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?