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 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

Remote-Field Eddy Current Inspection exciter coil ferromagnetic pipe sensing coil Remote Field Near 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 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)

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

3.2 Eddy Current Instrumentation

Single-Frequency Operation Vr 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

Nonlinear Harmonic Operation Vr 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

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

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 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

Conductivity versus Alloying and Temper IACS = International Annealed Copper Standard σIACS = 5.8107 Ω-1m-1 at 20 °C ρIACS = 1.7241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

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

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

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

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

3.3.2 Permeability

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)

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

3.3.3 Metal Thickness

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

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

3.3.4 Coating Thickness

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

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.)

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

3.3.5 Flaw Detection

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

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

Eddy Current Images of Small Fatigue Cracks probe coil crack Al2024, 0.025-mil crack Ti-6Al-4V, 0.026-mil-crack 0.5”  0.5”, 2 MHz, 0.060”-diameter coil

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

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]

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

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

Inhomogeneity AECC 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 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

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

3.4 Special Example

Residual Stress Assessment 10 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.

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

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:

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

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)

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!

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]

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