EM Lecture Spies Part2

Eddy Eddy Eddy Eddy CurrentCurrentCurrentCurrent TestingTestingTestingTesting

Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.

Eddy-Current Testing 1. Principle

Eddy-Current Testing 2. Eddy-current probes

Eddy-Current Testing 3. Signal interpretation

Eddy-Current Testing 4. Equipment

Eddy-Current Testing 5. Aircraft inspection

Eddy-Current Testing 6. Material characterisation

Eddy Eddy Eddy Eddy CurrentCurrentCurrentCurrent TestingTestingTestingTesting ---- ApplicationsApplicationsApplicationsApplications

Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS

Materials under Test:

Typical Applications:

Ferromagnetic and / or electrically conductive (metals, ...)

Defect Detection (Subsurface and Surface Cracks,Pores, Inclusions…)

Material Properties (Conductivity σ, Permeability µ,Hardness, Hardness Depth, ...)

Geometry (Wall/Layer/Film Thickness, …)

NDT&E of Materials Using Eddy Current Technique

Eddy Current (EC) Technique: General notes

- Time Alternating Field

Eddy Current (EC) Technique: Basic sensor arrangement

Transmitter coil

Receiver coili(t)=I⋅⋅⋅⋅cos(ωωωωt)

Specimen under test

- EC operating frequency

- electrical conductivity

- magnetic permeability

- sensor impedance

ω = 2πf

−σSensorLift Off

- Wall / LayerThickness

+ Wall / LayerThickness

µ Permeabilityσ Conductivity

Eddy Current (EC) Technique: Signal influencing variables

0.1 1 10 100 1000

CopperAluminumFerritic steel (µ=50)Austenitic steel (µ=1)

Eddy Current Standard Penetration Depth

Frequency (kHz)

Penetration depth(mm)

Penetration depth (mm)

Sensor & measurement hardware:

- noise level, dynamic range

- spatial resolution of the sensor

Sensitivity limitations

Presence of disturbing influences (application-specific):

- sensor lift-off / tilt

- inhomogenous geometry of the test specimen

(surface roughness, edges, ... )

- local variations of material properties (σ, µ) in the test specimen- . . .

The optimal sensor design strongly depends on the application

detectable defects detectable defect

Higher sensitivityBetter spatial resolution

Reduced inspection timeEasy operation

Inspectiontrace

Quantitative NDT using the EC technique

Demands on the inspection system:

Proper sensor design

High signal dynamic range and long-time operation stability

of the hardware

Efficient signal processing algorithm

in order to evaluate small signal changes

by large disturbing signals

Multiple eddy current operating frequencies (EC frequencies)

in order to extend the information content

Measurement dataobtained on calibration samples

Known values of targetfunction

Regressionanalysis

Filtercoefficients

Recalculation(numerical filtering)

Measurement data obtained on with known

values of target functiontest samples

Filtering results: target functionevaluation error

Calibration

Verification

NDE application

Filter coefficients

Measurement data obtainedon items to be tested

Filter coefficients

Recalculation(numerical filtering)

Filtering results: target function

values

Generation and Application of the Numerical Filtersfor Quantitative Multifrequency EC technique

Eddy Current Hardware

WS98 Board

Features

EC frequencies 10 Hz - 10 MHz

Operation with multiple frequencies

High long-time operation stability

16 Bit A/D-conversion, > 85 dB dynamic range

Digital Signal Processor with realtime algorithms

Ethernet interface

100 mm

64-Channel Eddy-CurrentInspection System

Front End / Main Electronics

Application examples

Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden Corrosion

Calculation of the Conductivity Gradient:Numerical Modeling of the Inverse Problem

EC Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden Corrosion

Inspection task:

Solution:

Non-destructive quantitative evaluation

of the corrosion damage depths

(or remaining thickness of aluminium)

in each layer of the bonded

multilayer structure

Multifrequency EC technique

0.7... 1.0mm

0.2... 0.4mm

EC sensor

Aluminium

Adhesion

corrosion

Calibration specimen

Aluminium sheet

thickness 0.7 mm

Shallow pits

with various depths

0.5 0.4 0.3 0.2 0.1 mm

0.3 mm

0.7 0.6 0.4 0.2 mm

corrosion depth [mm]

Results

Inspection situation:corrosion in the 1st and 3rdaluminium layer

Target function:corrosion in the1st aluminium layer

EC probe

Aluminium

Adhesion

corrosion

Inspection situation:corrosion in the 1st and 3rdaluminium layer

Target function:corrosion in the3rd aluminium layer

corrosion depth [mm]

Results

EC probe

Aluminium

Adhesion

corrosion

Determination of Layer Thickness using EC Technique: Increasing the Accuracy

Application-optimised sensor arrangement

Conclusion

Efficient signal processing & data interpretation

Barkhausen Barkhausen Barkhausen Barkhausen NoiseNoiseNoiseNoise and Eddy and Eddy and Eddy and Eddy CurrentCurrentCurrentCurrent MicroscopyMicroscopyMicroscopyMicroscopy

A A A A ScanningScanningScanningScanning Probe Probe Probe Probe TechniqueTechniqueTechniqueTechniqueforforforfor MicroscaleMicroscaleMicroscaleMicroscale Material Material Material Material CharacterizationCharacterizationCharacterizationCharacterization

BEMI - Introduction

BEMI provides high-resolution characterization of…• residual stress• coating thickness and homogeneity• microstructure• electrical / magnetic surface properties

features / advantages• nondestructive, even for coatings as thin as 25 nm• high local resolution (10 µm)

• high accuracy (coating thickness: ≤ 10 nm)• wide range of coating and substrate materials• contactless scanning option

Measured Quantities: Magnetic Barkhausen Noise

magnetic field strength

Barkhausen noiseamplitude

compressive stress

tensile stress• observed for ferromagnetic materials underalternating field magnetization (< 1 kHz)

• mainly caused by 180° Bloch wall jumps

� stress-dependent� microstructure-dependent� sensitive to lattice defects

Measured Quantities: Eddy Current Influenced Impedance

• established NDT method

� characterizes conductivity and permeability� suitable for coating thickness determination� sensitive to microstructure

ωLωL0

lift-off

surface defects

permeability changes

sub-surface defects

wall thickness variations

conductivity changes

ω→∞

BEMI Testing Device

• scanner control• eddy current hardware• barkhausen noise hardware

precision 3-d scanner

sensorsensor

samplesample

controlling PCcontrolling PC

sensor element:miniaturized inductive probe(modified VCR head)

• picks up Barkhausen noise• induces eddy currents

stationary electromagnet(Barkhausen only)

Fields of Application

Spatially High-Resolved Characterization of• Residual stress• Coating thickness and homogeneity• Electrical / magnetic surface properties

Advantages• High spatial resolution (≤ 10 µm)• Quick (0.2 – 2 s / position) and versatile• Coating thickness accuracy ≤ 10 nm• Multi-parameter target calibration• Ultra-light low-inertia probe support• Surface level compensation for non-contact

Probe Design

0.3 µm gap width

Probe similar to video recorder head

head surface

Resolution Test : Barkhausen Noise

0 200 400 600 800 1000 1200 1400x [µm]

Linear scan across micro-profiled test sample

Resolution Test : Eddy Current

6-µm gaps100 µm200 µmcopper

40 µm

60 µm

80 µm

100 µm

20 µm

10 µm

20-µm gaps

ferriteferriteperm-alloy

Different materialsunder test

Applications: Residual Stress (I)

thermally induced residual stresses in X20Cr13 steel: two laser-treated spots

comparison with X-ray method

Barkhausen noisearea scan

Applications: Residual Stress (II)

X-ray stress measurement

scan resolution: 5x5 pointstotal scanning time: 125 hours

(dark blue: 310 MPa, red: 396 MPa)

residual stresses in tempered Sendust (FeSiAl) – 2 µm film thickness, scan size: 2x2 mm²

Barkhausen noise area scan

scan resolution: 20x20 pointstotal scanning time: < 30 minutes

(maximum noise amplitude shown; dark blue: 1.26 V, bright yellow: 2.84 V)

Applications: Coating Thickness (I)

thickness of polyimide film on ferrite substrate

eddy current area scan40x30 points

optical image4x3 mm²

surface profilometry:

actual thickness: 400-900 nm

high correlation witheddy current signal

Applications: Coating Thickness (II)

100 nm

420 nm

860 nm

1540 nm

2600 nm

3120 nm

x [µm]

thickness

0 50 100 150

distance from surface [µm]

100 nm

420 nm

860 nm

1540 nm

2600 nm

3120 nm

thickness

thickness of Fe coating on Cu substrate

eddy current line scan50 points per sample

obtained thicknessaccuracy: ≤ 10 nm

(double RMSE)

Applications: Coating Thickness (II)

contactless eddy current scanning of a NiCo coated wafer

contact -25 µm -50 µm

lift-off performance of eddy current signal utilized for surface level interpolation

photo of wafer eddy current area scans of 24x24 mm region in center

Applications: Coating Thickness (III)

thickness of subsurface layers – NiFe/Cu/NiFe multilayer for GMI sensors

NiFe82/18

NiFe90/10

NiFe50/50

NiFe-GMI82/18

sample arrangement9 samples used for calibration

contactless eddy current scan (100x100 points)absolute thickness display after calibration

Applications: Residual Stress (III)

residual stresses at a crack tip in Charpy V-notch specimen

optical imageBarkhausen noise

area scan

stress-calibratedneural network output

Applications: Microstructure characterization

cementite needles in austenitic matrix („Spiegel iron“)

optical Image

eddy currentarea scan

3 x 0.8 mm2, 20 µm steps, 150 x 42 steps

Selected Applications

Eddy current scanof a „1 EURO“ coin

256x256 pixels24x24 mm²Parameter Im43 MHz

100x1003x3 mm²

Conclusions

• BEMI is a quick NDT method for residual stress and coating thickness characterization

• residual stress measurement using Barkhausen noise analysis

• coating thickness measurement using eddy current analysis

• local resolution: 10 µm, scanning speed: several points per second

• wide range of materials and coating thickness

• contactless scanning option

• eddy current method sensitive to sub-surface and intermediate layers

FluxFluxFluxFlux LeakageLeakageLeakageLeakage TestingTestingTestingTesting ---- BasicsBasicsBasicsBasics

Flux Leakage at a Gap at Various Magnetisations

Flux Leakage at Material Separations

Flux Leakage at Material Defects

Flux leakage

Gap at surface, perpendicular to magnetic field lines

Gap, parallel to magnetic field lines

Gap below surface, perpendicular to magnetic field lines

Flux Leakage at Crack: Tangential Field in the Outside

Flux Leage Testing

Advantages: Automated defect testing of rotationally symmetric parts (in production line) Testing equipment: Magnetising equipment Flux leakage probes Manipulating equipment Signal evaluation Marking equipment, sorting equipment Magnetisation: Permanent magnets Electromagnets, current generators

Flux Leakage Testing: Total Penetration

Testing object Detectable defect(transverse flaw)

JokeField direction

Current Exciting coil

Flux Leakage Testing: Magnetisation Using a Coil

Test object

Detectable defect(transverse flaw)

Field direction

Flux Leakage Testing: Magnetisation Using a Power Cable

Test object

Field direction

Power cable

Flux Leakage Testing: Magnetisation via Self-Penetration

Test object

Field direction

Contact electrode

Flux Leakage Testing: Additional Flux

Detectable defects(longitudinal/radial flaws)

Field directionTest object (pipe)

Current

Conductor (Cu)

Flux Leakage Testing: Magnetisation Using a Joke

Detectable defect (transverse flaw) Field direction

Test object (e.g. sheet)

Portable electromagnet(AC or DC)

FluxFluxFluxFlux LeakageLeakageLeakageLeakage TestingTestingTestingTesting ---- ApplicationApplicationApplicationApplication

Magnetic Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS

a: soft iron core

b: magnetic flux

c: magnetic coil

d: flaw indication

e: flux leakage

f: defect

Magnetic sensor

Magnetic fluxDefect

Flux leakage

Normal component

Tangential component

a: soft iron core

b: magnetic flux

c: magnetic coil

d: flaw indication

e: flux leakage

f: defect

Array 16 GMR-Sensors

Response curve of GMR Sensors

Physical Background of GMR layers

Array 16 GMR-Sensors

Response curve of GMR Sensors

Magnetic Flux Leakage from the crack in the ferromagnetic steel sheetmeasured by GMR-Sensor-Array

Comparison between magnetic particle testing (left)and magnetic flux leakage testing by using of GMR-Sensors (right)carried on the ferromagnetic cylindrical specimen (~1 µm wide)

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