E-Journal of Advanced Maintenance Vol.3, No.1 (2011) 25-38
Japan Society of Maintenology

A Simulation Technique of Non-Destructive Testing
using Magneto-Optical Film
Huy Minh LE1, Jinyi LEE2,*, Sehoon LEE3 and Tetsuo SHOJI4
1, 2

Department of Control, Instrumentation and Robot Engineering, Chosun University, Gwangju,
501-759, Korea.
3
Research Center for Real Time NDT, Chosun University, Gwangju, 501-759, Korea.
4
New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579, Japan.
ABSTRACTAn electro-magnetic field resulting from the application of a magnetic or electrical field to a metallic specimen
is distorted due to the existence of a crack. The distribution of the magnetic field is changed by this distortion of
the electro-magnetic field, and this change in the distribution can be visualized by the magneto-optical (MO)film in the magneto-optical nondestructive testing (NDT) method. The plane of a polarized light beam rotates
when the beam is transmitted through an MO film because of the Faraday effect, a kind of magneto-opticaleffect. In this paper, an NDT simulation technique, which uses not only the Faraday effect, but also the change
in the magnetic domains caused by an external magnetic field, the saturated magnetization effect, the effect ofthe magnetizer, the bias of the magnetic field and the temperature properties, is discussed. The simulation results
are verified by comparing them to experiment results.
KEYWORDS

ARTICLE INFORMATION

NDT, Crack, Faraday effect, Magneto-Optics, MO film, Leakage
Magnetic Flux.

Article history:
Received 15 December 2010
Accepted 28 April 2011

1. Introduction
Nondestructive Testing (NDT) is a noninvasive technique used to determine the integrity, safety
and maintenance of engineering systems. It makes assessments without doing harm, applying stress ordestroying the test object. NDT is used widely in industrial areas such as aerospace, power generation,
automotive, and railway. Some of the most widely used NDT methods are Ultrasonic Testing (UT),Radiographic Testing (RT), Electromagnetic Testing (ET), and Penetrant Testing (PT). Ultrasonic
Testing (UT) uses high frequency sound energy in examinations and measurements. A UT system
requires the use of a coupling liquid between the transducer and the specimen, making testing slow andtedious and making contact essential with the specimen. Radiographic Testing (RT) uses various typesof expensive and cumbersome equipment. X-ray is transmitted through a specimen to send to a receiveron the opposite side. The adjustments to the test setup to carry out this transmittal are complicated andtime consuming. Furthermore, this technique requires the operator to take specific precautions to avoid
radiation hazards. Penetrant Testing (PT) requires a liquid penetrant or dry powder, which is applied tothe surface of the specimen, as well as waiting time. Not only is this method is complicated, it is unable
to evaluate the depths of cracks. Electromagnetic Testing (ET) is receiving much attention as a faster,
implementation-easy and safe alternative to the other NDT techniques. ET induces electric currents ormagnetic fields or both inside a specimen, whose electromagnetic response can be observed. Somecommon methods of electromagnetic testing are as follows: Magnetic Particle Testing (MPT), Magnetic
Flux Leakage Testing (MFLT) and Eddy-Current Testing (ECT).
Hall sensors, magnetic resistance (MR) [1, 2] sensors, and giant magnetic resistance (GMR) [3,4] sensors are used as magnetic sensors to measure the distribution of a magnetic field in MFLT and
ECT. Another kind of sensor is the Magneto-Optical (MO) sensor, which is used in MFLT [5] andECT [6, 7]. With the MO sensor, the paint on specimen does not need to be removed because cracks*

Corresponding author, E-mail: jinyilee@chosun.ac.kr
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H. M. Le et al./ A Simulation Technique of Non-Destructive
Testing usingMagneto-Optical Filmcan be detected with no contact. The MO sensor has a wide detection area and high spatial resolutionof magnetic domains, so it can inspect structures at high speed. An automated real-time method forevaluating MO images can improve detection ability, reduce human errors, and increase inspection
accuracy and speed.
Crack properties such the crack direction, size and shape need to be evaluated from themagnetic field distribution to determine the required maintenance time. A finite element method(FEM) can calculate the magnetic field. In FEM, the magnetic source, cracks and air are divided intoeither small elements or nodes according to crack size, and the simultaneous equations describing theelectro-magnetic field at each element are solved [8]. Consequently, accuracy becomes proportional totime-consumption. Another numerical method uses a dipole model to analyze (DMA) [9~11] themagnetic field around cracks. In the DMA method, the crack is considered to be filled by dipoles withthe dipole moments oriented opposite to the direction of the applied field. Thus, the intensity of themagnetic flux leakage (MFL) at a point outside the specimen is the sum of the intensities of themagnetic fields generated by these dipoles at the same point. MFL around cracks, which is calculated
in DMA, can be used to model a crack easily. In this paper, we provide a simulation software using
DMA to model cracks. Also, the limitation of and solutions by the software are discussed bycomparing with them with the experimental results, which were obtained by using the MO sensor.

2. Principles
2.1.

Faraday effect and Magneto-Optical Sensor

The Faraday effect, a magneto-optical effect, is the interaction between light and magnetic field
in a medium. When the polarized light beam is transmitted through a film (Magneto-Optical Film,
MOF), the polarized plane of the light is rotated by an angle called the Faraday angle as shown in Fig.
1(a). This angle is proportional to the Verdet constant V, intensity of magnetization B, and thethickness of MOF t, as described by the equation (1) [12, 13]:(1)

θF= VBt

L-rotation (anticlockwise) when the direction of propagation and magnetization direction are parallel.
R-rotation (clockwise) when the direction of propagation and magnetization direction is anti-parallel.

(a)

(b)

Fig. 1. (a) The Faraday effect and (b) the structure of magneto-optical film

An MO sensor contains an MO film (GdBi)3(FeAl)5O12 , which is grown on a substrate(GdCa)3(MgZrGa)5O12 and a vaporized aluminum reflecting film as shown in the Fig. 1(b). The
Faraday rotation is almost doubled in this reflection-type optical system [14].
2.2.

The change in the magnetic domains cause by an external magnetic field

The rotation angle and the direction of the Faraday angle are affected by the strength anddirection of magnetization as indicated by equation (1). Usually, Verdet constant, V and thickness of
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H. M. Le et al./ A Simulation Technique of Non-Destructive
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MOF, t are constants. Therefore, the strength and direction of the magnetization can be estimated byestimating the rotation angle. In addition, the magnetic domains of an MOF have only two directions,vertical, up and down to the plane of the MOF because of magnetic anisotropy. Therefore, thepolarized light beam is rotated at a clockwise angle (θF) and anticlockwise angle (-θF) as shown in Fig.
2(a). Then the Analyzer obtains the electrical field of the light EB= Ecos(δ-θF) and ED= Ecos(δ+θF),
where E is the electrical field of the transmitted light, and EB, ED are the electrical fields of +θF and–θF , respectively, and δ is the angle between the polarizer and analyzer (Fig. 2(a)). Since EB> ED, the
MOF has alternating bright areas and dark areas. If there are no cracks on a specimen, the ratio of thebright area and dark area would be the same; if there are cracks on a specimen, the magnetic fieldaround the cracks would be different from that in no-crack area, so the domain widths are changed.
Therefore, cracks can be observed by using a polarizing optical system (Fig. 2(b)). Also, the domain
width of a MOF is approximately 3-50 μm without a magnetic field, which means smaller cracks canbe detected by using smaller magnetic domain widths of an MOF.
(b)

(a)

Fig. 2. Effect of MOF

2.3.

Saturated magnetization of Magneto-Optical element
(b)

(a)

Fig. 3. Saturated magnetization effectAs mentioned in the structure of an MOF, the MOF has a reflecting film which reflects the lightfrom the polarizer. The reflected light will form an image on the MOF, including black and whitecolors according to the intensity of the reflected light. The intensity can be described by the following
equation which describes the ratio of bright area and dark area [14]:
1
2

cos δ

θF

cos δ

θF

(2)

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H. M. Le et al./ A Simulation Technique of Non-Destructive
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where H(<|Hs|), Hs, E, δ, θF are the external magnetic flux intensity, the saturated magnetic intensityof the MOF, the electric field of the transmitted light, the angle between the polarizer and analyzer,
and the Faraday rotation angle, respectively. Eq. (2) can be expressed in the terms of two functions
δ andδ [10]:

δ
δ

1
2
1
2

δ

δ

(3)

δ

θF
δ

δ
θF

θF
δ

θF

Equation (3) is a linear equation with respect to H when δ is fixed. As shown in Fig. 3, if there1 ) then the width of dark and bright domain are equal (case ○2 ), Iis no magnetic field (H= 0, case ○increases, and then, the width of the bright domain increases, and the width of the dark domain
3 , ○
4 , ○
5 , ○
6 ), I is maximum,
decreases. When H= HS (saturation, cases ○δ θF and
7 ), due to thethere is only the bright domain. When the direction of H changes from zero (case ○residual magnetic field (B decrease, B> 0), I decreases, the width of the bright domain decreases, and8 ), as in case ○1 , the dark and brightthe width of the dark domain increases. When B= 0 (case ○9 (B< 0, H= -HS), the direction of magnetic field is inversed, and I
domain are equal. In the case of ○δ θF . There is only the dark domain.
is minimum,
2.4.

Magnetization of a specimen

(a)

(b)

(c)

(d)

Fig. 4. Several magnetic methods; (a) yoke type magnetizer, (b) cross type magnetizer,(c) vertical type magnetizer, (d) sheet type induced current (STIC)

An MOF has dark or bright areas depending on the direction of magnetization. So, if wecontrol the direction of magnetization, the dark and bright domains can be changed. When the
direction of magnetization is horizontal which is induced by a yoke type magnetizer (Fig. 4(a)) [15],the MOF shows dark and bright domain continuously at the both side of a crack. Because of the large
MFL around a crack, the areas of the domains are larger than those for a no-crack area. If the lengthof a crack is parallel to the magnetization direction, the MFL is minimized and the crack becomesdifficult to inspect. In this case, a cross type magnetizer is a good solution (Fig. 4(b)). A cross type
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H. M. Le et al./ A Simulation Technique of Non-Destructive
Testing usingMagneto-Optical Filmmagnetizer includes of two magnetic fields (induced by coil 1 and coil 2) that are perpendicular eachothers [16]. Another method uses a vertical magnetic field (Fig. 4(c)) [15]. The value of a verticalmagnetic field can be controlled by the value of the input current in the coils. If the magnetic density
in no-crack area (BZ, base) is slightly larger than the saturated magnetic density of the MO sensor (BS),the magnetic density in the crack area (BZ, crack) would be smaller than BS. Thus, the transaction of
crack will occur. The direction of crack length does not affect detection ability, so this method can use
for a thick specimen. A sheet type induced current (STIC) (Fig. 4(d)) [7] has been proposed for
detecting subsurface corrosion and cracks in paramagnetic metals; this system is usually called theMagneto-Optical Imager (MOI) [6]. The alternative magnetic field induces a current in the induction
foil (sheet type) on the specimen. Then, this sheet type induced current induces an eddy current
around the crack in the specimen. This eddy current induces a magnetic field into the specimen whichis perpendicular to the MOF. This magnetic field domain can be visualized by using MOI to observe
the crack.
2.5.

Bias of the magnetic field

The MO sensor responds and forms an image only if the magnetic flux density exceeds a certain
threshold value, Bs, which is a characteristic of the sensor. For example in MOI system, the eddy
current (Fig. 4(d)) induces a small magnetic field, so a bias coil is used to induce an additional DC
magnetic field (Fig. 5). This bias magnetic field permits adjustments to the MO sensor threshold andoptimizes the sensor response to varying field strengths. This bias magnetic field is controlled by theDC bias current, so it’s easy to control the level of the sensor threshold. Fig. 6 shows the experimentresults with hole type cracks using an MOI system. When we applied a (-) bias magnetic field, moreaccurate information about the cracks was obtained than when we had applied a (+) bias magnetic
field. In addition, magnetic field background noises such as the earth’s magnetic field affected thequality of the MO sensor. Using a bias magnetic field, it was easy to reduce this background noise
effect. Therefore, the sensitivity of the MO sensor can be adjusted by using a bias magnetic field.
Ø6.36, d0.7

Ø8.4, d0.9(-)bias(+)biasFig. 5. Bias magnetic field

2.6.

Fig. 6. The change of MO threshold using bias magnetic field

Temperature properties

The NDT technique that can detect a natural crack in the high-temperature testing regime using
an MOF was proposed [17]. Also, the behaviors of MOF in the low temperature regime have beenstudied. As shown in the Fig. 7, the domain width increases rapidly in the low temperature areas;
therefore, the spatial resolution of MOF is decreased at low temperature. However, the spatial
resolution of MOF is increased at high temperatures when the domain width is small. Fig. 8 shows therelation between the domain ratio and magnetic flux density for each temperature regime. The domain
ratio is the ratio of bright and dark areas, which is approximately 50% without magnetic induction atroom temperature. The expansion of the bright area or the increase of the domain ratio attributed totheir linear relation with the increase of magnetic induction. The domain ratio rapid changes at the
low temperature regime, so an MOF can saturate easily just with small magnetic induction at low
temperatures. It means that an MOF has high sensitivity at low temperature. For a paramagnetic metal
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H. M. Le et al./ A Simulation Technique of Non-Destructive
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specimen with small crack, the distortion of the magnetic field around the cracks is small, so theywould have to be detected at low temperatures. The domain ratio changes at around the boiling pointof water in Fig. 8. Therefore, the cracks on a specimen at several hundred degrees centigrade can bedetected using an MOF submerged in distilled water near the boiling point. Then, the crack can beevaluated using the MOF at any temperature.

Fig. 7. Average domain width
depends on temperature

2.7.

Fig. 8. Domain ratio due tothe magnetic flux density

Simulation technique

(a)

(b)

Fig. 9. Dipole model in the case of (a) horizontal magnetization and (b) vertical magnetization

Fig. 9(a) shows the dipole model and coordinate system for the case of horizontal
magnetization. The XY plane is represented on the surface of the specimen, the Y-direction is parallelwith the length direction of the crack, the magnetization direction is in the X-direction, and theZ-direction is presented as the vertical direction to the surface of specimen. DMA assumes thatmagnetic charges exist on both walls of a crack. The magnetic charge of area
(at the right wallin Fig. 9(a)) is. This magnetic charge induces a magnetic flux intensity at point r(x, y,
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H. M. Le et al./ A Simulation Technique of Non-Destructive
Testing usingMagneto-Optical Filmz) is

. The vertical directional components of the MFL in the Z-direction, is. The total magnetic flux intensity at point r(x, y, z) which is induced by magnetic charges on
the two walls of the crack can be described [10, 11]:
/

4
4

/

/2

/

/

/

/2

/

(4)

where, m, , lc, wc, dc and z are the magnetic charge per unit area, permeability of the specimen, andthe length, width, depth of the crack and the lift-off, respectively.
Fig. 9(b) shows the dipole model and coordinate system in the case of vertical magnetization.
The coordinate system is represented in the case of horizontal magnetization, as well. We assumed
following: the size of the specimen is larger than the size of the crack (
,
; ,
are
length and width of specimen), uniform magnetic charge per unit area, m on the surface of specimen
and at the bottom of the crack; and to be distributed on the whole surface of the specimen of width wS
and length lS (the magnetic flux intensity would be HZ, all). The crack area with width wc and length lc,
actually, has no magnetic charge (the magnetic flux intensity would be HZ, crack). At the bottom of thiscrack, m is assumed to be distributed over an area of width wc, and length lc , located at dc distance in) shown in Fig. 9(b),the Z-direction (the magnetic flux intensity would be HZ, under). For an area (the magnetic flux intensity at point r(x, y, z) in the Z-direction can be expressed by equation (5) [9,10]. From these suppositions, the distribution of the overall magnetic field can be calculated byequation (6). Furthermore, when the crack is moved (x offset, y offset) and rotated by θc from its center on
the XY plane, the crack position in the new plane XRYR (Fig. 10) can be expressed by equation (7)[18]. In the case of vertical magnetization, the Hz does not change. But in the case of horizontal
is changed tocos .
magnetization, theFig. 10. Model of crack and its relocated position

/

4
,

,

,

,

(5)
(6)
(7)

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H. M. Le et al./ A Simulation Technique of Non-Destructive
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Fig. 11(a) shows the distribution of HZ for the case of θc= 00 with horizontal magnetization. The
crack size is wc= 1 mm, lc= 10 mm, dc= 1 mm, lift-off z= 1 mm, m/4 = 1 in the area x= [-10:10]
mm , y= [-10:10] mm. For the case of θc= 900, we just need to exchange wc and lc as shown in Fig.
11(b). Fig. 11(c) shows the distribution of HZ for the case of θc= 00 with vertical magnetization. The= 1 in the area x= [-10:10]
crack size is wc= 1mm, lc= 10 mm, dc= 1 mm, lift-off z= 1 mm, m/4mm, y= [-10:10] mm, and the specimen size is width ws and length ls. For the case of θc= 900, we justneed to exchange the wc and lc, as shown in Fig. 11(d).
Magnetization direction

Magnetization direction

(b)
z

(a)
z

y

x

y

x
(d)

(c)

z

z

Magnetization directionx

Magnetization direction

y

x

y

Fig. 11. Distribution of Hz in the case of (a), (c) θc= 00 and (b), (d) θc= 900(a), (b) horizontal magnetization; (c), (d) vertical magnetization
(a)

(b)

Fig. 12. Image processing. (a) horizontal magnetization, (b) vertical magnetization

The software consisted of Visual Basic 6.0 and a 3-D tool (CW3DGraph 6.0 of Measurement
Studio by National Instruments Co.). The crack was visualized in 3-D, so the surface of the specimen
and depth of the crack were expressed by the XY plane and Z axis of the graphic tool, respectively.
Rectangular type, triangular type, elliptical type and step type cracks were simulated in the software.
Also, multiple, complex cracks were expressed by superposing several graphs, which had the same
XY plane and the same scale of the Z axis. These superposed graphs showed the different shapes andsizes of the cracks. Thus, the distribution of MFL can be calculated by using the above equations fromISSN – 1883 – 9894/11© 2011JSM and the authors. All rights reserved.

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H. M. Le et al./ A Simulation Technique of Non-Destructive
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Eq. (3) to Eq. (7) when multiple complex cracks are located at any position and direction on thespecimen. However, the z in each equation expresses the lift-off, the distance between the specimen
surface and the sensor.
Depending on the MFL distribution around a crack, different domains appear on MOF. In thecase of horizontal magnetization, the magnetic domain around a crack is larger than that in no-crack
area. Hence, the bright and dark domains appear large around a crack, and smaller stripe domains
appear far from a crack. In the case of vertical magnetization, the magnetic domain in a no-crack area
is saturated by MOF. Only the magnetic intensity in a crack area is lower than the saturated magnetic
intensity of MOF. So the MOF shows a dark domain in a crack area and a bright domain in a no-crack
area. The software has three colors, bright for H
H , dark for H
H , and dusky for -HS< HZ<
HS. These colors appear in the case of horizontal magnetization as shown in Fig. 12(a) [18]. The
bright color is around the dusky color which displays a crack in the case of vertical magnetization asshown in Fig. 12(b). Furthermore, the parameters such as magnetic charge per unit m, permeability of
specimen , saturated magnetic intensity of MOF HS, bias magnetic intensity Hb, and the temperature
regime T were introduced in the software. When bias magnetic intensity is applied, the total magnetic
intensity distribution is HZ + Hb. The temperature affects to the value of saturated magnetic intensityof MOF HS as shown in Fig. 8 and the spatial resolution of MOF as shown in Fig. 7. The spatial
resolution of the MOF was expressed as the meshing number of the 3-D graphic. So, when thetemperature T changes, the HS and meshing number are changed automatically. Thus, we can simplify
the principles in Table 1.
Table 1 Relation between the real condition and the simulation

Real Condition
Lift-off
Bias magnetic field
Saturated magnetic field of MOF

Magnetization direction

Temperature

Simulation
z
H
H
H
Hs
H
Bright color for H
Dark color for H
H
H
Dusky color for H
Two options in the software:
1. Vertical magnetization
2. Horizontal magnetization is affected by angle θc
Changing the sensitivity of MOF (Hs)
Changing the width of domain(meshing number of 3D graphic)

3. Experiments and discussion
Table 2 Size of cracks
Magnetization direction

Fig. 13. Cracks on specimen

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H. M. Le et al./ A Simulation Technique of Non-Destructive
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Fig. 13 shows a specimen of 250 x 250 x 5 mm in size, made of low carbon steel (SS41). Table
2 shows the rectangular cracks of different sizes with angels of θc= 00, 450, 600, 900. Cracks No. 5, No.
10, No. 15 and No. 20 are hole types, which were simplified as a square in the numerical analysis.
The cracks were machined by using an electro-discharge machine.

Fig. 14. (a) yoke type magnetizer, (b) cross type magnetizer
Table 3 Parameters of software
using for horizontal magnetization

Table 4 Parameters of software
using for vertical magnetization

Magnetization direction of yoke type magnetizer

Fig. 15. Simulation results withyoke type magnetizer

Fig. 16. Experimental results withyoke type magnetizer

Magnetic field was applied by using a yoke type magnetizer, as shown in Fig. 14(a). The yoketype magnetizer has 2 coils with each coil having 2000 turns. The two cores are 220 mm in height,
and 150 mm in length with the distance between the cores of 250 mm. The 0.1 A direct current was
supplied to each coil with the different direction each other. Figs. 15 and 16 show the simulation
results and experimental results with the parameters in Table 3. The experimental results andsimulation results were similar in the case of rectangular type cracks. As mentioned in the discussions
on the principles, bright and dark area existed around cracks. The cracks with the angle θc= 00 (No.
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H. M. Le et al./ A Simulation Technique of Non-Destructive
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1~No. 4 in Fig. 13) were easier to detect than the other cracks. On the other hand, the cracks with theangle θc= 900 (No. 16~No. 19 in Fig. 13) could not be detected because the magnetization direction
was perpendicular with the length directions of cracks (in the case of θc= 00), and parallel with thelength directions of cracks in the case θc= 900. In the case of θc= 450 and θc= 600, cracks were also
detected. Detection ability reduced according on the increase of the angle θc. Also, detection ability
increased according to the increase of the depth of crack as shown in No. 3, No. 8 and No. 13.
However, in the case of hole type cracks, the simulation results are different from the experimentresults because the hole type cracks were assumed as square type cracks in the numerical analysis
software. The experiment results appeared the brightness was lighter on the left side and darker on theright side because the north magnetizer pole and south magnetizer pole were on the left and right sidesof specimen, respectively.
Magnetization direction 1

Fig. 17. Simulation results withcross type magnetizer (pair 1)

Fig. 18. Experimental results withcross type magnetizer (pair 1)

Magnetization direction 2

Fig. 19. Simulation results withcross type magnetizer (pair 2)

Fig. 20. Experimental results withcross type magnetizer (pair 2)

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H. M. Le et al./ A Simulation Technique of Non-Destructive
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The cross type magnetizer has 4 cores of 200 mm in height, 150 mm in width, and 250 mm inpole distance, as shown in the Fig. 14(b). Each core is wound by 2000 turns of coil. The horizontal
and vertical magnetic fluxes are obtained after direct current is supplied to a pair of coils, which faceeach other. Figs. 17, 19 are the simulation results obtained with the parameters in Table 3. Figs. 18, 20
are the experiment results for input current of 0.1 A supplied to each pair of coils. When current was
input to the pair of coils (pair 1) which was line on the left side and right side of the specimen, thehorizontal magnetization from left side to right side (magnetization direction 1) occurred on specimen(Fig. 18). Only cracks No. 16~19 (Fig. 13) did not appear because theirs length directions wereparallel with the magnetization direction. This result is the same with the results of the yoke typemagnetizer above. By supplying current to another pair of coils (pair 2), which was in line on the backside and front side of specimen, we could obtain the horizontal magnetization from back side to front
side of the specimen (magnetization direction 2) as shown in Fig. 20. Cracks No. 1~4 (Fig. 13) did notappear because their direction of length were parallel to the magnetization direction. But other cracks
appeared by the same reasons that were discussed in yoke type magnetizer. However, in cases of No.
11~14 (Fig. 13), these cracks appeared more clearly in Fig. 19 than in Fig. 17 because their θc= 30o in
Fig. 17 was changed θc= 60o in Fig. 19. Therefore, we can switch the input current between each pairof coils to detect both cracks of θc= 0o and θc= 90o. These simulation results were similar with theexperiment results in Fig. 18 and Fig. 20. The information of cracks appeared but not clearly in No. 3,4 (Fig. 13) in Fig. 20 and No. 17, 18 (Fig. 13) in Fig. 18 because of the magnetic induction effectbetween the two coil and the residual magnetic field in each coils.
(a)

(b)

Fig. 21. (a) Simulation result and (b) experimental resultwith vertical magnetization

Fig. 21 shows the simulation result (a) and experimental result (b) in the case of verticalmagnetization. A rectangular crack of 0.5 mm of width, 10 mm of length and 4 mm of depth on SS41
specimen was used. The yoke type magnetizer (Fig. 14(a)) was used to induce vertical magnetization
with 0.17 A direct current supplied to two coils in the same direction. The parameters for simulation
are shown in Table 4. In the both results, only the information of crack appeared. In the case of thesimulation result (Fig. 21(a)), the information of crack appeared as dusky color, and the no-crack area
appeared as bright color as mention in the principle discussions. The magnetic intensity in the crack
area was HZ, crack, -HS< HZ, crack< HS and the magnetic intensity in no-crack area was HZ, no crack , HZ, nocrack ≥ HS. In the experiment result (Fig. 21(b)), the crack appeared with dark color but not smoothly
because of noises.

4. Conclusion
Numerical analysis software using a dipole model and a Magneto-Optical Film was used tosimulate the distribution of the magnetic flux around cracks. The software allowed 3-D visualizationof the magnetic distribution around a crack or multi-complex cracks on specimen of different shapes,
sizes, positions and length directions. The Faraday effect in MOF was integrated into the dipole modeland in the software. The software supported user input of important parameters such as lift-off
distance, magnetic charge per unit area, saturated magnetic intensity of MOF, bias magnetic intensity,
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H. M. Le et al./ A Simulation Technique of Non-Destructive
Testing usingMagneto-Optical Filmand temperature. Horizontal magnetization and vertical magnetization were implemented in thesoftware. The software performance was determined by comparing the simulation results with theexperiment results. Horizontal magnetization was induced by using a yoke type magnetizer, to detectcracks except the cracks whose direction of length were parallel with the magnetization direction. A
cross type magnetizer was used to induce horizontal magnetization, to detect cracks in all direction. In
the case of vertical magnetization, only the information about cracks was shown without the effect of
crack direction.

Acknowledgement
This research was supported by the MKE (The Ministry of Knowledge Economy), Korea, under the
ITRC (Information Technology Research Center) support program supervised by the NIPA (National
IT Industry Promotion Agency) (NIPA-2011-C1090-1121-0013). We are grateful for the supports.

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