Ultrasonics 51 (2011) 675–682

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Ultrasonics

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Improving the signal amplitude of meandering coil EMATs by using ribbonsoft magnetic flux concentrators (MFC)

R. Dhayalan, V. Satya Narayana Murthy, C.V. Krishnamurthy, Krishnan Balasubramaniam ⇑Centre for Nondestructive Evaluation and Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e i n f o

Article history:Received 30 May 2010Received in revised form 20 January 2011Accepted 25 January 2011Available online 2 February 2011

Keywords:Meander coil EMATLorentz forceMagnetic flux concentrator (MFC)Soft magnetic ribbons

0041-624X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.ultras.2011.01.009

⇑ Corresponding author.E-mail address: balas@iitm.ac.in (K. Balasubraman

a b s t r a c t

This paper presents a new method of improving the ultrasonic signal amplitude from a meander lineEMAT by using soft magnetic alloy ribbon (Fe60Ni10V10B20) as a magnetic flux concentrator (MFC). Theflux concentrator is a thin soft amorphous magnetic material (Fe60Ni10V10B20) which is very sensitiveto a small flux change. The MFC is used with the EMAT to improve the signal amplitude and it wasobserved that the peak signal amplitude increases by a factor of two compared to the signal withoutMFC. Two dimensional numerical models have been developed for the EMAT with MFC to quantify theimprovement of the received signal amplitudes. Model calculations and experiments have been carriedout for a wide range of ultrasonic frequencies (500 kHz–1 MHz) in different materials.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Electromagnetic acoustic transducers (EMATs) [1–5] are ad-vanced Ultrasonic transducers that can generate and detect soundwaves in an electrically conducting material without making phys-ical contact with it. Such noncontact sensors are attractive in appli-cations in the area of Nondestructive testing (NDT) of electricallyconducting materials. Their operation is based on the Lorentz forceprinciple and/or by magnetoelastic effects, depending on the typeof the material either ferromagnetic or not, bias field strengthand the EMAT configuration employed [6–8,37]. It can generatedifferent ultrasonic wave including modes that are difficult to gen-erate with conventional piezoelectric crystals, e.g., SH (horizontallypolarized shear) waves [9–11]. Since the coupling between thetransducer and the component is magnetic, it has many advanta-ges over the conventional ultrasonic techniques that require a cou-pling media. Hence, it is possible to eliminate any inconsistencyarising from the use of couplant during the inspection. This typeof transducer can be easily designed and fabricated based on spe-cific application required.

The two main important components of an EMAT are a coil car-rying a high current alternating pulse when in transmission modeand a permanent magnet to induce a strong static magnetic fluxwithin the skin depth of the test specimen directly below theEMAT. The pulsed alternating current fed to the transmitter EMATcoil induces eddy currents, ~Je, within the skin depth of the testpiece. In the presence of a large bias magnetic flux, ~BS, these eddy

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iam).

currents lead to body forces, ~FL, at the surface layer of the specimen[12–14]. Fig. 1(a) shows a single coil and magnet leading to thegeneration of Lorentz force inside the material. The Lorentz forces,~FL, on the eddy currents are transmitted to the solid by the elec-trons exchanging momentum with the atoms in the metal. Thedirection of the transient force in the solid below each elementof the ML coil alternates at the frequency of the driving currentand acts as the source of ultrasonic waves.

~FL ¼~Je �~BS ð1Þ

EMATs can be used as generators as well as detectors of ultrasound.EMATs are better as detectors than generators of ultrasonic waves[15] and in the receiving mode, the magnetic flux density ~BS fromthe EMAT’s permanent magnet interacts as a result of ultrasonicmotion with particles having a velocity v within the material. Thisproduces eddy currents ~Je within the sample according to Lorentzforce mechanism, expressed as [16],

~Je ¼ rð~v �~BSÞ ð2Þ

where r is the electrical conductivity of the sample. The currentdensity generated inside the sample in turn induces an e.m.f. inthe pick-up coil of the receiver EMAT which is normally placedabove the sample surface as shown in Fig. 2. This occurs as a resultof the interaction between an acoustic wave and a static magneticfield in the test sample. In conducting ferromagnetic materials, thisinteraction can produce Lorentz and magnetostrictive current den-sities. Since the amplitude of the received signal is very small (in therange of 10 lV–100 lV) high gain (above 60 dB), high input

Fig. 1. Schematic of (a) a single coil and magnet leading to the generation of Lorentzforce and (b) the meander coil configuration used.

Fig. 2. EMAT receiving mechanism.

676 R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682

impedance (50 O) and low noise electronic amplifiers (less than2 dB) are used to amplify the received signals [17].

In this paper, the improvement of the EMAT signal amplitudeshas been reported with the introduction of a new soft magneticribbon (Fe60Ni10V10B20) as magnetic flux concentrator (MFC). Oneof the most significant properties of this amorphous soft magneticmaterials is magnetoimpedance (MI) [18–20]. Such an effect in-volves a very large and sensitive change of the impedance of somesoft magnetic materials under the action of a static magnetic fieldand a small alternating current. The skin effect and domain wallmotion, depending on the frequency range, are the main physicalorigins of giant magnetoimpedance (GMI) [21]. MI is useful formagnetic-field detecting sensor applications [22]. These devicesare very sensitive and have quick responses to magnetic fields.

The soft magnetic ribbons (MFC) were used with the EMAT toimprove the signal amplitudes and it has been observed that theamplitude has increased for different wave modes in differentmaterials. Additionally, a 2D finite element model has been devel-oped for the EMAT to quantify the improvement of the signalamplitudes (peak to peak) of different wave modes with MFCand are compared to the experimentally obtained peak signalamplitudes with and without MFC for a wide range of frequencyfrom 500 kHz to 1 MHz.

The numerical simulation of an ML EMAT has been done in twosteps. First, an electromagnetic model has been developed for theLorentz force density calculation. The commercial finite elementsoftware COMSOL 3.5� was used for the electromagnetic modelingof the ML EMAT. This software package has predefined applicationscalled modes with built-in mathematical solutions to facilitatemodeling. The Lorentz force density calculation has been donefor a wide range of frequency varying from 500 kHz to 1 MHz. Sec-ond, the wave propagation simulation has been done by using an-other commercial explicit finite element package ABAQUS 6.6.8. Ituses an explicit integration scheme for solving the transient dy-namic and quasi-static analyses. The output of the COMSOL elec-

tromagnetic model has been utilized as the inputs for wavepropagation in the elastodynamic model. The numerical modelingof meander coil EMATs have been explained in more detail else-where [23,24] and here only the inclusion of the MFC in the modelwill be discussed. In this paper, 2D numerical models have beendeveloped to validate the experimental results obtained withEMAT with MFC.

2. Numerical simulation

In simulation part, the soft magnetic ribbon (MFC) is modeledwith a permanent magnet to examine the variation of the magneticflux density. Then the performance of MFC with EMAT has beenanalyzed and compared with the experimental results.

2.1. Modeling of MFC

A square permanent magnet of size 25 � 25 � 12.5 mm hasbeen used for this study and a thin soft magnetic material of30 lm thick and 10 mm length has been introduced with the mag-net. The permanent NdFeB (N38H) magnets with residual magneticstrength (Br = |Br|) of 1.254 T were used for both simulation studiesand experiments. This work involved two steps: first the perma-nent magnet was modeled alone and second, the magnet withthe MFC was modeled for the magnetic flux density (B) measure-ments. In the second case, the MFC was placed 0.5 mm below themagnet surface and the size of the MFC (10 � 0.02 mm) covers ex-actly the active area of the EMAT coil. Two dimensional magneto-static models have been developed for the spatial distributions ofthe magnetic flux density (B) arising from both the permanentmagnet and the permanent magnet with MFC. In such a model,the magnetic scalar potential Um for static magnetic field can bedefined as [25],

~H ¼ �rUm ð3Þ

where H is the magnetic field intensity. By using the Gauss lawof magneto statics, the divergence of magnetic flux density B at anypoint is described by [26],

r �~B ¼ 0 ð4Þ

The constitutive relation of magneto statics for both B and H can bewritten as,

~B ¼ l0ð~H þ ~MÞ ð5Þ

where l0 is the permeability of free space and M is the magnetiza-tion which can also be written as,

~M ¼ ~M0 þ ðlr � 1Þ~H ð6Þ

where M0 is the pre-magnetization vector of the magnet and lr isthe relative permeability. Combining all the equations, it can beshown that,

�r � ðl0lrrUm � l0~M0Þ ¼ 0 ð7Þ

In order to solve for the magnetic scalar potential (Um) based onEq. (7), the commercial finite element software COMSOL (FEMLAB3.2�) has been used. The permanent magnet and the magnet withMFC have been modeled using 2D static analysis with predefinedcoefficients of l = l0lr and c = l0M0 [27]. The pre-magnetizationconstant (M0) value for the NdFeB magnet (N38H) can be calcu-lated based on the magnetization hysteresis curve when H = 0and B = Br. Therefore Eqs. (6) and (7) reduce to M0 = Br/l0 [28]and the pre-magnetization value is calculated to be|M0| = M0 = 998 kA/m. The value of relative permeability for thesoft magnetic material (MFC) is lr = 2 � 106 as provided by themanufacture of the soft magnetic ribbons. The saturation magnetic

R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682 677

field for this material is 100 Oe and the electrical resistivity is134 lX cm. Additional details about the soft magnetic ribbon(MFC) are explained in the experimental part. The active regionof the EMAT coil has covered by the ribbon and the arrangementof the ribbon with the coil and magnet is shown in Fig. 3a.

After defining the material geometries, the appropriate compo-nents of M0 are applied for both the models. For the first case, themagnetic field is applied along the positive Y axis for the magnetand the Y axis component of M0 (M0Y) is set equal to M0 and theother component M0X is set to zero. For the second case, the mag-netic fields are applied along the positive Y axes like the previouscase the appropriate lr values are applied along the positive Y axesother axes set to zero for the MFC. With boundary conditions, the2D mappings of the magnetic flux densities have been modeled forboth cases. Fig. 4a and b shows spatial distribution of the magneticflux densities concentrated around the edges of the magnet and themagnet with MFC. Fig. 4c and d shows the observed magnetic fluxdensities below the surface (0.6 mm) of the permanent magnet andthe magnet with MFC respectively. The black lines just below themagnet and the magnet with MFC are the filed observation lines.

Fig. 3. Arrange of EMAT coil and the magnet (a) with soft m

Fig. 4. Magnetic flux density distributions of (a) the magnet alone and (b) the magnetvariation of the magnetic flux density with (c) the magnet alone and (d) the magnet wi

From Fig. 4c and d, it has been noted that the magnetic flux den-sity is increased for second case, the magnet with MFC. From thenumerical model, it has been observed that the maximum fluxdensity is about 0.3 T for the permanent magnet and 0.41 T forthe permanent magnet with MFC. In the first case, the magneticflux lines from the magnet diverges away from the edges and covera larger spatial area but in the second case, the soft magnetic rib-bon (MFC) concentrates the flux lines from the end face of the mag-net and cover the small active region of the EMAT coils.

3. Experimental details

For experimental studies, a set of meander coil EMATs havebeen developed which consists of a normal biasing magnet and acopper coil with periodicity of 3 mm. The coils were fabricatedby printed circuit board (PCB) technique which allows to fabricateany type of flat coil of arbitrary pattern. The printed circuits boardsare made by 150 lm polyester based flexible laminate with 30 lmthick copper clad. The permanent Neodymium–Iron–Boron (Nd–Fe–B) sintered magnets were used for bias magnetic field.

agnetic ribbon and (b) without soft magnetic ribbon.

with MFC. The cross-sectional field profiles across both the images represents theth MFC.

678 R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682

3.1. Meander coil EMAT

The transmitter coil is driven by an alternating current whichinduces eddy current inside the test specimen. The interaction ofeddy current and the perpendicular magnetic field induces Lorentzforce parallel to the surface, whose directions change alternativelywith the meandering period. Such a force distribution generatesRayleigh waves along surface and oblique bulk waves (longitudinaland transverse waves) into the specimen [29,30]. Fig. 5a shows thecross sectional view of the meander coil EMAT and the wavemodes generation inside a thick specimen. The meander coil EMATcan also be used to generate Lamb waves in thin plates. Twodifferent sizes of aluminum (400 � 200 � 2 mm and 300 � 755 �50 mm) and stainless steel plates (300 � 200 � 2 mm and250 � 125 � 30 mm) were used in the experiments.

Fig. 5b shows the schematic diagram of the instrumentation setup for the experimental studies. In order to generate a high currentRF tone burst, a RITEC RPR4000 gated amplifier was used and thehigh power output was applied to the transmitter excitation coil.An impedance matching network system was used between thehigh impedance of the pulser to the low impedance of the trans-mitter coil. By, adjusting the control setting present in the gatedamplifier, the output power to the coil was varied. The inbuiltbroadband receiver was used to amplify the received signal fromthe receiver EMAT. A 60 dB gain setting at the receiver has beenused for all the measurements. The receiver EMAT is connectedto another impedance matching network to match the low imped-ance receiver coil to the high impedance receiver. The output fromthe broadband receiver is fed to the Agilent DSO6032A Digital stor-age oscilloscope which is interfaced with PC for data acquisition.Before starting the actual experiment, all the instruments andthe power level of the RITEC with the control setting are testedand calibrated.

3.2. Magnetic flux concentrator (MFC)

The magnetic ribbon (Fe60Ni10V10B20) was prepared by the meltspinning technique in argon atmosphere. The ribbon (Fe60-

Ni10V10B20) has high content Fe with negligible or zero magneto-striction and it can be used for the design of low magnetic fieldsensors such as magneto impedance (DZ/Z%) sensors [31,32]. Oneof the most significant properties of amorphous soft magneticmaterial is magnetoimpedance (MI). Such an effect involves a verylarge and sensitive change of the impedance of some soft magneticmaterials under the action of a static magnetic field and a smallalternating current. The impedance of the magnetic material isvery sensitive to a small change in the magnetic field. Fig. 6b showsthe magnetoimpedance variation of this material with respect tothe magnetic field. A large change in magneto impedance can be

(a)Fig. 5. (a) Meander line EMAT (b) Schemat

observed under the action of static magnetic field. Fig. 6c showsthe variation of magnetoimpedance with respect to the frequency.At low frequency the variation of magnetoimpedance is around 5–10. The Fe60Ni10V10B20 alloy was prepared by arc melting the highpure constituent elements in argon atmosphere. The ingot wasmelted several times to obtain a homogenized mixture. The weightloss after the melting was less than 0.5%. Refs. [33,34] explains inmore detail the process used here for the fabrication of the amor-phous ribbons by using melt spinning method. The ribbons thatwere obtained are 30–40 lm thick, 1–2 mm wide and approxi-mately 2 m long. It is very easy to make the soft magnetic materialwith less cost. It has been used as the magnetic flux concentrator toincrease the flux lines from the bias magnet to the active part ofthe EMAT coils. A layer of ribbons were placed between the pick-up coil and the permanent magnet of the EMAT.

4. Results and discussion

Experiments were conducted to evaluate the influence of themagnetic ribbon (Fe60Ni10V10B20) as a MFC for a meander coilEMAT. The measurements were performed using EMATs, withMFC and without MFC in a through transmission (pitch–catch)method. A tone burst RF signal with a peak current value ofapproximately 50 A and a 6 ls pulse duration was applied to theexcitation coil. Different ultrasonic modes were observed as de-scribed below.

4.1. Generation of Lamb waves

First the Lamb waves generation have been done on 2 mm thickaluminum and stainless steel plates at 500 kHz. Fig. 7 shows themeasurement set up for Lamb wave generation for both experi-ment and simulation. At 500 kHz, the EMAT transmitter generatesthe fundamental Lamb wave modes (S0 and A0) and the receiverEMAT was placed 100 mm away from the source EMAT.

In both the simulation and experimental outputs, it was ob-served that the peak-to-peak value of amplitudes of the Lambwave modes (S0 and A0) showed a 100% increase in the case whereMFC was used when compared to the case where MFC was notused. The results are quantified in Table 1. Fig. 8a and b showsthe comparison of experimental and simulation outputs of Lambwave signals with and without MFC’s on the 2 mm thick aluminumand stainless steel plates.

4.2. Generation of Rayleigh waves

Fig. 9 shows the measurement setup for generating Rayleighwaves using meander coil EMAT with MFC and without MFC forboth simulation and experiment. A 50 mm thick aluminum and a

EMAT Receiver

Ritec Pulser Receiver RPR-4000

EMAT Transmitter

Test Specimen

Oscilloscope

(b)ic diagram of the experimental set-up.

0

5

10

15

20

25

30

35

0 5 10 15Frequency (MHz)

Z/Z%

(b)(a)

(c)Fig. 6. (a) Photograph of EMAT coil and the magnetic ribbon and variation of magneto impedance with (b) magnetic field and (c) frequency.

Fig. 7. Measurement set-up for Lamb waves generation on 2 mm thin plate.

Table 1Comparison of the amplitudes of different wave modes in different materials with and without MFC.

Material Method Condition with MFC R-wave(1 MHz)

Lamb wave (S0 mode)(500 kHz)

Lamb wave (A0 mode)(500 kHz)

Aluminium Experimental output – Amplitude(mV)

Without MFC 2.92 2.91 2.76With MFC 6.27 5.93 5.65Amplitude ratio(AR)

2.15 2.04 2.05

Simulation output – Displacement (m) Without MFC 2.24e�16 3.2e�17 1.81e�16With MFC 4.52e�16 6.42e�17 3.64e�16Amplitude ratio(AR)

2.02 2.01 2.01

Stainlesssteel

Experimental output – Amplitude(mV)

Without MFC 1.12 0.76 1.69With MFC 2.37 1.48 3.62Amplitude ratio(AR)

2.12 1.95 2.14

Simulation output – Displacement (m) Without MFC 8.32e�17 1.08e�17 7.04e�17With MFC 1.66e�16 2.18e�17 1.44e�16Amplitude ratio(AR)

2.00 2.02 2.04

R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682 679

30 mm thick stainless steel blocks are used for this measurement.The transmitter EMAT was located on the middle of the flat surfaceof the specimen and the receiver was placed 100 mm away fromthe centre of the transmitter, which was oriented along the axis

of the wave front generated by the source EMAT. The Rayleighwave generation was examined at the excitation frequency of1 MHz, Fig. 10a and b shows the comparison of Experimental andsimulation results of Rayleigh waves on two different materials

(b) Stainless steel

Experimental output Simulation output

(a) Aluminum

Fig. 8. Comparison of time histories of simulation and experimental results of Lamb wave modes (a) on an aluminum plate with and without out MFC and (b) on a stainlesssteel plate with and without MFC.

Fig. 9. Schematic diagram for Rayleigh wave measurement set up.

(a) Aluminum

(b) Stainless steel

Experimental output

Fig. 10. Comparison of time histories of simulation and experimental results of Rayleighblock with and without MFC.

680 R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682

with MFC and without MFC. It has been observed that the ampli-tudes of Rayleigh wave signals have been increased twice for EMATwith MFC and is quantified in Table 1.

Table 1 summaries the performance of meander coil EMAT withMFC and without MFC for both experimental and simulation re-sults. The various sound modes have been generated on differentmaterial for a wide range of frequencies and the signal amplitudeshave been measured for both bulk and guided wave modes. Theincreasing signal amplitudes with MFC indicate that the soft mag-netic material (Fe60Ni10V10B20) acts as a flux concentrator and in-creases the normal bias magnetic field. To quantify the

Simulation output

waves (a) on an aluminum block with and without MFC and (b) on a stainless steel

R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682 681

performance of MFC, the signal amplitude (peak to peak) of differ-ent wave modes were calculated and compared with MFC andwithout MFC. For example, comparing the peak signal amplitudesof Rayleigh (R-waves) waves on aluminum plate with MFC andwithout MFC

Amplitude ratioðARÞ ¼PeakAmplitudewithMFC

PeakAmplitudewithoutMFC¼ 6:27 mV

2:92 mV

� 2:15: ð8Þ

4.3. Transfer impedance ratio (TIR)

In nonmagnetic materials, the mechanisms for both generationand detection of elastic waves with EMATs are linear and obeystandard reciprocity relations. There are excellent treatments ofthese in the literature and little would be served by reproducingthem here [6,35]. For a totally isotropic elastic body described byelastic constant C and mass density qM, the wave equation is

qM@2~u@t2 ¼ C

@2~u@x2 þ~FðxÞ ð9Þ

where F(x) is the Lorentz force driving force per unit volume.Therefore,

~FðxÞ ¼~JeðzÞ �~BsðyÞ ð10Þ

The induced current density can be obtained by solving theMaxwell’s equations. This current density, given by j ¼

ffiffiffiffiffiffiffi�1p

,

~JeðzÞ ¼ð1� jÞ~n� ~H0

dcexp

zð1� jÞdc

� �ð11Þ

and results from the magnetic field H0 produced by the EMAT RFcoil in close proximity to the surface. In Eq. (10), dc = (2q/l0x)1/2,q is the metal resistivity, l0 = 4p � 10�7 H/m, and x is the angularfrequency [37,38].

When an elastic wave is reflected from a metal surface, it alsogenerates a current density and electric field inside the metaland, of course, a corresponding electric and magnetic field outsidethe surface. The magnetic field (B) is sensed by the received coil. Itcan be shown that the surface electric field is given by ~v �~B. Con-sequently neglecting some complexities due to a finite skin depth,the receiver coil output voltage is given by [36]

V ¼Z

x~u�~Bc � ~dl ð12Þ

where integration is over the entire length of wire in the receivercoil. only that wire is very close to the metal surface is effectivein receiving the elastic wave. When the magnetic field is perpendic-ular to the surface, the displacement is given by [6,7]

~u ¼~H0BxZA

j1� jbM

� �ð13Þ

where bM is the analytical function of thickness and ZA is the acous-tic impedance of the material. In the limit of small bM, the transferimpedance for free surface boundary conditions is given by,

Table 2Comparison of the transfer impedance ratio ZT (TIR) and received voltage f

Material Physical parameter Condition with MFC R-w

Aluminum ZT (TIR) (X) Without MFC 62.3With MFC 119.

VReceived (V) Without MFC 31.1With MFC 59.6

Stainless steel ZT (TIR) (X) Without MFC 22.3With MFC 42.7

VReceived (V) Without MFC 11.1With MFC 21.3

ZT ¼BRBT NRNTðARATÞ1=2

ZAð14Þ

where BR, BT, NR, NT, AR and AT are the magnetic fields, number ofturns and effective areas of coils of the transmitter and receiverEMATs respectively. The parameter bM is defined by

bM ¼5fqv2 ð15Þ

where f is the operating frequency. If the transmitter and receiverare identical, the Eq. (13) becomes

ZT ¼B2N2A

ZAð16Þ

So, the efficiency of an EMAT transmitter and receiver system canbe measured by the Transfer impedance ratio ZT (TIR) which isthe ratio of the received voltage by the receiver to the input drivingcurrent. For an ideal case without acoustic attenuation and the ef-fect of coil lift-off, the transfer impedance ratio ZT (TIR) of an EMATcan be expressed as [12],

ZTðTIRÞ ¼ VReceived

IDriving¼ N2B2A

ZAe�4pG=D ð17Þ

where N is the number of turns in each coil (three turns) and A is thearea covered by the coil (9 � 30 mm). The magnetic flux density B is0.3 T for EMAT without MFC and 0.41 T for EMAT with MFC and G isthe magnetic lift-off (1 mm). D is the coil spacing (1.5 mm) and IDriv-

ing is the input driving current (50 Å).To verify the performance of the MFC, the transfer impedance

ratio ZT (TIR) of the meander coil EMAT with MFC and withoutMFC have been calculated for both the bulk wave on aluminumand stainless steel samples. Table 2 summarizes the calculatedphysical parameters such as the transfer impedance ratio ZT (TIR)and the received voltage (VReceived) for different wave modes on dif-ferent materials with and without MFC. and guided waves andsummarized in Table 2. The magnetic flux density of the EMATwithout MFC is 0.3 T and the EMAT with MFC is 0.41 T.

The actual improvement for sole generation (or detection) ofEMAT with MFC is 1.3, and only when two EMAT transducers(through transmission method) are used, or pulse–echo test is em-ployed, then the amplitude increment will be a factor of two.Moreover these kind of improvement occurs only in non-ferromag-netic materials.

From the transfer impedance ratio ZT (TIR) calculations, it hasbeen observed that the peak signal amplitudes of all the receivedwave modes with MFC has been increased almost by a factor oftwo for both aluminum and stainless steel. Hence by using the softmagnetic alloy ribbon (Fe60Ni10V10B20) as a MFC with EMAT im-proves the amplitudes of the wave modes. The magnetic materialimproves the signal strength and it can be used for defect detectionto increase the sensitivity of the defect.

or different wave modes in different materials with and without MFC.

ave (1 MHz) Lamb wave (S0 mode)(500 kHz)

Lamb wave (A0 mode)(500 kHz)

7e�8 33.41e�8 58.47e�835e�8 63.93e�8 111.89e�88e�6 16.71e�6 29.24e�68e�6 31.97e�6 55.95e�6

3e�8 12.46e�8 21.23e�84e�8 23.84e�8 40.63e�87e�6 6.24e�6 10.62e�67e�6 11.92e�6 20.32e�6

682 R. Dhayalan et al. / Ultrasonics 51 (2011) 675–682

5. Summary and conclusions

In this paper a novel method for improving the EMAT signalamplitude has been presented by introducing a soft magneticmaterial (Fe60Ni10V10B20) as a magnetic flux concentrator (MFC).This soft magnetic material (MFC) has been used along with themeander coil EMAT to increase the bias magnetic field. Theimprovements of the magnetic field with MFC have been calcu-lated by numerical simulation models and validated by experimen-tal observations. The permanent magnet and the magnet with MFChave been modeled by using the commercial finite element soft-ware COMSOL (FEMLAB 3.2�). The magnetic flux density of themagnet with MFC increased from 0.3 T to 0.41 T which is appliedto the active region of the EMAT coil. A coupled 2D numerical mod-el has been designed for the meander coil EMAT which is workingunder the principle of Lorentz force mechanism. For this first anelectromagnetic model has been developed for calculating the Lor-entz force density. The force density calculation has been done fora wide range of frequency (500 kHz–1 MHz) for different soundwave generation. Then, the elastodynamic model has been devel-oped for wave propagation and the calculated force densities havebeen utilized as the sources for the ultrasonic wave propagationmodeling.

A set of meander coil EMATs have been developed by using3 mm periodicity copper coils and the permanent magnets (Nd–Fe–B) of 0.3 T flux density for the experimental studies. The exper-imental measurements have been done for various sound modeson two different materials for wide range of frequency. It has beenobserved that the strength of the received signals have been in-creased with MFC for all the wave modes. The magnetic materialacts as a flux concentrator which allows more flux lines for the ac-tive part of the meander coil. The improvement of the signal ampli-tudes have been quantified by comparing the signal amplitudes(peak to peak) of the wave modes with MFC and without MFC. Ithas been observed that the peak signal amplitudes of all the re-ceived wave modes with MFC have been increased almost by a fac-tor of two. The numerical simulation results show a goodagreement with the experimental measurements. The performanceof the EMAT with MFC is also verified by calculating the Transferimpedance ratio ZT (TIR) for the shear wave generation on alumi-num and stainless steel samples. From the Transfer impedance ra-tio ZT (TIR) calculation, it has been shown that the received signalamplitude is increased twice by the new magnetic field.

For the future, we noted that the similar magnetic ribbon (Fe60-

Ni10V10B20) with different composition of materials may improvethe signal amplitude significantly.

Acknowledgement

The authors of this paper would like to acknowledge Dr. G.Markandeyulu, Department of Physics and the Magnetics labora-tory of the Indian Institute of Technology Madras for providingthe experimental facilities for this work.

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