Application Of Eddy Current Testing Research Articles

Signal-distribution-based crack detection for divertor monoblock inspection using eddy current testing

This study evaluated the applicability of eddy current testing to the surface inspection of a divertor plasma-facing unit in a Tokamak fusion reactor. Artificial slits were introduced on the surfaces of some samples. The eddy current testing was performed to detect these slits. A quantitative detection evaluation method based on evaluating the distribution differences is proposed. The results reveal that the signals of slits near the edge are easily buried by the edge signals and difficult to detect directly from the signal amplitude image. With the aid of the proposed method, not only the slit in the middle of the sample surface but also the slits near the sample edges were successfully detected by eddy current testing.

Application of Eddy Current Testing for the Preparation of Samples with Artificial Defects

Application of Eddy Current Testing for the Preparation of Samples with Artificial Defects

Applicability of eddy current testing in CFRP using electrodes on CFRP surface and control of the testing range

In this study, a novel eddy current testing (ECT) method was developed to facilitate the testing of unidirectional carbon fiber reinforced plastic (UD-CFRP) and control the detection range. First, the effect of adding electrodes orthogonal to the fibers of UD-CFRP were analyzed. The results showed that the eddy current density could be significantly increased by adding electrodes. Moreover, it was found that the distribution of eddy current density in depth axis can be controlled by adjusting the distance of electrodes. Next, the same analysis was conducted for quasi-isotropic CFRP for comparison. The results showed that, unlike the results for the UD-CFRP, the distribution in depth direction could not be controlled by adjusting the distance of the electrodes. Finally, the impedance of the UD-CFRP with electrodes was measured, for confirming that adding electrodes facilitated ECT. It was also confirmed that the impedance increased as the distance between electrodes increased, and this trend was consistent with the numerical simulation results.

ECT simulation and measurements in ferromagnetic tube using DC homogeneous magnetic field

ABSTRACT The application of Eddy Current Testing (ECT) to ferromagnetic materials is challenging owing to the difficulties stemming to ferromagnetism. Due to the non-linear magnetic permeability, the use of a strong external DC magnetic field is often necessary in order to bring ferromagnetic materials close to magnetic saturation where permeability is minimised; strongly improving eddy current’s penetration in the material volume. In this work, the main application studied is ECT on a ferromagnetic tube with an Inside Diameter (ID) coil. The effect of different levels of the DC magnetic field on the coil impedance changes is investigated both experimentally and by simulation. It is shown that DC field has a systematic influence both to magnitude and phase of the Eddy Current signal. In addition, for the different defect depths, a critical level is observed, where the signals from discontinuities become visible and stand out from the noise of non-linearity. Moreover, for a range of magnetisation field level, the changes of phase angle are distinct for different defects’ depths allowing thus the application of phase analysis for defect characterisation.

Heterodyne Eddy Current Testing Using Magnetoresistive Sensors for Additive Manufacturing Purposes

In recent years additive manufacturing technologies have become widely popular. For complex functional components or low volume production of workpieces, laser powder bed fusion can be used. High safety requirements, e.g. in the aerospace sector, demand extensive quality control. Therefore, offline non-destructive testing methods like computed tomography are used after manufacturing. Recently, for enhanced profitability and practicality online non-destructive testing methods, like optical tomography have been developed. This paper discusses the applicability of eddy current testing with magnetoresistive sensors for laser powder bed fusion parts. For this purpose, high spatial resolution giant magnetoresistance arrays are utilized for testing in combination with a single wire excitation coil. A heterodyne principle minimizes metrology efforts. This principle is compared to conventional signal processing in an eddy current testing setup using an aluminum test sample with artificial surface defects. To evaluate the influence of the powder used in the manufacturing process on eddy current testing and vice versa, a laser powder bed fusion mock-up made from stainless steel powder (316L) is used with artificial surface defects down to $100~\mu \text{m}$ . This laser powder bed fusion specimen was then examined using eddy current testing and the underlying principles.

Numerical Simulation of Non-Destructive Remote Field Eddy Current Testing of Rolled Metal Tubes

The first publications describing the physical principles of the non-destructive remote field eddy current testing method appeared about 30 years ago. This method allows to significantly expand the field of application of eddy current testing. However, due to the lack of a theoretical justification, this method did not get widespread use around the world. Domestic publications in this area are completely absent, and the descriptions given in few foreign publications often contradict each other. There are no results of full-scale simulation using numerical methods in available domestic and foreign sources. The distinctive feature of this method under consideration is the ability of detecting defects on the external (with respect to the eddy current transducer) side of the tested object, which is impossible for the classical eddy current method due to the limited eddy current penetration depth. The basics of the method were considered, the distinctive features were presented, and the advantages and disadvantages of remote field eddy current testing of metals were pointed out. A numerical simulation with the subsequent analysis of the obtained results has been carried out, the transducer design for remote field eddy current testing is given. The influence of various factors on the change in the added voltage of the signal coil of the eddy current transducer in the presence of a defect in the external wall of the tube was considered. Expressions that determine the optimal ratio of the diameters of the transducer and the tested product were obtained. The values of the test parameters and the limiting wall thickness of the tested ferromagnetic product were determined.

FEATURES OF QUALITY CONTROL OF WELDED STRUCTURES USING THE MATRIX VIRTUAL SENSORS

The article is devoted to the analysis of the eddy current method of quality control welded structures using a matrix of sensors, modern devices based on it and determining the prospects for the development of this method in flaw detection.The main tasks in the application of the eddy current method are: quality control of metals and alloys, semiconductors, electrically conductive rods, plates, sheets, wire; inspection of welding defects, scheduled inspections of welded structures; diagnostics of pipes, fittings, railroad rails, bearing elements, fasteners and many other industrial products.The control method using the matrix of eddy current sensors is based on electronic control and reading of information from several eddy current sensors located in the form of a matrix on the test object (linear scanning). The matrix of eddy current sensors can be optimized for each specific case by changing the relative position of the individual sensors and the shape of the entire matrix.The advantages of monitoring with eddy current matrices are as follows: the monitoring time is significantly reduced, a large area is covered in one pass, the design of mechanical and automated scanning systems is simplified, the results of monitoring a given area are provided in real time, the interpretation of data is simplified, and the control reliability and reliability are improved.The most common application of eddy current testing with matrix transducers is the search and mapping of stress-corrosion lesions in the metal. The development of this type defects is extremely difficult to identify with traditional methods, but they are the cause of numerous accidents on gas pipelines all over the world.The use of computer equipment in eddy-current systems, which provides both automatic control of the entire measurement cycle and interpretation of the results, opens up great prospects. Well-known companies that manufacture wide range of matrix sensors are Olympus NTD Inc and Eddyfі.The output of the inspection results is carried out in the form of a C-Scan (2D or 3D), which is a color-coded two- or three-dimensional image of the test surface. It facilitates the analysis of results, which in particular includes measurements of the depth and length defects.

A feasibility analysis on the application of eddy current testing to the detection of the most detrimental defects in historical metallic tie-rods

Historical metallic tie-rods have a very important structural role in buildings characterized by arches and vaults because they balance the horizontal thrusts, avoiding the overturning of vertical supports. Nowadays, no information can be found in the literature about the non-destructive detection of possible defects in historical tie-rods and their effects on the in-service structural integrity. On the other hand, many Non-Destructive Techniques are commonly used to inspect modern metallic components and structures, but their viability for cultural heritage is still to be assessed.In this paper, the application of eddy current testing to the historical metallic tie-rods of the Milan cathedral (Italy) is evaluated, discussed and adapted. The considered historical structures are characterized by a very heterogeneous material, irregular surface geometry and high inherent defectiveness. Recently, in-service failures showed the most detrimental flaws in tie-rods consist of handmade straight scarf forged welds. A first experimental eddy current testing approach, based on off-the-shelf high and low frequency probes, was performed. Results were partially unsatisfactory and showed the challenge is not the maximization of sensitivity and spatial resolution, as in typical modern applications, but their suitable minimization. Consequently, a customized eddy current testing technique, based on a novel low frequency probe, was designed and optimized. Its performance was evaluated by numerical simulations as well as by experimental testing. Based on the satisfactory achievements, the customized eddy current testing technique was applied on site on seven tie-rods. The most significant discontinuities could be effectively detected.

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.

Characterization of Hydrogen Embrittlement in 2.25cr-1mo-0.25v Steel by Eddy Current Method

ABSTRACTEddy current method has been recently developed to characterize mechanical properties of materials and assess internal hydrogen content of high strength low alloy steels. The application of eddy current testing in evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V ferritic steel has been investigated using tensile test and electrochemical hydrogen charging test. The relationship between the embrittlement index and eddy current signal is well established. It found that there is a good linear relation between the hydrogen-induced plasticity loss and the eddy current signal. This shows eddy current testing is an effective method for evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V steel.

Simulation of Alloy Content Assessment in Fe-Si Alloy Steel by Eddy Current Testing

The present study, which is based on the finite element method, investigates the alloy element content assessment of these Fe-Si alloy steel sheets from the view point of eddy current testing. The relative permeability and conductivity were preset parameters. Eddy current signals were gathered by a differential type cylindrical probe and subsequent nondestructive tests were carried out with the preparation of four Fe-Si alloy steel sheets with different silicon contents. The result of the numerical simulation demonstrated that the signal increased with the increase of the silicon content in the Si-Fe alloy steel sheets. And the measured result confirmed the rationality of the application of eddy current testing in the average silicon content assessment.

Evaluating the III-Posedness of Inverse Problem to Size Flaws from Eddy Current NDT Signals Obtained with an Absolute Type Probe

This study discusses the applicability of eddy current testing to the non-destructive evaluation of the depth of a flaw. Three-dimensional finite element simulations are carried out to calculate eddy current signals due to a flaw, and subsequent calculations correlate the difference between signals due to two flaws with the difference between the depths of the flaws. Standard deviation of the difference between the depths of the flaws is used to discuss the ill-posedness of the problem. A flaw is modeled as a sufficiently long rectangular region with a constant width and uniform electrical conductivity; an absolute pancake probe is considered. This study reveals that lift-off and coil diameter do not have a large effect on the ill-posedness, which supports that it is reasonable to decide parameters in actual inspections so that signal-to-noise ratio is maximized. The results obtained also indicate the difficulty in evaluating the depth of a flaw deeper than 1 mm using the signals obtained using an absolute pancake probe.

Quantitative Analysis of Transverse Cracking of Rail Using Eddy Current Non-Destructive Testing

In this study, we analyzed transverse cracks in rails using eddy current testing. Quantitative analysis was performed on transverse crack defect signals ascertained by actual flaw detection and finite element simulation of the rail that had developed a central transverse crack. The conductivity and width of the finite element model were equivalent to those of the actual rail. By analyzing the distribution of eddy current on the rail surface, the relationship between the variation in the density of eddy current and size of transverse crack was obtained. Experimental and simulation results indicated that the application of eddy current testing in quantitative analysis of a rail flaw was feasible and would be a valuable reference in future research.

Detection of embedded fatigue cracks in Inconel weld overlay and the evaluation of the minimum thickness of the weld overlay using eddy current testing

This study evaluates the applicability of eddy current testing to the detection and sizing of fatigue cracks embedded in Inconel weld overlays. Welded plate specimens, which model head penetration welds and their weld overlays, are fabricated, and fatigue cracks are artificially introduced into the specimens. Eddy current inspections are performed using a uniform eddy current probe driven with 10 kHz, and all of the fatigue cracks are detected with clear signals. Subsequent numerical inversions estimate that the minimum thicknesses of the weld overlays are 1.47, 2.17, and 2.23 mm, whereas true thicknesses revealed through destructive testing are 1.51, 3.25, and 2.10 mm, respectively. Thicknesses are also evaluated using potential drop and ultrasonic testing methods; the results demonstrate that eddy current testing is the most efficient of the three methods.

Use of eddy current testing method in detection and evaluation of sensitisation and intergranular corrosion in austenitic stainless steels

This paper deals with the applicability of eddy current testing (ECT) technique to assess and quantify sensitisation and intergranular corrosion (IGC) in thermally aged AISI type 316 stainless steel. Chemical and electrochemical tests specified by ASTM A262 Practices A and E (Strauss test) and G108, and ECT, were used to quantify degree of sensitisation (DOS). The DOS was categorised based on severity of the crack after bend test in increasing order as unaffected, fissured, cracked, and broken. The eddy current (EC) amplitude increased with increasing DOS in both the as-aged and Strauss tested conditions. The EC amplitude in as-aged condition was smaller than in Strauss tested condition. Empirical relationships were established between ECT amplitude and other DOS parameters.

Eddy current inversions of defects in rough welds using a simplified computational model

This study demonstrates that the profiles of defects in Inconel welds with a rough surface can be sufficiently evaluated by eddy current testing with the aid of computational inversion techniques. A plate specimen with Inconel welding is fabricated and artificial notches are introduced between the welding metal and base material. Eddy current inspections are performed to measure signals caused by the artificial notches, and their profiles are reconstructed from the measured signals using a computational inversion algorithm based on the conjugate gradient method. Since it is difficult to model the specimen exactly, simulations to calculate eddy current signals due to a reconstructed profile are performed in a simplified configuration. The lengths and maximum depths of the notches are well estimated, which validates the applicability of eddy current testing to not only the detection but also evaluation of defects in Inconel welds.

Excitation with phase shifted fields-enhancing evaluation of deep cracks in eddy-current testing

This study enhances the applicability of eddy current testing to the evaluation of deep surface-breaking cracks in the front face of thick structures, while keeping the advantages of higher frequency inspection. The key idea is to suppress eddy currents on the surface of a test-piece, and thus realize the deeper penetration of eddy currents. Based upon this idea, a mutual induction eddy current testing probe, consisting of four coaxial rectangular tangential exciter coils and a pancake pick-up coil, has been designed. The exciters are driven by AC currents with different phases and amplitudes, which contrive to emulate the desired behavior. Numerical simulations and consecutive experimental verifications are presented in the paper. It is revealed that when the new probe is used for inspections under a frequency of 10 kHz there is a 20% difference between signals due to an electro-discharge machined (EDM) notch of 15 mm depth and those due to one of 20 mm depth. On the other hand, a conventional probe driven with the same frequency gives only 1% difference between the signals of the two EDM notches.

Detection of Flaws in a Printed Circuit Board by Using an Eddy-Current Testing Approach

Eddy-current testing (ECT) is a powerful and useful technique for detecting cracks and flaws in uniform conductive plates and pipes. We investigated the diagnostics of printed circuit boards (PCBs) as a new application of ECT. Recent high-density PCBs require a new cost-effective testing system for the production phase. ECT is an interesting approach for inspection of as-made printed wirings, because it uses non-contact testing, low stress, conductive testing, and fast scanning. This paper describes the detection characteristics for some flaws in printed wirings. We experimentally examined the use of ECT for inspecting disconnections, chipping defects, and imperfections in the thickness of printed wirings. The results confirm than the ECT approach is attractive and effective for PCB diagnostics.

Simulation of eddy current signals from multiple defects

The application of eddy current testing for structural integrity assessment of heat exchangers is a well established practice. Service induced multiple defects in heat exchanger tubes produce difficult-to-evaluate complicated impedance plane trajectories during periodic eddy current examinations. Finite element simulations carried out to analyse interactions in a two-defect system are discussed. Probable uncertainty in the interpretation of eddy current trajectories is highlighted. Systematic analysis of the results from these studies will simplify procedures for on-line evaluation of eddy current signals.