060301 tofd training
DESCRIPTION
NDE Technique >> TOFDTRANSCRIPT
www.zetec.com
Principles and Application Principles and Application of the TOFD UT of the TOFD UT
TechniqueTechnique
Advanced NDT Training Programme,November 2004
2
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
3
The Diffraction PhenomenonThe Diffraction Phenomenon
CRACK
Diffractedwave
Diffractedwave
Incidentwave
Reflectedwave
4
The Diffraction PhenomenonThe Diffraction Phenomenon• Huygens’ principle:
Each point of object act as new source of spherical waves
incoming wave makesobject vibrate
5
The Diffraction PhenomenonThe Diffraction Phenomenon
All directions
Low energy
Dependent of incidence angle
CRACK
Diffractedwave
Diffractedwave
Incidentwave
Reflectedwave
6
The Diffraction Phenomenon: SummaryThe Diffraction Phenomenon: Summary
• Incident wave reflected wave
• Incident wave diffracted waves emitted by defect
boundaries
• Cylindrical/spherical waves emitted in all directions
• Amplitude typically 10 to 20 dB below specular (direct)
reflection
7
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
8
Slot or crack
• Pulse-echo tip diffraction method (satellite-pulse observation technique)
Time
Amplitude
2
2
Tip diffraction1
1
Corner reflectionTOF
Angle
TOF, ANGLE & VELOCITY HEIGHT
Conventional Use of Diffraction Conventional Use of Diffraction
9
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
10
Transmitter Receiver
Lateral wave
Upper tip
Lower tip
Back-wall reflection
Principles of TOFD Principles of TOFD
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Principles of TOFD : Basic Set-UpPrinciples of TOFD : Basic Set-Up
• 2 probes (transmitter, receiver)
• Wide beam, Long. waves
• Symmetrically to the weld center
• Lateral wave (sub-surface LW)
• Back-wall reflection
• Diffraction signal detection (high receiver sensitivity)
12
Transmitter ReceiverLateral wave
LW
Upper tip Lower tip
Back-wall reflection
BW
Principles of TOFD: A-Scan SignalPrinciples of TOFD: A-Scan Signal
13
Lateral wave
LW
Upper tip Lower tip
Back-wall reflection
BW+
-
+
-
Principles of TOFD: Phase DifferencesPrinciples of TOFD: Phase Differences
14
Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement
• Based upon:
• Accurate flight-time measurements
• Simple trigonometric equations
• Achieved by UltraVision software
15
S
Initial pulse
Transmitter ReceiverS
d
LW BW
t0 t0
t
Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement
16
Transmitter ReceiverS S
d
t0 t0
0
22
.2.2
tc
dSt
Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement
17
c
TStt B
22
0 2.2
LB tt
STSc
.22 22
Transmitter ReceiverS S
d
t0 t0
PCS
T
SPCS .2 (Probe Center Separation)
c
Stt L
.2.2 0
Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement
18
Transmitter ReceiverS S
d
t0 t0
220
2
.2.2
Sttc
d
Principles of TOFD: Principles of TOFD: Flaw Depth MeasurementFlaw Depth Measurement
19
Transmitter Receiver2S
d1
12 ddh
d2
Since only flight-time measurements are used to calculate the height, very accurate height sizing is possible. In practice, 1 mm accuracy on real cracks is
achievable (0.1 mm on artificial reflectors)
Principles of TOFD: Principles of TOFD: Flaw Height MeasurementFlaw Height Measurement
20
Principles of TOFD: Principles of TOFD: Flaw CharacterizationFlaw Characterization
• In most cases, no correlation between amplitude and
importance of flaw
• Typical signature for each flaw type
• Interpretation of phase variations
• (Partial) loss and/or variation of LW, BW : indication for
surface-breaking cracks
21
Transmitter Receiver
Crack tip
Back-wall reflection
BW
Lateral wave is blocked
No lateral wave
Near-Surface Breaking Cracks Near-Surface Breaking Cracks
22
Transmitter ReceiverLateral wave
LW
Tip
Back-wall echo blocked
No back-wall echo
Far-Surface Breaking Cracks Far-Surface Breaking Cracks
23
(lack of inter-run fusion, lamination)(lack of inter-run fusion, lamination)Transmitter Receiver
Lateral wave
LW
Back-wall reflection
BW
Reflected signal
Reflected signal
Horizontal Planar Defects Horizontal Planar Defects
24
Principles of TOFD : SummaryPrinciples of TOFD : Summary
• Two probes, pitch & catch configuration
• Longitudinal waves
• Lateral wave (LW), back-wall echo (BW)
• Diffracted signals from defect edges
• Phase difference between tip & bottom signals
• Flaw depth and height are determined with high accuracy, based on
flight-time calculations
• Not based on amplitude
25
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
26
Practical ImplementationPractical Implementation
• General Set-Up
• Probe Considerations
• Data Processing and Presentation
• Manipulator
• Scanning Types
27
Position encoder
Magnetic wheels
UT probes
Weld
Z-Scan UT
Scanner
Probes
Z-Scan UT
Practical Implementation: Practical Implementation: General Set-UpGeneral Set-Up
28
• Propagation mode and angle
• Time domain resolution
• Beam characteristics
• Synthesis table
Practical Implementation: Practical Implementation: Probe ConsiderationsProbe Considerations
29
• Longitudinal Waves :
• Fastest waves, easy interpretation, no confusion with mode
converted waves (SW)
• Relation between signal phase and signal origin (tip, bottom)
• Stronger diffracted signals
Propagation ModePropagation Mode
30
• Relation between probe angle and amplitude of
generated diffracted signals
• Precision on flaw height measurement
• Inspected volume coverage
Compromise
In many cases 60 degrees is a good compromise
Probe AngleProbe Angle
31
Time Domain ResolutionTime Domain Resolution
• Measurements based on flight-time
• Requirement for short ultrasonic pulses (importance of
UT equipment : probe excitation parameters)
• Higher frequencies than standard UT (pulse-echo)
examinations
32
Beam CharacteristicsBeam Characteristics
• Wide beam to cover volume to be inspected
• High frequency small probe aperture lower
sensitivity
Compromise
Aperture
Beam Beam
33
Probe Selection TableProbe Selection Table
Wall-thickness
(mm)
Frequency
(MHz)
Diameter
(mm)
Angle
(°)
t 15 7.5 / 10 / 15 6 60 / 70
15 < t 35 7.5 / 10 6 60
35 < t 100 + 5 10 30 / 45 / 60
34
• Processing of all non-rectified signals requires powerful
computing capability
• Mass amount of complex signals requires a simple way
to visualize the data
• Calculations require easy to use tools
Practical Implementation: Practical Implementation: Data ProcessingData Processing
35
• Huge amount of data
• Need for phase information
Practical Implementation: Practical Implementation: Data ProcessingData Processing
36
Practical Implementation: Practical Implementation: PresentationPresentation
White+
Black-
Amplitude
Time
Time
One A-scan picture is replaced by one gray-coded line
37
B-scan
Near-surface Back-wall
A-scanLW
BW
Practical Implementation: Practical Implementation: PresentationPresentation
39
• Flaw depth is expressed by a complex mathematical
equation
• Basic tools are needed for
• Initial calibration
• Performing depth and height measurements
Practical Implementation: Practical Implementation: PresentationPresentation
40
A-scan
B-scan
PCSt0 t0
Tc
LW BW
PCS, thickness, sound velocity, probe delay, lateral wave or back-wall flight-time
Not all parameters have to be known
Practical Implementation: Practical Implementation: CalibrationCalibration
41
Cursors
Build-in calculator
t1,t2 d1, d2 and h are automatically calculated
A-scan
B-scan
c
d1d1h
t1 t2
l
P
Practical Implementation: Practical Implementation: Measurement ToolsMeasurement Tools
42
Very simple to use
Magnetic wheels
Manual (or motorized)
One axis position encoding
Basically 2 probes, must be able to hold more (PE)
Easy and precise adjustment of probe separation is needed
Position encoder
Magnetic wheels
UT probes
Weld
Practical Implementation: Practical Implementation: ManipulatorManipulator
43
• Non-parallel, along defect axis
• Parallel, across defect axis
Practical Implementation: Practical Implementation: Scanning TypesScanning Types
44
Weld
Non-parallelscan
Perpendicularto probe beam direction
DetectionInitial sizing
High speed inspection
Most frequently used for weld inspection
Scanning Types: Scanning Types: Non-Parallel ScanNon-Parallel Scan
45
• Limitations :
• Defect depth measurement only accurate when probes are
symmetrically positioned with regard to defect
• Uncertainty on lateral position of defect results in height sizing
error
Scanning Types: Scanning Types: Non-Parallel ScanNon-Parallel Scan
46
Influence of Defect Position UncertaintyInfluence of Defect Position Uncertainty
Transmitter ReceiverS S
d
t0 t0
x
47
Influence of Defect Position UncertaintyInfluence of Defect Position Uncertainty
Transmitter ReceiverS S
t2t1
Constant timelocus
(t1+t2=ct)
dmin dmax
In practice:Maximum error on absolute depth smaller than 10%Height sizing error on internal (small) defect is negligible.Caution for small defects situated at the back-wall.
48
Parallel ScanParallel Scan
Weld
Parallel scan
Parallelto probe beam direction
Accurate sizing and positioning
49
Back-wallB-scan
Lateral waveThis type of
scan yields a typical inverted
parabola
Flight-time will be minimal when probes are positioned symmetrically over defect
Parallel ScanParallel Scan
50
Parallel Scan : LimitationsParallel Scan : Limitations
• Weld inspection: weld cap often reduces or makes
impossible the extent of the scan.
51
Practical Implementation : SummaryPractical Implementation : Summary
• Simple, light weight set-up
• High speed inspection
• L-waves, wide beam, high frequency probes
• Data visualization and analysis tools
• Two scan types : non-parallel, parallel
52
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
54
British StandardBritish Standard
• Guide to calibration and setting-up of TOFD technique,
BS 7706 (1993)
• Detailed document with useful practical guidelines for
setting up TOFD examination
• Guide for interpretation of TOFD data
• Examples of typical weld defects
55
CENCEN
• TOFD technique as a method for defect detection and
sizing, CENV 583-6 (1997)
• Preliminary standard
• Recommended probe parameters with regard to different
wall thicknesses (frequency, crystal size, nominal angle)
56
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
57
• Advantages (+) :
• rapid, flexible
• reliable detection of both
volumetric and planar flaws
• amplitude insensitive
• accurate height sizing of flaws (±
1 mm)
• independent of weld configuration
• on-line interpretation, permanent
record
• Limitations (-) :
• “dead zones”
• masking of flaws
• influence of lateral defect position
uncertainty
• some cases require
complementary pulse-echo UT
• strongly attenuating materials?
Advantages and Limitations of TOFDAdvantages and Limitations of TOFD
59
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique
60
Zetec SolutionZetec Solution
• TOFD : YES
• BUT : let’s also benefit from the advantages offered by
the standard Pulse-echo (PE) technique
• SOLUTION: perform TOFD and PE simultaneously,
without reducing inspection speed
61
The Z-Scan UT system allows for simultaneous
acquisition and analysis of TOFD and PE
TOFDPE 45 SW PE 60 SW
Zetec SolutionZetec Solution
63
• Portable system (11.7 kg)
• Multi channel data acquisition and display
• Real-time averaging
• UltraVision software supports simultaneous Pulse-Echo and
TOFD examination
• Calibration for true depth on flat and cylindrical surfaces
• Parabolic cursors for improved length sizing
• Lateral wave straightening and removal for TOFD
• SAFT processing
Zetec SolutionZetec Solution
64
• The Diffraction Phenomenon
• Conventional Use of Diffraction
• Principles of TOFD
• Practical Implementation
• Codes and Standards
• Advantages and Limitations of TOFD
• Zetec Solution
• Demonstration
Time of Flight Diffraction TechniqueTime of Flight Diffraction Technique