Conventional Eddy Current Testing Research Articles

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Sensitivity Study & Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetite permeability.The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non- axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN. Here, the specificity of the TSPs and their clogging justifies qualifying the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main features of the simulation module in CIVA, its experimental validation, and the main trends in the distortion of the simulated signal as a function of variation in the clogging description.

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Numerical Tools and Experimental Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetic permeability. The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non-axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN, related for example to the inspection of the U-bend tubes or of a friction wear under the anti-vibration bars. Here, the specificity of the TSPs and their clogging justifies further investigations between CEA and IRSN to qualify the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main characteristics of the simulation module in CIVA.

Standard development for Eddy Current Arrays in lieu of Magnetic Particle Testing

The technology of Eddy Current Arrays has advanced considerably in recent years. In several trials, superior performance compared to magnetic particle testing was shown. In response, ASME has published new articles on Eddy Current Array (ECA) testing in the 2019 edition of the BPV, and IIW has started a working group to develop ISO standards for the use of eddy current array testing in lieu of magnetic particle testing. Significant interest has been indicated by stake holders, including manufacturers, end-users and regulators. In the paper, the work performed in the IIW working group will be presented. This includes a literature survey on the relative performance of eddy current array testing and magnetic particle testing. In particular, the performance in terms of probability of detection and characterization will be explored. The technical background of performance differences with conventional eddy current testing, magnetic particle testing and dye penetrant testing will be evaluated. Conclusions are drawn on the most important properties to be treated in a new ISO standard in the ISO 17635 framework. The scope of the IIW working group includes a general standard, a weld testing standard and acceptance criteria. Finally, the issues discussed in standard development will be presented and the resulting formulations explained. Comparisons will be made between the ASME and IIW/ISO resolutions.

Quality assessment of SUS316L fabricated by metal additive manufacturing with eddy current inspection

Guaranteeing the quality of additive manufacturing (AM) components after fabrication is very important. In this study, a discrete non-destructive evaluation (NDE) system that includes an eddy current testing (ECT) probe and a fluxgate sensor was developed. In conventional ECT, the defect detection depth is limited by the skin depth of the material under investigation and requires a high frequency of the order of MHz. However, in the NDE system, the high sensitivity of the fluxgate sensor and the use of an ECT coil make it possible to detect subsurface defects at a relatively low frequency. A double-excitation coil was used to excite eddy currents in the specimen at relatively low frequencies, whereas a planar differential coil was used as a pick-up loop connected to an input coil that was inductively coupled to the fluxgate sensor. Based on the developed NDE system, five stainless steel (SUS316L) specimens fabricated by powder bed fusion were inspected. A strong correlation was observed between the detected signals and internal porosity, which was confirmed using X-ray computed tomography images. Although this approach cannot detect the local existence of defects, it can be applied for quality assessment, such as density and distribution of porosity in metallic parts fabricated by AM.

Feature fusion for pulse-modulation eddy current imaging and evaluation of defects in layered conductors

In this paper, the imaging and evaluation of defects hidden within the stratified conductor via pulse-modulation eddy current testing (PMEC) is intensively scrutinised through theoretical and experimental investigation. Following the proposition regarding the optimal excitation signal of PMEC, a fused signal feature integrating the harmonic and pulsed characteristics of PMEC signals is proposed in an effort to further enhance the testing sensitivity and signal-to-noise ratio (SNR). By comparing with the features of conventional PMEC and pulsed eddy current testing (PEC), the superiority of the fused feature is identified via theoretical simulations. This is subsequently supported by the corrosion imaging results obtained from the experiments, further indicating the advantage of the fused feature over traditional features in terms of testing sensitivity and SNR.

A sensitivity enhancement PEC method for bottom flaws and corrosions detection

Eddy current testing (ECT) is an efficient tool to detect the bottom flaw and corrosion because of its characteristics of non-contact, high sensitivity, and flexible. Compared with conventional ECT, the pulsed eddy current (PEC) testing applies instantaneous power to the measured object, thereby obtaining obvious transient response. In this paper, a novel Gauss-pulse-modulation eddy current (GPMEC) method is proposed to enhance the sensitivity of sensor. The energy excited by excitation coil is higher at the carrier frequency through frequency analysis. Correspondingly, an appropriate carrier frequency is chosen based on skin depth. Moreover, the Gauss window function acts on pulse-modulation signal to further increase the energy distribution. Following this, the experiments for bottom flaws and corrosions detection are carried out. The theoretical analysis and experimental results demonstrate that GPMEC is advantageous over than conventional PEC for bottom flaws detection.

Study of Change in Stainless Steel Plate Thickness using S-Domain Parameters Derived from Transient Eddy Current Oscillations Method

Failures in the industrial components are unavoidable due to material degradation from various sources, thus continuous monitoring and timely inspection is mandatory for structural safety. In the present study, the recently developed Transient Eddy Current Oscillations (TECO) method is used to detect wall-thinning in a stainless-steel plate. Unlike the conventional Eddy Current Testing impedance plane data a new interpretation method has been implemented to identify a thickness variation using the S-domain parameters. Furthermore, time and frequency spectral features are analysed to get more insight into the results. The decay time and the resonant frequency are the basis for the discussion to assess the wall thickness.

Numerical Model of Eddy Current Inspection with DC Magnetic Field Associated

Most non-destructive techniques can be well represented in a virtual environment, in particular, eddy current testing (ECT) simulation is a useful and well-established tool to predict and represent real inspection situations permitting testing customization in a fast, cheap and efficient way. Conventional ECT generally works with low-intensity magnetic fields, however, for advanced variations of the technique, where external DC magnetic fields can be applied to locally decrease the magnetic permeability, there is no Finite Element Method (FEM) packages available to deal with such nonstandard model. Many authors [1] and [2] have presented this ECT solution for different industrial applications using external DC magnetization to carry nonlinear ferromagnetic materials to the saturation level of the magnetization curve to increase the ECT depth penetration. In general, ECT modelling calculation is benefited by properties of steady-state regime where all magnetic fields are oscillating at the same frequency not permitting through multi-frequency calculation. The present work proposes a simulation solution for such a case where DC magnetic field is associated with ECT. A theoretical model is presented together with experimental results validation.

A novel electromagnetic nondestructive testing method for carbon fiber reinforced polymer laminates based on power loss

Carbon fiber reinforced polymer (CFRP) is electrically conductive, so eddy-current testing (ECT) method can be used for non-destructive testing (NDT) of CFRP. However, due to the low bulk conductivity of CFRP, the high-frequency systems are considered more suitable in conventional ECT method. But high-frequency systems are more difficult to build and implement. Concerning this issue, a novel CFRP non-destructive testing method with working frequency up to 750 kHz is proposed in this paper. The method is based on power loss of the micro-probe, which makes use of Joule heat generated by the eddy-current in carbon fibers. The method is combination of ECT and thermography principle, which not only maintains higher signal-to-noise ratio at low frequency but also avoids the problems caused by high frequency. An experimental setup using C-Scan technique was built. Scanning experiments on orthogonally woven laminates, two-directional laminates, four-directional laminates and unidirectional laminates at various working frequencies including 750 kHz were carried out. Scanning results prove that the method can detect the cracks and fiber distribution of CFRP at low frequency up to 750 kHz, and has a good signal-to-noise ratio.

A Nondestructive Detection Method Based on Target Resonance

Conventional eddy current testing (ECT) considers the detected target as an RL series (RLs) circuit. Starts from the problem of grounding grid break point detection, this paper proposes a detection method based on RLC series (RLCs) circuit resonance since a break point introduces a capacitor for the detection target. When the RLCs target circuit is coupled with the transmitter coil, the input impedance-frequency curve changes significantly. This paper focuses on methodological exploration, and the influences of target coil capacitance, resistance and detection distance on detection system’s impedance curve are discussed. Simulation and experimental results show the proposed method can distinguish the RLs and RLCs target circuit, and the detection distance of the RLC target is much greater than the RL target detection distance under the same condition. Besides, the feature region, where the impedance curve changes significantly when RLCs circuit is coupled to the transmitting coil, is located near the resonance point of the testing target. Moreover, the detection distance increases with the decrease of the target resistance. In addition, the closer the coil distance is and the smaller the resistance is, the more obvious the feature region will be.

Eddy Current Testing Basics and Innovation

Eddy current testing is considered a theoretically challenging technique. Out of all the different nondestructive testing (NDT) methods, the electromagnetic testing (ET) method (of which eddy current testing is a technique) is probably the most difficult for understanding theory. This is perhaps why the last Materials Evaluation Back to Basics paper on eddy current testing is from 2006, which is a long time ago given the amount of innovation in the technique that has taken place since then (Hansen and Peoples 2006). In this paper we will show what has changed due to recent innovations. We first will present the physics, and then explain how modern equipment assists the user in distinguishing between different physical phenomena. Although this paper is on conventional eddy current testing, we will also mention some other ET techniques along with their advantages and disadvantages.

Evaluation of Coating Thickness Using Lift-Off Insensitivity of Eddy Current Sensor.

Defect detection in ferromagnetic substrates is often hampered by nonmagnetic coating thickness variation when using conventional eddy current testing technique. The lift-off distance between the sample and the sensor is one of the main obstacles for the thickness measurement of nonmagnetic coatings on ferromagnetic substrates when using the eddy current testing technique. Based on the eddy current thin-skin effect and the lift-off insensitive inductance (LII), a simplified iterative algorithm is proposed for reducing the lift-off variation effect using a multifrequency sensor. Compared to the previous techniques on compensating the lift-off error (e.g., the lift-off point of intersection) while retrieving the thickness, the simplified inductance algorithms avoid the computation burden of integration, which are used as embedded algorithms for the online retrieval of lift-offs via each frequency channel. The LII is determined by the dimension and geometry of the sensor, thus eliminating the need for empirical calibration. The method is validated by means of experimental measurements of the inductance of coatings with different materials and thicknesses on ferrous substrates (dual-phase alloy). The error of the calculated coating thickness has been controlled to within 3% for an extended lift-off range of up to 10 mm.

A New Method of Eddy Current Impedance-Extracting and Hardness-Classifying for Valves

This paper describes a new method of eddy current impedance extraction and hardness classification for valves, which is used in online classification. A finite element simulation is conducted to estimate the base magnetic response in specimens with various initial permeabilities, and a scatter diagram between the hardness calibration and initial permeability is plotted experimentally; thus, the hardness classification model is obtained by using the initial permeability as the intermediate variable. A lock-in amplifier is commonly used to measure the amplitudes of impedance components in conventional eddy current testing. This study replaces the lock-in amplifier with a cross-correlation algorithm with the objective of integrating the impedance processing with the subsequent classification. Moreover, an incremental and decremental support vector machine is used to classify the valve specimens based on the extracted impedance due to its demonstrated excellent performance for both linearly separable and nonlinearly indivisible problems. The cross-correlation algorithm and the incremental and decremental support vector machine are implemented on a field-programmable gate array, which has the advantage of parallel processing and can satisfy the online requirements. A comparison with the traditional impedance comparison method shows that the proposed method not only is suitable for online hardness classification but also achieves good performance in correlating impedance amplitude changes caused by a temperature increase of the coil and various heat treatment conditions with the hardness values.

A novel frequency-band-selecting pulsed eddy current testing method for the detection of a certain depth range of defects

Local wall thinning is one of the major defects in industrial structures and in some cases it generates in a certain depth range of the object structures. In this study, a novel nondestructive testing method called frequency-band-selecting pulsed eddy current testing (FSPECT) is proposed to detect a certain depth range of defects. This novel FSPECT method is based on the frequency-band-selecting strategy as well as the conventional square wave pulsed eddy current testing (PECT) method. In this study, FSPECT method is introduced and the frequency range selection principle of the excitation signal of FSPECT is explained in details at first. Then the simulation of FSPECT for detecting a certain depth range of defects has been performed. In addition, the sensitivities of the proposed FSPECT and the conventional PECT are compared and analyzed. Finally, a comparative experiment of FSPECT method and PECT method is implemented. The superiority of the proposed FSPECT method for detecting a certain depth range of defects is demonstrated through simulation and experiment.

Extraction Method of Crack Signal for Inspection of Complicated Steel Structures Using A Dual-Channel Magnetic Sensor.

Conventional eddy current testing (ECT) using a pickup coil probe is widely employed for the detection of structural cracks. However, the inspection of conventional ECT for steel structures is difficult because of the magnetic noise caused by the nonuniform permeability of steel. To combat this challenge, we have developed a small magnetic sensor probe with a dual-channel tunneling magnetoresistance sensor that is capable of reducing magnetic noise. Applying this probe to a complicated component of steel structures—such as the welds joining a U-shaped rib and deck plate together—requires the reduction of signal fluctuation caused by the distance (liftoff) variations between the sensor probe and the subject. In this study, the fundamental crack signal and the liftoff signal were investigated with the dual-channel sensor. The results showed that the liftoff signals could be reduced and differentiated from the crack signals by the differential parameters of the dual-channel sensor. In addition, we proposed an extraction technique for the crack signal using the Lissajous curve of the differential parameters. The extraction technique could be applied to the inspections not only for flat plates but also for welded angles to detect cracks without the influence of the liftoff signal.

Performance analysis of single-frequency near electrical resonance signal enhancement (SF-NERSE) defect detection

In the search for ever more sensitive non-destructive testing (NDT) techniques to detect progressively smaller defects in increasingly complex industrial materials, environments and geometries, conventional eddy-current testing (ECT) methods, have reached their sensitivity limit. Fortunately, advanced resonance-based techniques, demonstrated in recent years, promise to help extend that limit, but, before such techniques can be implemented with confidence, a detailed understanding of their sensitivity and stability must be achieved. In this study, statistical probability-of-detection (PoD) analysis was performed to assess the sensitivity of a novel single-frequency, near electrical resonance signal enhancement (SF-NERSE) technique relative to an equivalent conventional probe operation. This study was performed on 36 real fatigue defects in Titanium 6V-4Al (Ti6-4) with defects ranging from 0.10−6.48 mm in surface extent. In addition, a critical evaluation of background noise stability in the SF-NERSE technique (a common concern with NERSE-based methods) was also performed to establish the viability of such a technique in relation to industrial inspection and assessment criteria. The results of this study demonstrate a sensitivity enhancement of up to 20% for the SF-NERSE method over conventional operation and, through controls, confirmed that the effect is a result of the resonance-shifting phenomenon and not just an increase in operational frequency. In addition, an examination of background noise implies that a SF-NERSE method exhibits more stable background noise than conventional excitation. This study validates the SF-NERSE technique and methodology as a viable industrial inspection technique, able to significantly improve detection capabilities.

Lift-off effect reduction based on the dynamic trajectories of the received-signal fast Fourier transform in pulsed eddy current testing

Reduction of interferences such as the lift-off effect in pulsed eddy current (PEC) testing has been difficult. A new approach based on the dynamic trajectories of the fast Fourier transform (FFT) of the received signals is presented in this paper for reducing the lift-off effect in PEC. The PEC probe scans the surface of the test sample and a set of received signals for a corresponding set of scanning points is stored. FFT is performed on all these stored signals, and the real and imaginary parts for a particular FFT harmonic of all the stored scanning points are plotted in a complex plane to form a dynamic trajectory. The defect trajectory can be recognized from the lift-off trajectory by observing the angle between them, as in conventional eddy current testing. By moving the defect trajectory in the complex plane for each harmonic to a zero-lift-off point and performing an inverse fast Fourier transform (IFFT) based on the new trajectories for all the harmonics, the traditional PEC time domain differential signals for a defect can be reconstructed. Then, the acquired results are amplified with a coefficient related to the inspection settings and final differential signals closely approximating the original defect differential signals without lift-off are obtained. Experimental results demonstrate that the proposed method is highly effective, particularly for the simultaneous inspection of internal and surface defects, although the location of the defect in the depth direction is unknown before inspection.

Influence of metallic shields on pulsed eddy current sensor for ferromagnetic materials defect detection

Pulsed eddy current (PEC) with transient analysis is a significant advance over conventional eddy current testing using impedance analysis at a particular frequency. The fabrication and optimization of PEC sensor are important issues in PEC applications in nondestructive testing (NDT) field especially for ferromagnetic materials. In this work, the influence of metallic shields on circular PEC sensor for defect detectability in ferromagnetic metals has been investigated through finite element models (FEM) and experimental studies by comparing iron and aluminum shields with no-shield. Both simulations and experiments indicate that PEC probe with an aluminium shield can effectively enhance the amplitude of PEC response in both surface and subsurface defect detection. However, the sensitivity is decreased. In contrast, iron shield has better shielded performance and can effectively improve the sensitivity of both surface and subsurface defects detection in ferromagnetic materials.

Numerical investigation of the ability of eddy current testing to size surface breaking cracks

This study attempts to estimate the limitations of eddy current testing for evaluating the depth of a crack with the aid of numerical simulations. Three-dimensional finite element simulations were conducted to obtain eddy current signals resulting from a crack in a plate made of Ni-based alloy. The simulations modelled the crack as a sufficiently long rectangular region with a constant width and depth. A uniform conductivity was given inside the modelled crack because actual cracks do not always behave as electrically insulating walls. The conductivity of the modelled crack was chosen on the basis of a recent study reporting how real cracks can be modelled from the viewpoint of electromagnetic non-destructive evaluation. This study considered three conventional eddy current probes: a pancake probe, a vertical pancake probe and a plus point probe. The results of the simulations reveal that lower frequency does not always enhance the evaluation of a deeper crack if the crack is conductive; moreover, the maximum depth of a stress corrosion crack that conventional eddy current testing can evaluate quantitatively should be limited to a few mm at most. In contrast, the simulations also demonstrate that the ability of eddy current testing to size a fatigue crack would not differ significantly from that to size an artificial slit.

Eddy current techniques for super duplex stainless steel characterization

Super duplex stainless steel (SDSS) is a two-phase material where the microstructure consists of grains of ferrite (δ) and austenite (γ). SDSS exhibit an attractive combination of properties, such as: strength, toughness and stress corrosion cracking resistance. Nevertheless, SDSS attain these properties after a controlled solution heat treatment, leading to a similar volumetric fraction of δ and γ. Any further heat treatment, welding operation for example, can change the balance of the original phases, or may also lead to precipitation of a deleterious phase, such as sigma (σ). For these situations, the material corrosion resistance is severely impaired. In the present study, several SDSS samples with low σ phase content and non-balanced microstructure were intentionally obtained by thermally treating SDSS specimens. Electromagnetic techniques, conventional Eddy Current Testing (ECT) and Saturated Low Frequency Eddy Current (SLOFEC), were employed to characterize the SDSS samples. The results showed that ECT and SLOFEC are reliable techniques to evaluate σ phase presence in SDSS and can provide an estimation of the δ content.