1. 7 Electromagnetic Acoustic Transducers (EMATs)
7.1 EMAT Principles
7.2 EMAT Instrumentation
7.3 EMAT Applications

2. Quartz (silicon dioxide, SiO2)
Piezoelectricity
Quartz (silicon dioxide, SiO2)

3. Electromagnetic Acoustic Transducers
(EMATs)
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

4. 7.1 EMAT Principles

5. Principle #1: Lorentz Force
Ampère's law:
Faraday's law:
Ohm’s law:
Lorentz force: Je
conducting medium Hp He Ip
Transmission (I  F)
Reception (v  V): F B0 I v V

6. Principle #2: Magnetization
Fm magnetic force
µ0 permeability of free space
V volume
M magnetization
H magnetic field
y height
χ magnetic susceptibility
electromagnet Fm
specimen y
no bias
some bias
strong bias
Time
Signal
Time
Signal
Time
Signal
excitation current magnetization force

7. Principle #3: Magnetostriction
Spontaneous magnetostriction:
H = 0 2 4 6
Magnetic Field [104 A/m]
low-carbon steel
Magnetostriction [10-6]
-20
-10
-30
-40 10 Fe Co Ni
Induced magnetostriction: H

8. 7.2 EMAT Instrumentation

9. tangential polarization
EMAT Polarization
high coupling:
“surface” traction:
magnetic force:
tangential polarization
normal polarization Je B0
n I t

10. Normal-Beam EMATs spiral coil radially polarized shear wave
rectangular coil
linearly polarized shear wave
symmetric coil
longitudinal wave B0 S N S N B0 N S B0

11. Angle-Beam Shear EMATs
periodic permanent magnet
horizontally polarized shear wave S N L q B0
meander coil
vertically polarized shear wave

12. EMAT Electronics
EMATs with
permanent or electromagnets
driver
amplifier
oscillator + _ Vs
matching
network
specimen
7-turn, 10-mm-diameter spiral coil on ferritic steel
0.5 1
1.5 2
2.5 3
Frequency [MHz] 4 6 8 10 12 14 16 18 20
Impedance [Ω]
resistance
reactance

13. Impedance Matching  transformer (κ ≈ 1) I N V F2 V F 11 22 12 21 ,
ideal transformer (κ = 1)

14. 7.3 EMAT Applications

15. Texture Assessment by EMATs
cold-pressed 2024 aluminum, 1.4 MHz, EMAT
transmitter
receiver
Rayleigh
wave
Textured Specimen
cavg = 2,850 m/s, 0.2% per division, η = (cmax – cmin)/cavg
η  0% (annealed)
η = 0.45 %
η = 0.8 %
η = 1.6 %

16. High-Temperature Monitoring
SiC/Ti-6Al-4V composite
(Ogi et al., 2001) 60 55 50 45 40 35 30 25 20
200
400
600
800
1000
Temperature [K]
Stiffness [GPa]
C44
C66
230
210
190
170
150
130
C11
C33

17. Electromagnetic Acoustic Resonance
(Hirao and Ogi et al., 2003)
SCM 440 steel
pure titanium
Stress [MPa]
0.05
-0.05
-0.10
Birefringence [%]
as-received
quenched & tempered
annealed
-120
-80
-40
Stress [MPa]
-1.0
-1.1
-1.2
Birefringence [%]
load
unload 50
100
150
couplant
PZT
specimen
EMAT

18. Thank You!