Eddy Current Inspection Research Articles

Real Time Non-Destructive Testing Methods of Welding

This work presents a review of the three most efficient non-destructive testing methods. The methods are radiography, eddy current and ultrasonic inspection. These particular techniques were chosen because they are able to cover most of the industrial needs for welding joint inspection. The aim of this work is to present the physical background of operation for the given methods, discuss their benefits, limitations, and typical areas of application, and compare them with each other. In the first part of this work, all three methods and their variations are described in detail with schemes and figures which represent their working principles. It appears that, although all the given methods can detect all types of flaws in welded joints, they have their specific limitations. For example, ultrasonic testing is able to detect defects only in certain directions. The eddy current technique is also sensitive to defect direction, but it can be applied for inspecting conductive materials only. The main flaw of radiography is the resolution: it is not usable for very fine defects. The second part of the work is for comparing the testing methods and for drawing the conclusions. The methods are compared according to the possible materials, defect types and their position, as well as the possible areas of application. This part gives the background for choosing a proper welding joint testing method for certain applications in the welding industry.

Analysis of Spalling in Roughing Mill Backup Rolls of Wide and Thin Strip Hot Rolling Process

In this paper, the spalling causes of backup rolls in a roughing mill of a hot strip rolling mill were investigated. The roughing mill withstood extreme service conditions with long service cycles for backup and work rolls, a large variety of the same width rolling campaigns, severe and non‐uniform wear contours of backup and work rolls. Analysis of cyclic stress applied on backup rolls indicated that contact stress played a dominant role in rolling contact fatigue which led to spalling on the backup roll surface. A 3D finite element model of roll system was established and contact stresses between rolls in whole service periods of backup and work rolls were calculated. The simulation results showed that roll wear had significant impact on contact stress distribution. In particular, the severe and non‐uniform U‐typed wear contours of work rolls led to produce huge contact stress peak in two sides of the rolls, where existed large cracks by eddy current inspection of post‐service backup rolls. Backup rolls stood thousands of times stress cycle in a service period, they were liable to fatigue damage in these areas. A new backup roll contour was developed to improve contact stress distribution and no spalling accidents were happened any longer after applied to the roughing mill.

A Benchmark Problem for Eddy Current Nondestructive Evaluation

Currently, there are three TEAM Workshop benchmark problems involving eddy current nondestructive evaluation configurations, Nos. 8, 15, and 27. Regarding the first two, they have ceased to constitute challenging problems since they can be routinely solved with existing commercial software. In this paper, we present precision impedance measurements in a configuration involving a system of plates with through holes and a crack and propose them for benchmarking numerical codes. The benchmark represents a simplified version of eddy current inspection of fastener holes in aircraft structures, provides a lot of experimental data, and overall constitutes a more challenging problem.

Correction of eddy current measurements to obtain accordance with simulation results

Abstracts Forecasting an eddy current inspection task using simulation software is relevant for a better understanding of new problems. Simulation results have to be validated to prove how good the simulation is in compliance with the reality. A comparison of the simulated results and the direct output of an ET device very often lead to significant discrepancies. It can be shown that the reason for these discrepancies lies in the testing equipment itself and how these measurement errors can be corrected. Examples illustrate the strong correlation between simulation results and corrected measured values.

Coverage path planning for eddy current inspection on complex aeronautical parts

Non-destructive testing (NDT) plays a critical role in controlling the structural integrity (therefore the quality) of aeronautical parts, during fabrication as well as during maintenance. Eddy current (EC) testing is one of the most used NDT techniques in the aerospace industry. However, EC testing is still mainly performed by human operators and reliability as well as repeatability is not always guaranteed. To solve these issues, automating this NDT technique with a robotic system is investigated. In this paper, an EC probe equipped with a passive compliant system is assumed to be attached to the end-effector of a 6-DOF manipulator arm to carry on the inspection. Then, assuming that a 3D model of the inspected part is known a priori, a coverage path planning method using a zigzag (or rastering) pattern adapted to EC testing on aeronautical structures with a complex geometry is proposed. To reach this objective, the approach adopted in this work is to adapt existing coverage path planning techniques based on a “divide-and-conquer” strategy used for spraying applications to EC inspection. More precisely, three successive segmentations are applied to the surface to be inspected so that consistent rastering paths can be generated. Simulation results are shown for a complex part of an aeronautical structure to demonstrate the efficiency of the proposed method.

Integral equation method for evaluation of eddy-current impedance of a tilted, surface-breaking crack

An integral equation method for solving the eddy-current nondestructive evaluation problem for a flat, tilted, and surface-breaking crack in a conducting half-space is presented. The method involves use of a half-space Green's tensor and the Bowler potential. This potential describes the jump in the electric field over the crack and is expanded in basis functions related to the Chebyshev polynomials, being a more analytical approach than the commonly used boundary element method. In the method, the scatterer defines a transformation operator to be applied on the incoming field. This is practical in simulations of the eddy-current inspection where this operator is independent of the position of the probe. The numerical calculations of the change in impedance due to the crack are compared to a Finite Element model of the problem and good agreement is found.

Evaluation of the electromagnetic characteristics of type 316L stainless steel welds from the viewpoint of eddy current inspections

This study evaluated the electromagnetic characteristics of austenitic stainless steel welds from the viewpoint of eddy current testing. Seven welded plate specimens, which were welded using JIS Z3221:2010 YS316L welding metals, were prepared. Two welding metals and several welding conditions were adopted to discuss the generality. The results of eddy current examination of the specimens using three different types of eddy current probes, that is, an absolute pancake probe, a differential plus-point probe, and a uniform eddy current probe, confirmed that the welds cause a large noise especially when the absolute pancake probe was used. The analysis of the signals through finite element simulations revealed that the magnetic property of the welds is not negligible from the viewpoint of eddy current testing. Complementary experiments were conducted using a vibrating sample magnetometer to validate the results.

Computer Modeling Solves Practical Problems in NDE

Abstract Modeling and simulation are proving to be an effective alternative to the trial-and-error practices often used when developing and evaluating nondestructive tests for materials characterization. This article demonstrates the benefits of modeling in different cases where eddy current and ultrasonic inspection methods are employed.

Defect classification by pulsed eddy current technique in con-casting slabs based on spectrum analysis and wavelet decomposition

By comparison with traditional eddy current inspection, abundant response in wide frequency range and more utilized information both in the time-domain and frequency-domain can be realized by pulsed eddy current (PEC). However, the lift-off effect, measurement noise, and the surface oxidation are observed to negatively effects time-domain feature during PEC classification of the con-casting slabs (CCS). In this work, the spectrum analysis (SA) and SA after wavelet decomposition (SA-WD) are proposed for feature extraction. The principal component analysis (PCA) plus to PCA linear discriminant analysis (PCA-LDA) are aimed at counteracting the dimensional methods which are used for supplying the lower dimensional features. The Bayesian classifier is also applied to defect classification. The experimental results demonstrate that the cracks and cavities in surface and sub-surface can be classified satisfactorily by the proposed methods, which have the potential for gauging automatic in situ classification for CCS.

Certification of hardened surface layers by magnetic and electromagnetic methods

The possibilities of certification of hardened surface layers by measurement of coercive force, eddy current inspection and analysis of the field dependence of differential magnetic permeability μ d (H) are considered. The advantages of analysis of the pattern of peaks on the μ d (H) dependence for estimating the state of surface-hardened steels subjected to subsequent force loading are shown.

Model Based Capability Assessment of an Automated Eddy Current Inspection Procedure on Flat Surfaces

The probability of detection (POD) of a well-controlled, automated eddy current (EC) procedure is evaluated in a numerical model and compared with experiments. The procedure is applied in laboratory environment with a single absolute probe which is positioned for raster scan over flat surfaces containing fatigue cracks. The variability of the signal amplitude, due to small fatigue cracks in the Titanium alloy Ti-6Al-4 V, is expected to mainly originate from crack characteristics and the index distance of the raster scan. The POD model is based on the signal versus crack size ( versus a) result. The presented procedure provides a well-defined basis for a comparison between a simulated and an experimentally based POD assessment. Finite element analysis is used to model the EC method. A simplified fatigue crack model is first introduced and evaluated experimentally. Numerical computations are then used to build the corresponding model-based POD curve which shows good agreement with the experimental result. The model-based POD curve is generated both by means of a parametric and a nonparametric approach. Differences between model-based and experimental POD are discussed as well as the delta POD approach using transfer functions.

A New Approach to Improve the Probability of Detection of Eddy Current Inspection

Eddy current testing is now a widely used Non Destructive Test (NDT) method for determining the size of cracks in metallic structures. The probability of detection (POD) is a key metric in assessing the effectiveness of nondestructive evaluation techniques. The POD analysis using common method is studied and modified. The eddy current sensor effective is discussed to support the proposed method. The new method is shown better result in improve the POD than the common one. The estimates of POD as a function of crack size will be valuable for future model-assisted POD study.

Methodology of short fatigue crack detection by the eddy current method in a multi-layered metal aircraft structure

The field of non-destructive inspection (NDI) is an integral part of aircraft maintenance and service. It is optimal to apply the eddy current (EC) method on the complex multi-layered metal aircraft structure to detect cracks and other damage. A range of standard eddy current probes giving satisfactory results with respect to crack detection are available. However, these EC probes are not always suitable for the detection of short fatigue cracks hidden under a rivet head. This article presents a new methodology for eddy current inspection of a critical area on the wing. The development of the new methodology was inspired by a catastrophic glider accident in 2010. Because of this accident, all glider service was prohibited. The critical area is located in the structure of the wing spar, which consists of six layers. The position of the critical area is hidden under the countersunk rivet head in the third layer of the spar flange and under two layers of metal sheets. Thus, the actual location of the area is in the fourth layer, created by the countersunk rivet head. In addition, another rivet of a smaller diameter is inserted into the rivet in the flange. This article presents the work related to the development of a special EC probe that is able to measure in an operating frequency range between 200Hz and 100kHz with a single-value interpretation of the individual EC signals.

Statistical Assessment of Probability of Detection for Automated Eddy Current Nondestructive Evaluation Inspection

Eddy current inspection is widely used in nondestructive evaluation to detect cracks in metal structures. The impedance plane measurement response, collected using our motion controlled eddy current inspection system, is used in the analysis. A scalar reduction from the impedance plane response is used to minimize human-factor variation. We apply a noise interference model to the data from a large-scale experiment taking measurements on fastener rivet holes in multilayer structures with fatigue cracks, and estimate the probability of detection (POD). The estimates of POD as a function of crack size will be valuable for future model-assisted POD studies.

Investigation of the parameters of samples that are used for eddy-current inspections

In order to eliminate errors that result from different properties of a calibration sample and a tested object when selecting the conditions for an eddy-current inspection, the electric and the magnetic properties of various steel grades were investigated that are necessary to analyze the positions of the hodographs on the flawless areas of the calibration sample and the test object and to make corrections for the expected changes in these properties that are caused by the effects of a gap for defective areas of a tested object. The study was carried out using samples that were manufactured of steel grades of different chemical compositions with a set of artificially made defects of known sizes. The test technique was evaluated on the basis of a reference specimen from D16 duralumin. The magnetic and electrical properties of the steel grades that are necessary to calculate the hodographs of the signals from the transition-type and lay-on eddy-current converters and to select test conditions are reported.

Eddy Current Rail Inspection Using AC Bridge Techniques.

AC bridge techniques commonly used for precision impedance measurements have been adapted to develop an eddy current sensor for rail defect detection. By using two detection coils instead of just one as in a conventional sensor, we can balance out the large baseline signals corresponding to a normal rail. We have significantly enhanced the detection sensitivity of the eddy current method by detecting and demodulating the differential signal of the two coils induced by rail defects, using a digital lock-in amplifier algorithm. We have also explored compensating for the lift-off effect of the eddy current sensor due to vibrations by using the summing signal of the detection coils to measure the lift-off distance. The dominant component of the summing signal is a constant resulting from direct coupling from the excitation coil, which can be experimentally determined. The remainder of the summing signal, which decreases as the lift-off distance increases, is induced by the secondary eddy current. This dependence on the lift-off distance is used to calibrate the differential signal, allowing for a more accurate characterization of the defects. Simulated experiments on a sample rail have been performed using a computer controlled X-Y moving table with the X-axis mimicking the train’s motion and the Y-axis mimicking the train’s vibrational bumping. Experimental results demonstrate the effectiveness of the new detection method.

針形状磁気プローブを用いた狭空間内の微小傷に対するうず電流探傷

針形状磁気プローブを用いた狭空間内の微小傷に対するうず電流探傷

Research on spectral response of pulsed eddy current and NDE applications

Using pulsed excitation, pulsed eddy current (PEC) technique generates responses over a wide range of frequencies, containing more information than traditional eddy current inspection in the frequency domain. This paper investigates the spectral response of PEC under varying probe lift-off, material properties (electrical conductivity and magnetic permeability) and directional tensile stress. It is shown that material properties and lift-off exhibit different spectra patterns. Using normalisation in frequency domain, the lift-off effect can be reduced. Spectra for both electrical conductivity and magnetic permeability show a complex pattern. Tensile stress measurement in aluminium 2024 and 5083 is investigated, which show similar spectra pattern to electrical conductivity variation. Applications for corrosion characterisation, displacement measurement and profile inspection can be developed in the future in conjunction with time-domain features.

Eddy Current Modeling of Narrow Cracks in Planar-Layered Metal Structures

Eddy current testing (ECT) of narrow cracks is an important issue in several industrial areas. Efficient modeling tools are required in order to better interpret measurements, to design adapted probes, and to assess the reliability of testing procedures. A dedicated boundary element model (BEM) suitable for eddy current inspection of several narrow cracks affecting a planar-stratified conductive medium is proposed. The cracks' openings can be arbitrarily small, and their respective orientations be along parallel or orthogonal directions. The approach alleviates most of the computational difficulties of classical methods in such peculiar cases. Comparisons of the results with experimental and simulated data taken from the literature, completed with in-house experiments, illustrate its good accuracy and low computational burden.

Noninvasive detection of artificial heart valve outlet strut fracture using sweep frequency eddy current testing

Thepaper proposes utilizationof eddy current inspection fordetection ofdefects intheartificialheart valve. Artificial heart valves are prosthetic replacements which are used if the function of the natural heart valve is limited. A special type of prosthetic heart valve replacements is the Bj¨ ork Shiley convexo-concave (BSCC) heart valve. The fractures of the BSCC artificial heart valve usually affect the outlet strut. If the fracture of the strut is not detected in time the patient can die of heart attack. Detection of the fracture in the outlet strut is the concern here. Experimental measurements are performed in order to evaluate the detection capability of the technique and the sensitivity of response signal to dimensional parameters of a fracture. A BSCC heart valve prototype having the electromagnetic characteristics of the Co-Cr (Hayness 25) alloy is inspected with a probe. A non-conductive defect is positioned in the outlet strut and its dimensions are varied. The probe is positioned above the centre of the crack and the frequency of exciting signal is changed in a wide range. The presented results demonstrate that the sweep frequency technique can enhance eddy current inspection of a defect in BSCC heart valve.