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TOFD
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TOFDTime of Flight Diffraction
04/17/23 2
TOFD Technique Overview Calibration Applications Summary & Limitations
04/17/23 3
TOFD Overview Forward Diffraction
Technique First described by Silk
in 1977– Using diffracted signals
from crack tips
Traditional grey scale TOFD presentation
04/17/23 4
Advantages of TOFD Technique
– Wide coverage area using a pair of transducers – Accurate flaw sizing; amplitude-independent – Sizing technique using time-of-flight information – On-line volume inspection - very fast scanning – Setup independent of weld configuration – Sensitive to a variety of defects – No sensitivity to defect orientation – Amplitude-insensitive - acoustical coupling less critical
04/17/23 5
Diffraction
– Modification or deflection of sound beam– Sound striking defect causes oscillation– Ends of defect become point sources– Not related to orientation of defect– Weaker signal than reflected – needs higher gain– Sharp defects provide best emitters– Tips signals are located accurately– Time of flight of tip signals used to size
04/17/23 6
Diffraction
Each point of the defect generates new elementary spherical waves called diffraction
The incoming wave vibrates the defect.
04/17/23 7
Waves
FLAW
Diffractedwaves
Diffractedwaves
Incidentwave
Reflectedwave
All directions
Low energy
Independent of incidence angle
04/17/23 8
Slot or crack
Conventional Use of Diffraction Tip diffraction method (satellite-pulse observation technique)
Time
Amplitude
2
2
Tip diffraction1
1
Corner reflection
TOF
Angle
TOF, Angle and velocity Height
04/17/23 9
Signals Signals Received
– Lateral wave– Subsurface– Back-wall echo– Mode converted (shear wave) echo
Define top and bottom of part Note phase change
Basic Principles of the TOFD Technique
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TOFD: Typical Setup
Transmitter Receiver
Lateral wave
Upper tip
Lower tip
Back-wall reflection
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A-Scan SignalsTransmitter Receiver
Lateral wave
LW
Upper tip Lower tip
Back-wall reflection
BW
Some Typical DefectsUpper surface breaking crack
Back wall breaking crackHorizontal planar defect
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Upper Surface Breaking CrackTransmitter Receiver
Crack tip
Back-wall reflection
BW
Lateral wave is blocked
No Lateral wave
04/17/23 15
Back Wall Surface Breaking CrackTransmitter Receiver
Lateral wave
LW
Tip
Back wall echo blocked
No back wall echo
04/17/23 16
Horizontal Planar Defect(Lack of Inter-Run Fusion, Laminations)
Transmitter ReceiverLateral wave
LW
Back wall reflection
BW
Reflection echo
Reflected signal
04/17/23 17
Data Visualization
White+
Black-
Amplitude
Time
Time
One A-scan picture is replaced by one gray-coded line
04/17/23 18
Data Visualization
D-scan
Uppersurface
Back wall
A-scanLW
BW
04/17/23 19
Calibration Tools A-scan
D-scan
PCS
Tc
LW BW
Parameters:PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall
Not all of the parameters need to be known
04/17/23 20
Measurement Tools A-scan
D-scan
Cursors
t1 t2
l
Pt1,t2 d1, d2 and h are automatically calculated
d1d1h
Build-in calculator
04/17/23 21
Defect Position Influence
Transmitter ReceiverS S
d
t0 t0
x
04/17/23 22
Defect Position Uncertainty
Transmitter ReceiverS S
t2t1
Constant timelocus
(t1+t2=ct)
dmin dmax
In practice:Maximum error on absolute depth position lies below 10 %.Error on height estimation of internal (small) defect is negligible.Caution for small defects situated at the back wall.
04/17/23 23
Transverse Scan
Upper surface
Back-wall
B-scan
Lateralwave
This type of scan yields a typical inverted parabola
Time will be minimum when probes are symmetrically positioned over the defect
04/17/23 Source: Ginzel
What do TOFD scans really look like?
Lack of Fusion
Porosity
Incomplete Penetration
Slag
TOFD images show the lateral wave and backwall, plus SW signals after and reflections from all defects
04/17/23 25
What do TOFD scans really look like?
Lateral wave is clearly seen in a good TOFD scan. Typically used for calibration.
On clean material, defects show up well.
Backwall is always strong. Watch for perturbations.
04/17/23 26
Recommended Solution
TOFD: YES BUT: do not forget the good things offered by the
standard Pulse-Echo technique SOLUTION: do both TOFD and PE
simultaneously, without reducing the scan speed
04/17/23 27
Typical Requirements for TOFD and PE Applications
Small, lightweight, 1 to 16 channels PE and TOFD software Lateral wave straightening Real-time averaging Multi-channel data acquisition and display Linearization for true depth on flat or cylindrical
surfaces Processing (data compression,..)
04/17/23 28
Calibration A-scan
D-scan
PCS
Tc
LW BW
PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall
Typical multi-channel UT instrument is very user friendly and guides you with a software Wizard
04/17/23 29
Weld 1(PL4882)
Toe crack
Lack of side wall fusion
Lack of root fusion
Porosity
12.5 mm
04/17/23 30
Weld 1 (plate 4882) with Pulse-Echo
TOFD and PE clearly show the defects embedded in this weld:
•Lack of Fusion (root)
•Lack of Sidewall Fusion
•Porosity
•Toe Crack
04/17/23 31
Parallel Scan
D-scan
Uppersurface
Back-wall
A-scanLW
BW
04/17/23 32
Recommended Solution
The system allows for simultaneous acquisition and analysis (inTomoview
only) of TOFD and PE
TOFDPE 45 SW PE 60 SW
04/17/23 33
Linearized Lateral Wave
Transmitter Receiver
Lateral wave
Couplant thickness variation Change in time of flight
04/17/23 34
Linearized Lateral Wave
Transmitter Receiver
Lateral wave
Misalignment variations Change in time of flight
04/17/23 35
Linearized Lateral Wave
Transmitter Receiver
Lateral wave
Small mechanical variations of probe separation Change of time of flight
04/17/23 36
TOFD Advantages Excellent PoD for mid-wall defects Good detection of mis-oriented defects Can characterize surface-breaking defects Excellent sizing for defects in transverse TOFD mode,
especially with signal processing Tolerable sizing for defects in linear mode Works very well in conjunction with pulse-echo
04/17/23 37
TOFD Limitations Dead zone of ~3mm at outer surface Potential dead zone at inner surface Prone to “noise” Over emphasizes some benign defects, e.g.
porosity, laminations, interlamellar LoF Not easy to interpret
04/17/23
A Few Final Words on TOFD Is the best defect sizing technique available when correctly
set-up Use in conjunction with pulse-echo for code and PoD
reasons
04/17/23 39
SignalsLateral Wave
Back-wall Echo
04/17/23 40
Calculator
TOFD probe separation can be calculated with basic mathematical formula or Excel calculator tools
04/17/23 41
Calculator
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04/17/23 48
Calculator
-13.6
-28.9
-55
-45
-35
-25
-15
-5
5
-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80
X-axis (mm.)
Z-a
xis
(mm
.)
Note volume coverage less than ideal at this PCS (missing upper third). -12dB beam transmit only used for coverage calculation
7MHz 100mmPCS 1.5 cycles assumed
04/17/23 49
Calculator
Increase refracted angle to 65° improves coverage without compromising resolution (for these specific conditions) PCS remains the same.
-6.6
-23.3
-55
-45
-35
-25
-15
-5
5
-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80
X-axis (mm.)
Z-a
xis
(mm
.)
04/17/23 50
Flaw Tip
Flaw lengths parallel to the surface can be measured from the TOFD image by fitting hyperbolic cursors similar to SAFT correction but SAFT post-processes the data
04/17/23 51
TOFD - SAFT
Pre-SAFT processing – Post-SAFT Processing
04/17/23 52
Limitations of TOFD Cannot detect all defects Limited coverage results from two potential
dead zones– Dead zone near the surface
» as a result of the lateral wave
– Dead zone at the backwall» resulting from the width of the backwall reflection
Other Typical Defects
04/17/23 54
Near Surface Crack
The crack blocks the Lateral WaveAnd the lower tip appears on the A-scan
21
1
2
04/17/23 55
Incomplete Root Penetration
21
Note the two signals from the top & bottom
12
3
4
1 2 3 4
04/17/23 56
Lack of Root Penetration
Note the inverted phase between LW and defect
1
2
3
1
23
04/17/23 57
Lack of Fusion - Side Wall
Note the two signals from the top & bottom
1
2
3
4
1
2
3
4
04/17/23 58
Porosity
Porosity may image in many forms whether individual or cluster
12
3
1
2
04/17/23 59
Transverse Crack
In the LW we can observe the wide beam effect on the crack
1
2
3
4
1
2
3
1
2
3
04/17/23 60
Concave Root
Distortion of back-wall echo
1
23
1
2
3
04/17/23 61
Lack of Fusion - Interpass
1
2
3
04/17/23 62
Choosing an Angle» Optimum Upper tip q ≈
64°» Optimum Lower tip q ≈
68°
From Charlesworth & Temple
Angle selected is a compromise for depth
May require selecting several “zones” for best results
04/17/23 63
04/17/23 64
Diffracted rather than reflected signals Longitudinal waves B-scan type imaging (side view) Accurate sizing capability (height) Fast scanning Interpretation of defects Less sensitive to defect orientation
TOFD Advantages
04/17/23 65
Blind area - near surface, backwall Weak signals Flaw classification limitation Interpretation of defects Sensitive to grain noise
TOFD Limitations
04/17/23 66
Codes and Standard British Standard European ASME
04/17/23 67
TOFD Guides Developed BS 7706 (1993) Guide to calibration and
setting-up of the ultrasonic time-of-flight diffraction (TOFD) technique for detection, location, and sizing of flaws. British Standards Institute 1993.
pr EN 583-6 (1995) Nondestructive testing- ultrasonic examination - Part 6: Time-of-flight diffraction technique as a method for defect detection and sizing.
04/17/23 68
ASME Adaptations to TOFD
ASME VIII Code Case 2235 (2000 Edition) Ultrasonic examination to be in accordance
with ASME Section V, Article 4 “Alternatively, for techniques that do not use
amplitude recording levels, …”. This has opened the door for TOFD to be
used on Section VIII pressure vessels
04/17/23 69
Root CrackTOFD technique Root crack defect
X-Ray
04/17/23 70
PorosityTOFD technique Porosity defect
04/17/23 71
Slag inclusionTOFD technique Slag inclusion defect
04/17/23 72
Lack of Root FusionTOFD technique Lack of root fusion defect
04/17/23 73
Root ConcavityTOFD technique Root Concavity defect
04/17/23 74
Incomplete Root PenetrationTOFD technique Incomplete root penetration defect
04/17/23 75
Over PenetrationTOFD technique Over penetration defect
04/17/23 76
Irregular Root PenetrationTOFD technique Irregular root penetration defect
04/17/23 77
Excess Weld CapTOFD technique Excess weld cap defect
X-Ray