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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
Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Eddy-Current Testing 2. Eddy-current probes
Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Eddy-Current Testing 3. Signal interpretation
Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Eddy-Current Testing 4. Equipment
Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Eddy-Current Testing 5. Aircraft inspection
Author: Otto-von-Guericke-Universität, Institut für Werkstofftechnik und WerkstoffprüfungEditor: Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V.
Eddy-Current Testing 6. Material characterisation
Eddy Eddy Eddy Eddy CurrentCurrentCurrentCurrent TestingTestingTestingTesting ---- ApplicationsApplicationsApplicationsApplications
IZFP
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
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
- Time Alternating Field
Eddy Current (EC) Technique: Basic sensor arrangement
Transmitter coil
Receiver coili(t)=I⋅⋅⋅⋅cos(ωωωωt)
U(t)
Specimen under test
- EC operating frequency
- electrical conductivity
- magnetic permeability
- sensor impedance
fσµ
σ,µ
IU
Z EC=
ω = 2πf
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
+σ
−σSensorLift Off
- Wall / LayerThickness
+µ
−µ
+ Wall / LayerThickness
µ Permeabilityσ Conductivity
Eddy Current (EC) Technique: Signal influencing variables
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
0.01
0.1
1
10
100
0.1 1 10 100 1000
CopperAluminumFerritic steel (µ=50)Austenitic steel (µ=1)
r
r
Eddy Current Standard Penetration Depth
NDT&E of Materials Using Eddy Current Technique
Frequency (kHz)
Penetration depth(mm)
Penetration depth (mm)
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
detectable defects detectable defect
Higher sensitivityBetter spatial resolution
Reduced inspection timeEasy operation
vs.
Inspectiontrace
Inspectiontrace
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
64-Channel Eddy-CurrentInspection System
Front End / Main Electronics
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Application examples
Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden Corrosion
NDT&E of Materials Using Eddy Current Technique
Calculation of the Conductivity Gradient:Numerical Modeling of the Inverse Problem
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Aluminium
Aluminium
Adhesion
Adhesion
corrosion
corrosion
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
EC Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden 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
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
EC Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden Corrosion
0
0.4
0.5
corrosion depth [mm]
0.3
0.2
0.1
Results
Inspection situation:corrosion in the 1st and 3rdaluminium layer
Target function:corrosion in the1st aluminium layer
EC probe
Aluminium
Aluminium
Aluminium
Adhesion
Adhesion
corrosion
corrosion
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
EC Inspection of Layered Aluminum Aircraft Structuresto Detect and Size Hidden Corrosion
Inspection situation:corrosion in the 1st and 3rdaluminium layer
Target function:corrosion in the3rd aluminium layer
corrosion depth [mm]
Results
0
0.2
0.4
0.6
EC probe
Aluminium
Aluminium
Aluminium
Adhesion
Adhesion
corrosion
corrosion
NDT&E of Materials Using Eddy Current Technique
IZFP
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
ωL0
R-R0
ω=0
ω→∞
ω
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
BEMI Testing Device
• scanner control• eddy current hardware• barkhausen noise hardware
• scanner control• eddy current hardware• barkhausen noise hardware
precision 3-d scanner
precision 3-d scanner
sensorsensor
samplesample
controlling PCcontrolling PC
sensor element:miniaturized inductive probe(modified VCR head)
cm
• picks up Barkhausen noise• induces eddy currents
stationary electromagnet(Barkhausen only)
stationary electromagnet(Barkhausen only)
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
scans
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Probe Design
10 mm
0.3 µm gap width
Probe similar to video recorder head
head surface
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Resolution Test : Barkhausen Noise
0 200 400 600 800 1000 1200 1400x [µm]
0
1
2
3
4
5
6
7
8
MM
AX
[V]
Linear scan across micro-profiled test sample
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Resolution Test : Eddy Current
6-µm gaps100 µm200 µmcopper
glass
40 µm
60 µm
80 µm
100 µm
20 µm
10 µm
5 µm
20-µm gaps
ferriteferriteperm-alloy
Different materialsunder test
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Applications: Residual Stress (I)
thermally induced residual stresses in X20Cr13 steel: two laser-treated spots
comparison with X-ray method
Barkhausen noisearea scan
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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)
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Applications: Coating Thickness (II)
50
1250
2450
3650
4850
0
0.5
1
1.5
2
2.5
3
100 nm
420 nm
860 nm
1540 nm
2600 nm
3120 nm
x [µm]
thickness
0
0.5
1
1.5
2
2.5
3
0 50 100 150
distance from surface [µm]
eddy
cur
rent
sig
nal [
arb.
uni
t]
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)
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Applications: Coating Thickness (III)
thickness of subsurface layers – NiFe/Cu/NiFe multilayer for GMI sensors
nm
25 n
m
25 n
m
25 n
m
100
nm
100
nm
100
nm
200
nm
200
nm
200
nm
100
/ 30
/ 100
nm
NiFe82/18
NiFe90/10
NiFe50/50
NiFe-GMI82/18
x
y
sample arrangement9 samples used for calibration
contactless eddy current scan (100x100 points)absolute thickness display after calibration
GMI
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Applications: Residual Stress (III)
residual stresses at a crack tip in Charpy V-notch specimen
optical imageBarkhausen noise
area scan
stress-calibratedneural network output
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Selected Applications
Eddy current scanof a „1 EURO“ coin
256x256 pixels24x24 mm²Parameter Im43 MHz
100x1003x3 mm²
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage at a Gap at Various Magnetisations
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage at Material Separations
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage at Crack: Tangential Field in the Outside
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage Testing: Total Penetration
Testing object Detectable defect(transverse flaw)
JokeField direction
Current Exciting coil
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage Testing: Magnetisation Using a Coil
Joke
Test object
Coil
Detectable defect(transverse flaw)
Field direction
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage Testing: Magnetisation Using a Power Cable
Test object
Detectable defect(transverse flaw)
Field direction
Power cable
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage Testing: Magnetisation via Self-Penetration
Test object
Detectable defect(transverse flaw)
Field direction
Contact electrode
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Flux Leakage Testing: Additional Flux
Detectable defects(longitudinal/radial flaws)
Field directionTest object (pipe)
Current
Conductor (Cu)
Spies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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 Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
H
Hn
t
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
Magnetic Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Array 16 GMR-Sensors
Response curve of GMR Sensors
Physical Background of GMR layers
Magnetic Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
Array 16 GMR-Sensors
Response curve of GMR Sensors
Magnetic Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
-6
-4
-2
0
2
4
Magnetic Flux Leakage from the crack in the ferromagnetic steel sheetmeasured by GMR-Sensor-Array
Magnetic Flux Leakage TestingSpies / FHG-ITWM & University of SaarlandElectromagnetic NDT Methods / Master CNDMS
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)