks, as well as ever-increasing electronic devices applying and/or producing electromagnetic waves has excited the global concern. Evidently, the novel threats are springing up as a sleeping giant awaiting to irrupt; thus, the immediate counteraction is inescapable against the augmenting harmful electromagnetic waves. Till date, based on the permeability and permittivity of the materials in the microwave region, diverse microwave absorbing structures have been fabricated to overcome the aforementioned problem. It is worth noting that nanostructures are under the spotlight as a hot spot generated from their incomparable surface area-to-volume ratio tuning polarizability, magnetic features, electrical conductivity, eddy current loss, and other absorbing mechanisms. It should be noted that the secondary pollution produced at the threshold of the absorbing structures is tunable by the impedance matching. Hitherto, the size, shape, and morphology of nanostructures have been manipulated to improve microwave attenuation. The more surface area-to-volume ratio brings the more defect, dipole, and interfacial polarization as well as enhances the interfacial interactions at heterogeneous interfaces providing more multiple reflections and scattering. On the other hand, the intrinsic properties of the absorbing media and their unique interactions at grain boundaries can promote microwave attenuation which have recently attracted a great deal of attention. The obtained results manifest that the morphology and medium influence on microwave absorbing characteristics are the tip of the iceberg investigated by diverse approaches. In this study, a comprehensive prospect is presented using recent researches related to the size, shape, defect, morphology, and medium effect on the microwave absorbing mechanisms paving the way for microwave attenuation. The achieved works have attested that the mentioned parameters are the vital factors influencing the microwave absorbing properties. Graphical abstract

The present study proposes a novel eddy-current-based tuned inerter damper (EC-TID) that integrates the tuned inerter damper (TID) with nonlinear eddy current damping. The inclusion of nonlinear eddy current damping is expected to improve the control performance of optimal TID. In particular, the mechanical model and configuration of proposed EC-TID are introduced in detail. A closed-form solution for EC-TID optimal design in both undamped and damped structures is established based on effective damping ratio enhancement (EDRE) effect. This closed-form solution ensures equivalence of nonlinear eddy current damping through employment of statistical linearization techniques (SLT) with both force-based and energy-based equivalent criteria. The EC-TID control effectiveness obtained through Monte Carlo simulation under white noise excitation, which include effective damping ratio and EDRE effect, verifies the accuracy of the closed-form solution established via SLT, and highlights the importance of a large critical velocity of EC-TID in achieving higher accuracy. Moreover, it has been found that the closed-form solution for EC-TID and eddy-current-based tuned viscous mass damper (EC-TVMD) optimal design exhibit significant similarities when their equivalent damping ratios are identical. Several numerical studies have been conducted to investigate the control performance of EC-TID under real seismic excitations. The results demonstrate that both TID and EC-TID exhibit superior EDRE effects in mitigating the seismic response of structures compared to viscous and eddy current dampers with the same damping parameters. Additionally, EC-TID offers improved performance over TID, including slightly higher effectiveness, pre-designed maximum damping force, and reduced deformation. Noted that EC-TID is more effective than EC-TVMD in reducing structural seismic responses when their equivalent damping ratio is set at 0.03. This finding contrasts with a recent study where TID and tuned viscous mass damper (TVMD) exhibited comparable effectiveness under the same conditions.

This paper presents a new method for nondestructive testing—a pulsed multifrequency excitation and spectrogram eddy current testing (PMFES-ECT), which is an extension of the multifrequency excitation and spectrogram eddy current testing. The new method uses excitation in the form of pulses repeated at a specified time, containing several periods of a waveform consisting of the sum of sinusoids with a selected frequency, amplitude and phase. This solution allows the maintenance of the advantages of multifrequency excitation and, at the same time, generates high energy pulses similar to those used in pulse eddy current testing (PECT). The effectiveness of the new method was confirmed by numerical simulations and the measurement of thin Inconel plates, consisting of notches manufactured by the electric-discharge method.