where expertise comes together - since 1996 -

The Largest Open Access Portal of Nondestructive Testing (NDT)

Conference Proceedings, Articles, News, Exhibition, Forum, Network and more

where expertise comes together
- since 1996 -

886 views
Technical Discussions
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1304

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1304
02:22 Dec-04-1999
What defines the TOFD Technique?


Mark Davis recently made a comment about the answer to the question on the number of probes used in TOFD. Like Mark, I too was a bit concerned by the answer provided that only one probe be needed for TOFD.
However, I thought more on this and came to no conclusion other than this leads to the point that a better definition of TOFD may be needed.

To use the acronym TOFD literally we could assume that the Time Of Flight Diffraction technique monitors the arrival times of diffracted sound wave pulses. In this case then any technique looking at arrival times off diffraction boundaries would satisfy the condition and would constitute a TOFD technique. This would include the simple two probe "delta" technique or the pulse-echo techniques popularised by Harumi's Tip Echo Working Group of the Japanese Society for NDT or the more familiar transmit-receive configurations we NORMALLY associate with TOFD.

Charlesworth and Temple (Engineering Applications of Ultrasonic Time-of-Flight Diffraction, Research Studies Press, England, 1989, page 4) state "The question then arises; if, in the final analysis pulse-echo inspection, while usually based on a search for specular reflections, is actually relying in some cases on diffracted waves for accurate sizing, would it not be advantageous to design a technique which is aimed directly at those diffracted waves and which deliberately avoids the specular reflections which may mask them?" They then go on to credit Maurice Silk with the invention of the technique (in 1979) which does this. Throughout their subsequent description they infer that two probes are used (one transmitting the other receiving) and that the probes are placed on either side of the defect to be sized. They also indicate that a "typical" signal in TOFD consists of a lateral wave (the first pulse from the wave travelling by the most direct route) followed by zero or more diffracted pulses from defects and then the backwall echo.

But these generalisations for the TOFD set-up break down as geometries other than simple butt welds on flat plate are investigated. Applications to T-K and Y node welds may not always have the probe pairs exactly opposite each other and techniques for thick sections usually require small angles with greater probe-centre-spacing with the effect of eliminating the detection of a lateral wave.

So what constitutes the TOFD technique as different from a pulse-echo technique looking at diffracted signals??

I have a difficult time trying to imagine how I would set up a single probe to provide me with a lateral wave and backwall signal BUT as I pointed out, the lateral wave will not be detected when testing some thick sections…so the presence of a lateral wave is not the defining method.

Some would advocate that TOFD is a sizing method using diffracted waves that are forward scattered only. The Delta Technique (described in the ASNT Handbook second edition vol 7 page 278) also uses forward scattered sound and is not considered TOFD. Also, the relative direction of some diffractions in a T, K or Y node are actually backscattered to the receiver on the other leg, yet this IS a common TOFD configuration.

Y.Aikawa (Ultrasonic Defect Sizing - Japanese Handbook- second edition. Japanese Society for NDT, 1996, page 54) used both a forward AND a backscattered technique and just called his sizing technique Tip Echo (not TOFD).

Are there any suggestions as to what is unique about a TOFD technique that would separate it from other items we also use for sizing and detection?
Ed



 
 Reply 
 

Product Spotlight

NEW - TD Focus-ScanRX

The NEW Next Generation Advanced UT platform, TD Focus ScanRX - Also available as a card stack solut
...
ion. Key Improvements 1. Data acquisition is significantly faster than current design 2. Better aesthetic – closely aligns with HandyScan RX 3. Improved IP rating (Target IP66) 4. Ruggedized housing 5. Connectors are protected from impact and ingress 6. Integrated stand and separate retractable handle easy to keep clean) 7. Touchscreen with ruggedized display glass 8. 3-Axis encoder input
>

HD-CR 35 NDT Computed Radiography System

Portable high-resolution CR scanner for all radiography applications - weld testing, profile images
...
and aerospace. No matter what type of radiographic testing you are performing, the unique TreFoc Technology of the HD-CR 35 NDT imaging plate scanner always guarantees the highest image quality.
>

FMC/TFM

Next generation for Phased Array UT is here now with FMC/TFM! Have higher resolution imaging, impr
...
oved signal to noise ratio, characterize, size and analyze defects better with access to several wave mode views and save raw FMC data for higher quality analysis.  Some of the benefits are:
  • Beautiful Image! Easier to understand what you're looking at
  • Completely focused in entire image or volume
  • Much easier to define setups before inspection
  • Easier to decipher geometry echoes from real defects
  • Oriented defects (e.g. cracks) are imaged better
  • See image from different wave modes from one FMC inspection
  • FMC data can be reprocessed/analyzed without going back to the field
>

NDT.net launches mobile-friendly design

NDT.net has revamped its website providing a mobile-friendly design.The front page received a comp
...
letely new design and all other sections are now reacting responsively on mobile devices. This has been a major step to make our website more user- friendly.
>

Share...
We use technical and analytics cookies to ensure that we will give you the best experience of our website - More Info
Accept
top
this is debug window