Effect of fatigue crack orientation on the sensitivity of ... · Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels

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


Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels


Outline

Introduction

Experimental procedure and data analysis

FE modelling and analysis

Results and discussion

Conclusions

2/21



Introduction

Risk assessment programs

3/21

Damage tolerance models

loading conditions

material properties

flaw characteristics

Nondestructive testing (NDT) methods

Reliability quantification by probability of detection (PoD)

ෝ𝒂 versus 𝒂



Introduction

Martensitic stainless steels

4/21

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



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

5/21

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



Outline

6/21

Introduction




Conclusions



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.

7/21

1.89 mm

0.61

mm

𝐷𝑒𝑝𝑡ℎ = −0.0006 + 0.3475 𝐿𝑒𝑛𝑔𝑡ℎ , 𝑟2= 0.9845



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

8/21



Outline

9/21

Introduction




Conclusions



3D model and material properties used for

the assembly of the probe and sample

10/21

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)



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



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



Outline

13/21

Introduction




Conclusions



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



Simulation

Measurement

Verifying simulated signals by

comparing them to measurements

15/21

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



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



Relative position of the probe and notches

at which the amplitude is maximum

17/21

2 m

m

0.5

mm 1

.0 m

m

1.5

mm

Orientation 0°



Induced current density distribution as

the notch length varies - 0°

18/21

Notch length = 0.5 mm Notch length = 1.5 mm Notch length = 3 mm



Induced current density distribution as

the notch length varies - 90°

19/21

Notch length = 2 mm Notch length = 4 mm Notch length = 6 mm



Outline

20/21

Introduction




Conclusions



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

Documents

Effect of fatigue crack orientation on the sensitivity of ... · Effect of fatigue crack orientation on the sensitivity of eddy current inspection in martensitic stainless steels