A model-based study of the effect of semi-elliptical ... · Periodical inspections for detecting...

A model-based study of the effect of semi-elliptical surface notch

geometry on the signal of a Split-D eddy current probe

Ehsan Mohseni𝑎, Martin Viens𝑎, Demartonne Ramos França𝑏

𝑎Département de Génie Mécanique, École de Technologie Supérieure (ETS)

𝑏Engineering Technologies Department, John Abbott College

NDT in Canada 2016 & 6th International CANDU In-Service Inspection Workshop

November 15-17, 2016

Burlington, Ontario, Canada

6th International CANDU ISI Workshop/NDT in Canada 2016 Conference

A model-based study of the effect of semi-elliptical surface notch geometry on the signal of a Split-D eddy current probe


Outline

Introduction

Impedance measurement for the split-D probe

FE model of a differential split-D probe

Results of FEM and measurements together

Conclusions

2/21



Introduction

Eddy current testing (ECT) using reflection differential surface probes

Periodical inspections for detecting tiny fatigue cracks

Finite element modeling for this probe configuration

Quantitative evaluation of ECT signals for efficient subsequent repairs

3/21



Objectives

Small foot print

Low sensitivity to lift-off

The best for calibration purposes

Resembling the signal of fatigue cracks

Lower fabrication cost relative to fatigue cracks

Assessing the reliability of FEM for predicting the EC signals

Establishing a size dependent signal archive at the selected test frequency to train an artificial intelligence algorithms as a basis for inversion purposes.

Repair of defects according to their characteristics

How the signal of a surface notch can represent the signal from a fatigue crack

4/21



Summary of the study

A FEM of reflection differential split-D surface probe is prepared and the scans are simulated

The signal is recorded for

scanning of three

semielliptical notches

having different sizes Validation of the simulated

signals of the notches

Effect of the variation of the notch length, depth and opening on the EC signals is studied.

5/21



Outline

Introduction



Results of FEM and measurements Together

Conclusions

6/21



Impedance measurement for the split-D

probe Nortec 500S along with a reflection

differential split-D probe are used

The probe’s frequency range is 500kHz-3Mhz

Calibrating the probe for perpendicularity

Scanning an aluminum 7075-T6 blockcontaining three semi-elliptical notchesusing an encoded X-Y table

The scan is performed perpendicularly tothe notch’s length

Acquiring horizontal and vertical readings ofNortec 500S

Compensating for the selected gains in therecorded data

NotchLength, L

(mm)

Depth, D

(mm)

Opening,

W (mm)

A 2.84 1.11 0.1

B 1.62 0.63 0.1

C 0.81 0.31 0.1

L

DW

Scan direction

7/21



Probe characteristics

In order to prepare the 3D model for theprobe:

Nikon XTH 225 micro-CT scan is used toscan the probe

Dimensions of the probe’s constituents aremeasured

Accordingly, the CAD model is built andimported into COMSOL Multiphysics.

8/21



Outline

Introduction




Conclusions

9/21



FE model for the split-D probe scanning

the notches A half scale 3-D CAD model is built and imported into Comsol based on the probe’s

geometry.

Firstly, large scale air and sample domains are modeled and afterwards, they are

truncated according to attenuation of vector potential’s field.

Multi-turn coil domain along with MF physics in frequency domain are used:

2 1 1

0 r 0 r e( j )A ( B) J eJ NI / S

Before truncation

After truncation

10/21




the notches Material properties are assigned according to

measurements and data sheets

Second order tetrahedral elements are applied

to the entire geometry of the problem making

sure that there are at least six elements across

the first three standard penetration depths

Elements are locally finer around the notch

Direct solver is chosen

Each scan is simulated in a single run for 1.3 mm

displacements of the probe along the scanning

path having increments of 0.1 mm. 0 mm 1.3 mm

Probe’s displacement

11/21




the notches

The impedance (ΔZ) of the probe at each position of the scan is calculated through

Accordingly, the probe’s signal for each scan is plotted on the impedance plane.

The simulations are ordered as follows:

2 1( ) /R R D V V IZ

Scans of the three notches A, B and C are simulated in order to compare the measured and simulated signals

Scans are simulated for the three aforementioned notches while their openings are varied from 0.01 to 0.095 mm

The ratio of the notch depth to the standard penetration depth of eddy currents (D/δ) is varied from 1.88 to 6.88 by increments of 2.5. Besides, the ratio of the notch length to the driver coil’s diameter (L/D) is varied from 1 to 3 with increments of 0.5 for each depth of the notch.

1st

2nd

3rd12/21



Outline

Introduction




Conclusions

13/21



-1.2E-1

-1.0E-1

-8.0E-2

-6.0E-2

-4.0E-2

-2.0E-2

0.0E+0

-5.0E-3 5.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch A

Validating the simulation results for

Notches A, B and C

-1.2E-1

-1.0E-1

-8.0E-2

-6.0E-2

-4.0E-2

-2.0E-2

0.0E+0

-5.0E-3 5.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch B-1.2E-1

-1.0E-1

-8.0E-2

-6.0E-2

-4.0E-2

-2.0E-2

0.0E+0

-5.0E-3 5.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch C

Deviations of the material properties used in simulations from the actual values Nominal dimensions of the notch are inserted into the model

Sensitivity of the simulation outputs to the modelling parameters, such as mesh size and distribution, increases

14/21



Effect of the variations of the notch

opening on the probe’s signal

-1.1E-1

-9.4E-2

-7.4E-2

-5.4E-2

-3.4E-2

-1.4E-2

6.0E-3

-1.5E-2 -5.0E-3 5.0E-3 1.5E-2

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch A-5.0E-2

-4.0E-2

-3.0E-2

-2.0E-2

-1.0E-2

0.0E+0

-1.0E-2 0.0E+0 1.0E-2

Im(Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch B

0.095 mm 0.08 mm

0.06 mm 0.04 mm

0.02 mm

-2.0E-2

-1.5E-2

-1.0E-2

-5.0E-3

0.0E+0

5.0E-3

-5.0E-3 0.0E+0 5.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

Notch C

The shape of the signals remains unchanged as the opening of the notches varies

2 2

max max(Im( )) (Re( )) , (Im( ) / Re( ))Arctan ΔZ ΔZ ΔZ ΔZ ΔZ

15/21



Changes in the signal amplitude and

phase as the notch opening varies

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.03 0.06 0.09 0.12

Sig

na

l A

mp

litu

de

(Ω)

Notch opening (mm)

-100

-96

-92

-88

-84

-80

0 0.03 0.06 0.09 0.12

Sig

na

l P

ha

se

(Deg

rees

)

Notch opening (mm)

Notch A Notch B Notch C

0.5

1

1.5

2

2.5

3

3.5

4

0 0.03 0.06 0.09 0.12

No

rma

lize

d S

ign

al

Am

pli

tud

e

Notch opening (mm)

Both the signal amplitude and phase decrease with a quasi-linear trend as the notch gets tighter

For explaining the relative slope of these curves, it is necessary to consider the probe position with respect to the notch when the probe impedance reaches its maximum

16/21

2 2

max max(Im( )) (Re( )) , (Im( ) / Re( ))Arctan ΔZ ΔZ ΔZ ΔZ ΔZ



The probe position with respect to the notch

when the probe impedance is maximized

Notch

Eddy current

surface

Situation for notch A

Eddy current

Notch

Situation for notch C

Notch A Notch C

17/21



Effect of the variation of notch depth and

length on the probe’s signal

-1.7E-2

-1.2E-2

-7.0E-3

-2.0E-3

3.0E-3

-3.0E-3 0.0E+0 3.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

D = 0.3-6.5E-2

-5.5E-2

-4.5E-2

-3.5E-2

-2.5E-2

-1.5E-2

-5.0E-3

5.0E-3

-5.0E-3 0.0E+0 5.0E-3

Im(Δ

Z)

(Ω)

Re (ΔZ) (Ω)

D = 0.7

L/D = 1 L/D = 1.5L/D = 2 L/D = 2.5L/D = 3

g

h

i

j

-1.2E-1

-1.0E-1

-8.0E-2

-6.0E-2

-4.0E-2

-2.0E-2

0.0E+0

-6.0E-3 0.0E+0 6.0E-3

Im (Δ

Z)

(Ω)

Re (ΔZ) (Ω)

D = 1.1

o

n

m

l

K

Diameter > Length Diameter ≈ Length Diameter < Length

a

b

c

d

e

f

a,b,c,d,e,f,g,h,k

i,l J,m,n,o

Driver’s coil diameter= 1.8 mm

18/21



Outline

Introduction



Results of FEM and measurements Together

Conclusions

19/21



Conclusions The impedance trajectories generated by the numerical modelling of the split-D probe

within Comsol Multiphysics closely match the eddy current impedance measurements of

the same semi-elliptic surface notch using the commercial Nortec 500S.

The signal amplitude increases as the notch opening becomes wider. Variations of both

phase and amplitude of the impedance trajectories versus changes of the notch opening

is linear for all the notches investigated here

For notches shorter than the probe's diameter, the signal amplitude is highly sensitive to

variations of the notch opening. Due to this high sensitivity, numerical simulations might

not be the most suitable approach to reproduce signals from very tiny fatigue cracks.

On the other hand, larger notches have lower sensitivities to opening changes, and

hence could be efficiently used for estimating signals from fatigue cracks having a similar

geometry.

There are three possibilities related to the shapes of signals from the split-D probe

scanning semi-elliptical surface notches, and these shape variations are only influenced

by the ratio of the notch length to the driver’s coil diameter.

Moreover, numerical signals obtained for different sizes of notch enable one to establish a

training set for inversion algorithms.

20/21

Thank you for your

attention



Information related to the probe

Dimensions and material properties of the commercial split-D probe’s components.

Wire diameter of receiver D-coils 0.063 mm Gap between cores 0.224 mm

Wire diameter of driver coil 0.055 mm Shielding inner radius 0.967 mm

Number of windings of D-coils 20 Shielding outer radius 1.264 mm

Number of windings of driver coil 37 Shielding height 3.000 mm

Height of receiver and driver coils 1.260 mm Relative permeability of cores and shielding 2500

Core radius 0.627 mm Conductivity of cores and shielding 0.2 S/m

Core height 2.000 mm Test frequency 500 kHz

Nortec 500S Flaw Detector from Olympus

Reflection differential Split-D probe (500 kHz-

3MHz)

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