02:04 Jun-21-2007 Jack Tensen R & D, project leader Lismar Engineering b.v., Netherlands, Joined Mar 2000 6
I am working with TR probes (KK) and I have some questions about these probes. The two active elements in the probe are separated by a thin UT absorbing plate and this plate is located at right angels on the surface of the test object. The advantage of such a probe is of course that the surface reflection is blocked by the absorbing plate and the result is a good near surface resolution. In many cases the elements are not looking straight to the test object but under an angle (roof angle) so the overlapping beam plots areas are located more to the surface area (like focusing) to improve the near surface sensitivity. But there are also TR probes with no roof angle (0-degrees types). I like to know for what applications these type of probes are used and what is the advantage in such cases? Because to me the main advantage for TR probes is the typical near surface behavior? For looking deeper in the object a normal PE probe is also suitable so what is the advantage of a this TR 0 degree probe? Another thing is that the isolating plate has a (negative) effect on the beam shape. The overlapping area is an ellipse. What is the best lateral movement over the test object to get the best results, I mean hold the probe in such a way that the isolating plate or thin line (if you look in the probes delay line) is in line with the scan direction ? Or is better to do it at right angles with the scan direction? The probe is used for automatic testing of large areas (X-Y scanning, meander pattern). Y axis is fast scanning and in the X direction it is indexing along this axis. If the proper probe position is chosen for the Y axis what is the best accompanying index step distance?
Re: TR probes Yes, there are zero degree roof angle TR (dual element) transducers -- my company sells them in several styles. They are used for inspection of thick material, typically thicker than about 20 mm, where the pseudo-focus effect would be irrelevant. They have two advantages over typical single element contact transducers in such cases: they have high temperature tolerance for testing hot materials (typical contact transducers limited to 50 C, duals can go much hotter), and they can sometimes offer greater effective sensitivity to irregularly shaped flaws or corroded backwalls since you can use high pulser energy and receiver gain without problems from excitation pulse ringdown or backing noise.
When scanning with a dual, you would normally orient the sound barrier perpendicular to the direction of scanning. Many users would index by 50% of the transducer diameter (or focused beam diameter when using focused tranducers), but you can determine the effective beam width of your transducer from an appropriate flat bottom hole test standard and adjust the index accordingly if your test requirements so permit. Just be sure there is some beam overlap if you need 100% coverage.