7 electromagnetic acoustic transducers (emats) 7.1emat principles 7.2emat instrumentation 7.3emat...
TRANSCRIPT
7 Electromagnetic Acoustic Transducers (EMATs)
7.1 EMAT Principles
7.2 EMAT Instrumentation
7.3 EMAT Applications
Piezoelectricity
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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!