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Effect of fatigue crack orientation on the sensitivity of
eddy current inspection in martensitic stainless steels
Hamid Habibzadeh Boukani, Ehsan Mohseni, Martin Viens
Département de Génie Mécanique, École de Technologie Supérieure (ETS)
NDT in Canada 2017 Conference
June 6-8, 2017
Quebec City, Quebec, Canada
NDT in Canada 2017 Conference
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Outline
Introduction
Experimental procedure and data analysis
FE modelling and analysis
Results and discussion
Conclusions
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Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Introduction
Risk assessment programs
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Damage tolerance models
loading conditions
material properties
flaw characteristics
Nondestructive testing (NDT) methods
Reliability quantification by probability of detection (PoD)
ෝ𝒂 versus 𝒂
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Introduction
Martensitic stainless steels
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High strength, hardness, and
toughness
Moderate resistance to
corrosion
• Bearings
• Shafts
• Compressors
• gas generators
• valves
widely used method for the detection of surface-breaking cracks
eddy current split-D probe
Low cost
Eddy current testing (ECT)
Principal degradation
FATIGUE
High S/N
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
AIS
I 410
finite element modelling (FEM) is employed to expand the extent of our study to larger sizes and thus to gain a better insight into
this observation
Objectives
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In the framework of a PoD study of automated ECT, ෝ𝒂 versus 𝒂 at 3 different
crack orientations with respect to the scan direction, as the influential parameters for
the PoD evaluation, is studied
Due to limitations in crack length interval in
experiments
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Outline
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Introduction
Experimental procedure and data analysis
FE modelling and analysis
Results and discussion
Conclusions
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Test unit and samples used in the study
Nortec 500S along with a reflection differential split-Dprobe are used
The probe’s frequency range is 500kHz-3Mhz
Starter flaws using electrical discharge machining (EDM)process on the surface of samples
cyclically loaded in order to grow fatigue cracks out of thestarter flaws.
Samples containing fatigue cracks of 0.76 to 2.95 mm inlength
According to destructive tests, Their depth linearlyincreases with their length.
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1.89 mm
0.61
mm
𝐷𝑒𝑝𝑡ℎ = −0.0006 + 0.3475 𝐿𝑒𝑛𝑔𝑡ℎ , 𝑟2= 0.9845
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
ECT automated scans and signal analysis
Calibration on a reference flaw:
• device gain
• the impedance plane angle
• perpendicularity of the probe to the sample’s surface
Initial lift-off of 0.03 mm
Raster scans at frequencies of 500 kHz and 1 MHz
Scan index of 0.5 mm
Gains are compensated for each axis
signals’ characteristics are extracted
ECT signal length (𝑽𝒑𝒑) : main parameter under study
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Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Outline
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Introduction
Experimental procedure and data analysis
FE modelling and analysis
Results and discussion
Conclusions
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
3D model and material properties used for
the assembly of the probe and sample
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Shield
Cores
Coils
3-D modeling in Comsol multyphysics:
• A half-scaled CAD model for orientations of 0°and 90° owing to their plane symmetry
• Full model for orientation of 45°
• Cracks represented by semi-elliptical notcheshaving 0.02 mm opening
• Dimensions of the probe’s Interior componentsaccording to X-ray tomography reconstruction
• Initial lift-off of 30 μm
Material properties: measurements and data sheets
Component Relative permeability Electrical conductivity
Cores and shield 2500 1(S/m)
Sample 300 1.9e6(S/m)
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Physics, mesh and solver
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Physics:
• MF physics within AC/DC module
• Multi turn domains for coils
• Magnetic insulation boundary condition forencompassing air domain
Mesh:
• Second order tetrahedral elements
• 8 boundary layer mesh on the surface of the sample
• Each layer has the thickness of first standardpenetration depth
• Finer elements for the notch geometry
Solver:
• Iterative stationary solver
2( ( )) / ( )0 r 0 r ej A A J 1j
2 1( ) /R R D V V IZ
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Details of simulated scans
1.3 mm displacements of the
probe along the scan path
Probe’s displacement increments
of 0.1 mm
Probe is centered by the notch
at the beginning of the scan
0 mm Depends on the notch length
Start of the scan
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90°
45°
0°
0 mm 1.3 mm
0 mm 1.4 mm
End of the scan
OrientationNotch length
Variations(mm)Steps (mm)
90° 2 - 6 0.5
45° 1.5 - 4.5 0.5
0° 0.5 - 3 0.5
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Outline
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Introduction
Experimental procedure and data analysis
FE modelling and analysis
Results and discussion
Conclusions
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Measured signal amplitude versus crack
length
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0
0.2
0.4
0.6
0.8
1
0.5 1.5 2.5 3.5
No
rma
lize
d S
ign
al A
mp
litu
de
Crack Length (mm)
90 deg
45 deg
0 deg
0
0.2
0.4
0.6
0.8
1
0.5 1.5 2.5 3.5
No
rma
lize
d S
ign
al A
mp
litu
de
Crack Length (mm)
90 deg
45 deg
0 deg
The same behaviour at both frequencies
Amplitude is independent of the orientation for crack length below 1.8 mm
Amplitude changes versus length variation becomes plateau after 1.8 mm for 0° orientation
500 kHz 1000 kHz
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Simulation
Measurement
Verifying simulated signals by
comparing them to measurements
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Shape discrepancies :
• Deviation of notch geometry fromthe fatigue crack
• Material properties
• Probe manufacturing imperfections
-3.0E-1
-2.5E-1
-2.0E-1
-1.5E-1
-1.0E-1
-5.0E-2
0.0E+0
-4.0E-2 6.0E-2 1.6E-1
Im (Δ
Z)
(Ω)
Re (ΔZ) (Ω)
-3.0E-1
-2.5E-1
-2.0E-1
-1.5E-1
-1.0E-1
-5.0E-2
0.0E+0
-4.0E-2 6.0E-2 1.6E-1
Im(Δ
Z)
(Ω)
Re (ΔZ) (Ω)
Good amplitude agreement
0°
orientation
L=2.94 mm L=1.50 mm
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Simulated and measured signal amplitude
versus crack length
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For each orientation, Amplitude variations becomes less than 5 % after a certain notch length:
• 0°- 2.5 mm
• 45°- 3.5 mm
• 90°- 4 mm
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 2 4 6 8No
rma
lize
d s
ign
al
am
pli
tud
e
Notch length (mm)
0 deg
90 deg
45 deg
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8No
rma
lize
d s
ign
al
am
pli
tud
e
Length (mm)
0 deg-sim
45 deg-sim
90 deg-sim
0 deg-exp
45 deg-exp
90 deg-expSimulation
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Relative position of the probe and notches
at which the amplitude is maximum
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2 m
m
0.5
mm 1
.0 m
m
1.5
mm
Orientation 0°
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Induced current density distribution as
the notch length varies - 0°
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Notch length = 0.5 mm Notch length = 1.5 mm Notch length = 3 mm
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Induced current density distribution as
the notch length varies - 90°
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Notch length = 2 mm Notch length = 4 mm Notch length = 6 mm
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Outline
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Introduction
Experimental procedure and data analysis
FE modelling and analysis
Results and discussion
Conclusions
Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels
NDT in Canada 2017 Conference
Conclusions Depending on the flaw orientation, the signal amplitude increases with the
crack length up to a critical (flaw length)/(drive coil diameter) ratio (L/D)which is specific to the flaw orientation.
This critical ratio grows as the orientation increases from 0° to 90°
The variation of the signal amplitude as the crack length increases is almostindependent of the crack orientation until a L/D value equal to the unity andthen the slope of amplitude versus crack length variations becomesorientation dependent.
Accordingly, the probability of detection of fatigue is almost independent ofcrack orientation once the L/D ratio is below the unity.
The results of our FEM study show a good agreement with the measurementsoutcome in terms of amplitude. Therefore, this model is a reliable means tocarry out model-based studies of probability of detection.
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Thank you for your
attention