TOFD Introduction
Time of flight Diffraction (TOFD) is an accurate defect sizing
UT technique not depend on sound amplitude reflected
from defect.
TOFD Uses longitudinal (compression) waves.
TOFD Pair of transducers are considered as part of AUT
system arrangements ( one or two channels of the multi
channel system).
TOFD could be used with both of Pulse echo or phased
Array Techniques.
Sizing of defect not related to its orientation.
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The Diffraction Phenomenon
Huygens' principle:
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TOFD CONCEPT & THEORY
The Time of Flight Diffraction technique employs the
use of two compressional wave transducers using
angles between 30°and 70°set at a calculated
distance (Probe Centre Spacing “PSC”) apart in the
pitch and catch mode.
When the compressional wave strikes a defect
diffracted waves are generated from the extremities
of the defect (oscillation of defect ends).
The diffracted waves are detected by the receiving
transducer.
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TOFD CONCEPT & THEORY
The first signal received (1)
is from the lateral wave.
This is a creeping wave at
the surface with the same
velocity of the compressional
or longitudinal waves.
The second (2) and third (3)
signals are the diffracted
waves from the tips of the
flaw. The fourth (4) signal
is the back-wall representing
one complete sound path.
The signals past the back-
wall are mode converted
Shear wave.
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TOFD Transducer Arrangement
Dual Transducer Technique.
Transmitter and Receiver mounted typically opposite of and
symmetrically about the volume of interest.
Typically mounted at fixed probe centre spacing (PCS).
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Crack Tip Diffraction
Diffraction occurs at both top and bottom
tips
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Volumetric Discontinuities
Reflection produces response from the top of
the discontinuity
Creeping wave produces response from the
bottom of the discontinuity.
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Manual TOFD Hand Scanner
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TOFD Standing Wave Pattern
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Typical TOFD Display
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TOFD Waveform with Mid-wall Discontinuity
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TOFD Depth Measurement
Constant depth loci form an ellipse with the beam
index points the focal points.
Consequently, some minor error for lateral
position offset. 13
Amplitude Independent
•Depth calculations based on pulse transit time
•therefore, so long as gain is sufficient for detection
•size calculation will not change
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TOFD Implementation –Weld Inspection
Application
Beam angle, transducer size, and PCS set to flood weld volume.
Uni-axial scan along weld length “B or D Scanning mode”.
Dead zone near test surface due to inability to resolve flaws from
lateral wave.
Maximum thickness (single scan) is about 2-2.5 inches.
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Near-Surface Dead Zone
For near-surface flaws, time difference between
lateral and flaw is not sufficient to separate the two,
therefore, near-surface flaws are hidden by lateral
wave.
increased PCS worsen the effect.
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Improved Near-Surface Detection and Sizing
Resolution
• Narrower PCS.
• Higher frequency probes.
• heavier probe damping (broader bandwidth).
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Depth Zone Coverage
Deeper zones generally use:
1. larger transducers.
2. lower frequency.
3. lower refracted angle.
4. larger PCS.
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TOFD Imaging
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2-Dimensional Raster Scan TOFD
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TOFD Image Showing No Indications
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Creep Macro-Crack
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TOFD technique advantages
TOFD technique offers the following advantages in
comparison to radiography:
I- Radiation hazard-free area – This allows work to be performed
by other personnel in the vicinity of the examination area
without any radiation safety precautions.
II- Fast, accurate results – This process provides information at
the examination area without the need to develop film,
determine radiograph quality conformance (i.e. density and
sensitivity) and film interpretation.
III- Less expensive than radiography per foot of weld.
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TOFD technique advantages
IV- Volumetric information – As opposed to radiography that
provides a 2-D view of a weldment, and radiography’s
detection difficulty with cracks not directly oriented to the
cone of radiation, this process can provide accurate length
and depth information of recorded indications.
V- Permanent record – The scan image files can be saved for
future reference and evaluation.
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Codes and standard
British Standard.
European
ASME
ASTM
API 1104 “19th Edition”
1. 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.
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Codes and standard
2. pr EN 583-6 (1995) Non-destructive testing
ultrasonic examination - Bart 6: Time-of-flight
diffraction technique as a method for defect
detection and sizing.
3. 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
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Codes and standard
4. In1998 ASTM E-1961 was published describing
the various aspects involved in pipeline
inspection using mechanised UT.
5. In the 19th edition API 1104 they revised its
description of UT requirements to include
mechanised systems.
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TOFD Limitations
1. Blind area - near surface, back-wall.
2. Weak signals.
3. Couplant loss.
4. Flaw classification limitation.
5. Interpretation of defects.
6. Sensitive to grain noise.
7. Coarse grained materials.
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