Eddy Current

3 Eddy Current NDE

3.1 Inspection Techniques

3.2 Instrumentation

3.3 Typical Applications

3.4 Special Example

3.1 Inspection Techniques

Coil Configurationsvoltmeter

testpiece

oscillator

excitationcoil

sensing coil

~

voltmeter

testpiece

oscillator

coil

Zo

~

Hall or GMR detector

voltmeter

testpiece

oscillator

excitationcoil

~~

differential coils

coaxial rotatedparallel

Remote-Field Eddy Current Inspection

Remote Field Remote FieldNear Field

exciter coilferromagnetic pipe sensing coil

ln(Hz)

z

low frequency operation (10-100 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.

0

1

rf

/0

zz zH H e

TimeS

igna

l

Main Modes of Operation

single-frequency time-multiplexed multiple-frequency

frequency-multiplexed multiple-frequencypulsed

Time

Sig

nal

Time

Sig

nal

Time

Sig

nal

excited signal (current) detected signal (voltage)

2D

Nonlinear Harmonic Analysis

single frequency, linear response

nonlinear harmonic analysis

Time

Sig

nal

Time

Sig

nal

H

B

ferromagnetic phase(ferrite, martensite, etc.)

3.2 Eddy Current Instrumentation

Single-Frequency Operation

low-passfilter

low-passfilter

oscillatordriver

amplifier

+

_

90º phaseshifter

A/Dconverter

display

probe coil(s)

driverimpedances

processorphase

balanceV-gainH-gain

Vr

Vm

Vq

m s s r o q ocos( ), cos( ), sin( )V V t V V t V V t

m r s s o s o s s1

cos( ) cos( ) cos( ) cos(2 )2

V V V t V t V V t

m q s s o s o s s1

cos( ) sin( ) sin( ) sin(2 )2

V V V t V t V V t

o om r s s m q s scos( ), sin( )

2 2

V VV V V V V V

Nonlinear Harmonic Operation

low-passfilter

low-passfilter

n dividerdriver

amplifier

+

_

90º phaseshifter

A/Dconverter

display

probe coil(s)

driverimpedances

processorphase

balanceV-gainH-gain

oscillatorVr

Vm

Vq

m s1 s1 s2 s2 s3 s3cos( ) cos(2 ) cos(3 ) ...V V t V t V t

r o cos( )V V n t om r s scos( )

2 n nV

V V V

q o sin( )V V n t om q s ssin( )

2 n nV

V V V

Specialized versus General Purpose

Nortec 2000S system Agilent 4294A system*

frequency range* 0.1 – 10 MHz 0.1-80 MHz

probe coil three pencil probes single spiral coil

relative accuracy ≈ 0.1-0.2% ≈ 0.05-0.1%

frequency scanning manual electronic

measurement time ≈ 50 minutes for 21 points ≈ 3 minutes for 81 points

*high-frequency application

I1

V2

11

V1

I2

2212 21,

Probe Considerations

V Z I

*wireZ i L R

sensitivity

thermal stability

eddy current

ferrite-core coil

high coupling

high coupling

eddy current

air-core coil

high coupling

low coupling

eddy current

flat air-core coilhigh coupling

flexible, low self-capacitance, reproducible, interchangeable, economic, etc.

I

V

1 11 12 1

2 12 22 2

V Z Z I

V Z Z I

*

12 12Z i L

topology

3.3 Eddy Current NDE Applications

• conductivity measurement• permeability measurement• metal thickness measurement• coating thickness measurements• flaw detection

3.3.1 Conductivity

Conductivity versus Probe Impedance constant frequency

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5Normalized Resistance

Nor

mal

ized

Rea

ctan

ce

Stainless Steel, 304

CopperAluminum, 7075-T6

Titanium, 6Al-4V

Magnesium, A280

Lead

Copper 70%,Nickel 30%

Inconel

Nickel

Conductivity versus Alloying and Temper IACS = International Annealed Copper Standard

σIACS = 5.8107 Ω-1m-1 at 20 °C

ρIACS = 1.724110-8 Ωm

20

30

40

50

60

Con

duct

ivit

y [%

IA

CS

]

T3 T4

T6

T0

2014

T4

T6T0

6061

T6

T73T76

T0

70752024

T3 T4

T6

T72

T8

T0

Various Aluminum Alloys

Apparent Eddy Current Conductivity

• high accuracy ( 0.1 %)

• controlled penetration depth

specimen

eddy currents

probe coil

magnetic field

0

0.2

0.4

0.6

0.8

1.0

0.10 0.2 0.3 0.4 0.5

lift-offcurves

conductivity

curve(frequency)

Normalized Resistance

Nor

mal

ized

Rea

ctan

ce

= 0

= s

1

23

4


Nor

mal

ized

Rea

ctan

ce

Lift-Off Curvature

inductive(low frequency)

capacitive(high frequency)

“Horizontal” Component

“Ver

tica

l” C

ompo

nent

lift-off

.

conductivity

σ2

σ1

σ

ℓ = s ℓ = 0

“Horizontal” Component

“Ver

tica

l” C

ompo

nent

.

conductivity

lift-off

σ2

σ1

σ

ℓ = s ℓ = 0

Inductive Lift-Off Effect

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0.1 1 10 100Frequency [MHz]

Rel

ativ

e Δ

AE

CC

[%

] .

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0


Rel

ativ

e Δ

AE

CC

[%

] .

63.5 μm

50.8 μm

38.1 μm

25.4 μm

19.1 μm

12.7 μm

6.4 μm

0.0 μm

-10

0

10

20

30

40

50

60

70

80


AE

CL

[μ

m]

.

-10

0

10

20

30

40

50

60

70

80


AE

CL

[μ

m]

. .

63.5 μm

50.8 μm

38.1 μm

25.4 μm

19.1 μm

12.7 μm

6.4 μm

0.0 μm

4 mm diameter 8 mm diameter

1.5 %IACS 1.5 %IACS

Instrument Calibration

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0


AE

CC

Cha

nge

[%]

. 12A Nortec 8A Nortec 4A Nortec 12A Agilent 8A Agilent 4A Agilent 12A UniWest 8A UniWest 4A UniWest 12A Stanford 8A Stanford 4A Stanford

Nortec 2000S, Agilent 4294A, Stanford Research SR844, and UniWest US-450

conductivity spectra comparison on IN718 specimens of different peening intensities.

3.3.2 Permeability

Magnetic Susceptibility

0

0.2

0.4

0.6

0.8

1.0

0.10 0.2 0.3 0.4 0.5

lift-off

frequency(conductivity)

Normalized ResistanceN

orm

aliz

ed R

eact

ance

permeability


Nor

mal

ized

Rea

ctan

ce

0

1

2

3

4

0 0.2 0.4 0.6 0.8 1 1.2

2

3

1

µr = 4permeability

moderately high susceptibility low susceptibility

paramagnetic materials with small ferromagnetic phase content

increasing magnetic susceptibility decreases the apparent eddy current conductivity (AECC)

frequency(conductivity)

Magnetic Susceptibility versus Cold Work

10-4

10-3

10-2

10-1

100

101

0 10 20 30 40 50 60Cold Work [%]

Mag

neti

c S

usce

ptib

ilit

y

SS304L

IN276

IN718

SS305

SS304SS302

IN625

cold work (plastic deformation at room temperature) causesmartensitic (ferromagnetic) phase transformation

in austenitic stainless steels

3.3.3 Metal Thickness

Thickness versus Normalized Impedance

thickness loss due to corrosion, erosion, etc.

probe coil

scanning

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5 0.6

thickplate


Nor

mal

ized

Rea

ctan

ce

thinplate

lift-off

thinning

-0.2

0

0.2

0.4

0.6

0.8

1

0 1 2 3Depth [mm]

Re

{ F

}

f = 0.05 MHz f = 0.2 MHz f = 1 MHz

aluminum (σ = 46 %IACS)

/ /( ) x i xF x e e

Thickness Correction

1.0

1.1

1.2

1.3

1.4

0.1 1 10

Frequency [MHz]

Con

duct

ivit

y [%

IAC

S]

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

Vic-3D simulation, Inconel plates (σ = 1.33 %IACS)

ao = 4.5 mm, ai = 2.25 mm, h = 2.25 mm

3.3.4 Coating Thickness

Non-conducting Coating

non-conductingcoating

probe coil, ao

t

d

ℓ

conducting substrate

ao > t, d > δ, AECL = ℓ + t

-10

010

2030

4050

6070

80


AE

CL

[μm

]

-10

010

2030

4050

6070

80


AE

CL

[μm

]

63.5 μm

50.8 μm

38.1 μm

25.4 μm

19.1 μm

12.7 μm

6.4 μm

0 μm

ao = 4 mm, simulated

lift-off:

ao = 4 mm, experimental

Conducting Coating

conductingcoating

probe coil, ao

t

d

ℓ

conducting substrate (µs,σs)

approximate: large transducer, weak perturbation

equivalent depth:

e1

AECC( )2 s s

ff

21

( ) AECC4 s s

zz

se 2

analytical: Fourier decomposition (Dodd and Deeds)

numerical: finite element, finite difference, volume integral, etc.

(Vic-3D, Opera 3D, etc.)

zJe

z = δe

Simplistic Inversion of AECC Spectra

AE

CC

Cha

nge

[%]

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.001 0.1 10 1000

Frequency [MHz]

AE

CC

Cha

nge

[%]

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.001 0.1 10 1000

Frequency [MHz]

uniform

Gaussian

0.254-mm-thick surface layer of 1% excess conductivity

3.3.5 Flaw Detection

Impedance Diagram


0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5

conductivity(frequency)

crackdepth

flawlessmaterial

ω1

lift-off

Nor

mal

ized

Rea

ctan

ce

ω2

apparent eddy current conductivity (AECC) decreasesapparent eddy current lift-off (AECL) increases

Crack Contrast and Resolution

probe coil

crack

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5Flaw Length [mm]

Nor

mal

ized

AE

CC

semi-circular crack

-10% threshold

detectionthreshold

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

Eddy Current Images of Small Fatigue Cracks

Al2024, 0.025-mil crack Ti-6Al-4V, 0.026-mil-crack

0.5” 0.5”, 2 MHz, 0.060”-diameter coil

probe coil

crack

Crystallographic Texture J E

1 1 1

2 2 2

3 3 3

0 0

0 0

0 0

J E

J E

J E

generally anisotropic hexagonal (transversely isotropic)

1 1 1

2 2 2

3 2 3

0 0

0 0

0 0

J E

J E

J E

cubic (isotropic)

1 1 1

2 1 2

3 1 3

0 0

0 0

0 0

J E

J E

J E

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

2 2a 1 2( ) ½ [ sin (1 cos )]

2 2n 1 2( ) cos sin

M 2

1 2

2 2m 1 2( ) sin cos

x1

x3

x2basal plane

θ

surface plane

σn

σm

σM

Electric “Birefringence” Due to Texture

1.00

1.01

1.02

1.03

1.04

1.05

0 30 60 90 120 150 180

Azimuthal Angle [deg]

Con

duct

ivit

y [%

IAC

S]

highly textured Ti-6Al-4V plate equiaxed GTD-111

1.30

1.32

1.34

1.36

1.38

1.40

0 30 60 90 120 150 180

Azimuthal Angle [deg]

Con

duct

ivit

y [%

IAC

S]

500 kHz, racetrack coil

Grain Noise in Ti-6Al-4V

as-received billet material solution treated and annealed

heat-treated, coarse

heat-treated, very coarse heat-treated, large colonies equiaxed beta annealed

1” 1”, 2 MHz, 0.060”-diameter coil

Eddy Current versus Acoustic Microscopy

5 MHz eddy current 40 MHz acoustic

1” 1”, coarse grained Ti-6Al-4V sample

InhomogeneityAECC Images of Waspaloy and IN100 Specimens

homogeneous IN100

2.2” 1.1”, 6 MHz

conductivity range 1.33-1.34 %IACS

±0.4 % relative variation

inhomogeneous Waspaloy

4.2” 2.1”, 6 MHz

conductivity range 1.38-1.47 %IACS

±3 % relative variation

Conductivity Material Noise

1.30

1.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

1.48

1.50

0.1 1 10Frequency [MHz]

AE

CC

[%

IAC

S]

Spot 1 (1.441 %IACS)



Spot 4 (1.382% IACS)

as-forged Waspaloy

no (average) frequency dependence

Magnetic Susceptibility Material Noise1” 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

3.4 Special Example

Residual Stress Assessment

106102

intact (no residual stress)

with opposite residual stress

Fatigue Life [cycles]

104 1080

500

1000

1500

endurancelimit

service load

life timenatural

life timeincreased

Alt

erna

ting

Str

ess

[MP

a]

Residual stresses have numerous origins that are highly variable.Residual stresses relax at service temperatures.

Surface-Enhancement TechniquesLow-Plasticity Burnishing (LPB)Shot Peening (SP) Laser Shock Peening (LSP)

Depth [mm]0 0.2 0.4 0.6 1.0 1.2

200

0

-200

-400

-600

-800

-1000

Res

idua

l Str

ess

[MP

a]

SP Almen 12ASP Almen 4A

LSPLPB

Ti-6Al-4V

0 0.2 0.4 0.6 1.0 1.2Depth [mm]

Col

d W

ork

[%] 40

30

20

10

0

50

SP Almen 12ASP Almen 4A

LSPLPB

Ti-6Al-4V

Piezoresistive Effect

Electroelastic Tensor:

1 0 11 12 12 1

2 0 12 11 12 2

3 0 12 12 11 3

/ /

/ /

/ /

E

E

E

11 120/

/a

ipip E

Isotropic Plane-Stress ( and ) :1 2 ip 3 0

parallel, normal, circular

F F

Adiabatic Electroelastic Coefficients:*11 11 th *12 12 th

-40-20

020406080

Time [1 s/div]

Axial Stress [ksi]

Time [1 s/div]1.3971.3981.399

1.41.4011.4021.403

Conductivity [%IACS]

IN 718, parallel

Material Types

parallel

-0.004

-0.002

0

0.002

0.004

-0.001 0 0.001 0.002

ua / E

normal

Copper

Ti-6Al-4V

parallel

-0.004

-0.002

0

0.002

0.004

-0.002 0 0.002 0.004

ua / E

normalparallel

-0.004

-0.002

0

0.002

0.004

-0.001 0 0.001 0.002

ua / E

normal

Al 2024

parallel

-0.004

-0.002

0

0.002

0.004

-0.001 0 0.001 0.002

ua / E

normal

Al 7075

Waspaloy

parallel

-0.004

-0.002

0

0.002

0.004

-0.002 0 0.002 0.004

ua / E

normal

IN718

parallel

-0.004

-0.002

0

0.002

0.004

-0.002 0 0.002 0.004

ua / E

normal

XRD and AECC Measurements

-2000

-1500

-1000

-500

0

500

0 0.2 0.4 0.6 0.8Depth [mm]

Res

idua

l Str

ess

[MP

a]

Almen 4A Almen 8A Almen 12A

Almen 16A

-1

0

1

2

3


Con

duct

ivit

y C

hang

e [%

] Almen 4A Almen 8A Almen 12A

Almen 16A

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8C

old

Wor

k [%

]


Almen 16A

Depth [mm]

before (solid circles) and after full relaxation for 24 hrs at 900 °C (empty circles)

-2000

-1500

-1000

-500

0

500

0 0.2 0.4 0.6 0.8Depth [mm]

Res

idua

l Str

ess

[MP

a]


Almen 16A0

10

20

30

40

50

0 0.2 0.4 0.6 0.8

Col

d W

ork

[%]


Almen 16A

Depth [mm]

-1

0

1

2

3


Con

duct

ivit

y C

hang

e [%

] Almen 4A Almen 8A Almen 12A

Almen 16A

Waspaloy

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

0.1

0.2

0.3

0.4

0.5

0.6

App

aren

t Con

duct

ivit

y C

hang

e [%

] 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!

XRD versus Eddy Current

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.01 0.1 1 10Frequency [MHz]

AE

CC

Cha

nge

[%]

eddy current

0.0 0.5 1.0 1.5Depth [mm]

Col

d W

ork

[%]

.

0

5

10

15

20

XRD

.

-1400

-1200

-1000

-800

-600

-400

-200

0

200

0.0 0.5 1.0 1.5Depth [mm]

Res

idua

l Str

ess

[MP

a]

eddy current XRD

inversion of measured AECC in low-plasticity burnished Waspaloy

0

10

20

30

40

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Depth [mm]

Col

d W

ork

[%]

.

Almen 4A (XRD)

Almen 8A (XRD)

Almen 12A (XRD)

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Depth [mm]

Res

idua

l Str

ess

[MP

a]

.

Almen 4A (AECC)

Almen 8A (AECC)

Almen 12A (AECC)

Almen 4A (XRD)

Almen 8A (XRD)

Almen 12A (XRD)

50 MHz

XRD versus High-Frequency Eddy Current

shot peened IN100 specimens of Almen 4A, 8A and 12A peening intensity levels

Eddy Current

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