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Tofd 2

TOFD is an ultrasonic technique that uses the time difference between diffracted signals from defect tips to determine defect size and depth. The document compares TOFD to radiography and pulse echo techniques for examining defects in thick walled stainless steel welds. TOFD was able to accurately determine defect sizes with less scanning time compared to pulse echo. While all techniques could detect defects, TOFD and radiography had the lowest percentage variations in measured defect size compared to actual sizes. The study concludes that TOFD compares well to conventional NDE and can replace radiography for inspecting thick walled welds.

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223 views22 pages

Tofd 2

TOFD is an ultrasonic technique that uses the time difference between diffracted signals from defect tips to determine defect size and depth. The document compares TOFD to radiography and pulse echo techniques for examining defects in thick walled stainless steel welds. TOFD was able to accurately determine defect sizes with less scanning time compared to pulse echo. While all techniques could detect defects, TOFD and radiography had the lowest percentage variations in measured defect size compared to actual sizes. The study concludes that TOFD compares well to conventional NDE and can replace radiography for inspecting thick walled welds.

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TIME-OF-FLIGHT DIFFRACTION

TECHNIQUE AN ALTERNATIVE
TO RADIOGRAPHY
EXAMINATION OF THICK
WALLED STAINLESS STEEL
WELDMENT
R.Subbaratnam, Dr. B. Venkatraman and
Dr.Baldev Raj
Indira Gandhi Centre for Atomic Reasearch,
Kalpakkam, INDIA 603 102
TOFD – An Introduction
 Developed by Dr Silk in 1980’s
 Two probes used a transmitter and a receiver
 The time between the Lateral Wave and the Time
between the diffracted waves between the top and
bottom of the discontinuity is taken for arriving the
location, depth and orientation of the discontinuities
 Now it have been taken by the international codes like
ASME to replace Radiography
TOFD – An Introduction
 The defect sizing technique based on the
measurement of time difference between the
diffracted signals from the defect tips is called time-
of-flight diffraction technique.
 The diffracted waves originate at the discontinuity
tips and thus their separation in space, and hence in
time, directly relates to the size of the defect.
 Sound energy from the lower tip will arrive later than
that from the upper tip and this time delay relates to
the defect dimensions.
 The sound energy travels along the surface (lateral
wave) and that reflected from the back wall provides
information about the location of the defect within the
Geometrical model of TOFD
 Schematic  Typical A-scan
representation
Advantages of TOFD
 Use of higher frequencies

 Accurate Sizing

 Angle of examination is not the criteria for detection as


that in Pulse Echo Tech.

 Number of Scanning is less and hence, less scanning


time

 Focusing at different thicknesses of specimen by varying


the transducer separation distance
Advantages of TOFD over pulse echo UT
and RT
 Rapid scanning is possible and weldments can be
scanned in single pass making this technique more
efficient and faster.
 Discontinuity size and depth can be very accurately
determined. Since the technique is based on the
detection of diffracted signals, it is not affected by
the orientation of the discontinuity and angle of
examination.
 Longitudinal angle beam being used by TOFD
makes it possible to examine thick austenitic
stainless steel weldments.
 Real time discontinuity monitoring is possible and
the data can be stored for further reference and
Experimental details
 Material: 316 L Austenitic stainless steel
 Weld process: Shielded Metal Arc Welding (SMAW)
 Induced defects: Lack of Penetration, Slag and
Porosity
352
220 mm

25 mm 2 mm Land

Flush Ground 2 mm Root Gap

Edge preparation
Radiographic Parameters
 X-ray Source: Seifert 200MF
 Voltage: 180 kV
 Exposure: 60 mA – mins.
 SFD: 800 mm
 Film: Agfa D7
 Technique: Single Wall Single Image (SWSI)
 Image Quality Indicator: Plaque type
 Processing: Manual
 Sensitivity Achieved: 2-2T
 Radiographic Density : 2.0 – 2.5
Ultrasonic Parameters
Pulse Echo (Manual) [PEM]
 Equipment: USD 10, M/s Krautkramer, Germany
Pulse Echo (Automatic) [PEA]
 Equipment: MICROPLUS of AEA Technology, UK
 Software: μ-scan
TOFD
 Equipment: MICROPLUS of AEA Technology, UK
 Software: TOFD
 Probes: WSY 45, 4 MHz - For all Tech.
Radiographic Images

 Lack of penetration

 Slag inclusion

 Porosity
Ultrasonic Images - Pulse echo [PEA]

Lack of penetration Slag inclusion

Porosity
Ultrasonic Images - TOFD

Lack of penetration Slag inclusion

Lack of penetration Slag inclusion


Porosity
Across weld Across weld
COMPARISON BETWEEN
RADIOGRAPHY, PEA & TOFD
Length and Depth of Discontinuities and
Porosity Area Arrived by NDE Methods Used
LOP 1 LOP 2 Slag 1 Slag 2 Porosity

Discontinuity/
Exam method
Length Depth Length Depth Length Depth Length Depth Length Depth Area (mm2)

Actual 29.5 -- 34.5 -- 34.5 -- 29.5 -- 24.5 -- --

RT 30.0 -- 35.0 -- 35.0 -- 30.0 -- 25.0 -- 120 & 70

PE Manual 35.0 10.6 -12.6 45.0 13.1 40.0 10.4 -11.8 35.0 11.8 -13.2 30.0 8.1 -10.76 140 & 80

PE Automatic 28.0 12.3 38.0 12.3 38.0 10.5 28.0 12.6 25.0 8.9 -10.5 130 & 75

TOFD 29.0 12.76 38.0 12.76 37.0 10.12 29.0 13.31 22.0 9.0 - 9.9 125 & 75
Percentage Variations on Sizes of
Discontinuities
Discontinuity/
LOP 1 LOP 2 Slag 1 Slag 2 Porosity
Exam method

RT 1.7% 1.45% 1.8% 1.7% 2.4%

PE Manual 18.6% 30.3% 12.7% 18.7% 22.5%

PE Automatic -5.1% 10.1% 10.1% -5.1% 2.0%

TOFD -1.7% 10.1% 7.3% -2.7% -11%


Percentage variation for LOP1

TOFD

PEA
Techniques

PEM

RT

-5 0 5 10 15 20
Percentage variation
Percentage variation for LOP2

TOFD

PEA
Techniques

PEM

RT

0 5 10 15 20 25 30
Percentage variation
Percentage variation for Slag1

TOFD

PEA
Techniques

PEM

RT

0 2 4 6 8 10 12
Percentage variation
Percentage variation for Slag2

TOFD

PEA
Techniques

PEM

RT

-5 0 5 10 15 20
Percentage variation
Percentage variation for Porosity

TOFD

PEA
Techniques

PEM

RT

-10 -5 0 5 10 15 20 25
Percentage variation
Reasoning for Variation
 The variation on LP and Slag with respect to
both Pulse Echo techniques are due to
Beam Divergence
Orientation of the discontinuity
acoustic beam interaction with the discontinuity

 In the case of variation only on porosities in


case of TOFD is due to diffracted signals. But it
was observed in the authors lab that specimens
with single pore or SDH was clearly identified.
Conclusion
 The experimental work shows TOFD compares
well with the conventional techniques such as
radiography and PE ultrasonics for defect
detection in thick walled weldments.
 TOFD could estimate the size of defects with less
scanning times compared to automated pulse
echo.
 RT can be Replaced by TOFD than PE
Techniques

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