3 Eddy Current NDE3.1Inspection Techniques3.2Instrumentation3.3Typical Applications3.4Special Example

3.1 Inspection Techniques

Coil Configurations~differential coilscoaxialrotatedparallel

Remote-Field Eddy Current Inspectionln(Hz)zlow 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.

Main Modes of OperationTimeSignalsingle-frequencytime-multiplexed multiple-frequencyfrequency-multiplexed multiple-frequency

pulsedTimeSignalTimeSignalTimeSignal

Nonlinear Harmonic Analysissingle frequency, linear responsenonlinear harmonic analysisferromagnetic phase(ferrite, martensite, etc.)

3.2 Eddy Current Instrumentation

Single-Frequency Operationlow-passfilterlow-passfilterdriveramplifier90 phaseshifterdisplayprobe coil(s)driverimpedancesprocessorphasebalanceV-gainH-gain VrVmVq

Nonlinear Harmonic Operationlow-passfilterlow-passfiltern dividerdriveramplifier90 phaseshifterdisplayprobe coil(s)driverimpedancesprocessorphasebalanceV-gainH-gain oscillatorVrVmVq

Specialized versus General Purpose*high-frequency application

Nortec 2000S systemAgilent 4294A system*frequency range*0.1 10 MHz0.1-80 MHzprobe coilthree pencil probessingle spiral coil relative accuracy 0.1-0.2% 0.05-0.1%frequency scanningmanualelectronicmeasurement time 50 minutes for 21 points 3 minutes for 81 points

Probe Considerationssensitivity thermal stabilityflexible, 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

Conductivity versus Alloying and Temper IACS = International Annealed Copper Standard IACS = 5.8107 -1m-1 at 20 CIACS = 1.724110-8 m

Apparent Eddy Current Conductivity high accuracy ( 0.1 %) controlled penetration depth

Lift-Off Curvatureinductive(low frequency)capacitive(high frequency)

Inductive Lift-Off Effect4 mm diameter8 mm diameter1.5 %IACS1.5 %IACS

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

3.3.2 Permeability

Magnetic SusceptibilityNormalized ResistanceNormalized Reactance231r = 4permeabilitymoderately high susceptibilitylow susceptibilityparamagnetic materials with small ferromagnetic phase contentincreasing magnetic susceptibility decreases the apparent eddy current conductivity (AECC)frequency(conductivity)

Magnetic Susceptibility versus Cold Workcold work (plastic deformation at room temperature) causesmartensitic (ferromagnetic) phase transformationin austenitic stainless steels

3.3.3 Metal Thickness

Thickness versus Normalized Impedancethickness loss due to corrosion, erosion, etc. scanningaluminum ( = 46 %IACS)

Thickness CorrectionVic-3D simulation, Inconel plates ( = 1.33 %IACS)ao = 4.5 mm, ai = 2.25 mm, h = 2.25 mm

3.3.4 Coating Thickness

Non-conducting Coating non-conductingcoatingao > t, d > , AECL = + t ao = 4 mm, simulatedlift-off:ao = 4 mm, experimental

Conducting Coating conductingcoatingapproximate:large transducer, weak perturbationequivalent depth:analytical:Fourier decomposition (Dodd and Deeds)numerical:finite element, finite difference, volume integral, etc.(Vic-3D, Opera 3D, etc.)zJez = e

Simplistic Inversion of AECC Spectra0.254-mm-thick surface layer of 1% excess conductivity

3.3.5 Flaw Detection

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

Crack Contrast and Resolutionsemi-circular crack-10% thresholddetectionthreshold

Eddy Current Images of Small Fatigue Cracksprobe coilcrack

Crystallographic Texture1conductivity normal to the basal plane2conductivity in the basal planepolar angle from the normal of the basal planemminimum conductivity in the surface planeMmaximum conductivity in the surface planeaaverage conductivity in the surface plane

Electric Birefringence Due to Texture highly textured Ti-6Al-4V plateequiaxed GTD-111500 kHz, racetrack coil

Grain Noise in Ti-6Al-4V1 1, 2 MHz, 0.060-diameter coil

Eddy Current versus Acoustic Microscopy1 1, coarse grained Ti-6Al-4V sample

InhomogeneityAECC Images of Waspaloy and IN100 Specimens

Conductivity Material NoiseFrequency [MHz]AECC [%IACS]as-forged Waspaloyno (average) frequency dependence

Magnetic Susceptibility Material Noise1 1, stainless steel 304

3.4 Special Example

Residual Stress Assessment

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

Piezoresistive EffectElectroelastic Tensor:Adiabatic Electroelastic Coefficients:

Material Types

XRD and AECC Measurementsbefore (solid circles) and after full relaxation for 24 hrs at 900 C (empty circles)Waspaloy

Thermal Stress Relaxation in WaspaloyWaspaloy, Almen 8A, repeated 24-hour heat treatments at increasing temperatures

XRD versus Eddy Current.Depth [mm]Residual Stress [MPa] inversion of measured AECC in low-plasticity burnished Waspaloy

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