7 Electromagnetic Acoustic Transducers (EMATs)

7.1 EMAT Principles

7.2 EMAT Instrumentation

7.3 EMAT Applications

Piezoelectricity

+

-+

-

-

+

- -

+

+

- +-

- -

+

+

+

+ + + + + + +

- - - - - - -+ + + + + + +

- - - - - - -

SiSi

Si

O OO

Quartz (silicon dioxide, SiO2)

E bV

FA

SE

eD ESe K

Electromagnetic Acoustic Transducers

Key Features:

non-contact/no couplant

multiple wave modes (including SH)

Disadvantages:

low sensitivity

requires special electronics

material dependent

Advantages:

easy automation

high speed scanning

high reproducibility

high-temperature inspection

minimal wear

less surface preparation required

easy to customize

(EMATs)

7.1 EMAT Principles

Principle #1: Lorentz Force

( )Q F E v B

H J

t

B

E

J E

Ampère's law:

Faraday's law:

Ohm’s law:

Lorentz force:

Je

conducting medium

Hp

He

Ip

Transmission (I F) Reception (v V):

Je

F

B0

I

B0

Je

v

V

Principle #2: Magnetization

Fm magnetic force

µ0 permeability of free space

V volume

M magnetization

H magnetic field

y height

χ magnetic susceptibility

specimen

electromagnet

y

20

0 2mVdH dH

F V Mdy dy

excitation current magnetization force

no bias strong biassome bias

Time

Sig

nal

Time

Sig

nal

Time

Sig

nal

Fm

Principle #3: Magnetostriction

Spontaneous magnetostriction:

H = 0

Induced magnetostriction:

H

1,2,3 3

e

12

3

e 1

2,3 2 3

e

0 2 4 6Magnetic Field [104 A/m]

low-carbon steel

Mag

neto

stri

ctio

n [

10-6

]

-20

0

-10

-30

-40

10

Fe

Co

Ni

7.2 EMAT Instrumentation

EMAT Polarization

en dA Q I J vhigh coupling:

0n τ I B“surface” traction:

m 0Q F v Bmagnetic force:

tangential polarization normal polarization

Je

B0n I

B0n I

Je

Normal-Beam EMATs

spiral coilradially polarized shear wave

rectangular coillinearly polarized shear wave

symmetric coillongitudinal wave

B0

S

N

N SB0

SN

B0

Angle-Beam Shear EMATs

sin

periodic permanent magnethorizontally polarized shear wave

SN

B0

meander coilvertically polarized shear wave

S

N

B0

EMAT Electronics

EMATs withpermanent or

electromagnets

driveramplifier

oscillator+

_Vsmatching

networkmatchingnetwork

specimen

0 0.5 1 1.5 2 2.5 3Frequency [MHz]

02468

101214161820

Impe

danc

e [Ω

]

resistance

reactance

7-turn, 10-mm-diameter spiral coil on ferritic steel

Impedance Matching

VgV

gZ

Z

2

g *max g g g

gwhen ,

8

VP Z Z R R X X

R

transformer (κ ≈ 1)

12 21

22 11

2 2 21 22( )d

V Ndt

1 1 11 12( )d

V Ndt

I1

N1 N2 V2

11

V1

I2

2212 21,

2 2

1 1

V N

V N

2 2 11 22( )d

V Ndt

1 1 11 22( )d

V Ndt

2 1

1 2

I N

I N

22 2

21 1

NZ

Z N

ideal transformer (κ = 1)

7.3 EMAT Applications

Texture Assessment by EMATscold-pressed 2024 aluminum, 1.4 MHz, EMAT

η 0% (annealed) η = 0.45 % η = 0.8 % η = 1.6 %

cavg = 2,850 m/s, 0.2% per division, η = (cmax – cmin)/cavg

Textured Specimen

transmitter receiver

Rayleighwave

High-Temperature Monitoring

60

55

50

45

40

35

30

25

20200 400 600 800 1000

Temperature [K]

Sti

ffne

ss [

GP

a]

C44

C66

230

210

190

170

150

130200 400 600 800 1000

Temperature [K]

Sti

ffne

ss [

GP

a]

C11

C33

SiC/Ti-6Al-4V composite

(Ogi et al., 2001)

Electromagnetic Acoustic Resonance(Hirao and Ogi et al., 2003)

0 50 100 150

SCM 440 steel pure titanium

-120 -80 -40 0Stress [MPa]

-1.0

-1.1

-1.2B

iref

ring

ence

[%

]

loadunload

couplantPZT

specimen specimen

EMAT

Stress [MPa]

0.05

0

-0.05

-0.10

Bir

efri

ngen

ce [

%]

as-receivedquenched & temperedannealed

Thank You!