3 Eddy Current NDE 3 1 Inspection Techniques

3 Eddy Current NDE 3. 1 Inspection Techniques 3. 2 Instrumentation 3. 3 Typical Applications 3. 4 Special Example

3. 1 Inspection Techniques

Coil Configurations voltmeter oscillator excitation coil voltmeter ~~ oscillator ~ ~ Zo coil sensing coil testpiece Hall or GMR detector testpiece differential coils parallel coaxial rotated

Remote-Field Eddy Current Inspection ferromagnetic pipe exciter coil Remote Field Near Field sensing coil Remote Field ln(Hz) 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. z

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

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

3. 2 Eddy Current Instrumentation

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

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

Specialized versus General Purpose Nortec 2000 S system Agilent 4294 A 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

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

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 1 Titanium, 6 Al-4 V Normalized Reactance 0. 8 Inconel Stainless Steel, 304 0. 6 Copper 70%, Nickel 30% 0. 4 Lead 0. 2 Copper Magnesium, A 280 Nickel Aluminum, 7075 -T 6 0 0 0. 1 0. 2 0. 3 Normalized Resistance 0. 4 0. 5

Conductivity versus Alloying and Temper IACS = International Annealed Copper Standard σIACS = 5. 8 107 Ω-1 m-1 at 20 °C ρIACS = 1. 7241 10 -8 Ωm 60 Conductivity [% IACS] 2014 2024 7075 6061 50 T 0 40 T 6 T 72 T 6 30 T 6 T 8 T 0 T 73 T 76 T 4 T 3 T 4 20 Various Aluminum Alloys T 6

Apparent Eddy Current Conductivity magnetic field probe coil specimen Normalized Reactance 1. 0 0. 8 lift-off curves 0. 6 0. 4 conductivity (frequency) curve 0. 2 0 0 eddy currents 0. 1 0. 2 0. 3 0. 4 Normalized Resistance • high accuracy ( 0. 1 %) • controlled penetration depth Normalized Reactance s = s 2 4 s = s 1 3 l= 0 l= s s, l 2 1 Normalized Resistance 0. 5

Lift-Off Curvature inductive (low frequency) lift-off ℓ =0 ℓ =s lift-off ℓ =0 σ2 σ2 conductivity σ σ1 “Vertical” Component. ℓ =s capacitive (high frequency) conductivity σ σ1 “Horizontal” Component

Inductive Lift-Off Effect 4 mm diameter 1. 5 %IACS 8 mm diameter 1. 5 %IACS

Instrument Calibration conductivity spectra comparison on IN 718 specimens of different peening intensities. Nortec 2000 S, Agilent 4294 A, Stanford Research SR 844, and Uni. West US-450

3. 3. 2 Permeability

Magnetic Susceptibility paramagnetic materials with small ferromagnetic phase content moderately high susceptibility low susceptibility 1. 0 4 µr = 4 3 permeability 3 2 2 1 frequency (conductivity) 1 0 Normalized Reactance permeability 0. 8 lift-off 0. 6 frequency (conductivity) 0. 4 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 1 Normalized Resistance 1. 2 0 0. 1 0. 2 0. 3 0. 4 Normalized Resistance increasing magnetic susceptibility decreases the apparent eddy current conductivity (AECC) 0. 5

Magnetic Susceptibility versus Cold Work cold work (plastic deformation at room temperature) causes martensitic (ferromagnetic) phase transformation in austenitic stainless steels Magnetic Susceptibility 101 SS 304 L SS 302 SS 304 100 10 -1 10 -2 SS 305 10 -3 IN 718 IN 625 IN 276 10 -4 0 10 20 30 Cold Work [%] 40 50 60

3. 3. 3 Metal Thickness

Thickness versus Normalized Impedance scanning probe coil thickness loss due to corrosion, etc. 1 0. 8 1 thinning 0. 6 0. 4 thick plate 0. 2 f = 0. 05 MHz f = 0. 2 MHz f = 1 MHz 0. 8 lift-off Re { F } Normalized Reactance aluminum (σ = 46 %IACS) 0. 6 0. 4 0. 2 thin plate 0 -0. 2 0 0. 1 0. 2 0. 3 0. 4 0. 5 Normalized Resistance 0. 6 1 2 Depth [mm] 3

Thickness Correction Vic-3 D simulation, Inconel plates (σ = 1. 33 %IACS) ao = 4. 5 mm, ai = 2. 25 mm, h = 2. 25 mm Conductivity [%IACS] 1. 4 1. 3 thickness 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 1. 2 1. 1 1. 0 0. 1 1 Frequency [MHz] 10

3. 3. 4 Coating Thickness

Non-conducting Coating probe coil, ao non-conducting coating ℓ t d conducting substrate ao > t, d > δ, AECL = ℓ + t ao = 4 mm, simulated 63. 5 μm 50. 8 μm 38. 1 μm 25. 4 μm 19. 1 μm 12. 7 μm 6. 4 μm 0 μm 1 10 100 Frequency [MHz] 80 70 60 50 40 30 20 10 0 -10 0. 1 AECL [μm] lift-off: AECL [μm] 80 70 60 50 40 30 20 10 0 -10 0. 1 ao = 4 mm, experimental 1 10 100 Frequency [MHz]

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

Simplistic Inversion of AECC Spectra 0. 254 -mm-thick surface layer of 1% excess conductivity uniform Gaussian

3. 3. 5 Flaw Detection

Impedance Diagram 1 Normalized Reactance 0. 8 conductivity (frequency) lift-off 0. 6 crack depth ω1 flawless material 0. 4 ω2 0. 2 0 0 0. 1 0. 2 0. 3 0. 4 Normalized Resistance apparent eddy current conductivity (AECC) decreases apparent eddy current lift-off (AECL) increases 0. 5

Crack Contrast and Resolution Vic-3 D simulation ao = 1 mm, ai = 0. 75 mm, h = 1. 5 mm probe coil austenitic stainless steel, σ = 2. 5 %IACS, μr = 1 f = 5 MHz, δ 0. 19 mm crack 1 -10% threshold Normalized AECC 0. 8 0. 6 0. 4 0. 2 0 semi-circular crack detection threshold 0 1 2 3 Flaw Length [mm] 4 5

Eddy Current Images of Small Fatigue Cracks probe coil crack 0. 5”, 2 MHz, 0. 060”-diameter coil Al 2024, 0. 025 -mil crack Ti-6 Al-4 V, 0. 026 -mil-crack

Crystallographic Texture generally anisotropic hexagonal (transversely isotropic) cubic (isotropic) x 1 θ x 3 σM σn σm basal plane x 2 surface plane 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

Electric “Birefringence” Due to Texture 500 k. Hz, racetrack coil equiaxed GTD-111 1. 05 1. 40 1. 04 1. 38 Conductivity [%IACS] highly textured Ti-6 Al-4 V plate 1. 03 1. 02 1. 01 1. 00 1. 36 1. 34 1. 32 1. 30 0 30 60 90 120 150 180 Azimuthal Angle [deg]

Grain Noise in Ti-6 Al-4 V 1” 1”, 2 MHz, 0. 060”-diameter coil as-received billet material solution treated annealed heat-treated, coarse heat-treated, very coarse heat-treated, large colonies equiaxed beta annealed

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

Inhomogeneity AECC Images of Waspaloy and IN 100 Specimens inhomogeneous Waspaloy homogeneous IN 100 4. 2” 2. 1”, 6 MHz 2. 2” 1. 1”, 6 MHz conductivity range 1. 38 -1. 47 %IACS conductivity range 1. 33 -1. 34 %IACS ± 3 % relative variation ± 0. 4 % relative variation

Conductivity Material Noise as-forged Waspaloy 1. 50 1. 48 1. 46 AECC [%IACS] 1. 44 1. 42 1. 40 1. 38 1. 36 Spot 1 (1. 441 %IACS) 1. 34 Spot 2 (1. 428 %IACS) Spot 3 (1. 395 %IACS) 1. 32 Spot 4 (1. 382% IACS) 1. 30 0. 1 1 Frequency [MHz] no (average) frequency dependence 10

Magnetic Susceptibility Material Noise 1” 1”, stainless steel 304 intact 0. 51× 0. 26× 0. 03 mm 3 edm notch f = 0. 1 MHz, ΔAECC 6. 4 % f = 0. 1 MHz, ΔAECC 8. 6 % f = 5 MHz, ΔAECC 0. 8 % f = 5 MHz, ΔAECC 1. 2 %

3. 4 Special Example

Residual Stress Assessment Alternating Stress [MPa] 1500 1000 with opposite residual stress service load 500 intact (no residual stress) natural life time 0 10 2 endurance limit increased life time 10 4 10 6 Fatigue Life [cycles] 108 Residual stresses have numerous origins that are highly variable. Residual stresses relax at service temperatures.

Surface-Enhancement Techniques Laser Shock Peening (LSP) 200 50 0 40 Cold Work [%] Residual Stress [MPa] Shot Peening (SP) -200 -400 Ti-6 Al-4 V SP Almen 4 A SP Almen 12 A LSP LPB -600 -800 -1000 0 0. 2 0. 4 0. 6 Depth [mm] 1. 0 Low-Plasticity Burnishing (LPB) Ti-6 Al-4 V SP Almen 4 A SP Almen 12 A LSP LPB 30 20 10 1. 2 0 0 0. 2 0. 4 0. 6 Depth [mm] 1. 0 1. 2

Piezoresistive Effect parallel, normal, circular F Electroelastic Tensor: F Adiabatic Electroelastic Coefficients: and ): Conductivity [%IACS] Isotropic Plane-Stress ( Axial Stress [ksi] d 80 60 40 20 0 -20 -40 1. 403 1. 402 1. 401 1. 4 1. 399 1. 398 1. 397 Time [1 s/div] IN 718, parallel Time [1 s/div]

Material Types Al 2024 Ti-6 Al-4 V 0 0 parallel normal 0 -0. 002 -0. 004 -0. 001 0 0. 002 0. 004 tua / E 0. 002 0. 004 parallel normal 0. 002 Ds / s 0 0. 004 0 0 0. 001 0. 002 tua / E IN 718 Waspaloy Ds / s 0 0. 002 0. 004 parallel normal 0 0. 002 parallel normal 0 -0. 002 -0. 004 -0. 001 0 0. 002 0. 004 tua / E 0 0. 001 0. 002 tua / E Copper Ds / s 0 0. 002 0. 004 parallel normal Ds / s 0 0. 004 Al 7075 0 0. 001 0. 002 tua / E

XRD and AECC Measurements Waspaloy Almen 4 A Almen 8 A Almen 12 A Almen 16 A -1000 -1500 0 0. 2 0. 4 0. 6 Depth [mm] 30 10 0 0. 8 500 50 0 40 -500 Almen 4 A Almen 8 A Almen 12 A Almen 16 A -1000 -1500 0 0. 2 0. 4 0. 6 Depth [mm] Almen 4 A Almen 8 A Almen 12 A Almen 16 A 20 0 0. 2 1 0 -1 0. 8 30 Almen 4 A Almen 8 A Almen 12 A Almen 16 A 20 0 2 Almen 4 A Almen 8 A Almen 12 A Almen 16 A 1 Frequency [MHz] 10 3 10 0. 8 0. 4 0. 6 Depth [mm] Conductivity Change [%] Cold Work [%] 40 -500 -2000 3 Conductivity Change [%] 0 -2000 Residual Stress [MPa] 50 Cold Work [%] Residual Stress [MPa] 500 0 0. 2 0. 4 0. 6 Depth [mm] 0. 8 2 1 Almen 4 A Almen 8 A Almen 12 A Almen 16 A 0 -1 0. 1 1 Frequency [MHz] before (solid circles) and after full relaxation for 24 hrs at 900 °C (empty circles) 10

Thermal Stress Relaxation in Waspaloy, Almen 8 A, repeated 24 -hour heat treatments at increasing temperatures Apparent Conductivity Change [% ] 0. 6 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 0. 5 0. 4 0. 3 0. 2 0. 16 0. 25 0. 4 0. 63 1 1. 6 2. 5 4 6. 3 Frequency [MHz] The excess apparent conductivity gradually vanishes during thermal relaxation! 10

XRD versus Eddy Current inversion of measured AECC in low-plasticity burnished Waspaloy 20 1. 2 eddy current XRD 0 . . Residual Stress [MPa] 15 0. 8 Cold Work [%] AECC Change [%] 1. 0 200 0. 6 0. 4 0. 2 10 5 0. 0 -0. 2 0. 01 -200 -400 -600 -800 -1000 XRD eddy current -1200 0. 1 1 Frequency [MHz] 10 0 0. 5 1. 0 Depth [mm] 1. 5 -1400 0. 5 1. 0 Depth [mm] 1. 5

XRD versus High-Frequency Eddy Current shot peened IN 100 specimens of Almen 4 A, 8 A and 12 A peening intensity levels