You are only as good as the knowledge you acquire through the experience of others. The caliber of those doing this training will be directly reflected in your abilities and a direct reflection of the teaching abilities of those who trained you. There were three key persons in my career that turned the lights on for me. George Lawrence, Emory Roberts, and Hector Diaz. I consider Emory to be the best I've ever seen for manual UT. George was a walking encyclopedia of UT theory, and Hector was the best AUT/TOFD hand I've ever seen. I was very very fortunate to have had the opportunity to pick their brains for their collective experience and knowledge and they were professional and conscientious enough to freely give of that knowledge to people who wanted to learn. They themselves were a collection of knowledge of those who mentored them and were not afraid to state so. There were others who filled in one hole or another in my knowledge base for UT, but those three are the cornerstones of my abilities.
Alternatively, if you are unfortunately saddled with being trained by people who are legends in their own minds, you will end up with the antithesis of the personal experience I describe.
I've seen a lot of techs(?) over the years who can only turn a machine on and go through the motions without any real understanding of what it is they are doing. As long as the machine tells them it's so, well that's the way it is. They either never took the initiative to learn the physics and mechanics behind UT or they were unfortunately trained by a con artist that only muddied their understanding of the method.
I would be very very careful if I were being trained for any method. Look up the books, find anything you can on the method. Independently verify what it is you are being told by that trainer. In short, take the initiative and maintain a questioning attitude. If the person training you can't back up in writing what they are trying to teach you with authoritative text, they are probably just a legend in their own mind.
With that said, I'll try to address the technical aspects of your post.
Maxpayne stated it this way "backing bar will have a characteristic reflector from the root zone"
Which is correct in so far as it goes. Long experience will eventually reveal the "characteristic reflector".
However, what is it and why is it.
Looking at it from the back diffraction standpoint (similar to TOFD or aka tip echo method) an understanding of the echodynamics of the particle motion will help your ability to discern the difference between a benign backing bar signal and a flaw signal.
ASME Section V 2007 edition has attempted to alleviate the general misunderstanding of what that sound is doing, and the wave mechanics behind it. Appendix N of article 4 has given a fairly good basic version. NDT.net also has a lot of good information in this regards from the Euro perspective.
If you dig out this particular appendix note that for TOFD the positive and negative phase of the waveform is presented as a white and black line. Look at the images and you will note that the entrant surface and the back surface are 180 out of phase with each other.
http://www.kettering.edu/~drussell/Demos/reflect/reflect.htmlA look at these waveforms will be enlightening. The hard backsurface of the steel creates a 180 phase change, passage through the opening of a fully penetrated backing bar will create the centerline 180 reflection, and from either edge of the opening a tip diffraction from either edge of the tie in (one on the front side of the main signal and one on the back depending on the distances involved they may both be on one side or the other depending on the time) the upper tip will be negative, the bottom of it will be positive, however; all of them will be surrounded by the 180 out signal as the base metal will give you a hard edge reflector as well. There are many other factors involved, but if you've read this, you now have a starting point to pick up the rest for yourself. (this post is getting long enough as it is)
Expanding your timeline with the RF screen for verification purposes, wheither or not your looking at a lack of pen, or a root area lack of fusion will be as clear as a neon sign once you've trained yourself to recognize the "characteristic reflector" as maxpayne put it.
For those who have long experience in it, they can typically recognize it with a fully rectified screen.
While that may sound like hocus pocus to the prospective UT technician, the physics of it don't change for any of us.
this is all information that machine will not give you. The machine assumes a straight line, and does not take into account beam spread, echodynamics/wavemechanics, nor does it take into account mode conversion (another tell tale sign of a backing bar when your scanning at 60 or 70 typically) and a host of other factors.
Learn the method, not the machine is the best advice I can give you. Doing anything else just makes you an operator, and not a technician.
Regards,
Gerald
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