20:21 Jan-20-2010 Re: Half-skip and full-skip Yes your picture could be called pitch and catch technique. Also tadem and dual probe I think.
Full skip is the distance travelled when the angle beam has travelled down then up.
Half skip is half.
I.e If you measure the distance between the 2 probes shown above this will be full skip distance as you see the beam has travelled all the way down, hit the bottom (back wall reflection?) then travelled all the way up to the surface again.
05:32 Jan-21-2010 Re: Half-skip and full-skipIn Reply to anjafo79 at 20:21 Jan-20-2010 .
The English verb "to skip" describes a regular cyclical up-and-down movement - witness "skipping rope", "skipping stones" on water. In ultrasonics I would confine its use to the single probe pulse-echo technique. Take away the right-hand side probe from your diagram and it shows a 2-leg full-skip, one and half skips is 3 legs back down to the lower surface.
Your diagram is pitch-catch, but contrary to anjafo, I would not call it tandem, that being when both probes are facing the same direction, one transmitting and the other receiving only signals reflected from a discontinuity, particulalrly used for detecting perpendicular, specular weld defects such as Lack of Sidewall Fusion (LoSWF).
09:53 Jan-21-2010 Re: Half-skip and full-skipIn Reply to Nigel Armstrong at 05:32 Jan-21-2010 .
Thank you anjafo and Nigel!
I understood the "skip" terminology (thanks also to Ed Ginzel who replyed to me directly).
I am agree with Nigel that picture is not TANDEM.
The problem is how to describe that technique is pitch-and-catch and we can obtain information about shadowing of backwall reflection by vertical cracks (through transmission method) and find diffraction signal from tips.
Maybe it is TOFD, but this technique can be implemented also with shear wave (without TOFD`s lateral wave)... And probes are moving in cross-section.
So aI called it "pitch-and-catch mirror-echo techique" in a paper... Will the NDT specialist understand it?
11:09 Jan-23-2010 Re: Half-skip and full-skipIn Reply to Andrey Bazulin at 09:53 Jan-21-2010 .
I would have no problems equating the diagram with the term "pitch-and-catch" with the expectation of monitoring the backwall echo for an amplitude drop caused by an obstacle in either leg of the sound path, but "mirror-echo" would confuse me. My interpretation for "mirror-echo" would be "specular" which leads on to pulse-echo and looking for a reflected amplitude, so the combined terms are, in my opinion, contradictory. I understand that the backwall can be considered the mirror and the received sound energy the echo from the backwall, but traditional UT speak is that the defect acts as the mirror reflecting the signal back to the single probe.
Judging from your post, Andrey you mean this diagram to describe a shear-wave parallel-scan pitch-catch examination. Without the parallel-scan only a small portion of the cross-sectional material would be interrogated due to the small beamspread of the shear wave probe. I think that the diagram would benefit by indicating on it the parallel scan (bidirectional arrow).
What objective does the test have that cannot be met by more conventional examinations. I ask as I was thinking of the parallel-scan required to interrogate the full cross-section of the weld means that a small lack of sidewall fusion on the same side of the weld as the transmitter and close to the surface would block as much energy as a large defect close to the top surface in the far side of the weld. And what role would defect orientation play in evaluation.
P.S. Pitch-catch definition from the NDT encyclopaedia:
"An ultrasonic testing technique involving the use of two separate probes; one probe being used to transmit the ultrasonic energy into the body and the other being positioned so as to receive the reflected energy from a discontinuity. In variations of the technique, more than two probes may be used. Also called double probe technique".
I should also have refined my description of the tandem technique to the transmitting and receiving probes located usually in a jig to maintain constant separation and orientation facing in the same direction and on the same side of the weld.
06:44 May-25-2013 Re: Half-skip and full-skipIn Reply to Andrey Bazulin at 16:17 Jan-20-2010 (Opening).
A SHEAR WAVE FROM AN ANGLE BEAM TRANSDUCE PROGRESS THROUGH A FLAT TEST PIECE BY REFLECTING FROM THE SURFACE AT POINTS CALLED "NODES". THE LINEAR DISTANCE BETWEEN TWO SUCCESSIVE NODES ON SAME SURFACE IS CALLED "SKIP DISTANCE".
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