Magnetic flux leakage (MFL) techniques have been widely used for non-intrusively inspecting steel installations by applying magnetization. In the situations where defects may take place on the near and far surfaces of the structure under inspection, current {MFL} techniques are unable to determine their approximate size. Consequently, an extra transducer may have to be included to provide the extra information required. This paper presents a new approach termed as pulsed magnetic flux leakage (PMFL) for crack detection and characterisation. The probe design and method are introduced. The signal features in timeâ��frequency domains are investigated through theoretical simulations and experiments. The results show that the technique can potentially provide additional information about the defects. Lastly, potential applications are suggested

Sophian, Ali

Tian, Gui Yun

Zairi, Sofiane

English

The International Islamic University Malaysia Repository

Sensors and Actuators A 125 (2006) 186–191Pulsed magnetic flux leakage techniques forcrack detection and characterisationAli Sophian ∗, Gui Yun Tian, Sofiane ZairiSchool of Computing and Engineering, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UKReceived 21 December 2004; received in revised form 16 June 2005; accepted 15 July 2005Available online 29 August 2005AbstractMagnetic flux leakage (MFL) techniques have been widely used for non-intrusively inspecting steel installations by applying magnetization.In the situations where defects may take place on the near and far surfaces of the structure under inspection, current MFL techniques are unableto determine their approximate size. Consequently, an extra transducer may have to be included to provide the extra information required. Thispaper presents a new approach termed as pulsed magnetic flux leakage (PMFL) for crack detection and characterisation. The probe design andmethod are introduced. The signal features in time–frequency domains are investigated through theoretical simulations and experiments. Ther©K1pmgdcmietwlcastte0desults show that the technique can potentially provide additional information about the defects. Lastly, potential applications are suggested.2005 Elsevier B.V. All rights reserved.eywords: Magnetic flux leakage; Pulsed magnetic field; NDT&E; Defect detection and characterisation. IntroductionMagnetic flux leakage techniques are widely used foripe and tank floor inspection [1–7]. This technique requiresagnetisation of the specimen under test. The magnetisationenerates magnetic flux flowing in the specimen in a certainirection, which is ideally perpendicular to the axis of therack to be detected. The presence of any flaws will imple-ent as an abrupt change of magnetic permeability to the fluxn the specimen. The permeability of the flawed part is gen-rally lower than flawless parts, providing high resistanceo the flux and forcing it to take a different route. In caseshere the other routes are magnetically saturated, some fluxeaves the specimen to the surrounding space temporarilyausing flux ‘leakage’. This leakage is readily detectable bymagnetic sensor located in the proximity of the specimenurface. The defect parameters that affect the distribution ofhe leakage flux are the ratio of depth of the defect to thehickness of the pipe wall, length, width, sharpness at thedges and sharpness at the maximum depth [2]. In practice,the magnetisation device is usually a permanent magnet oran electromagnet [8,9]. For dc inspection, Hall devices, mag-netoresistives and SQUIDs [10] can be used to measure theleakage field. For ac measurements, pick-up coils are anotheralternative. The advantage of MFL techniques is its simplicityand low cost. The technique is more robust to the variation ofmagnetic properties in magnetic materials compared to eddycurrent techniques, which belong to electromagnetic NDTtechniques as well. Like many electromagnetic techniques,MFL is also non-contact, which is a very useful feature foron-line dynamic inspection. Unlike eddy currents, however,MFL only works with magnetic materials.Improvement in accuracy in NDT techniques is desiredin many applications, such as pipe inspection where goodaccuracy in defect detection and characterisation can helpreduce unnecessary costly pipe replacements. To meet thisrequirement, pulsed MFL technique is proposed in this paper.The technique potentially offers richer information aboutstructural defects compared to the other MFL techniques.Our study on this proposed technique is presented in thispaper. In the following sections, simulation and experimentson PMFL are reported, followed by conclusions and further∗ Corresponding author. Tel.: +44 1484 472497; fax: +44 1484 472413.E-mail address: a.sophian@hud.ac.uk (A. Sophian). work.924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2005.07.013A. Sophian et al. / Sensors and Actuators A 125 (2006) 186–191 1872. Pulsed MFLThe dc MFL technique provides limited information onthe defects detected in terms of location and sizing. Gen-erally, it has to be ensured that only one type of defect ispresent and these defects can only happen on one side ofthe inspected structure to allow accurate inference of thedefect size, because the technique only relies on one mea-surement feature, i.e. the magnetic field leakage intensity.In some implementations they are complemented with sen-sors of other modality to allow discrimination of near andfar surface defects. With ac MFL, the inspection is generallysensitive to only one side of the sample depending on theexcitation frequency chosen. With pulsed MFL, the probe isdriven with a square waveform and the rich frequency com-ponents can provide information from different depths due tothe skin effects. It is expected that we could inspect thickersamples for far side defects and at the same time has good sen-sitivity for near surface defects. In addition, we should alsohave additional information such as the location and size ofdefects. The development of the pulsed MFL system is anextension of our work on pulsed eddy current NDT systems[11,12].To explore the potential of the technique, a probe wasdesigned and built using a U-shaped ferrite yoke. Fig. 1 showsFpthe dimensions of the probe in mm. A Hall device sensorfrom Honeywell’s SS490 family [13] has been chosen andis positioned halfway between the yoke’s poles to measurethe magnetic field density normal to the sample surface. TheHall sensor has a sensitivity of 3.125 mV/G. A coil of wire iswound around the top horizontal part of the yoke and drivenwith a rectangular waveform for excitation. During operation,the excitation current is controlled to avoid the ferrite yokegetting magnetically saturated. Data acquisition is performedusing a 14-bit digitisation at 100 kHz sampling rate.3. SimulationFinite element modelling (FEM) has been widely usedfor the study of electromagnetic NDT techniques, includingMFL [4]. In this work, a FEM package called FEMLAB isused to study the effects of surface and sub-surface crackson the magnetic field and to predict the system outputs. Thepackage uses the finite difference method to perform tran-sient analysis that is required for our purpose. Fig. 2 showsthe meshed model used in the simulation. Finer meshes arecreated surrounding the slot to give more accurate results.The width of all the slots used in the simulation is 1 mm. Inthis article, the depth of the surface slots refers to the lengthoistoaabtsig. 1. (a) Illustration of the probe and a test ample and (b) pulsed MFLrobe layout and dimensions.f the slot from the surface to the bottom tip of the slot andn the simulation it is varied from 1 to 3 mm. The depth ofub-surface slots is the distance between the top surface ofhe sample to the top of the slots that always have an openingn the bottom surface of the sample. The sub-surface slotsre located 0.5 and 1 mm below the surface in the simulation.Fig. 3 shows the calculated normal magnetic field densitybove the right hand side edge of the each slot. Fig. 3a andshow the results from surface and sub-surface slots respec-ively. In the figures, time = 0 is when the excitation pulsetarts rising. It is shown by the shapes of the signals that theFig. 2. FEM simulation.188 A. Sophian et al. / Sensors and Actuators A 125 (2006) 186–191Fig. 3. FEM simulation results: (a) surface slots and (b) sub-surface cracks (symbols are for identification, not actual data points).Fig. 4. Comparison of different excitations.technique can potentially discriminate the depths of the slotsdetected and also the position of the defect by using temporalinformation of the signals.Fig. 4 shows the plots of the calculated normal magneticfield against the distance x to the central major axis of a sur-face slot with different excitation waveforms. The surfaceslot’s depth is 3 mm. The plots show that the transient per-forms slightly better than the 10 kHz frequency excitation andsignificantly better than both the dc and the low frequencyexcitations.4. Experimental resultsThe experiments are designed to give us some initialresults that illustrate the capabilities of the technique. Thesamples have surface slots with depths varying from 1 to9 mm and sub-surface slots with location depths ranging from0.5 to 7 mm. The width of the slots is approximately 1 mm.The coil is driven with a square waveform with a pulse widthof 40 ms. The plots of signals shown in this section are: initialexperiment results show that the highest signal peak ampli-tudes are obtained at different normal distances from the slot’scentral major axis depending whether the slot is on the topsurface or on the bottom surface of the sample. When theslot is located below the surface, the measured field is morespread out due to field dispersion. Therefore, the positive peakto negative peak distance is larger than the slot width. Thisis illustrated by experimental results plotted in Fig. 5. Theslots’ width is approximately 3 mm. The depth of the surfaceslot is 3 mm and the buried slot is located 1 mm below thesurface. The thickness of the sample is 10 mm. The resultswere obtained by manually scanning the probe over the slotswith a 1 mm step and x = 0 being the central major axis ofthe slots. The probe is unmoved every time a measurement istaken. The signal amplitudes are taken for the plot. It is knownthat with MFL techniques, the polarity of the magnetic fieldFig. 5. Scanning results of sub-surface and surface slots.A. Sophian et al. / Sensors and Actuators A 125 (2006) 186–191 189Fig. 6. Results with surface slots with depths of 1, 2 and 3 mm; the rising edgeof the excitation pulse initiates at time = 1 ms (symbols are for identification,not actual data points).changes if the sensor is scanned over a crack. The plots showthat with the 1 mm scanning step used, the distances betweenthe positive and negative peaks are 4 and 8 mm for the surfaceand sub-surface slots, respectively.As can be seen in Fig. 5, the amplitude of the signal varieswith the relative position of the probe to the slot axis. Fromnow on, the output signals used are obtained when the probeis such positioned that the highest signal amplitude is mea-sured. The positive peaks are taken with the assumption thatthe negative peaks have the same absolute amplitudes dueto symmetry. Fig. 6 shows the resulting signals from sur-face slots with different depths. It shows that the technique isable to differentiate different depths of the inspected slots byusing the amplitudes of the signals, provided that the locationof the slot is known. It should be noted that all plots of theexperimental signal output against time have been arrangedso that the rising edge of the excitation pulse coincides withtime = 1 ms.Fig. 7 shows the comparison of the signals obtained forboth surface and sub-surface slots. It clearly shows that thesignal of the sub-surface has a different characteristic whereit initially increases slowly and after some point in timeincreases at a faster rate. In other words, the inflexion pointsof sub-surface slot signals happen later than those of surfaceslot signals. These can be more clearly seen by taking the firstderivative of the signals.saadhiosFig. 7. Surface and sub-surface slot signals; the rising edge of the excitationpulse initiates at time = 1 ms (symbols are for identification, not actual datapoints).approximately the same time, while the inflexion point of thedeeper-located sub-surface slot happen at a later time than inthe case of a sub-surface slot that is closer to the surface. Allthese results indicate that the inflexion point can be used todiscriminate the depth location of a detected slot.Fig. 9 shows the frequency analysis of the signals. Theplots support our statement that the temporal information,which is represented as phase in the frequency analysis, isuseful for characterising the defects. The low frequency com-ponents, below 50 Hz, seem to discriminate not only betweensurface and sub-surface but also discriminate the distance ofthe sub-surface slots below the surface. Location discrimina-tion seems to also be achievable using the frequencies around200 Hz. It is, therefore, clear that determination of the slot’sdistance from the surface and the discrimination of which sur-FriExperimentally it was found that the probe could not detectub-surface slots 1 mm below the surface. To demonstrate thebility to discriminate surface and sub-surface and also to beble to discriminate the location depths of the sub-surfaceiscontinuities, another probe with a bigger yoke having aorizontal length of 93 mm is used. The results are shownn Fig. 8, which demonstrates that again the inflexion pointsf the sub-surface slot signals happen later than the surfacelot signals. The inflexion points for surface signals occurs atig. 8. Surface and sub-surface slot signals using a bigger-yoked probe; theising edge of the excitation pulse initiates at time = 1 ms (symbols are fordentification, not actual data points).190 A. Sophian et al. / Sensors and Actuators A 125 (2006) 186–191Fig. 9. Frequency analysis of the surface and sub-surface signals: (a) magnitude, (b) phase (symbols are for identification, not actual data points).face the slot is located, can be achieved conveniently usingthe pulsed MFL technique.5. ConclusionsA variant of pulsed MFL technique has been proposedand investigated. The numerical analysis and experimentalstudies has shown PMFL clearly has advantages in terms ofdefect location and sizing by using features in time–frequencydomain, including the arrival time of the inflexion point, sig-nal magnitude and phase variation of frequency components.It has been shown that the technique is able to discriminatecracks’ locations in addition to give relative depths of thecracks. The probe design should be tailored to the applica-tion in hand as the dimension of the yoke determines the depthof penetration, where generally a larger yoke offers deeperpenetration. The scanning results also show the potential ofexploiting a linear array of magnetic sensors between theyoke poles for better understanding of the sample conditions.The simulation results show that the transient or pulsed MFLperforms the best overall for both surface and sub-surfacecracks inspection. The advantages of PMFL make it poten-tially suitable for many applications for ferromagnetic mate-rials, including detection of cracks in ferromagnetic metalstrips where defects can be present on both sides while accessiiuTtAiReferences[1] B.P.C. Rao, T. Jayakumar, Discontinuity characterisation using elec-tromagnetic methods, J. Non-Destruct. Test. Eval. 2 (2) (2002)23–29.[2] S. Mukhopadhyay, G.P. Srivastava, Characterisation of metal lossdefects from magnetic flux leakage signals with discrete wavelettransform, NDT&E Int. 33 (1) (2000) 57–65.[3] W. Mao, L. Clapham, D.L. Atherton, Effects of alignment of nearbycorrosion pits on MFL, NDT&E Int. 36 (2) (2003) 111–116.[4] M. Katoh, N. Masumoto, K. Nishio, T. Yamaguchi, Modeling of theyoke-magnetization in MFL-testing by finite elements, NDT&E Int.36 (7) (2003) 479–486.[5] J.C. Drury, A. Marino, A comparison of the magnetic flux leakageand ultrasonic methods in the detection and measurement of corro-sion pitting in ferrous plate and pipe, in: Proceedings of the 15thWorld Conference on Non-destructive Testing, Roma, 2000.[6] C.R. Coughlin, L. Clapham, D.L. Atherton, Effects of stress on MFLresponses from elongated corrosion pits in pipeline steel* 1, NDT&EInt. 33 (3) (2000) 181–188.[7] M. Afzal, S. Udpa, Advanced signal processing of magnetic fluxleakage data obtained from seamless gas pipeline, NDT&E Int. 35(7) (2002) 449–457.[8] K. Mandal, T. Cramer, D.L. Atherton, The study of a racetrack-shaped defect in ferromagnetic steel by magnetic Barkhausen noiseand flux leakage measurements, J. Magnet. Magnet. Mater. 212 (1–2)(2000) 231–239.[9] M. Goktepe, Non-destructive crack detection by capturing localflux leakage field, Sens. Actuators A: Phys. 91 (1–2) (2001) 70–72.[[[[[[s limited to one side of the strip. For future work, the capabil-ties of the PMFL for corrosion detection/characterisation bysing feature fusion techniques [14,15] will be investigated.he simulation technique will also be improved to explorehe potential of this technique further.cknowledgementThe authors would like to thank EPSRC for partially fund-ng the project.10] H.-J. Krause, M.v. Kreutzbruck, Recent developments in SQUIDNDE, Physica C: Supercond. 368 (1–4) (2002) 70–79.11] A. Sophian, G.Y. Tian, D. Taylor, J. Rudlin, Design of a pulsededdy current sensor for detection of defects in aircraft lap-joints,Sens. Actuators A: Phys. 101 (1–2) (2002) 92–98.12] A. Sophian, G.Y. Tian, D. Taylor, J. Rudlin, A feature extractiontechnique based on principal component analysis for pulsed eddycurrent NDT, NDT&E Int. 36 (1) (2003) 37–41.13] Honeywell, SS490 Series Miniature Ratiometric Linear Solid StateSensors, 2000.14] G.Y. Tian, A. Sophian, Reduction of lift-off effects for pulsed eddycurrent NDT, NDT&E Int. 38 (4) (2005) 319–324.15] G.Y. Tian, A. Sophian, Defect classification using a new feature forpulsed eddy current sensors, NDT&E Int. 38 (1) (2005) 77–82.A. Sophian et al. / Sensors and Actuators A 125 (2006) 186–191 191BiographiesDr. Ali Sophian received BE (Hons) degree in 1998 and PhD in 2004,both in electronics engineering from the University of Huddersfield, Eng-land. He is currently a research fellow at the School of Computingand Engineering, the University of Huddersfield. His research interest,includes eddy current NDT system design, signal processing and embed-ded systems. He has been working on electromagnetic NDT systems sincehe started his PhD project that was co-sponsored by the TWI Ltd.Dr. Gui Yun Tian obtained his BSc in metrology and instrumentation andMSc in precision engineering at the University of Sichuan (Chengdu, PRChina) in 1985 and 1988, respectively. After working in the Universityof Sichuan for 3 years as an associate professor, he then pursued his PhDdegree in the UK from 1995 and was awarded his degree at the Universityof Derby in 1998. He is currently a reader in the School of Computingand Engineering, University of Huddersfield, UK and a visiting professorin the University of Sichuan. He maintains diverse and active researchin the area of sensors, intelligent instrumentation, non-destructive testing,digital signal processing, computer vision and microelectromechanicalsystem (MEMS), which have been funded by the EPSRC, the RoyalSociety, the Royal Academy of Engineering and World-Class Industries.He has published over 100 books and papers in English or Chinese inthe above areas and successfully supervised several researchers at thevarious levels such as post doctors, PhD and master studying. He is asenior member of IEEE and a regular reviewer for international journalsand conferences.Dr. Sofiane Zairi obtained his BSc (Hons) in applied physics and semi-conductors from the University of Monastir in 1996. He acquired hisMSc in microelectronics from INSA of Lyon in 1997, France. Then, hehas got his PhD in integrated sensors from the School of Centrale Lyon,France in 2001. Later on, he joined a post-doc project at the Universityof Strathclyde, Glasgow, UK. This project funded by the SHEFC coun-cil, where versatile cell library of microsystems, pertaining the realms ofMOEMS and RF MEMS, has been fabricated. Dr. Zairi has shown a greatinterest to the fields of embedded microsystems, and software modellingusing HDL and FEM-based analysis methods. He is currently a researchassistant at the School of Computing & Engineering, at the University ofHuddersfield.

Pulsed magnetic flux leakage techniques for crack detection and characterisation

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Pulsed magnetic flux leakage techniques for crack detection and characterisation

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