1. 3 Eddy Current NDE 3.1 Inspection Techniques 3.2 Instrumentation
3.3 Typical Applications
3.4 Special Example

2. 3.1 Inspection Techniques

3. Coil Configurations ~ ~ ~ ~ voltmeter testpiece oscillator excitation
sensing coil ~
Hall or GMR detector
voltmeter
testpiece
oscillator
excitation
coil ~
voltmeter
testpiece
oscillator
coil Z o ~ ~
differential coils
parallel
coaxial
rotated

4. Remote-Field Eddy Current Inspection
exciter coil
ferromagnetic pipe
sensing coil
Remote Field
Near Field
ln(Hz)
low frequency operation ( Hz)
Exponentially decaying eddy currents propagating mainly on the outer surface cause a diffuse magnetic field that leaks both on the outside and the inside of the pipe. z

5. Main Modes of Operation
single-frequency
time-multiplexed multiple-frequency
Signal
Signal
Time
Time
pulsed
frequency-multiplexed multiple-frequency
Signal
Signal
Time
Time
excited signal (current)
detected signal (voltage)

6. Nonlinear Harmonic Analysis
single frequency, linear response
Time
Signal
ferromagnetic phase
(ferrite, martensite, etc.) H B
nonlinear harmonic analysis
Time
Signal

7. 3.2 Eddy Current Instrumentation

8. Single-Frequency OperationVr
low-pass
filter
A/D
converter
oscillator Vq
driver
amplifier
90º phase
shifter
low-pass
filter
driver
impedances
processor
phase
balance
V-gain
H-gain + _ Vm
probe coil(s)
display

9. Nonlinear Harmonic OperationVr
low-pass
filter
oscillator
A/D
converter Vq
driver
amplifier
90º phase
shifter
low-pass
filter
n divider
driver
impedances
processor
phase
balance
V-gain
H-gain + _ Vm
probe coil(s)
display

10. Specialized versus General Purpose
Nortec 2000S system
Agilent 4294A system*
frequency range*
0.1 – 10 MHz
MHz
probe coil
three pencil probes
single spiral coil
relative accuracy
≈ %
≈ %
frequency scanning
manual
electronic
measurement time
≈ 50 minutes for 21 points
≈ 3 minutes for 81 points
*high-frequency application

11. Probe Considerations sensitivity thermal stability topology
flexible, low self-capacitance, reproducible, interchangeable, economic, etc.

12. 3.3 Eddy Current NDE Applications
• conductivity measurement
• permeability measurement
• metal thickness measurement
• coating thickness measurements
• flaw detection

13. Conductivity

14. Conductivity versus Probe Impedance
constant frequency
0.2
0.4
0.6
0.8 1
0.1
0.3
0.5
Normalized Resistance
Normalized Reactance
Stainless
Steel, 304
Copper
Aluminum, 7075-T6
Titanium, 6Al-4V
Magnesium, A280
Lead
Copper 70%,
Nickel 30%
Inconel
Nickel

15. Conductivity versus Alloying and Temper
IACS = International Annealed Copper Standard
σIACS = 5.8107 Ω-1m-1 at 20 °C
ρIACS = 10-8 Ωm 20 30 40 50 60
Conductivity [% IACS] T3 T4 T6 T0
2014
6061
T73
T76
7075
2024
T72 T8
Various Aluminum Alloys

16. Apparent Eddy Current Conductivity
0.2
0.4
0.6
0.8
1.0
0.1
0.3
0.5
lift-off
curves
conductivity
curve
(frequency)
Normalized Resistance
Normalized Reactance
specimen
eddy currents
probe coil
magnetic field s, l
s = s2
s = s1
= 0
= s 1 2 3 4
Normalized Resistance
Normalized Reactance
• high accuracy ( 0.1 %)
• controlled penetration depth

17. Lift-Off Curvature inductive (low frequency) capacitive
(high frequency)
“Horizontal” Component
“Vertical” Component .
conductivity
lift-off σ 2 1 ℓ = s
= 0
“Horizontal” Component
“Vertical” Component
lift-off .
conductivity σ 2 1 ℓ = s
= 0

18. Inductive Lift-Off Effect
4 mm diameter
8 mm diameter
1.5 %IACS
1.5 %IACS

19. Instrument Calibration
conductivity spectra comparison on IN718 specimens of
different peening intensities.
Nortec 2000S, Agilent 4294A, Stanford Research SR844, and UniWest US-450

20. Permeability

21. Magnetic Susceptibility
paramagnetic materials with small ferromagnetic phase content
moderately high susceptibility
low susceptibility 1 2 3 4
0.2
0.4
0.6
0.8
1.0
0.1
0.3
0.5
lift-off
frequency
(conductivity)
Normalized Resistance
Normalized Reactance
permeability
µr = 4
permeability 3
Normalized Reactance 2 1
frequency
(conductivity)
0.2
0.4
0.6
0.8 1
1.2
Normalized Resistance
increasing magnetic susceptibility decreases the apparent eddy current conductivity (AECC)

22. Magnetic Susceptibility versus Cold Work
cold work (plastic deformation at room temperature) causes
martensitic (ferromagnetic) phase transformation
in austenitic stainless steels
10-4
10-3
10-2
10-1
100
101 10 20 30 40 50 60
Cold Work [%]
Magnetic Susceptibility
SS304L
IN276
IN718
SS305
SS304
SS302
IN625

23. Metal Thickness

24. Thickness versus Normalized Impedance
scanning
probe coil
thickness loss due to corrosion, erosion, etc.
0.2
0.4
0.6
0.8 1
0.1
0.3
0.5
thick
plate
Normalized Resistance
Normalized Reactance
thin
lift-off
thinning
aluminum (σ = 46 %IACS)
-0.2
0.2
0.4
0.6
0.8 1 2 3
Depth [mm]
Re { F }
f = 0.05 MHz
f = 0.2 MHz
f = 1 MHz

25. Vic-3D simulation, Inconel plates (σ = 1.33 %IACS)
Thickness Correction
Vic-3D simulation, Inconel plates (σ = 1.33 %IACS)
ao = 4.5 mm, ai = 2.25 mm, h = 2.25 mm
1.0
1.1
1.2
1.3
1.4
0.1 1 10
Frequency [MHz]
Conductivity [%IACS]
1.0 mm
1.5 mm
2.0 mm
2.5 mm
3.0 mm
3.5 mm
4.0 mm
5.0 mm
6.0 mm
thickness

26. Coating Thickness

27. Non-conducting Coating
probe coil, ao t d ℓ
conducting substrate
non-conducting
coating
ao > t, d > δ, AECL = ℓ + t
ao = 4 mm, simulated
ao = 4 mm, experimental
lift-off:
-10 10 20 30 40 50 60 70 80
0.1 1
100
Frequency [MHz]
AECL [μm]
63.5 μm
50.8 μm
38.1 μm
25.4 μm
19.1 μm
12.7 μm
6.4 μm
0 μm

28. conducting substrate (µs,σs)
Conducting Coating
probe coil, ao t d ℓ
conducting substrate (µs,σs)
conducting
coating
z = δe Je z
approximate: large transducer, weak perturbation
equivalent depth:
analytical: Fourier decomposition (Dodd and Deeds)
numerical: finite element, finite difference, volume integral, etc.
(Vic-3D, Opera 3D, etc.)

29. Simplistic Inversion of AECC Spectra
0.254-mm-thick surface layer of 1% excess conductivity
uniform
Gaussian

30. Flaw Detection

31. Impedance Diagram Normalized Resistance 0.1 0.3 0.5
0.2
0.4
0.6
0.8 1
0.1
0.3
0.5
conductivity
(frequency)
crack
depth
flawless
material ω1
lift-off
Normalized Reactance ω2
apparent eddy current conductivity (AECC) decreases
apparent eddy current lift-off (AECL) increases

32. Crack Contrast and Resolution
ao = 1 mm, ai = 0.75 mm, h = 1.5 mm
austenitic stainless steel, σ = 2.5 %IACS, μr = 1
Vic-3D simulation
f = 5 MHz, δ  0.19 mm
probe coil
crack
0.2
0.4
0.6
0.8 1 2 3 4 5
Flaw Length [mm]
Normalized AECC
-10% threshold
detection
threshold
semi-circular crack

33. Eddy Current Images of Small Fatigue Cracks
probe coil
crack
Al2024, mil crack
Ti-6Al-4V, mil-crack
0.5”  0.5”, 2 MHz, 0.060”-diameter coil

34. Crystallographic Texture
generally anisotropic
hexagonal (transversely isotropic)
cubic (isotropic) x1 x3 x2
basal plane θ
surface plane σn σm σM
σ1 conductivity normal to the basal plane
σ2 conductivity in the basal plane
θ polar angle from the normal of the basal plane
σm minimum conductivity in the surface plane
σM maximum conductivity in the surface plane
σa average conductivity in the surface plane

35. Electric “Birefringence” Due to Texture
500 kHz, racetrack coil
highly textured Ti-6Al-4V plate
equiaxed GTD-111
1.00
1.01
1.02
1.03
1.04
1.05 30 60 90
120
150
180
Azimuthal Angle [deg]
Conductivity [%IACS]
1.30
1.32
1.34
1.36
1.38
1.40 30 60 90
120
150
180
Azimuthal Angle [deg]
Conductivity [%IACS]

36. Grain Noise in Ti-6Al-4V 1”  1”, 2 MHz, 0.060”-diameter coil
as-received billet material
solution treated and annealed
heat-treated, coarse
heat-treated, very coarse
heat-treated, large colonies
equiaxed beta annealed

37. Eddy Current versus Acoustic Microscopy
1”  1”, coarse grained Ti-6Al-4V sample
5 MHz eddy current
40 MHz acoustic

38. Inhomogeneity AECC Images of Waspaloy and IN100 Specimens
homogeneous IN100
2.2”  1.1”, 6 MHz
conductivity range  %IACS
±0.4 % relative variation
inhomogeneous Waspaloy
4.2”  2.1”, 6 MHz
conductivity range  %IACS
±3 % relative variation

39. Conductivity Material Noise
as-forged Waspaloy
1.30
1.32
1.34
1.36
1.38
1.40
1.42
1.44
1.46
1.48
1.50
AECC [%IACS]
Spot 1 (1.441 %IACS)
Spot 2 (1.428 %IACS)
Spot 3 (1.395 %IACS)
Spot 4 (1.382% IACS)
0.1 1 10
Frequency [MHz]
no (average) frequency dependence

40. Magnetic Susceptibility Material Noise
1”  1”, stainless steel 304
f = 0.1 MHz, ΔAECC  6.4 %
f = 5 MHz, ΔAECC  0.8 %
intact
f = 0.1 MHz, ΔAECC  8.6 %
f = 5 MHz, ΔAECC  1.2 %
0.51×0.26×0.03 mm3 edm notch

41. 3.4 Special Example

42. Residual Stress Assessment10 6 2
intact (no residual stress)
with opposite residual stress
Fatigue Life [cycles] 4 8
500
1000
1500
endurance
limit
service load
life time
natural
increased
Alternating Stress [MPa]
Residual stresses have numerous origins that are highly variable.
Residual stresses relax at service temperatures.

43. Surface-Enhancement Techniques
Shot Peening (SP)
Laser Shock Peening (LSP)
Low-Plasticity Burnishing (LPB)
Depth [mm]
0.2
0.4
0.6
1.0
1.2
200 -
400
600
800
1000
Residual Stress [MPa]
SP Almen 12A
SP Almen 4A
LSP
LPB
Ti-6Al-4V
0.2
0.4
0.6
1.0
1.2
Depth [mm]
Cold Work [%] 40 30 20 10 50
SP Almen 12A
SP Almen 4A
LSP
LPB
Ti-6Al-4V

44. Piezoresistive Effect
parallel, normal, circular F d
Electroelastic Tensor:
-40
-20 20 40 60 80
Time [1 s/div]
Axial Stress [ksi]
1.397
1.398
1.399
1.4
1.401
1.402
1.403
Conductivity [%IACS]
IN 718, parallel
Isotropic Plane-Stress ( and ) :
Adiabatic Electroelastic Coefficients:

45. Material Types Ti-6Al-4V parallel -0.004 -0.002 0.002 0.004 tua / E
0.002
0.004
tua / E
Ds / s0
normal
parallel
-0.004
-0.002
0.002
0.004
-0.001
0.001
tua / E
Ds / s0
normal
Al 2024
parallel
-0.004
-0.002
0.002
0.004
-0.001
0.001
tua / E
Ds / s0
normal
Al 7075
Waspaloy
parallel
-0.004
-0.002
0.002
0.004
tua / E
Ds / s0
normal
IN718
parallel
-0.004
-0.002
0.002
0.004
tua / E
Ds / s0
normal
parallel
-0.004
-0.002
0.002
0.004
-0.001
0.001
tua / E
Ds / s0
normal
Copper

46. XRD and AECC Measurements
Waspaloy
-2000
-1500
-1000
-500
500
0.2
0.4
0.6
0.8
Depth [mm]
Residual Stress [MPa]
Almen 4A
Almen 8A
Almen 12A
Almen 16A -1 1 2 3
0.1 10
Frequency [MHz]
Conductivity Change [%] 20 30 40 50
Cold Work [%]
-2000
-1500
-1000
-500
500
0.2
0.4
0.6
0.8
Depth [mm]
Residual Stress [MPa]
Almen 4A
Almen 8A
Almen 12A
Almen 16A 10 20 30 40 50
0.2
0.4
0.6
0.8
Cold Work [%]
Almen 4A
Almen 8A
Almen 12A
Almen 16A
Depth [mm] -1 1 2 3
0.1 10
Frequency [MHz]
Conductivity Change [%]
Almen 4A
Almen 8A
Almen 12A
Almen 16A
before (solid circles) and after full relaxation for 24 hrs at 900 °C (empty circles)

47. Thermal Stress Relaxation in Waspaloy
Waspaloy, Almen 8A, repeated 24-hour heat treatments at increasing temperatures
0.1
0.16
0.25
0.4
0.63 1
1.6
2.5 4
6.3 10
Frequency [MHz]
0.2
0.3
0.5
0.6
Apparent Conductivity Change [% ]
intact
300 °C
350 °C
400 °C
450 °C
500 °C
550 °C
600 °C
650 °C
700 °C
750 °C
800 °C
850 °C
900 °C
The excess apparent conductivity gradually vanishes during thermal relaxation!

48. XRD versus Eddy Current
inversion of measured AECC in low-plasticity burnished Waspaloy
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.01
0.1 1 10
Frequency [MHz]
AECC Change [%]
eddy current
0.0
0.5
1.0
1.5
Depth [mm]
Cold Work [%] . 5 10 15 20
XRD
-1400
-1200
-1000
-800
-600
-400
-200
200 .
Residual Stress [MPa]
eddy current
XRD
0.0
0.5
1.0
1.5
Depth [mm]

49. XRD versus High-Frequency Eddy Current
shot peened IN100 specimens of Almen 4A, 8A and 12A peening intensity levels