Diapositive 1

1
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)
Contents

• Background
• Purpose of the study
• Development of an inspection method
• Design of the wide aperture phased-array
• Experimental results
• Effect of an external load
• Conclusion

2
Background
Characterization of closed cracks
Difficulties to detect tips diffraction echoes on
closed cracks with standard transducers
Results of previous experimental studies on
closed cracks under load
Several limiting factors - crack opening local
contacts between matching lips - material
parameters (noise level)
Cf. ref.  Advanced Phased Array Transducer for
Detection of Closed Crack Tip Diffraction ,O.
Dupond, EDF RD, EPRI London
2004.
Inaccurate sizing or no tip diffraction detection
in some local areas
Improvement of closed crack characterization
Natural roughness of the crack induces contact
discontinuities even on closed cracks
Detection of small diffracting points along the
crack using a high-spatial resolution transducer
3
Purpose of the study
- Development of a method that improves the
sizing of closed cracks

• High-spatial 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-spatial resolution phased
array (wide aperture)

• Evaluation of the phased array performances for
  crack sizing under load

4
Inspection method
First step Detection of the defect using the
corner effect
Generation of 45-SW focused at the backwall
surface
Second step Characterization of the crack from
the base to the top
Displacement of the focus point in depth by
applying suitable delay laws (multi-sequence
mode)
Scan axis
Ultrasonic beam simulation
Focusing depth displacement
crack
Detection of diffraction points along the crack
5
Design of the wide aperture phased array
Requirements
- 45 Shear Waves - wide aperture - natural
focusing
Phased array designed with CIVA software

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

Main features
121 sectorial elements
Immersed phased array
Top view
45 SW
6
Beam characteristics
CIVA simulation of the ultrasonic beam at the
nominal focal depth
121 active elements with no 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

7
Experimental results on a mock-up

• Mock-up austenitic stainless steel, 30 mm
  thick
• Transducer driven by a MULTIX UT acquisition
  system
• (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
Accurate sizing of the crack
8
Effect of an external load
UT examination with the advanced phased array

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

Test description

• Mock-up austenitic stainless steel, 29-mm
  thick (400x70x29)
• Transducer driven by a MULTI2000 System
• Scanning of the component

29 mm
Assembly test for loading
Without external load
Under load
4-point bending system
Mock-up
Tightening bolts
0
0
D
D
bolts
? -0.6 mm
180 kN
Fatigue crack
Closed crack
9
Acquisition without load

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

?
?
? 0 mm
characterization of the crack along the y axis
crack
Focusing at 9 mm from the backwall surface
y
x
Tip -18 dB
Tip diffraction
z
z
8.5 mm
Corner
Reconstructed Bscan
Dscan
Reference side drilled hole ? 2 mm at a depth
of 20 mm
- Tip diffraction detection at -18 dB (SNR 18
dB) height measurement of the
crack 8.5 mm
- Detection of intermediate echoes close to the
top of the crack
10
Acquisition under load (2)

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

?
?
? -0.6 mm
crack
Focusing at 9 mm from backwall surface
y
x
z
z
Intermediate
Intermediate -17 dB
7 mm
Corner
Reconstructed Bscan
Dscan
Reference side drilled hole ? 2 mm at a depth
of 20 mm

• increase of the intermediate echo amplitude (4
  dB)
• tip diffraction echo still detected but with a
  lower amplitude (-7dB)
• measurement of the height of the crack 8.5 mm
  (independent of the applied load)

11
Benefits of the wide aperture Beam
simulations
? 100 mm active aperture
? 50 mm active aperture
100 mm
50 mm
20 mm
Ultrasonic beam simulation
Ultrasonic beam simulation
20 mm in depth
20 mm in depth
Æ
1.5 mm
Æ
0.9 mm
3.5 mm
17 mm
Beam in the incident plane
Beam in the perpendicular plane
Beam in the incident plane
Beam in the perpendicular plane
Reducing the aperture loss of resolution
12
Effects of the active aperture Acquisition with
a ?100 mm active aperture

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

True Bscan
Dscan
y
x
Tip -25 dB
Tip
z
z
Intermediate -17 dB
Intermediate
8.5 mm
7 mm
8.5 / RSB 7 dB
corner
13
Effects of the active aperture Acquisition with
a ?50 mm active aperture

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

Dscan
y
True Bscan
x
Tip -33 dB
z
z
Intermediate -24 dB
Intermediate
7 mm
corner
y
z
Loss of sensibility
- decrease of the tip diffraction amplitude -
fewer diffracted points at the tip crack height
is (under) evaluated to 7 mm
7 mm
8.5 mm
Segmented Dscan
The ?100 mm active aperture allows a more
accurate depth sizing of the crack under load
14
Analyze of the crack discontinuities (1)
Experimental observations
Mock-up under load / Wide aperture phased array
y
z
Top diffraction
Dscan
Intermediate diffraction
Corner echo
Step axis
Reconstructed Bscan
Top diffraction
?depth 1.5mm
depth
?ampl. 12 dB
Intermediate diffraction
8.5
7
Corner echo
No correlation between UT acquisitions and crack
profile
15
Analyze of the crack discontinuities (2)
Use of simulation tools (CIVA) to evaluate the
size of the discontinuities detected during the
loading experiments
Experimental observations reproduced in terms of
amplitude for
zoom
L1 z 0.1
L3z 0.7
y
z
L2 1.5
Areas in contact under compression no defect
Opened areas described as planar defects
16
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 Q0.1mm
  

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