Magnetic Anomaly Detection in Ferromagnetic Material.pdf

8/10/2019 Magnetic Anomaly Detection in Ferromagnetic Material.pdf

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PIERS ONLINE, VOL. 7, NO. 8, 2011 736

Magnetic Anomaly Detection in Ferromagnetic Material

J. Atzlesberger and B. G. Zagar

Institute for Measurement Technology, Johannes Kepler University, Austria

AbstractThis paper discusses a non-destructive testing (NDT) technique based on the mag-netic flux leakage (MFL) method, which is able to detect magnetic anomalies in ferromagneticmaterial. The goal is the detection of very small inhomogeneities not only on the surface butalso anywhere in the cross section of magneto-conductive objects by scanning their surfaces usinglow-cost GMR (giant magneto resistance) sensors at rather high scanning speeds (up to 1 m/s) inorder to enable an automatic inspection and to minimize test time. The measured magnetic fluxdensity variations due to the expected inhomogeneities are down to only some nT (dependingon the inhomogeneity size and position) on a pedestal of some mT, therefore the system mustrealize a very high resolution and all parameters affecting the systems sensitivity (geometricalarrangement, sensors and electronics, signal acquisition, signal processing, etc.) have to be op-timized in order to get a robust and highly sensitive measuring system. As a pilot survey, sometest specimen were prepared (by inserting artificial inhomogeneities into otherwise homogeneous,

isotropic magneto conductive objects) and inspected with a prototype measuring system.

1. INTRODUCTION

Since many years non-destructive testing (NDT) methods are of a great concern in measurementand testing technology. The oldest NDT method for inspecting ferromagnetic material was alreadyapplied in 1870. For the detection of flaws in railway tracks and in gun barrels a compass needle asindicator was used [1]. Moving the compass across blemished material, a deflection of the compassneedle can be seen. In 1922 William E. Hooke discovered, that swarf on a magnetic fixed steelsheet form a pattern that was according to the pattern built by visible surface cracks on the steelsheet [1]. This was the hour of birth of the magnetic particle inspection (MPI) [2]. Since thendifferent magnetic NDT methods were developed for the detection of anomalies in ferromagnetic

material. These anomalies can be caused for example by thickness variation of the inspected testspecimen [3], corrosion [4], residual stresses [5], cracks [6], flaws, non-metallic inclusions, materialblemishes and/or surface-defects [7].

2. MEASURING PRINCIPLE

One possible magnetic anomaly detection principle is the magnetic flux leakage (MFL) method [8]as shown in Fig. 1. An electro magnet magnetizes the ferromagnetic specimen. Any inhomogeneityinside the specimen produces a variation of the magnetic flux density on and near the surface ofthe specimen and this variation can be detected by scanning the specimens surface using appropri-ate magnetic field sensors. In the past flux gate sensors, hall-sensors or AMR sensors (anisotropicmagneto resistance) were used. In 1988 the giant magneto resistive effect (GMR) [9] was discoveredin Fe/Cr/Fe trilayers [10, 11] and this effect is much more sensitive than the AMR effect. Further-

more GMR sensors can be built much smaller than flux gates and therefore allow a better spatialresolution thus GMR sensors were used for the measurement set-ups described in this paper. The

Figure 1: Schematic test set-up of the magnetic flux leakage method using an electro magnet for the magneticexcitation and a GMR sensor which measures the magnetic flux density near the surface of the ferromagneticspecimen.


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(a)The calculated magnetic flux density of a Helmholtzcoil pair (Eq. (1)) at an excitation current ofI= 3A,N= 50

turns andR= a= 205mm.

(b)Measured magnetometer characteristic at asupply voltage of U = 10V.

Figure 2: Distribution of the magnetic flux density between two coaxial air coils of a Helmholtz arrangementand the characteristic of the used magnetometer.

advantages of MFL is that there is no couple medium needed (vs. ultrasonic testing), that thereincurs no radiation burden (vs. X-ray inspection) and due to the small sensor sizes array processingis easily possible.

3. SENSOR TYPESDifferent types of GMRs (magnetometer, gradiometer, spin valve sensors, etc.) were consideredwhereat this paper deals with a magnetometer and a gradiometer.

3.1. Magnetometer

The used GMR magnetometer is a magnetic field sensor which measures the absolute value ofthe magnetic field strength and accordingly the magnetic flux density1. In order to obtain themagnetometers characteristic (see Fig. 2(b)) the sensor was positioned in the center of a Helmholtzcoil arrangement (which consists of two coaxial air coils on the x-axis with equal magnetic fluxorientation through the two coils) and the excitation current through the two coils was varied. Ifthe origin of the coordinate system is placed symmetrically between the two air coils, the magneticflux density on the x-axis can be calculated using Biot-Savarts law [12] and superpositioning themagnetic flux densities produced by the two air coils

B(x) = 0INR

2

2R2 +

a2

+x2 32 ex+

0INR2

2R2 +

a2 x

2 32 ex. (1)

With I, excitation current through the two coils, N number of turns per coil, R the radius ofand a the distance between the two coils. For a = R, the first and the second derivative of themagnetic flux density (on thex-axis) with respect to location are equal to zero in the origin thereforepositioning the sensor (actually in the center of the Helmholtz coil pair) doesnt need to be veryprecise as it can be seen from the horizontal magnetic flux density distribution in the range closeto the central point (x= 0) (see Fig. 2(a)).

Equation (1) evaluated in the center of the Helmholtz coil arrangement (x= 0) results in

Btot =0 8 NI

125R ex. (2)

So the magnetic flux densities B(x) and Btot only depend on the geometric parameter R (radiusand the distance between the two coils), the number of windings Nof the coils and are linearlyrelated to the excitation current I.

3.2. Gradiometer

The gradiometer measures the gradient of the magnetic flux density, so the characteristic in Fig. 3(b)was obtained by positioning the sensor in the center of a Maxwell coil arrangement2 and varyingthe excitation current through the two coils. Superpositioning the magnetic flux densities of both

1In air the magnetic flux density B is linear to the magnetic field strength H and can be calculated as B = Hwith the

magnetic permeability = 0 = 4107 V sAm

, but in general is a tensor.2A Maxwell coil arrangement consists of two coaxial coils with different magnetic flux orientations through the two coils.


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coils and calculating the magnetic flux densitys first spatial derivative (gradient) with respect tolocation results in

gradB

= 3

4

0INR2 (2x a)

R2 +a2 x

2 52 ex 3

4

0INR2 (2x+a)

R2 +a2

+x2 52 ex. (3)

The gradient of the magnetic field is linearly related to the excitation current Iand depends onthe coil radius R, the coil separation aand the number of windings N. Fora=

3R, the first and

the second derivative of the magnetic flux density gradient on the x-axis with respect to locationare equal to zero in the center between the two coils as shown in Fig. 3(a).

4. LOCK-IN-AMPLIFIER

For measuring very small flux density variations and increasing the SNR (signal-to-noise ratio) alock-in-amplifier was used. The schematic diagram of an analog lock-in-amplifier is shown in Fig. 4.For a sinusoidal reference signal with frequency fref and a sensor signal Uin(t) the output signalUout(t) for an analog lock-in-amplifier can be calculated as

Uout(t) = 1

T t

tT

sin(2frefs+ )Uin(s)ds. (4)

The maximum of the output voltage is obtained, if the reference and the input signal are in-phase(= 0) and can be adjusted by using the phase shift block in Fig. 4. A lock-in-amplifier can beconsidered as a band-pass filter with its center frequency equal to the reference frequency and itsbandwidth inversely proportional to the integration time T. For a digital lock-in-amplifier Eq. (4)can be rewritten to

Uout(n) = 1

N

nnN

sin

2

fref

fsn+n

Uin(n), (5)

with sample frequency fs. For finding out the smallest detectable flux density variation of themagnetometer, the excitation current I through the two coils of the Helmholtz arrangement was

varied sinusoidal with a frequency off= 75 Hz and the magnetic flux density was measured. Thedigitalized unbiased magnetometer signal (sample frequency = 10 kHz) was subsequently filteredusing a software implemented lock-in-amplifier (Eq. (5) with N= 10000). A flux density variationof 1.25 nT could be measured even under the influence of interference as large as 100 nT.

(a) The calculated magnetic flux density of a Maxwellcoil pair (Eq. (3)) at an excitation current ofI= 3A,N = 50

turns andR= a= 205mm.

(b) Measured gradiometer characteristic at a supplyvoltage of U= 10V.

Figure 3: Distribution of the magnetic flux density between two coaxial air coils of a Maxwell arrangementand the characteristic of the used gradiometer.

Figure 4: Schematic diagram of a lock-in-amplifier.


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(a) Gradiometer signal of a line scan at an x-distanceof 47.5 mm.

(b) Two-dimensional gradiometer scan of the hotrolled steel sheet containing artificial inclusions.

Figure 5: One- and two-dimensional gradiometer scan of a hot rolled steel sheet. The steep flanks in thegradiometer signal in (a) indicate the holes and the spiral drills positions.

(a) Horizontal and sheer plan of the specimen. (b) Measurement result of the circular test specimen.

Figure 6: The horizontal and sheer plan of a specimen containing a surface parallel eroded slot and themeasurement result obtained by scanning the specimens surface with a GMR magnetometer.

5. PRELIMINARY MEASUREMENT RESULTS

5.1. Static Detection of Surface Parallel Blind Holes

A hot rolled steel sheet with a thickness of 3.3 mm containing drilled blind holes and a brokenmagnetizable spiral drill with a diameter of 500 m was scanned using a GMR gradiometer typesystem (pointwise measurement). In Fig. 5 a one- and a two-dimensional gradiometer scan areshown. The holes and the air gap between the specimen and the spiral drill (which is partly filledwith magnetizable swarf and therefore the effective air gap is even smaller) simulate elongated,nonmagnetic, subsurface inclusions.

5.2. Dynamic Detection of a Surface Parallel Slot

A circular steel sheet with a thickness of 12 mm containing a central eroded slot (width = 195 m)was prepared. The horizontal, the sheer plan and the measurement result are shown in Fig. 6.The circular specimen was fixed in a mounting plate which was rotated by a variable speed motorand during the rotation an electro magnet magnetized the specimen and the magnetic flux densitynear the specimens surface was measured with a magnetometer. The average circumferential speedwas 1 m/s. The dashed line in Fig. 6(b) indicates the detected slot which simulates an elongated,nonmagnetic, subsurface defect.

6. CONCLUSION

An effective measurement system using GMR sensors for detecting magnetic anomalies was intro-duced. Magnetizable objects inside of ferromagnetic material and even inclusions with a very small

cross section (as compared to the specimens total cross section) oriented parallel to the magneticflux lines could be detected. For increasing the signal-to-noise ratio and to investigate the magne-


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tometers sensitivity limit a software implemented lock-in-amplifier was used and a magnetic fluxdensity variation of only 1.25 nT could be verified.

ACKNOWLEDGMENT

The authors gratefully acknowledge the partial financial support for the work presented in thispaper by the Austrian Center of Competence in Mechatronics (ACCM).

REFERENCES

1. Gauss, G., et al., Oberflachenrissprufung nach dem Magnetpulver-Verfahren,Zerstorungsgreie Werkstuck- und Werkstoffprufung, 133177, 2nd Edition, 1993.

2. Lovejoy, D., Magnetic Particle Inspection A Practical Guide, Kluwer Academic Publisher,1993.

3. Niese, F., Wall thickness measurement sensor for pipeline inspection using EMAT technologyin combination with pulsed eddy current and MFL, ECNDT, 2006.

4. Coughlin, C. R., et al., Effects of stress on MFL responses from elongated corrosion pits inpipeline steel, NDT& E International, Vol. 33, 118188, 2000.

5. Ricken, W., GMR and eddy current sensor in use of stress measurement,Sensors and Actu-ators, 4245, 2001.

6. Goktepe, M., Non-destructive crack detection by capturing local flux leakage field, Sensors

and Actuators, 7072, 2001.7. Atzlesberger, J. and B. G. Zagar, Detection of inhomogeneities in magneto-conductive ob-

jects, Proc. of the SSD11, Sousse, Tunisia, Mar. 2011.8. Blitz, J., Electrical and Magnetic Methods of Non-destructive Testing, 2nd Edition, Chapman

& Hall, London, 1997.9. Hauser, H., et al., Magnetoresistors, Magnetic Sensors and Magnetometers, 129171, 2001.

10. Binasch, G., et al., Enhanced magnetoresistance in layered magnetic structures with antifer-romagnetic interlayer exchange, Physical Review, Vol. 39, 48284830, 1989.

11. Grunberg, P., et al., Layered magnetic structures: Evidence for antiferromagnetic coupling ofFe layers across Cr interlayers, Physical Review Letters, Vol. 57, 24422445, 1986.

12. Strassacker, G. and P. Strassacker,Analytische und Numerische Methoden der Feldberechnung,Teubner, Germany, 1993.

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Magnetic Anomaly Detection in Ferromagnetic Material.pdf