Low Frequency Eddy Current Testing Research Articles

Research on Sensing Shot Peening Residues Using Different Responsivity Between Magnet and Nonmagnet in Super Low-Frequency Eddy Current Testing

Research on Sensing Shot Peening Residues Using Different Responsivity Between Magnet and Nonmagnet in Super Low-Frequency Eddy Current Testing

Low-frequency incremental permeability for the evaluation of deep carburization treatments: Theoretical understanding

Precise evaluation of deep carburization treatments is essential to prevent degradation of high-performance mechanical components. A nondestructive magnetic method called low-frequency incremental permeability testing appears to be particularly suitable for this purpose. This method consists of slowly magnetizing the specimen to be controlled and then superimposing a relatively low-frequency eddy current testing process on the specimen (<2 kHz). The sensor’s coil impedance is monitored along with the magnetization cycle. When plotted versus the magnetic excitation, the impedance produced butterfly-shaped curves that were dependent on the carburization depth. Operating at low frequency increases the scanned layer thickness up to the untreated inner part of the specimen and induces characteristic variations in the butterfly-shaped curves. A model to solve for the indirect relationship between the magnetic properties and the carburization treatment properties is presented in this manuscript. This model combines the Dodd and Deeds analytical expression for a flat sensor coil above a two-layer conductor with Jiles-Atherton theory for consideration of the hysteresis. The magnetic and electromagnetic responses are well considered, providing accurate reconstructions of experimental data. The model predictions reveal magnetic indicators that are highly correlated with the carburization depth and provide optimal conditions for nondestructive testing observation.

Thickness Measurement at High Lift-Off for Underwater Corroded Iron-Steel Structures Using a Magnetic Sensor Probe

Infrastructure facilities that were built approximately half a century ago have rapidly aged. Steel sheet piles, the inspection object in this study, are severely corroded, resulting in cave-in damages at wharfs. To solve such a problem, non-destructive inspection techniques are required. We previously demonstrated plate thickness measurement using extremely low-frequency eddy current testing. However, when the steel sheet piles are located in water, shellfish adhere to their surface, causing a lift-off of several tens of millimeters. Therefore, this large lift-off hinders the thickness measurement owing to fluctuations of magnetic signals. In this study, sensor probes with different coil diameters were prototyped and the optimum size for measuring steel sheet piles at high lift-off was investigated. Using the probes, the magnetic field was applied with a lift-off range from 0 to 80 mm, and the intensity and phase of the detected magnetic field were analyzed. Subsequently, by increasing the probe diameter, a good sensitivity was obtained for the thickness estimation with a lift-off of up to 60 mm. Moreover, these probes were used to measure the thickness of actual steel sheet piles, and measurements were successfully obtained at a high lift-off.

Magnetic thickness measurement for various iron steels using magnetic sensor and effect of electromagnetic characteristics

The diagnosis and prevention of the deterioration of iron-steel infrastructure has become an important social issue in recent years. The thickness measurement technique (extremely low-frequency eddy current testing (ELECT)) using a magnetic sensor for detecting steel corrosion at extreme frequency ranges has been previously reported. Using the calibration curves based on the correlation between the phase of the detected magnetic signal and the plate thickness, the plate thickness reduction caused by corrosion can be estimated from the detected phase signal. Iron-steel materials have large changes in electromagnetic characteristics; therefore, the reference calibration data for each type of iron-steel are required for plate thickness estimation. In this study, the effect of electromagnetic characteristics on the magnetic thickness measurement was investigated to improve the thickness estimation. Four types of iron-steel plates (SS400, SM400A, SM490A, and SMA400AW) with thicknesses ranging from 1 mm to 18 mm were measured by ELECT, and the phase change at multiple frequencies of each plate were analyzed. The shift in the phase and linearity regions of the calibration curves for each type of steel plate was observed. To analyze this shift phenomenon, the electromagnetic characteristics (permeability μ and conductivity σ) of each type of steel were measured. Compared with the permeability μ and conductivity σ of each steel plate in the applied magnetic field strength range, the product (σμ) for various steel plates decreased in the following order: SM400 &amp;gt; SS400 &amp;gt;SMA400AW &amp;gt; SM490A. The product of μ and σ is related to the skin depth, indicating the electromagnetic wave attenuation and eddy current phase shift in the material. Therefore, each shift in the calibration curve of each type of iron steel is explained by the changes in the parameters σ and μ.

Research on the Technology of Conductor Corrosion Detection based on Eddy Current Testing

The detection of conductor corrosion state provides technical guarantee for mastering the corrosion type, corrosion degree and service life of power transmission lines, realizing the timeliness and accuracy of power line replacement, and reducing the hidden danger of line safety. In this paper, a method based on eddy current testing for corrosion detection of power wire is proposed. High and low frequency eddy current testing is used to measure the residual cross-sectional area of wire, and the output database of flaw detection is established to deduce the residual tensile force of wire. Finally, by introducing the artificial single line sample and the actual running conductor for dynamic comparison, the residual tension data of the conductor can be corrected. The results show that the method adopted in this paper is feasible and accurate, which further guarantees the safe and reliable operation of overhead transmission lines.

Rectangular wave eddy current testing using for imaging of backside defects of steel plates

Accurate, easy, and fast inspection of defects on the backside of thick steel plates is essential for the maintenance of infrastructures. Low frequency eddy current testing (LF-ECT) is a promising method to detect defects of the backside of steel plates, with a thickness of approximately 10 mm. However, it is possible that the signal from the backside defect is smaller than that from the surface magnetic noise, causing difficulty identifying the backside defect. In this study, we propose a method to reduce the surface noise by employing a square wave inverter to generate a harmonic signal (rectangular wave ECT, or RECT), and the result demonstrates that the surface noise is successfully reduced using the harmonic signal.

Low-frequency eddy current testing using HTS coil driven by PWM inverter

Low-frequency eddy current testing using HTS coil driven by PWM inverter

Detection of Slit Defects on Backside of Steel Plate Using Low-Frequency Eddy-Current Testing

The evaluation of opposite-side defects in thick ferromagnetic objects (for example, steel plates) is essential for safety maintenance. Eddy-current testing (ECT) can detect defects in metallic specimens; however, owing to the skin effect, its use is limited to the evaluation of surface or subsurface defects. Therefore, in this paper, we propose a non-contacting inspection system that consists of two excitation coils and a detection coil to detect slit defects on the backside of a steel plate using low-frequency ECT (LF-ECT). Using the proposed system, we examine whether LF-ECT mainly detects the leakage or the flux induced by the eddy current, and also examine which direction of the excitation currents in the two excitation coils is suitable for detecting slit defects. The results suggest that the method to drive the current in the same direction and to detect the imaginary part of the magnetic flux density, which corresponds to the flux induced by the eddy current, is suitable for detecting slit defects. Subsequently, we examine the detection limit of the height of the slit defect when the thickness of the steel plate is 10 mm and the width and length of the slit defect are constant, at 50 and 1 mm, respectively. The results demonstrate that the slit defect can be detected by using the proposed system when the height is greater than or equal to 4 mm. Therefore, the proposed system is a promising tool to detect critical defects on the backside of steel plates.

Low Frequency Eddy Current Testing of Insulators and Composites

Eddy current testing (ECT), a non-destructive testing method widely used to evaluate defects within conductive materials, is explored in this study as it applies to insulators and non-uniformly conductive materials. Previous work has shown that at high frequencies, differences in electric permittivity can be detected with ECT. In this study, a new design of an ECT sensor that employs two resonance-tuned coils is evaluated. Results show that material inconsistencies in insulators are detectable due to spatial variations in permittivity and magnetic permeability, and that detection is possible at lower frequencies than previously demonstrated. In addition to determining signal dependence on individual electromagnetic parameters, sensitivity for defect detection in a carbon fiber-reinforced polymer (CFRP) composite is qualitatively determined. Although low signal-to-noise ratio is observed with a small-diameter coil, by increasing the coil diameter, the signal to noise ratio is increased while preserving adequate spatial resolution to detect defects in the sample. This study expands on previous studies of the application of ECT to insulators, and demonstrates that defect detection is possible in CFRPs.

Application of low-frequency eddy current testing to the inspection of a double-walled tank in a reprocessing plant

This paper discusses the applicability of simple low-frequency eddy current testing to the detection of deeply embedded flaws. The study specifically considered a double tank in a reprocessing plant for extracting plutonium-uranium from spent nuclear fuels. The tank was modelled by two type 304 austenitic stainless steel plates situated with an air gap of 80 mm, and the change in the thickness of one of the plates was detected through the other plate and the air gap. Axisymmetric two-dimensional finite element simulations were conducted and found that a simple circular coil with a large diameter enabled to detect the thickness based on the magnetic flux density at the centre of the coil although the plates were as thick as 30 mm. The results of the numerical simulations were validated by experiments.

Thickness Measurement of an Iron Plate Using Low-Frequency Eddy Current Testing With an HTS Coil

We applied low-frequency eddy current testing (ECT) using a high-temperature superconducting (HTS) coil in order to measure the thickness of an iron plate. Using this method, we measured changes in coil impedance when the iron plate was placed below the coil. Although low-frequency measurements were necessary to avoid the skin effect, changes in coil impedance became very small at low frequencies. For this reason, an HTS coil was used in order to sensitively measure these small changes. First, changes in the inductance L and the resistance R of the coil were measured when the iron plate was positioned 18 mm below the HTS coil, and the thickness of the plate was changed in increments from 6 to 22 mm. The results show that we were able to estimate the thickness of the plate up to 22 mm based on the changes in R when an excitation frequency of 4 Hz was used. Next, the effect of the liftoff between the iron plate and the HTS coil on the changes in Land R was studied while the liftoff was changed from 18 to 28 mm. The results show that the liftoff could be estimated from the changes in L. We were therefore able to determine the liftoff and the thickness of the iron plate by measuring changes in L and R, respectively. Results obtained from numerical simulation using the finite element method were in agreement with the experimental results.

A214 極低周波渦電流探傷法の配管減肉モニタリングへの適用性評価(OS2 保全・設備診断技術(3))

A214 極低周波渦電流探傷法の配管減肉モニタリングへの適用性評価(OS2 保全・設備診断技術(3))

Probability of detection for surface-breaking holes with low-frequency eddy current testing – a non-linear multi-parametric approach

Probability of detection for surface-breaking holes with low-frequency eddy current testing – a non-linear multi-parametric approach

Evaluation of Sensitivity of Remote Field Eddy Current Testing and Low-Frequency Eddy Current Testing for Inspecting Grooves of Metal Plate

We compare remote field eddy current testing and low-frequency eddy current testing from the viewpoint of sensitivity to back-side grooves in a metal plate. We carry out experiments using three metal plates with different depths of artificial grooves. A U-shape magnetic shield gives rise to a remote field eddy current as occurs in pipe inspection. The experiments show that the two methods have almost the same sensitivity to back-side grooves if a relatively low frequency and relatively far coil interval are used, which is also validated by three-dimensional finite-element simulations. We conduct numerical investigations to find the reason for this condition. [doi:10.2320/matertrans.M2012323]

Difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing

We examined the difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing. The detection limits were measured by using a magneto-resistive sensor. Comparing the frequency of an applied magnetic field, the detection limit at 50 Hz is deeper than that at 1 kHz. Comparing the sample structure, the detection limit in the multi-layered samples is deeper than that in the continuous samples. These results are likely due to the differences in the skin depth and conductivity of the sample.

Electromagnetic field calculations for the low frequency eddy current testing of tubular products

Low frequency eddy current technique, known as remote field eddy current (RFEC) technique, is being widely used for the nondestructive testing (NDT) of metallic tubes. Despite the early recognition, advantages and successful application in the in-situ inspection of oil well casings this technique lacked the development of an adequate theoretical base to help understand its complex phenomenon. Theoretical simulation of a practical RFEC geometry (moving probe inside a flawed tube) is described in this paper through the application of finite element numerical analysis technique. The predicted RFEC output traces show a higher and constant detection sensitivity irrespective of flaw depth, flaw location (ID or OD) and location of the sensor within the remote field zone.