challenges in improving the performance of eddy current

Original Paper

Measurement and Control1ndash19 The Author(s) 2018Article reuse guidelinessagepubcomjournals-permissionsDOI 1011770020294018801382journalssagepubcomhomemac

Challenges in improving theperformance of eddy current testingReview

Ahmed N AbdAlla1 Moneer A Faraj2 Fahmi Samsuri2Damhuji Rifai3 Kharudin Ali3 and Y Al-Douri4

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

KeywordsEddy current testing non destructive testing lift-off defect detection

Date received 13 May 2018 accepted 22 August 2018

Introduction

Pipelines are regarded as a preferable way of transport-ing oil refined oil products or natural gas in large quan-tities over land The network of pipes has advantagesover other means of transportation (such as traintruck)because of its high cost12 Thus it is essential to con-tinuously check the conditions of the pipeline on a regu-lar basis

The eddy current testing (ECT) method has longbeen utilized for nondestructive testing (NDT)3 andwidely applied to inspect conductive material structurein order to identify their structural integrity Bettaet al4 suggested that coils be used to detect the mag-netic field (MF) as the primary indicator of a crack inthe pipeline However this method is limited due topoor sensitivity at low frequency for the inspection ofthe subsurface defect and thickness of materialsdemands sensitivity at low frequency To overcome thepoor-sensitivity limitation of the traditional eddy cur-rent probe NDT technology has the advantage ofemploying magnetometer (MR) sensors to obtain maxi-mum information from the component being tested5

Many types of research have introduced many struc-tures of ECT for the purpose of material inspection Leeet al6 proposed that the bobbin coil is used to induce the

eddy current in small piping and the MF can be pickedup by utilizing a Hall sensor array This technique allowsthe distribution of the distorted electromagnetic (EM) fieldaround outside diameter of the stress corrosion crackingso it can be imaged without the need for a rotating appa-ratus In another study Postolache et al7 proposed tworectangular planar excitation coils and an array of fiveAA002 giant magneto resistive (GMR) sensors for defectdetection of an aluminum plate They found that the opti-mization of the ECT probe had enhanced the ability ofinspection with rapid scanning time

Ye et al8 used a pair of orthogonal (axial and cir-cumferential) coils to generate an eddy current in steam

1Faculty of Electronic and Information Engineering Huaiyin Institute of

Technology Huairsquoan China2Faculty of Electrical amp Electronics Engineering University Malaysia

Pahang Pekan Malaysia3Faculty of Electrical amp Automation Engineering Technology TATI

University College Kemaman Malaysia4Nanotechnology and Catalysis Research Center (NANOCAT)

University of Malaya 50603 Kuala Lumpur Malaysia

Corresponding author

Ahmed N AbdAlla Faculty of Electronic and Information Engineering

Huaiyin Institute of Technology Huairsquoan 223002 China

Email dramaidecngmailcom

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generator tube wall and a GMR array to detect theinduced MF The probe showed superior inspectionaccuracy and sensitivity to defects of the axial and cir-cumferential directions Zeng et al9 suggested a multi-line as an excitation coil while a GMR sensor could beplaced on the line of symmetry to detect any changes inthe MF The simulation result shows improvement inthe detection sensitivity for multilayer subsurface flaws

Rifai et al10 also described the application of GMRsensors in ECT Also discussed in detail were the limita-tion of utilizing the coil as a sensor and compensationtechniques that have been used in the ECT In anotherstudy Ali et al11 discussed in detail the method of theECT and the parameters that impacted the signal fun-damental in relation to the hardware and softwareGarcıa-Martın et al12 delibertated an overview of theprimary variables and the principles of ECT sensors

Therefore the structure of the ECT probe should beidentified to provide and evaluate the tested materialswithout destroying them13 This paper will review theeddy current probe structure and error compensation ofthe eddy current probes to obtain the ideal measuringdepth of defect on carbon steel pipeline The lift-off effectwill be discussed in detail to ensure that the measuringdefect is right This study will also introduce differentconventional and intelligently lift-off compensations

The principle of ECT

The diagram illustrating the operation principle of theECT is shown in Figure 1 According to Faradayrsquos lawthe driving coil is excited to produce a MF known aseddy currents14ndash16 The emf induced in the receivingcoils are generated by the eddy currents

Four steps can demonstrate the MF connectionbetween the primary and secondary coils

Alternating current through the coil produced bythe principal MF

Alternating primary MF produces the EC in theconductive sample

EC produces a secondary MF in an opposingdirection

Flaws in the sample perturb the EC and decreasethe secondary MF which results in the variation ofimpedance changes of the coil

The advantages of ECT include the fact that theyare economical and environmentally friendly as a non-contact method with high detectability inspectionspeeds and offers good discrimination On the otherhand the main disadvantages are it is sensitive todefects near the surface and is only applicable to con-ductive materials1217

Eddy current probe

Various configurations are presented for the excitationsource and detection sensor However in many in-service applications the inductive coils are utilized bothas a field source and field sensors As a result the eddycurrent probes are commonly classified according totheir configuration and mode of operation The probeconfiguration is closely related to the way the coil orcoilsrsquo connection covers the testing area of interest Theprobe operation mode is commonly classified intoreflection differential absolute and hybrid modeswhereas some of the standard configurations includethe outside diameter probes inside diameter (bobbin)probes bolt hole probes and surface probes18

An inductive probe can include one or more coils Inconventional eddy current probes these coils typicallycomprise lengths of wire wound in a helical manner likea solenoid The winding will commonly have more thanone layer to increase the value of inductance As men-tioned above there are many ways in which these coilscan be constructed based on the specified applicationConventional ECTs are transmitndashreceive probes multi-pancake andor rotating pancake probes and bobbinprobes Each method has its respective strengths andweaknesses in consideration of their characteristics suchas the test speed flaw detection sensitivity and probestructure complexity19

Bobbin probes

Figure 2 shows the two types of the bobbin probeswhich are called the differential and absolute bobbinprobes Differential bobbin probe has two coils posi-tioned at 180 out of phase The probe has excellentsensitivity to detect abrupt anomalies and small defectssuch and relatively unaffected by lift-off pitting corro-sion fretting wear and probe wobble However theprobe is not sensitive to metallurgical and gradualchanges2021

Absolute bobbin consists of a single bobbin coil anda second identical reference coil The second identicalreference coil is used for EM shielding of the inspectedtubing and electronic balancing The probe has excel-lent sensitivity to detect axial cracks and is highly sensi-tive to material property variations and graduallyvarying wall thinning6 The main disadvantage of theabsolute coil is that the defect is typically superimposedover a lift-off as the large signal

Figure 1 Principle for the eddy current testing operation

2 Measurement and Control

Rotating probes

The rotating eddy current probes are used for high-resolution imaging of the steam generator tubes asshown in Figure 3822 The rotating probe is sensitive todefects of all orientations and has a high resolution andimproved sensitivity to characterize and size defectsHowever the mechanical rotation of the coils causesserious wear leading to frequent probe failure and affectthe inspection time and subsequently the cost willincrease significantly23

Array probe

The array probe types include the smart probe X-probe C-probes and intelligent probe The array probeworks as a transceiver probe and can cover the

direction of 360 The transmitting coils are activelydriven by the AC source with a different range of fre-quencies The receiving coils generate an induced vol-tage equal to the change of magnetic flux through thecoil The array probe response for different orientationdefects has a higher signal-to-noise ratio (SNR) and is10 times faster than the rotating probe Another disad-vantage is that the hand array probe is very costlybecause of its complicated excitation and data acquisi-tion parts2425 as shown in Figure 4

Rotating field probe with bobbin coil

Figure 5 shows the excitation part which consists ofthree coils with identical 120 axes degrees apart andbalanced alternating currents with adjustable fre-quency phase and amplitude The rotating MF is gen-erated without mechanical rotating support2126

Comparison of eddy current probes

The advantages and disadvantages of eddy currentprobes for the NDT as described in Table 130

EM NDT techniques

The EM methods of NDT comprise a full spectrum oftechniques ranging from static (DC) methods to high-frequency (10THz) methods The next section presentsthe most comprehensive inspection technique that usedin the EM NDT31

Figure 2 Tube inspection probes (a) absolute and (b) differential

Figure 3 Rotating probe

Figure 4 Array probe

AbdAlla et al 3

Pulsed eddy current

Pulsed excitation produces transient signals with a widerange of frequency components Hence it containsmore information compared to a single-frequency exci-tation3233 The pulsed eddy current (PEC) signals havecommon features in the transient characteristics suchas the peak amplitude time-to-peak amplitude andtime-to-zero crossing The peak amplitude will deter-mine the defect size The defect depth or material thick-ness will be identified by the peak amplitude34ndash36 Theearliest study of PEC for crack detection in layeredstructures with installed fasteners was conducted byHarrison3738 Giguere et al39 also studied the detectionof cracks beneath rivet heads using the transient ECtechniques Figure 6 shows some experimental resultsof PEC to test multilayer sample33

ECT with MF measurement method

The most recent research introduced the GMR sensorIt is widely used in many applications because the sensi-tivity of these sensors is independent of the MF it hasa high bandwidth only requires a low power supplythe dimensions of GMR are small and the output signalis high compared to the other MR sensors Thereforethe GMR-based EC testing exhibits significant advan-tages in detecting complex geometry such as a layeredcomponent inspection40 The directional property ofthe GMR sensor had been used to locate edge cracks in

aluminum specimen4041 A needle-type GMR imagingtechnique named the SV-GMR system was designedfor the inspection of a bare polychlorinated biphenylstructure to measure the magnetic fluid density in a liv-ing body1042

High-resolution GMR elements are fabricated in asmall package of sensors arrays An inspiring applica-tion of this array probe was found in the evaluation ofmetal medical implants for invisible cracks43 A lineararray of 20 GMR elements was packaged to image ahole defect in a steel plate using 1Hz excitationDesigns of GMR array probes in identical elementshad been studied to detect subsurface cracks44 High-density GMR arrays were especially promising forrapid scanning of a large area as well as high-resolutionimaging745 Another type of GMR array sensors thatuse two-directional elements was investigated in the ECtesting to detect surface cracks of unknown orientationThey measure both X-component and Y-component ofthe MF at the same point5 A fast Fourier transforma-tion to enhance the ECT probe based on the GMRarray sensors for pipe inspection was utilized by Duet al46

The transient excitation of the coil probes with twoMR sensors or two Hall sensors in differential modehave been studied by Lebrun et al4748 and used forcharacterizing crack parameters Kim et al49 intro-duced a method for the assessment of aircraft struc-tures The system produced pulse excitation thatenergized a planar multi-line coil The GMR field sen-sor was used to detect the transient field Tai et al50

studied similar transient features for an inversionscheme to qualify the conductivity and thickness of thesamples Table 2 shows the difference between the eddy

Table 1 ECT probes for tube or pipe assessment inspection

Types Advantages Disadvantage

Bobbin probe618 Rapidly inspects speed determine defect depth andlength lower price higher dependability andtoughness and sensitive to axial flaws

one dimension scan data and insensitive tocircumferential defect

Rotating probe21 Depth C-scan visualization for inner tube surfacesensitivity to the flow of all orientations and higherSNR

Low dependability and toughness costly andinspection speed is slow

Array probe2728 Sensitivity to flaws in all directions inspection speedis fast and size defect with the depth and length ofthe C-scan visualization for inner tube surface

Very expensive with complex instrumentation

Rotating field probe829 Sensitive to defects of all directions no needrotating probe mechanically high inspection speed

Need field test approve

SNR signal-to-noise ratio

Figure 5 3D model of rotating field probe with bobbin coil

Figure 6 PEC to test multilayer sample33


Table 2 Overviews of the utilities AC signal in eddy current testing with different probe designs

Author Analysis toolsoftwaresimulation

Type of sample Excitation coil Sensing Finding

DrsquoAngelo et al51 Multilayerperceptronneural network(NN)

Aluminum alloy(2024 T3)

Coil with aparallelepipedshape

GMR sensor Mechanical rotation ofEC probes around walltube is needed toinspect the inside pipe

Zhang et al52 Finite elementmethod (FEM)

Conductive plate Rectangularexcitation coil

Cylindrical pickupcoil

The analytical resultsagree with FEM resultsand the forwardmodel of quantitativedetection for ECTofmultilayer conductivestructure

Postolache et al53 ANNmultilayerperceptronfinite elementsimulation

Aluminum plate ECP1 has apancake type (largediameter and smallheight) and ECP2has a smalldiameter and longlength

The GMR sensor inboth cases

Implementation of theNN classificationmethod improves thedependability of defectclassification

Zeng et al9 Three-dimensionalfinite elementmesh

Aluminum andsteel fasteners

A multi-line coil GMR sensorlocated on the lineof symmetry

Simulation resultsestablish that theproposed methodimproved thesensitivity of themethod in detectionof multilayersubsurface flaws

Yang et al54 Finite elementsimulationimage fusiontechnique

Steel fastener Planar multi-linecoil

GMR sensorinstalled on the lineof symmetry

Eliminate the difficultyof detecting cracksunder steel fastenersby using thecharacteristics oftangential componentsBy and Bx of theinduced magnetic field

Yang et al55 FEM Aluminum andsteel rivets

Two unidirectionalplanar coilsoriented inorthogonaldirections

GMR sensor The experimental andsimulation outcomesindicate that themethod can detect allorientations of a flawunder the fastener

Kim and Lee56 Experimental Inconel 690 tubes Multiple coils withthree deferentangle

Multiple coils withthree deferentangle

The experimentalresults prove that thesuggested probes aremore sensitive tocircumferential defectsand sensitive to axialdefects by employingboth the new probesand the conventionalbobbin

Ye et al8 C-scan imageFEM

An Inconel 690steam generatortube

Pair of orthogonal(axial andcircumferential)coils

A circumferentialarray of 16 GMRsensors

The probe has highinspection speed andsensitivity to defectsof the axial andcircumferentialdirections However ithas very complicatedexcitation and dataacquisition system

Paw et al57 Design ofexperimentsoftware

Carbon steel pipe Encircling coil for adifferential probe(ECDP) andencircling coil foran absolute probe(ECAP)

Encircling coil for adifferential probe(ECDP) andencircling coil foran absolute probe(ECAP)

The probe cannotdetect the transversedefect

(Continued)

AbdAlla et al 5

Table 2 (Continued)



Shim et al58 The primarysimulated waterof a pressurizedwater reactor

Alloy 690 steamgenerator tubes

A conventionalbobbin coil probeand a conventional3-coil motorizedrotating probe

A conventionalbobbin coil probeand 3-coilmotorized rotatingprobe

The general corrosionrates of alloy withdifferent 690TT tubenoises can bepredicted from thetube noise valuemeasured by arotating pancake coilprobe

Ribeiro et al59 A conformaltransformation

Aluminum plate A rectangular crosscoil

GMR sensor The conformaltransformation usedto model the crack ofan aluminum plate topreview the acquiredvoltage signals

Yin and Xu60 Peakfrequencies ofthe sensorsignal have beenutilized tocalculate thethickness of theplate

Aluminum plates Triple-coil sensormeasurements aremade by excitingthe middle coil

Triple-coil sensormeasurements aremade by takinginduced voltagesfrom the bottomand the top coils

The results show thatthe technique is highlyimmune to lift-offvariations

Lee et al6 FEM Piping of titaniumalloy

The bobbin coil State Hall sensorarray

This technique allowsthe distribution of thedistorted EM fieldaround outsidediameter stresscorrosion cracking tobe imaged without theneed of a rotatingapparatus

Joubert et al61 C-scan images Bore hole Bobbin coil The row of pickupcoils of the sensingarray

The acquiredexperimental resultsshowed a greatsensing capability ofthe designed probe inthe 10ndash800 kHzfrequency range

Menezes et al62 Numericalsimulation FEM

Aluminum plate Pancake coil GMR sensor and apermanent magnetto put the sensorworking in itslinear range

The magnetic fluxdensity is closelycorrelated with thecrackrsquos depths wheremaximum amplitudedisturbance dependson the crack depth

Xie et al63 FEMthe NCSFalgorithm

A plate of 7075aluminum alloy

A large uniformcoil is designed onthe bottom andtop layers

64 sensing coilelements on twomiddle layers

All of the results showthat the array is notjust sensitive to microcracks howeveradditionally able tocrack length size

Suresh et al64 ANSYS-basedFEM

Ferro tube Bobbin coilcore ofiron

A hall sensor The efficiency of thesuggested bobbin coilis effective to inspectthe small diametertube

Ye et al65 FEM Two-layeraluminum

Two orthogonalcoils

Two linear arraysof GMR sensorsare located aboveand below theorthogonal coils

Experimental resultspresent that the probecan detect flawsregardless of theirorientation Thedifferential schemereduces the effect ofthe background fieldand improves SNR

(Continued)


current methods which rely on the sensor element thatutilities alternative current signal in ECT

Multi-frequency techniques

NDT widely uses multi-frequency techniques (MFTs)The MFT expanded the capabilities of using single-frequency testing which allows simultaneous tests

The multi-frequency process uses a composite signaland subtracts the undesirable signal12 The main unde-sirable signal caused by changes in the temperature var-iation material geometrical and probe lift-off69 MFTsare usually accomplished by combining the resultsobtained at different frequencies in the spatial domainLiu et al69 presented integrate two- (multi) frequencyinjection with dimensional spatial domain named as apyramid fusion method The SNR improved due toreduction in noise sources which demonstrated thepotential of signal enhancement via fusion method orraster scanning70 A two-dimensional (2D) surface pro-duced changes over of the impedance or impedance byraster scanning images12 Image processing techniquescan be applied to detect cracks using ECT Bartels andFisher71 proposed a multi-frequency eddy currentimage processing technique for the non destructivematerials evaluation SNR improvement up to 1100over traditional two-frequency techniques a sequenceof complex valued images generated from 2D ECT tomaximize the SNR The linear combination of theimages by Bartels and Fisher71

d x yeth THORN=X2Nf

i=1

cifi x yeth THORN

where Nf is the number of test frequencies and fi areextracted from the 2D images Results on experimentaldata demonstrate

Factors contributing to eddy currentsignals

The signal from an eddy current probe includes a col-lection of responses from defects sample geometry andprobe lift-off7273 Therefore it may be difficult to sepa-rate a single influence Adequate assessment of flaws orany other surface properties is likely when other factorsare understood7 The primary factors influencing theresponse of an eddy current probe are explained in thissection

Frequency

Eddy current response is strongly affected by the fre-quency chosen for the investigation This factor shouldbe appropriately selected by the operator based on thecrack detection sample such as lower frequencies forbulk characterization and higher frequencies for sur-face characterization

Many authors such as Ditchburn et al74 andThollon et al75 utilize this range and they suggestedthe range of 100Hzndash10MHz as standard inspection fre-quencies in ECT19 However a few authors such asOwston76 characterized high frequency at 25MHz forthin metallic coatings and detecting surface defects Inthe inspection of ferromagnetic materials low-frequency tests are applied to penetrate into the testspecimen and compensate for their high permeability

Table 2 (Continued)



Lee et al66 FEM The steel (SS400)plate

The two probesmeasure thecurrent and thevoltage todetermine the coilimpedance

A coil winding anda ferromagneticcore are used as asensing element

A low frequency hasbeen applied for deepinternal inspection of aferromagnetic material

Rifai et al67 FEM Carbon steel pipe Pair of orthogonal(axial andcircumferential)coil

An array of GMRsensors

It has verycomplicated excitationand data acquisitionsystem

Xin et al21 3D FEM Steam generatortubes

Three identicalwindings locatedon the samephysical axes 1201apart

The responsesignal can be pickedup by bobbin coil inthe center

The probe is sensitiveto flaws of allorientations in thetube wall and the linescan data enable rapidinspection rates

Abdalla et al68 Fuzzyinterferencesystem

Carbon steel pipe Pair of orthogonal(axial andcircumferential)coil

Hybrid absoluteand differentialprobe

The Mamdani-typefuzzy use to integratedabsolute anddifferential probe

GMR giant magneto resistive EC eddy current ECP eddy current probe ECT eddy current testing ANN artificial neural network NCSF

normalized crack signal fitting SNR signal-to-noise ratio

AbdAlla et al 7

Ramos et al77 have studied the detection of subsurfaceflaws relating to the characterization of depth profilesof the subsurface defects in aluminum plates78

However the high frequency applied for the inspectionof small discontinuities occured in the near-surface7980

Table 3 summarizes the impact of different frequencyvalues on the depth of penetration of severalmaterials81

Conductivity of test material

Electrical conductivity and the magnetic permeabilityof the test objects of the material depend on the micro-structure for example grain structure the presence ofa second phase work hardening and heat treatmentGreater conductivity of a material such as copper andaluminum will lead to greater flow of the eddy currentsand hence the probe coil resistance

In great conductive materials defects or cracks pro-duce a high signal as an impedance plane as illustratedin Figure 8 Furthermore the phase lag between thelift-off line and the defect is f1 f282 which is signifi-cant as indicated in Figure 7

However the penetration depth of highly conductivematerials at a fixed frequency is lower than in lowerconductive materials such as stainless steel and steelThe conductivity of the different materials can be mea-sured using the International Annealed CopperStandard (IACS) Table 4 summarizes the conductivityof common elements83

Magnetic permeability

ECT signals are significantly affected by ferromagneticmaterials due to the increase in flux produced by thesignificant relative permeability of certain materialssuch as stainless steel or carbon steel84 The permeabil-ity of material changes the coupling of the coil with theconductive specimen and subsequently affects the reac-tance of the coil Permeability has a significant effecton the ECT compared to conductivity where the crackdetection has no potential when permeability changesrandomly8586

Lift-off

ECT is strongly affected by the amount of lift-offwhich can be defined as the separation distancebetween the excitation coil surface and the conductingmaterial surface This distance changes the mutualinductance of the circuits as the lift-off increases theamplitude of the eddy current induces emf as the sec-ondary coil decreases which can result in the misinter-pretation of the signals as flaws At a significant lift-off no detectable emf will be induced in the secondarycoil due to the sample chosen818788 This effect is par-ticularly prominent when using sinusoidal excitationswhich lose sensitivity beyond 5mm89 Although it isnot required to have a zero lift-off it is imperative totry and maintain a consistent lift-off since the variationin coupling between probe and test piece will

Table 3 Typical depths of penetration

Metals 368 d

1 kHz 4 kHz 16 kHz 64 kHz 256 kHz

Copper 0082 0041 0021 0010 0005Uranium 0334 0167 0084 0042 0021Zirconium 0516 0258 0129 0065 00327075 T-6 0144 0072 0036 0018 0009Steel 0019 0009 00048 00024 000126061 T-6 0126 0063 0032 0016 0008Magnesium 0134 0067 0034 0017 0008

Figure 7 Lift-off curves and crack displacement at impedanceplane82

Table 4 Conductivity and resistivity of conductive materials

Material Conductivity ( IACS)

Copper 10000Gold 7000Aluminum 6061 4200Brass 2800Copper-nickel 90ndash10 910Cast steel 1070Inconel 600 172Silver 10500Stainless steel 304 239Zircalloy-2 240


significantly affect the received signal Table 5 lists pre-vious studies that have considered the lift-off issue

Figure 8 illustrates the offset position of the tubeinside the bobbin coils Lift-off is explained using a coilwhose axis is normal to the test piece However lift-offalso occur when the test is conducted using encirclingor bobbin probes The vibration of the rod or the tubeinside the probe generates noise which presents difficul-ties when inspections are conducted117

There are methods for lift-off compensation whenthe eddy currents are used in order to detect cracks andlift-off becomes an undesired variable For instanceYin et al100 researched dual excitation frequencies andcoil design to minimize the lift-off effect Researchabout processing the data was also conducted to mini-mize the lift-off effect Lopez et al118 proposed the useof wavelets to remove the eddy current probe wobblenoise from the steam generatorrsquos tubes Reduction inthe lift-off effect was also attempted by optimizing thecoil design and sensor array119

Tian et al101 had researched the reduction of lift-offeffects via normalization techniques The technique canbe applied to the measurement of metal thicknessbeneath the non-conductive coatings and to the mea-surement of microstructure and strainstress where theoutput is highly sensitive to the lift-off effect Table 5illustrates previous studies that considered the lift-offissue

Optimization of ECT probes design

According to the state of the art the probability ofdetection techniques of eddy current techniques can beimproved by the optimizing the probe design In recentyears several studies have focused on optimization ofthe ECT probe design for defect detection Rochaet al120 proposed an ECT probe design based onvelocity-induced eddy currents to detect surface defectsCommercial simulation software was used for the opti-mization and design of the probe Their experimentalresults confirmed that the proposed probe design wasable to detect defects in the conductive materials where

motion is involved120 A biorthogonal rectangular probewas developed by Zhang et al52 Simulation resultsshowed the new biorthogonal rectangular probe has alesser effect on the lift-off with higher sensitivity detec-tion Meanwhile Cardoso et al121 optimized the sensorconfiguration in the eddy current probe design A finiteelement modeling simulation was used to measure theaccuracy detection of the probe design Comparison ofexperimental and simulation date proved the accuracyof the proposed probe Research by Rosado et al122

reported the influence of the geometrical parameters ofan eddy currents planar probe in ECT The findingsshowed that modifications of the studied parameterscould substantially improve the probe performance Ina different study Ghafari et al123 proposed a methodol-ogy for determining the optimal sensor parameters forECT probe design Simulation results showed the pro-posed sensor geometry improved the probe sensitivity todepth changes in a crack

Chen and Miya124 proposed ECT probes based onsimplified detectability analysis method and a ring cur-rent model The optimal probe designs are developed inview of the combinations of these excitation and pickupcoils by comparing their detectabilities evaluated withthe simplified analysis method Aldrin et al125 pre-sented a comprehensive approach to perform model-based inversion of crack characteristics using bolt holeeddy current techniques Signal processing algorithmswere developed for wide range of crack sizes andshapes including mid-bore corner and through thick-ness crack types Inversion results for select mid-borethrough and corner crack specimens are presentedwhere sizing performance was found to be satisfactoryin general but also depend on the size and location ofthe flaw Table 6 summarizes previous studies on theoptimization of the ECT probe design

Application of artificial intelligent in eddycurrent

GMR sensor is used in ECT to pick up the MF in thepresence of the defect As the mutual inductance

Figure 8 Wobble simulation a bobbin coil in an offset position to a tube87

AbdAlla et al 9

Table 5 Review of lift-off compensation techniques

Author Technique software orhardware

Sensor type Signal excitation Sample type Research Area

Fan et al90 Model-based inversionalgorithm

Air-core coil Sinusoidal signal Three plates ofcopper aluminumand stainless steel

Lift-off elimination formodel-based inversionmethod

Dziczkowskiet al91

Excrementalmathematical model

Coil Sine excitation atlow frequencies isapplied

Conductivematerial

To eliminate the lift-offbetween the coil and thespecimen under test

Lu et al92 Dodd and Deedsmethod

Coaxiallyarrangedcoils

Frequencysweeping

The metal plate Reducing the lift-off effecton permeabilitymeasurement formagnetic plates frommulti-frequency inductiondata

Ribeiroet al93

Lift-off correctionbased on the spatialspectral behavior ofeddy current

GMR The probeexcitation currentwas set to 100 mAat 5 kHz

A duralumin2024 T3 plate

Increase the signal-to-noise ratio ofmeasurements

Kral et al72 Linear transformermodel to investigatethe effect of the lift-off

GMR sensor Pulse signal Metallic plate Investigate the origin andthe characteristicsassociated with the lift-offpoint of intersection(LOI)

Wu et al94 Signal analysis base onmulti-frequency phasesignature

Coil Sinusoidal signalwith multi-frequency

Copper andaluminum plates

Presented a simple modelfor metal thicknessmeasurement that isunaffected by lift-off effectto obtain the thickness

Yin and Xu60 Using peak frequenciesof the sensor signal toestimate the thickness(h)

Bottom andtop coils

Swap frequency Aluminum plates Designed a triple-coilsensor operating as twocoil pairs and in a multi-frequency mode tomeasure plate thickness

Huang andWu95

Relative magnetic fluxchanging rate (s)

Coil Pulse signal and thesquare wave usedas excitationcurrent

Four 16Mn steelplates

Ferromagnetic objecttesting used to removethe lift-off effect withoutextra measurement ofreference signals

Yu et al96 Measure the defectdimension base onslope of the linearcurve of the peakvalue

Hall sensor Pulse signal 7075 and 2024aluminum alloyplates

Reduce the lift-off noisefor detection of defectdepth or width

Zhu et al97 Hough transform wasused

Coil Sinusoidal signal Annealing copperplate

Investigate the lift-offeffect in the normalizedimpedance plane

LopesRibeiroet al98

The theory of thelinear transformer

GMR Sinusoidalexcitation

Aluminum plates oftype AL3105-H12

Measurement of thethickness of a metallicnon-ferromagnetic plate

Tian andSophian99

Normalizationtechnique

Coil Pulse signal Two plates ofaluminum

Minimize lift-off impact Itcould be utilized formetal thicknessmeasurement and formicrostructure analysis

Yin et al100 Analytical model thatdescribes theinductance

Air-coredcoil

Multi-frequency Conducting plate The phase signature ofsuch a sensor is virtuallylift-off independent

Tian et al101 Analytical model basedon the extendedtruncated regioneigenfunctionexpansion

Hall sensor Pulse signal Plate conductorwith arbitrarynumber of layers

Analyze LOI with respectto different PECconfigurations and build

(Continued)


Table 5 (Continued)



Wei et al102 U-shaped ACFMsystem

Coils The signalgenerator providesa 6 kHz sinevoltage waveformsignal

Mild steel sheetworkpiece with acrack Thelongitudinal

Determine an optimal lift-off value for a specific U-shaped ACFM system

Gotohet al103

The 3D edge-basedhexahedral nonlinearFEM

A transversetype Hallelement

The excitingfrequency is set to1 kHz

Nickel-coated steelplate

To inspect the thicknessof nickel-layer on steelplate without the impactof lift-off

LopesRibeiroet al98

A linear transformermodel

GMR sensor Sinusoidal signal Aluminum plates Measures the thickness ofmetallic

Gotohet al104

Electromagneticacoustic transducer

Transducercoil

A 250 kHzsinusoidal burstcurrent drives theEMAT coil

Steel plate Suitable lift-off value forthis specific EMAT systemshould be around 2 mm

Li et al105 The relative variationof magnetic flux (s)

Coil Pulse signal Metal plate Measurement of lift-off isextracted from themiddle part of the PECTsignal

Anganiet al17

Transient eddy currentoscillations

Hall sensor Pulses of the 50duty cycle with therepetition rate of2 s

Stainless steel plate Eliminate the falseindications due to the lift-off variations in thethickness measurement ofthe test material

Wanget al106

The slope of the lift-off curve (SLOC)feature of the eddycurrent sensor (ECS)

Sensor coil The workingfrequency (1 MHz)

Copper film Immunity to lift-offvariation

Wanget al107

Pulsed eddy current(PEC) responses

Hall sensor Pulse signal Nonmagnetic plate Study behaviors of lift-offpoint of intersectionpoints due to a plate withvarying conductivity andthickness

He et al108 Principal componentanalysis and supportvector machine

Coil The excitationpulse used 196 V100 Hz

Two 3003vAlndashMnalloy plate

Defect-automatedclassification

Aminehet al109

Blind deconvolutionalgorithm and awavelet networkinversion method

A tinyinductioncoil sensor

Alternating current Metal surface Lift-off evaluation andsurface crack signalrestoration

HoshikawaandKoyama110

Improvements inprobe design (h)

Tangentialcoil

Alternative current Brass plate To monitor the probe lift-off to avoid the probe notdetecting flaws in thematerial

Lu et al111 Multi-frequencyinductance spectraldata

Air-core coil Sinusoidal signal Metallic plates Proposed a new index tocompensate lift-offvariation linked to thethickness At different lift-offs the accuracy of thethickness measurementsproved to be more than98


Air-core coil Sinusoidal signal Magnetic plates Proposed a new modifiedpermeabilitymeasurement techniquefor magnetic plates frommulti-frequency inductiondata The permeabilityerror caused by lift-offcan be reduced within75

(Continued)

AbdAlla et al 11

between the coil and the specimen is sensitive to thelift-off any changes in lift-off must be compensated forin order to achieve the accurate depth of defectmeasurements

The fuzzy logic is instrumental for enhancing lift-offcompensation Artificial intelligent used is in manytypes of research in ECT There are two common fuzzyinference methods Mamdanirsquos fuzzy inference methodand TakagindashSugenondashKang a method of fuzzy infer-ence The output from Mamdani fuzzy inference sys-tem (FIS) can be easily transformed into a linguisticform as the inference result before defuzzification135

The main difference between Mamdani-type FIS andSugeno-type FIS resides in the way the crisp output isgenerated from the fuzzy inputs136 While Mamdani-type FIS uses the technique of defuzzification of afuzzy output Sugeno-type FIS uses weighted averageto compute the crisp output137 so the Sugenorsquos output

membership functions are either linear or constant butMamdanirsquos inference expects the output membershipfunctions to be fuzzy sets which are appropriate forcomparing the peak of sensors Based on artificial intel-ligent technology a lot of research has been done bymany researchers about ECT as shown in Table 7

Conclusion

This paper introduces an overview of the eddy currentprobes structure and error compensation techniques toobtain an ideal measuring depth of defect for the mate-rial under test There are two techniques to sense thechange in MF air coil and magnetoresistance sensorAir coil is less sensitive at low frequency which requiredto inspect the surface of pipe and plate This particularpaper also reviews the manufacturer in the industry toproduce the commercial probes for practical tube

Table 5 (Continued)



Wen et al113 Lift-off point ofintersection has beenapplied to determinethickness or evaluatedefects

Air-core coil PEC Metallic plates The effective signalfeature indicates that thefrequency and amplitudeat the LOI are decreasedwith sample thicknessincreasing In addition thefrequency LOI can beused to measure thesample thicknessfunctioning similarly totime domain LOI bymonitoring the frequencyand amplitude

Kral et al72 Linear transformermodel

Magnetoresistivesensor probe

Pulsed excitation No ferromagneticmetallic plates

The timederivative ofthe

magnetization curvesobtained for differentgaps between theexcitation coil and theplate was proposed

Yin et al114 Simplified modelrelated to thicknessconductivity and lift-offs

Air-core coil Sinusoidal signal Nonmagneticmetallic plate

Present the model withtwo independentparameters for a range ofthickness conductivityand lift-offs

Wei et al115 Design alternatingcurrent fieldmeasurement (ACFM)system

U-shapedprobe

Sinusoidal signal nonmagneticmetallic plate

Determine the optimum-induced frequency basedon the signal acquisitionand the measurementaccuracy of an ACFMsystem

Fan et al116 PEC spectral responseto serve as robustfeatures for thicknessevaluation

Hall sensor PEC Nonmagneticmetallic plate

Propose to apply thephase of PEC spectralresponse from a Hallsensor rather than pickupcoilndashbased probe tothickness measurementfor the elimination of lift-off effect

GMR giant magneto resistive ACFM alternating current filed measurement FEM finite element method PECT pulsed eddy current thermography


Table 6 Summary previous studies on optimization of ECT probe design

Authors Research area Optimization techniquesoftware tool

Observations

Betta et al126 Optimized complex signals forECT

Signals analyzed intransformed domains

The effect of suitable signaloptimizations carried out to retrieveexcitation frequencies linearly spacedin the skin depth domain instead ofusual signals with a harmonic contentlinearly spaced in the frequencydomain

Xiangchao and Feilu127 Optimization of PECT systemfor defect detection on aircraft-riveted structures

Phase-shift and logic-operation

The experimental results testify theavailability of the optimized system andthe necessity to optimize the PECTprobe design parameters

Pereira and Clarke128 Modeling and designoptimization of an eddy currentsensor for superficial and sub-superficial crack detection ininconel claddings

Finite-element modelswere used as a tool foreddy current sensordesign to optimize theirgeometry and operatingfrequency for detectionof the defect

A good agreement was found betweensimulated and experimental resultsshowing that this is a robust strategyfor special sensor design

Cardoso et al121 Improved magnetic tunneljunctions design for thedetection of superficial defectsby eddy currents testing

Optimize the sensorconfiguration

The experimental results obtainedshowed very good agreement with thesimulations for micron size defectdetection

Karthik et al129 Coil positioning for defectreconstruction in a steel plate

Genetic algorithmoptimization method

The pancake exciting coil can readmaximum voltage generated by thedefect

Deabes et al130 Optimized fuzzy imagereconstruction algorithm forECT systems

Fuzzy inference system(FIS)

The results show that the proposedoptimized algorithm has high accuracyand promising features

Li et al131 Multi-parametric indicatordesign for ECT sensoroptimization used in oiltransmission

A L9(34) orthogonal

designThe experimental results indicated thatthe sensor structure optimized withthe CFIECT could derive a greatersensitivity distribution and a betterimaging reconstruction results

Vacher et al15 Eddy current nondestructivetesting with the giant magneto-impedance sensor

Fast semi-analyticalmodels

Improved the probe sensitivityperformances at low frequencies andsmall size of defect detection

Chen and Miya124 A new approach for optimaldesign of ECT probes

Simplified detectabilityanalysis method and aring current model

The high performance of the newprobe designs is assured

Thollon and Burais132 Geometrical optimization ofsensors for eddy currentsNondestructive testing andevaluation

Genetic algorithm The optimized probe design showedhigh performance for claddingthickness measurement

Qi et al133 Parameters optimization andnonlinearity analysis of gratingeddy current displacementsensor using the neural networkand genetic algorithm

Combined an artificialneural network (ANN)and a genetic algorithm(GA) for the sensorparameters optimization

The calculated nonlinearity error is025 These results show that theproposed method performs well forthe parameters optimization of theGECDS

Huang et al27 Design of an eddy current arrayprobe for crack sizing in steamgenerator tubes

Numerical simulations Experiments show that the proposedprobe provides both a highdetectability and a remarkablecapability of reconstructing the shallowcracks of a tube

Rao et al134 Optimization of eddy currentprobes for detection of gartersprings in pressurized heavywater reactors

Two-dimensional finiteelement method is usedto optimize eddy currentprobe design parameters

The eddy current probe can detect thedisplacement of garter springs

CFIECT combination fuzzy index of the eddy current testing GECDS grating eddy current displacement sensor ECT eddy current testing

AbdAlla et al 13

inspection In addition compare the performancedetectability and the working principle of the bobbinprobe rotating probe and array probe for the pipeinspection was presented The lift-off effects are themain factors affecting the ECT signal causing erro-neous data interpretation The lift-off effect is discussedin detail ensuring that the measuring defect is rightFinally briefly review different conventional and intel-ligent lift-off compensation

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest withrespect to the research authorship andor publication of thisarticle

Funding

This work was supported by Huaiyin institute of Technologyand University Malaysia Pahang under grant numberRDU170379

Table 7 Intelligent technique in defect measuring

Author Proposed technique Application Remarks

Buck et al138 ANN with statistical techniqueof PCA

Steam generator data insimultaneous measurement

High sensitivity

DrsquoAngelo and Rampone139 Neural network To classify the aerospacestructure defects

The efficiency parameters werevery high near to one

Preda and Hantila140 FEMpolarization methodNNmethod

Reconstruction of defectshapes with PEC

NN-based inversion approachproduces an accurate and quickrebuilding of defect shape

Rosado et al141 Digital signal processing (DSP)suggested in FPGA

Multi-frequency for ECT inreal-time processing

Utilizing DSP to generate multi-frequency stimulus

Habibalahi et al142 Neural network data fusion Improving pulsed eddycurrent stress measurement

The ability of NN data fusion toenhance stress measurementreliability

He et al108 PCA-based feature extractionmethods (ANN)


Defect can be classifiedsatisfactorily when lift-off rangedfrom 0 to 14 mm

Rosado et al143 An ANN using data obtainedwith FEM

To estimate the defectproperties

The ANN has poorgeneralization ability outside therange

Babaei et al144 FEMANN Calculating the dimension ofrectangular cracks

The results of NN are veryguaranteeing to compare withtargets

Peng145 Multilayer feed-forward error-back propagation neuralnetwork

Assessment of crackexpansion direction

The estimation error by usingthe training BPNN presents tobe less than 2 which meets therequirement

Rosado146 Artificial neural network andnonlinear regressions

Profile reconstruction andestimate depth and width ofdefect

Finite element model wasapplied to produce syntheticECT data for some faultyscenarios

Postolache et al53 ANN including competitiveneural network and multilayerperceptronFEM

Estimation and classificationthe geometricalcharacteristic

The accuracy of defectclassification was increased byusing ANN

Zhang and Yang147 PSO algorithmANN-basedforward model

Rebuilding of crack geometryfrom ECT signal

Reconstruct crack profile rapidlyand accurately from ECT signals

Morabito and Versaci148 Fuzzy neural approach To localizing hole inconducting plates

The fuzzy system is effectivewith a limited number of inputs

Guohou et al149 DempsterndashShafer evidencetheorythe fuzzy inference

Evaluate the defect inconductive structure

Using the D-S approach todetermine the defect parameter

Upadhyaya et al150 ANN and fuzzy logic technique Flaw detection and tubedefect parameter estimation

The estimation of tube defectparameters performed with ahigh accuracy

Fan et al151 Both kernel principalcomponent analysis (KPCA) anda support vector machine (SVM)

Study the influences ofexcitation frequency for agroup of defects wererevealed in terms ofdetection sensitivity contrastbetween defect features andclassification accuracy

Determine the optimalexcitation frequency for a groupof defects

ANN artificial neural network PCA principal component analysis FEM finite element method PEC pulsed eddy current FPGA field-

programmable gate array ECT eddy current testing BPNN back propagation neural network PSO particle swarm optimization


ORCID iD

Ahmed N AbdAlla httpsorcidorg0000-0002-8633-1833Moneer A Faraj httpsorcidorg0000-0002-1945-9634

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19 Xu EX and Simkin J Total and reduced magnetic vector

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using rotating magnetic field eddy-current probe IEEE T

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field sensors based on giant magnetoresistance (GMR)

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sensors for nondestructive evaluation In Proceedings

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lingham WA International Society for Optics and

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array system for robotic pipe inspection In Proceedings

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application to the detection of deep cracks Mater Eval

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IEEE T Instrum Meas 2018 67 821ndash83052 Zhang S Tang J and Wu W Calculation model for the

induced voltage of pick-up coil excited by rectangular

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1810 New York IEEE53 Postolache O Ramos HG and Ribeiro AL Detection

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excitation frequency signal for depth crack defect in eddy

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2016 297 26ndash3159 Ribeiro AL Ramos HG and Postolache O A simple for-

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destructive inspection of aluminum plates using uniform

field probes Measurement 2012 45 213ndash217

60 Yin W and Xu K A novel triple-coil electromagnetic sen-

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defect in small diameter steam generator tube Measure-

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surement with rotating current excitation for evaluating

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defect in ferromagnetic material by low frequency eddy

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testing platform system for pipe defect inspection based

on an optimized eddy current technique probe design

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sors 2018 18 E210869 Liu Z Tsukada K Hanasaki K et al Two-dimensional

eddy current signal enhancement via multifrequency data

fusion J Res Nondestruct Eval 1999 11 165ndash17770 Aliouane S Hassam M Badidi Bouda A et al Electro-

magnetic acoustic transducers (EMATs) design


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idn591htm71 Bartels KA and Fisher JL Multifrequency eddy current

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IEEE T Instrum Meas 2013 62 2043ndash204973 Yadegari A Moini R Sadeghi S et al Output signal pre-

diction of an open-ended rectangular waveguide probe

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2010 43 1ndash774 Ditchburn R Burke S and Posada M Eddy-current non-

destructive inspection with thin spiral coils long cracks in

steel J Nondestruct Eval 2003 22 63ndash7775 Thollon F Lebrun B Burais N et al Numerical and

experimental study of eddy current probes in NDT of

structures with deep flaws NDTampE Int 1995 28 97ndash10276 Owston C Eddy-current testing at microwave frequen-

cies Non-Destruct Test 1969 2 193ndash19677 Ramos HG Postolache O Alegria FC et al Using the

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pp 1361ndash1366 New York IEEE78 Postolache O Ramos HG Ribeiro AL et al GMR based

eddy current sensing probe for weld zone testing In Pro-

ceedings of the sensors Christchurch New Zealand 25ndash

28 October 2009 pp 73ndash78 New York IEEE79 Faraj MA Samsuri F Abdalla AN et al Adaptive

neuro-fuzzy inference system model based on the width

and depth of the defect in an eddy current signal Appl

Sci 2017 7 66880 Faraj MA Fahmi S Damhuji R et al Investigate of the

effect of width defect on eddy current testing signals

under different materials Ind J Sci Technol 2017 10 1ndash581 Mook G Hesse O and Uchanin V Deep penetrating

eddy currents and probes Mater Test 2007 49 258ndash26482 Placko D and Dufour I Eddy current sensors for non-

destructive inspection of graphite composite materials

In Proceedings of the industry applications society annual

meeting conference Houston TX 4ndash9 October 1992

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and applications Boca Raton FL CRC Press 201684 Abbas S Khan TM Javaid SB et al Low cost

embedded hardware based multi-frequency eddy cur-

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Islamabad Pakistan 15ndash17 January 2016 pp 135ndash140

New York IEEE85 Chen X and Lei Y Electrical conductivity measurement

of ferromagnetic metallic materials using pulsed eddy cur-

rent method NDTampE Int 2015 75 33ndash3886 Almeida G Gonzalez J Rosado L et al Advances in

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ing manual on eddy current method Atomic Energy of

Canada Limited 1981 1 1ndash208

88 Mokros SG Underhill PR Morelli J et al Pulsed eddy

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ment Google Patents 2004 httpspatentsgooglecom

patentUS2002000851190 Fan M Cao B Yang P et al Elimination of liftoff effect

using a model-based method for eddy current characteri-

zation of a plate NDTampE Int 2015 74 66ndash7191 Dziczkowski L Elimination of coil liftoff from eddy cur-

rent measurements of conductivity IEEE Transactions on

Instrumentation and Measurement 2013 62 3301ndash330792 Lu M Zhu W Yin L et al Reducing the lift-off effect on

permeability measurement for magnetic plates from mul-

tifrequency induction data IEEE T Instrum Meas 2018

67 167ndash17493 Ribeiro AL Alegria F Postolache OA et al Liftoff cor-

rection based on the spatial spectral behavior of eddy-

current images IEEE T Instrum Meas 2010 59 1362ndash

136794 Wu Y Cao Z and Xu L A simplified model for non-

destructive thickness measurement immune to the lift-off

effect In Proceedings of the instrumentation and measure-

ment technology conference (I2MTC) Binjiang China

10ndash12 May 2011 pp 1ndash4 New York IEEE95 Huang C and Wu X Probe lift-off compensation method

for pulsed eddy current thickness measurement In Pro-

ceedings of 2014 3rd Asia-Pacific conference on antennas

and propagation Harbin China 26ndash29 July 2014 pp

937ndash939 New York IEEE96 Yu Y Yan Y Wang F et al An approach to reduce lift-

off noise in pulsed eddy current nondestructive technol-

ogy NDTampE Int 2014 63 1ndash697 Zhu W-l Shen H-x and Chen W-x New method for sup-

pressing lift-off effects based on Hough transform In

Proceedings of the international conference on measuring

technology and mechatronics automation Hunan China

11ndash12 April 2009 pp 653ndash656 New York IEEE98 Lopes Ribeiro A Ramos HG and Couto Arez J Liftoff

insensitive thickness measurement of aluminum plates

using harmonic eddy current excitation and a GMR sen-

sor Measurement 2012 45 2246ndash225399 Tian GY and Sophian A Reduction of lift-off effects for

pulsed eddy current NDT NDTampE Int 2005 38 319ndash

324100 Yin W Binns R Dickinson SJ et al Analysis of the lift-

off effect of phase spectra for eddy current sensors

IEEE T Instrum Meas 2007 56 2775ndash2781101 Tian GY Li Y and Mandache C Study of lift-off invar-

iance for pulsed eddy-current signals IEEE T Magn

2009 45 184ndash191102 Wei L Guoming C Xiaokang Y et al Analysis of the

lift-off effect of a U-shaped ACFM system NDTampE Int

2013 53 31ndash35103 Gotoh Y Matsuoka A and Takahashi N Electromag-

netic inspection technique of thickness of nickel-layer on

steel plate without influence of lift-off between steel and

inspection probe IEEE T Magn 2011 47 950ndash953104 Huang S Zhao W Zhang Y et al Study on the lift-off

effect of EMAT Sensor Actuat A-Phys 2009 153 218ndash

221105 Li J Wu X Zhang Q et al Measurement of lift-off using

the relative variation of magnetic flux in pulsed eddy

current testing NDTampE Int 2015 75 57ndash64

AbdAlla et al 17

106 Wang H Li W and Feng Z Noncontact thickness mea-

surement of metal films using eddy-current sensors

immune to distance variation IEEE T Instrum Meas

2015 64 2557ndash2564107 Fan M Cao B Tian G et al Thickness measurement

using liftoff point of intersection in pulsed eddy current

responses for elimination of liftoff effect Sensor Actuat

A-Phys 2016 251 66ndash74108 He Y Pan M Chen D et al PEC defect automated clas-

sification in aircraft multi-ply structures with interlayer

gaps and lift-offs NDTampE Int 2013 53 39ndash46109 Amineh RK Ravan M Sadeghi SH et al Removal of

probe liftoff effects on crack detection and sizing in

metals by the AC field measurement technique IEEE T

Magn 2008 44 2066ndash2073110 Hoshikawa H and Koyama K A new eddy current sur-

face probe without lift-off noise In Proceedings of the

10th APCNDT Brisbane 2001 httpswwwndtnet

articleapcndt01papers224224htm111 Lu M Yin L Peyton AJ et al A novel compensation

algorithm for thickness measurement immune to lift-off

variations using eddy current method IEEE T Instrum

Meas 2016 65 2773ndash2779112 Lu M Xu H Zhu W et al Conductivity lift-off invar-

iance and measurement of permeability for ferrite metal-

lic plates NDTampE Int 2018 95 36ndash44

113 Wen D Fan M Cao B et al Lift-off point of intersec-

tion in spectral pulsed eddy current signals for thickness

measurement IEEE Sens Let 2018 2 1ndash4114 Yin W Peyton AJ and Dickinson SJ Simultaneous

measurement of distance and thickness of a thin metal

plate with an electromagnetic sensor using a simplified

model IEEE Sensors Letters 2004 53 1335ndash1338115 Wei L Guoming C Wenyan L et al Analysis of the

inducing frequency of a U-shaped ACFM system

NDTampE Int 2011 44 324ndash328116 Fan M Cao B Sunny AI et al Pulsed eddy current

thickness measurement using phase features immune to

liftoff effect NDTampE Int 2017 86 123ndash131117 Theodoulidis T Analytical modeling of wobble in eddy

current tube testing with bobbin coils J Res Nondestruct

Eval 2002 14 111ndash126118 Lopez LANM Ting DKS and Upadhyaya BR Remov-

ing eddy-current probe wobble noise from steam gen-

erator tubes testing using wavelet transform Prog Nucl

Energ 2008 50 828ndash835119 Shu L Songling H Wei Z et al Improved immunity to

lift-off effect in pulsed eddy current testing with two-

stage differential probes Russ J Nondestruct Test 2008

44 138ndash144120 Rocha TJ Ramos HG Ribeiro AL et al Magnetic sen-

sors assessment in velocity induced eddy current testing

Sensor Actuat A-Phys 2015 228 55ndash61121 Cardoso FA Rosado LS Franco F et al Improved

magnetic tunnel junctions design for the detection of

superficial defects by eddy currents testing IEEE T

Magn 2014 50 1ndash4122 Rosado LS Cardoso FA Cardoso S et al Eddy cur-

rents testing probe with magneto-resistive sensors and

differential measurement Sensor Actuat A-Phys 2014

212 58ndash67123 Ghafari E Costa H and Julio E RSM-based model to

predict the performance of self-compacting UHPC

reinforced with hybrid steel micro-fibers Constr Build

Mater 2014 66 375ndash383124 Chen Z and Miya K A new approach for optimal

design of eddy current testing probes J Nondestruct

Eval 1998 17 105ndash116125 Aldrin JC Sabbagh HA Zhao L et al Model-based

inverse methods for sizing cracks of varying shape and

location in bolt-hole eddy current (BHEC) inspections

In Proceedings of the AIP conference AIP Publishing

httpsaipscitationorgdoipdf10106314940557126 Betta G Ferrigno L Laracca M et al Optimized com-

plex signals for eddy current testing In Proceedings of

the instrumentation and measurement technology confer-

ence (I2MTC) Montevideo Uruguay 12ndash15 May

2014 pp 1120ndash1125 New York IEEE127 Xiangchao H and Feilu L Optimization of PECT sys-

tem for defect detection on aircraft riveted structures

In Proceedings of the 2011 international conference on

computer science and network technology (ICCSNT)

Harbin China 24ndash26 December 2011 pp 996ndash999

New York IEEE128 Pereira D and Clarke TG Modeling and design optimi-

zation of an eddy current sensor for superficial and sub-

superficial crack detection in inconel claddings IEEE

Sens J 2015 15 1287ndash1292129 Karthik VU Mathialakan T Jayakumar P et al Coil

positioning for defect reconstruction in a steel plate In

Proceedings of the 31st international review of progress in

applied computational electromagnetics (ACES) Williams-

burg VA 22ndash26 March 2015 pp 1ndash2 New York IEEE130 Deabes WA Amin HH and Abdelrahman M Opti-

mized fuzzy image reconstruction algorithm for ECT

systems In Proceedings of the IEEE international con-

ference on fuzzy systems (FUZZ-IEEE) Vancouver

BC Canada 24ndash29 July 2016 pp 1209ndash1215 New

York IEEE

131 Li N Cao M He C et al A multi-parametric indicator

design for ECT sensor optimization used in oil transmis-

sion Sensors 2016 17 2074ndash2088132 Thollon F and Burais N Geometrical optimization of

sensors for eddy currents non destructive testing and

evaluation IEEE T Magn 1995 31 2026ndash2031133 Qi H-l Zhao H Liu W-w et al Parameters optimization

and nonlinearity analysis of grating eddy current displa-

cement sensor using neural network and genetic algo-

rithm J Zhejiang Univ-Sc A 2009 10 1205ndash1212134 Rao BPC Shyamsunder MT Bhattacharya DK et al

Optimization of eddy-current probes for detection of

garter springs in pressurized heavy water reactors Nucl

Technol 1990 90 389ndash393135 Mamdani E and Assilian S An experiment in linguistic

synthesis with a fuzzy logic controller Int J Hum-Com-

put St 1999 51 135ndash147136 Blej M and Azizi M Comparison of Mamdani-type and

Sugeno-type fuzzy inference systems for fuzzy real time

scheduling Int J Appl Eng Res 2016 11 11071ndash11075137 Hamam A and Georganas ND A comparison of Mam-

dani and Sugeno fuzzy inference systems for evaluating

the quality of experience of Hapto-audio-visual applica-

tions In Proceedings of the IEEE international work-

shop on Haptic audio visual environments and games

Ottawa ON Canada 18ndash19 October 2008 pp 87ndash92

New York IEEE


138 Buck JA Underhill PR Morelli JE et al Simultaneousmultiparameter measurement in pulsed eddy currentsteam generator data using artificial neural networksIEEE T Instrum Meas 2016 65 672ndash679

139 DrsquoAngelo G and Rampone S Shape-based defect classi-fication for non destructive testing In Proceedings of

the metrology for aerospace (Metroaerospace) Bene-vento 4ndash5 June 2015 pp 406ndash410 New York IEEE

140 Preda G and Hantila FI Nonlinear integral formulationand neural network-based solution for reconstruction ofdeep defects with pulse eddy currents IEEE T Magn

2014 50 113ndash116141 Rosado LS Ramos PM and Piedade M Real-time pro-

cessing of multifrequency eddy currents testing signalsdesign implementation and evaluation IEEE T Instrum

Meas 2014 63 1262ndash1271142 Habibalahi A Moghari MD Samadian K et al Improv-

ing pulse eddy current and ultrasonic testing stress mea-surement accuracy using neural network data fusionIET Sci Meas Technol 2015 9 514ndash521

143 Rosado LS Janeiro FM Ramos PM et al Defectcharacterization with eddy current testing usingnonlinear-regression feature extraction and artificialneural networks IEEE T Instrum Meas 2013 621207ndash1214

144 Babaei A Suratgar AA and Salemi AH Dimension esti-mation of rectangular cracks using impedance changesof the eddy current probe with a neural network J Appl

Res Technol 2013 11 397ndash401145 Peng X Eddy current crack extension direction evalua-

tion based on neural network In Proceedings of the

sensors Taipei Taiwan 28ndash31 October 2012 pp 1ndash4

New York IEEE146 Rosado L Janeiro FM Ramos PM et al Eddy currents

testing defect characterization based on non-linear

regressions and artificial neural networks In Proceed-

ings of the instrumentation and measurement technology

conference (I2MTC) Graz 13ndash16 May 2012 pp 2419ndash

2424 New York IEEE147 Zhang S and Yang H Inversion of eddy current NDE

signals using artificial neural network based forward

model and particle swarm optimization algorithm In

Proceedings of the international conference on informa-

tion and automation (ICIArsquo09) Macau China 22ndash24

June 2009 pp 1314ndash1319 New York IEEE148 Morabito E and Versaci M A fuzzy neural approach to

localizing holes in conducting plates IEEE T Magn

2001 37 3534ndash3537149 Guohou L Pingjie H Peihua C et al Application of

multi-sensor data fusion in defects evaluation based on

Dempster-Shafer theory In Proceedings of the instru-

mentation and measurement technology conference

(I2MTC) Binjiang China 10ndash12 May 2011 pp 1ndash5

New York IEEE150 Upadhyaya B Yan W Behravesh M et al Development

of a diagnostic expert system for eddy current data anal-

ysis using applied artificial intelligence methods Nucl

Eng Des 1999 193 1ndash11151 Fan M Wang Q Cao B et al Frequency optimization

for enhancement of surface defect classification using

the eddy current technique Sensors 2016 16 E649

AbdAlla et al 19

generator tube wall and a GMR array to detect theinduced MF The probe showed superior inspectionaccuracy and sensitivity to defects of the axial and cir-cumferential directions Zeng et al9 suggested a multi-line as an excitation coil while a GMR sensor could beplaced on the line of symmetry to detect any changes inthe MF The simulation result shows improvement inthe detection sensitivity for multilayer subsurface flaws

Rifai et al10 also described the application of GMRsensors in ECT Also discussed in detail were the limita-tion of utilizing the coil as a sensor and compensationtechniques that have been used in the ECT In anotherstudy Ali et al11 discussed in detail the method of theECT and the parameters that impacted the signal fun-damental in relation to the hardware and softwareGarcıa-Martın et al12 delibertated an overview of theprimary variables and the principles of ECT sensors

Therefore the structure of the ECT probe should beidentified to provide and evaluate the tested materialswithout destroying them13 This paper will review theeddy current probe structure and error compensation ofthe eddy current probes to obtain the ideal measuringdepth of defect on carbon steel pipeline The lift-off effectwill be discussed in detail to ensure that the measuringdefect is right This study will also introduce differentconventional and intelligently lift-off compensations

The principle of ECT

The diagram illustrating the operation principle of theECT is shown in Figure 1 According to Faradayrsquos lawthe driving coil is excited to produce a MF known aseddy currents14ndash16 The emf induced in the receivingcoils are generated by the eddy currents

Four steps can demonstrate the MF connectionbetween the primary and secondary coils

Alternating current through the coil produced bythe principal MF

Alternating primary MF produces the EC in theconductive sample

EC produces a secondary MF in an opposingdirection

Flaws in the sample perturb the EC and decreasethe secondary MF which results in the variation ofimpedance changes of the coil

The advantages of ECT include the fact that theyare economical and environmentally friendly as a non-contact method with high detectability inspectionspeeds and offers good discrimination On the otherhand the main disadvantages are it is sensitive todefects near the surface and is only applicable to con-ductive materials1217

Eddy current probe

Various configurations are presented for the excitationsource and detection sensor However in many in-service applications the inductive coils are utilized bothas a field source and field sensors As a result the eddycurrent probes are commonly classified according totheir configuration and mode of operation The probeconfiguration is closely related to the way the coil orcoilsrsquo connection covers the testing area of interest Theprobe operation mode is commonly classified intoreflection differential absolute and hybrid modeswhereas some of the standard configurations includethe outside diameter probes inside diameter (bobbin)probes bolt hole probes and surface probes18

An inductive probe can include one or more coils Inconventional eddy current probes these coils typicallycomprise lengths of wire wound in a helical manner likea solenoid The winding will commonly have more thanone layer to increase the value of inductance As men-tioned above there are many ways in which these coilscan be constructed based on the specified applicationConventional ECTs are transmitndashreceive probes multi-pancake andor rotating pancake probes and bobbinprobes Each method has its respective strengths andweaknesses in consideration of their characteristics suchas the test speed flaw detection sensitivity and probestructure complexity19

Bobbin probes

Figure 2 shows the two types of the bobbin probeswhich are called the differential and absolute bobbinprobes Differential bobbin probe has two coils posi-tioned at 180 out of phase The probe has excellentsensitivity to detect abrupt anomalies and small defectssuch and relatively unaffected by lift-off pitting corro-sion fretting wear and probe wobble However theprobe is not sensitive to metallurgical and gradualchanges2021

Absolute bobbin consists of a single bobbin coil anda second identical reference coil The second identicalreference coil is used for EM shielding of the inspectedtubing and electronic balancing The probe has excel-lent sensitivity to detect axial cracks and is highly sensi-tive to material property variations and graduallyvarying wall thinning6 The main disadvantage of theabsolute coil is that the defect is typically superimposedover a lift-off as the large signal

Figure 1 Principle for the eddy current testing operation


Rotating probes


Array probe







EM NDT techniques





AbdAlla et al 3

Pulsed eddy current




























































(Continued)

AbdAlla et al 5

Table 2 (Continued)
































Two orthogonalcoils



(Continued)






d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19

Rotating probes


Array probe







EM NDT techniques





AbdAlla et al 3

Pulsed eddy current




























































(Continued)

AbdAlla et al 5

Table 2 (Continued)
































Two orthogonalcoils



(Continued)






d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19

Pulsed eddy current




























































(Continued)

AbdAlla et al 5

Table 2 (Continued)
































Two orthogonalcoils



(Continued)






d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19







































(Continued)

AbdAlla et al 5

Table 2 (Continued)
































Two orthogonalcoils



(Continued)






d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19

Table 2 (Continued)
































Two orthogonalcoils



(Continued)






d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19





d x yeth THORN=X2Nf

i=1

cifi x yeth THORN




Frequency



Table 2 (Continued)




















AbdAlla et al 7










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19










Lift-off



Metals 368 d




















AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19













AbdAlla et al 9







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19







Dziczkowskiet al91



Conductivematerial




Frequencysweeping


Ribeiroet al93












Bottom andtop coils


Huang andWu95











LopesRibeiroet al98





Tian andSophian99





Air-coredcoil





(Continued)


Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19

Table 5 (Continued)







Gotohet al103






LopesRibeiroet al98



Gotohet al104


Transducercoil





Anganiet al17




Wanget al106




Wanget al107







Aminehet al109






Tangentialcoil






(Continued)

AbdAlla et al 11





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19





Conclusion


Table 5 (Continued)














U-shapedprobe










Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19



Observations




















A L9(34) orthogonal



















AbdAlla et al 13




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19




Funding






High sensitivity












































ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19

ORCID iD


References














































634ndash638










































2258ndash2264

AbdAlla et al 15





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19





















































191ndash200











































































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19































































































AbdAlla et al 17












































































York IEEE





















New York IEEE






































AbdAlla et al 19












































































York IEEE





















New York IEEE






































AbdAlla et al 19





































AbdAlla et al 19

challenges in improving the performance of eddy current

Documents