Diapositive 1

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IMPROVEMENT OF CRACK SIZING USING PHASED ARRAY
TECHNIQUES
Ph. Bredif (1), O. Dupond (2), C.Poidevin (1), L.
de Roumilly (1) (1) CEA (French Atomic Energy
Commission) (2) EDF (Electricité de France)
OUTLINE

• Background
• Development of an inspection method, design of
  the wide aperture phased array
• Experimental results
• Effect of an external loading
• Effect of the aperture of the probe
• Size closed crack tip diffracting areas
• Conclusion

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2
Background
Difficulties to detect tips diffraction echoes on
closed cracks with standard transducers
Several limiting factors - defect parameters
crack opening between matching lips - material
parameters (noise level)
inaccurate sizing or no tip diffraction detection
in some local areas
Improvement of closed crack characterization
Main limiting factor local contacts between
matching lips
Natural roughness of the crack induces contact
discontinuities even on closed cracks
Cf. ref.  Advanced Phased Array Transducer for
Detection of Closed Crack Tip Diffraction ,O.
Dupond, EDF RD, EPRI London
2004.
Detection of small diffracting points along the
crack using a high-spatial resolution transducer
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Purpose of the study
- Development of a method that improves the
sizing of closed cracks

• High resolution probe (small focused spot)
  
• Displacement of the focal area to scan the
  defect from its base to the top

phased array technique adaptative focusing in
depth by applying suitable delay laws
- Design of a high-resolution phased array

• wide aperture
• natural focusing

• Evaluation of the phased array performances for
  crack sizing

• Effect of loading
• Benefit of the wide aperture

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Inspection method
First step Detection of the defect
Generation of a focused beam at the backwall
surface
Delay laws
Ultrasonic beam simulation
Scan axis
transducer
crack
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Inspection method
First step Detection of the defect
Generation of a focused beam at the backwall
surface
Second step characterization of the crack from
its emerging part to the top

• Scanning of a local area around the defect

• For each position of the probe, displacement of
  the focusing point by applying suitable delay
  laws (multi-sequence mode)

• Diffraction detection along the crack height

Ultrasonic beam simulation
transducer
45 SW
crack
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Design of the wide aperture phased array
Requirements
- 45 Shear Waves - wide aperture - natural
focusing in depth around the cracks
Phased array designed with CIVA software
Main features

• - circular sectorial aperture Ø 100 mm
• - frequency 4.5 MHz
• 121 elements
• - natural focusing (Fermat surface) at 25 mm in
  depth

121 sectorial elements
Immersed phased array
Mechanical focusing
Top view
45 SW
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Beam characteristics
CIVA simulation of the ultrasonic beam at the
nominal depth focusing
121 active elements with null delay laws
25 mm
Fermat surface
Ø 0.8 mm
Beam in the perpendicular plane
Advantages of the designed phased-array
transducer

• Very high-spatial resolution Focused spot
  diameter at a depth of 25 mm 0.8 mm at -6 dB

• Electronic focusing displacement in
  depth of the small focused spot along the 45 SW
  axis

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Experimental results on a mock-up

• Mock-up austenitic stainless steel, 30 mm
  thick
• Transducer driven by a MultiX System (M2M), 128
  parallel channels
• Scanning in a local area around the crack, with
  focusing depth displacement from 30 mm (backwall
  surface) to 15 mm

30 mm
Reconstructed Bscan for each focal depth
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Effect of an external loading
UT examination with the advanced phased array

• Characterization of crack without loading
• Characterization of crack under 180 kN load

Test description

• Mock-up austenitic stainless steel, 29 mm
  thick (400x70x29)
• Transducer driven by a MultiX System
• Scanning of the component

29 mm
Assembly test for loading
Without external load
Under load
4 points bending system
Mock-up
Tightening bolts
0
0
D
D
bolts
? -0.6 mm
180 kN
Fatigue crack
Closed crack
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Acquisition without load

• No loading ? 0 mm
• Raster scanning over the mock-up

?
?
? 0 mm
characterization of the crack along the y axis
x
y
crack
z
crack
Dscan
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Acquisition under load (2)

• Loading 180 kN (? -0.6 mm)
• Raster scanning over the mock-up

?
?
? -0.6 mm
x
y
crack
z
crack
Dscan
True Bscan (focusing at 9 mm from backwall)
Dscan (focusing at 9 mm from backwall)
y
x
z
z
Intermediate
Intermediate -17 dB
7 mm
Corner
Reference side drilled hole ? 2 mm at 20 mm in
depth
Increasing the applied load

• increase of intermediate echo
• tip diffraction echo still detected (SNR 10
  dB)
• measurement of the height of the crack 8.5 mm
  (independent of the applied load)

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Effects of the active aperture
Beam simulations
? 100 mm active aperture
100 mm
20 mm
Ultrasonic beam simulation
20 mm in depth
Æ
0.9 mm
3.5 mm
Beam in the incident plane
Beam in the perpendicular plane
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Benefits of the wide aperture (1)
Acquisition with a ?100 mm active aperture

• Loading 180 kN (? -0.6 mm)
• Raster scanning of the mock-up
• Reference side drilled hole ? 2 mm at 20 mm in
  depth

True Bscan
Dscan
y
x
Tip -25 dB
Tip -25 dB
z
z
Intermediate -17 dB
Intermediate -17 dB
8.5 mm
7 mm
8.5 / RSB 7 dB
corner
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Benefits of the wide aperture (2)
Acquisition with a ?50 mm active aperture

• Loading 180 kN (? -0.6 mm)
• Raster scanning of the mock-up
• Reference side drilled hole ? 2 mm at 20 mm in
  depth

True Bscan
Dscan
y
x
Tip -33 dB
z
z
Intermediate -24 dB
Intermediate
7 mm
corner
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Analyze of the crack discontinuities (1)
Purpose Estimate the size of the local
discontinuities detected on the top of the crack
(intermediate and tip diffraction)
Mock-up under load Wide aperture phased
array
True Bscan
x
z
Tip -25 dB
?depth 1.5mm
Intermediate -17 dB
8.5 mm
7 mm
Corner echo
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Analyze of the crack discontinuities (2)
Modelling the closed crack
(Experiment)
y
Dscan
Tip -25 dB
area partially closed
z
Intermediate -17 dB
Opened area
corner
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Conclusion

• - Development of a method based on a specific
  phased-array
• wide aperture ?100 mm
• natural focusing at a depth of 25 mm
• 121 sectorial elements, 4.5 MHz

High-spatial resolution transducer
- Experimental results obtained on a crack under
load (laboratory conditions)

• Apparition of intermediate echoes amplitude
  higher than top diffraction echoes
• Top diffraction echoes still detected

Crack correctly sized

• Benefits of the wide aperture crack may be
  undersized with smaller spatial resolution

• Evaluation of the size of the discontinuities
  detected at the top of the crack 0.1mm
  

The specific ?100 mm aperture phased array
improves significantly crack sizing,
independently of the applied load
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