TOFD L2
Basic Principle
The diffraction phenomenon
Incident
wave Diffracted
wave
Reflected CRACK
wave
Diffracted
wave
The diffraction phenomenon
Incident
wave Diffracted
wave
All directions
Reflected CRACK
wave Low energy
Dependent of
Diffracted incidence angle
wave
The 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
Principles of TOFD
Transmitter Receiver
Lateral wave
Upper tip
Lower tip
Back-wall reflection
Principles of TOFD : A-scan signals
Lateral wave
Transmitter Receiver
Back-wall reflection
LW BW
Upper tip Lower tip
Principles of TOFD : phase difference
Lateral wave
Back-wall reflection
+
LW + BW
- -
Upper tip Lower tip
Non-rectified A-scan presentation is needed to show the phase changes
Principles of TOFD : Flaw depth measurement
Based upon:
Accurate flight-time measurements
Simple trigonometric equations
Achieved by TomoView software
Principles of TOFD: Flaw depth measurement
S S
Transmitter Receiver
t0 t0
Initial pulse
LW BW
t
Principles of TOFD: Flaw height measurement
2S
Transmitter Receiver
d1
d2
h d 2 d1
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: Flaw 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
Near-surface breaking crack
Transmitter Receiver
Lateral wave is blocked
Back-wall reflection
BW
No lateral wave Crack tip
Far-surface breaking crack
Transmitter Receiver
Lateral wave
Back-wall echo blocked
LW
No back-wall echo
Tip
Horizontal planar defect
(lack of inter-run fusion, lamination)
Transmitter Receiver
Lateral wave
Reflected signal
Back-wall reflection
LW BW
Reflected signal
Practical implementation: Probe
selection
Propagation mode
Probe angle
Time domain resolution
Beam characteristics
Synthesis table
Propagation mode
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
Probe angle
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
Time 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
Beam characteristics
Wide beam to cover volume to be inspected
High frequency small probe aperture lower sensitivity
Aperture
Beam Beam
Compromise
TOFD L2
Practical Implementation
Data processing and visualization
Amplitude
White
+
Time
- Black
Time
One A-scan picture is replaced by one gray-coded line
Data processing and visualization
A-scan
LW
B-scan BW
Near-surface Back-wall
Data processing and visualization
Data processing and visualization
Flaw depth is expressed by a complex mathematical equation
Basic tools are needed for
Initial calibration
Performing depth and height measurements
Calibration
PCS
t0 A-scan
t0
T
c LW BW
PCS, thickness, sound velocity,
probe delay, lateral wave or
back-wall flight-time
Not all parameters have to be
known
B-scan
Measurement tools
A-scan
d1
d1
h
t1 t2
Cursors
Build-in calculator
l
t1,t2 d1, d2 and h are P
automatically calculated
D-scan
Manipulator
Position encoder Very simple to use
Weld Magnetic wheels
Manual (or motorized)
One axis position encoding
Basically 2 probes, must be
UT probes able to hold more (PE)
Magnetic wheels
Easy and precise
adjustment of probe
separation is needed
Scanning types
Non-parallel, along defect axis
Parallel, across defect axis
Scanning types : Non-parallel scan
Most
frequently
Weld used for
Non-parallel weld
scan inspection
Perpendicular
to probe beam Detection
direction
Initial sizing
High speed
inspection
Scanning types : Non-parallel scan
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
Influence of defect position uncertainty
S S
Transmitter Receiver
t0 t0
x
Parallel scan
Weld
Parallel scan
Parallel Accurate
to probe sizing and
beam positioning
direction
Parallel scan
Flight-time will
be minimal
when probes
are positioned
symmetrically
over defect
Lateral
wave Near surface
Back-wall
B-scan
This type of scan yields a typical inverted parabola
Parallel scan : limitations
Weld inspection: weld cap often reduces or makes impossible the
extent of the scan.
Practical 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
TOFD L2
Codes and Standards
Codes and standards
British Standard
European Norm
American Standard
British 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
CEN
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)
Advantages & limitations of TOFD technique
Advantages (+) : Limitations (-) :
rapid, flexible “dead zones”
reliable detection of both masking of flaws
volumetric and planar flaws
influence of lateral defect
amplitude insensitive position uncertainty
accurate height sizing of flaws some cases require
(± 1 mm) complementary pulse-echo
independent of weld UT
configuration strongly attenuating
on-line interpretation, materials ?
permanent record
Is there any 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
PE 45 SW TOFD PE 60 SW
The system allows for simultaneous
acquisition and analysis of TOFD and PE
TOFD L2
Interpretation
1
The crack blocks the Lateral Wave and the lower tip
appears on the A-scan
1
2
Note the two signals from the top & bottom
1
2
3
Note the inverted phase between LW and defect
1
2
3
Note the two signals from the top & bottom
2 1
Porosity may image in many forms whether
individual or cluster
1
2
In the LW we can observe the wide beam effect
on the crack
1
2
2
3
3
Distortion of back-wall echo
1
2
LOF - Underpass
