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Technical Discussions
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
15:36 Aug-31-2015
To Focus or Not to Focus
Download

About 2-3 weeks ago, one of the forum members sent me email asking me about the topic of PAUT focussing. It seems that he was at a well-known training school and the instructors there told him that he should be focussing the beam for weld inspection. No doubt he had to do this to pass the course, but it did not seem right to him so he asked me for my thoughts.
I used Civa to explain why it should not be done.
The idea that training schools are telling students to do things that are not accomplishing useful results bothered me so I thought it might be helpful to put a discussion topic out to NDT.net, with some extra details (with thanks to Civa for making the explanation much more "visual").
Perhaps typical of my explanations, the item is rather long, so not suitable to fit in the normal sized discussion box. Therefore I have uploaded the thoughts as a pdf file.
I would be interested to learn of other people's experience with focussed beams for general weld inspection (apart from the zonal technique).

 
 Reply 
 
Philippe Rioux
Philippe Rioux
17:47 Aug-31-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 15:36 Aug-31-2015 (Opening).

hi Ed,
What do you think about inspecting on-service welds for detection using no focussing. Afterwards, better sizing using focusing where the flaw has been found.
Thus, we use focus advantages for dimensions.

Would you also recommend DDF for weld inspection? I am not sure though it is actually used sectorial scan (instead of just Linear-scan). I think that DDF could improve Sectorial scan "uniformity" .

Thanks for the PDF explanations by the way.
Very good to see that some people take the time to analyse some "no brainer".

Phil

 
 Reply 
 
Roberto Otero
Engineering,
OMEGA SERVICIOS SPA, Chile, Joined Aug 2004, 15

Roberto Otero

Engineering,
OMEGA SERVICIOS SPA,
Chile,
Joined Aug 2004
15
17:58 Aug-31-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 15:36 Aug-31-2015 (Opening).

Hi Ed! Excelent topic!... I think this idea (and concepts) should have appeared explicitly in PA related Codes. Especially when an advanced technique ( as PA) is taught in different parts of the world with different degrees of rigor.

 
 Reply 
 
Roberto Otero
Engineering,
OMEGA SERVICIOS SPA, Chile, Joined Aug 2004, 15

Roberto Otero

Engineering,
OMEGA SERVICIOS SPA,
Chile,
Joined Aug 2004
15
18:14 Aug-31-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 15:36 Aug-31-2015 (Opening).

And related to the dilemma proposed… I think a good strategy would be to inspect first without any focusing and on those cases and situations that need it… implement focusing to better size .

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
20:05 Aug-31-2015
Re: To Focus or Not to Focus
In Reply to Roberto Otero at 18:14 Aug-31-2015 .

Roberto et Philippe, you both seem to have the same opinion, and I concur with both of you. Any "initial" inspection in accordance with typical UT weld-inspection standards should be done without focussing. This would make relating PAUT results much more comparable to techniques and acceptance criteria used by manual UT (like ISO 11666). In fact, if no focussing is used, I see no reason why ISO 11666 cannot be used to evaluate welds inspected by PAUT. Workmanship acceptance criteria based solely on amplitude and length keeps it simple for all concerned.
If the extra effort is being put into a PAUT inspection to focus a beam on specific flaws, it suggests that sizing the flaw height and ligament is required. Reasonable precautions to avoid oversizing should then be taken (including focussing). But this implies fracture mechanics is being used so the entire process would typically be "qualified" to quantify the error.

 
 Reply 
 
massimo carminati
Consultant, AUT specialist
IMG Ultrasuoni Srl, Italy, Joined Apr 2007, 691

massimo carminati

Consultant, AUT specialist
IMG Ultrasuoni Srl,
Italy,
Joined Apr 2007
691
17:13 Sep-11-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 15:36 Aug-31-2015 (Opening).

Dear Ed,

it is always complicated to add something to your posts, but this time I would like to try anyway.

I believe that the main thing is to choose the proper aperture and use the natural beam shape instead of focusing. In other words, and if the combination of PA Unit and PA probe available allows me to do it, I would choose an aperture so that the region of interest is just after the near field; in other words, is like if I had a wide selection of conventional probes with different element size.

On weld inspection, and this is particularly true with narrow gaps, when I focus (knowing that I am in the near field!) i normally use the "projection" or "vertical plane" so that each focal law is focussed at a different material depth according to the zone it covers.

I hope this could help.

Massimo

 
 Reply 
 
Roberto Otero
Engineering,
OMEGA SERVICIOS SPA, Chile, Joined Aug 2004, 15

Roberto Otero

Engineering,
OMEGA SERVICIOS SPA,
Chile,
Joined Aug 2004
15
17:27 Sep-14-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 20:05 Aug-31-2015 .

Ed, I think this thread has still many elements to exchange ... which have not been mentioned...
I understand the idea behind when you say …. "reasonable precautions to avoid oversizing should then be taken"…is based on your experience and many previous papers, international Projects, etc... because it has been demonstrated that there is an oversizing inherent trend to the PA technique when we measure length and an under sizing trend when we measured height and ligament of defects... (based in P. Ciorau papers, for example)
In my experience I've had to deal with several "solutions" to take into account these trends ... some of which are not implemented in commercial PA equipment … such as ...
-Take into account the beam spread correction versus depth for the selected VPA in the sizing process without focusing...
-Perhaps using SAFT processing for those cases in which a “monostatic” (one element emitting and the rest of elements receiving) methodology has been employed in beam forming process…or the DDF technique (focusing in receiving mode for longitudinal waves)...
Are there other suggestions?

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
14:22 Sep-18-2015
Re: To Focus or Not to Focus
In Reply to Roberto Otero at 17:27 Sep-14-2015 .

Roberto, indeed there can be much spin off from the discussion on the need (or lack of need) to focus.
I posted this because I had learned that students were being told to carry out general weld inspection with a focussed beam. It is my opinion that this is not good guidance.
Generally people using workmanship acceptance criteria need only to detect an indication and estimate its length. This is the same whether we use manual UT with mono-element probes or we use PAUT with many focal laws. If we use the same principles (i.e. aperture and frequency of PAUT being similar to probe dimensions and frequencies of manual UT), it is reasonable to use the same acceptance criteria (if PAUT is not focussed).
Once the acceptance criteria changes and we are asked to "size" a flaw, then we have a "Part 2" to the story. Part 1 is detecting what might be in the material, Part 2 is sizing it for vertical extent and remaining ligament.
I have watched discussions on this forum where people are using the so-called 6dB drop for vertical sizing. But it is not really the same as we did in the 1970s and 1980s when using monoelements. Then we used a dB drop technique and collected the soundpaths and exit point positions on our test piece and then used our beam-spread overlay that we had constructed on the IOW block. This was the method to compensate for beam-spread oversizing. It was slow and not VERY accurate, but it significantly reduced the vertical size compared to not using the overlay. Today's technicians do not make overlays and still expect that the 6dB drop should provide some estimate of vertical extent...they are dreaming or simply uniformed.
In one of your earlier posts you suggested a two part strategy. First detection then sizing (when vertical sizing is required). But what method to use for sizing? That is often a Procedural Policy. Many sizing technique can be used. In some Codes, there is an expectation to demonstrate the accuracy of the technique you use. I like the DNV requirement to demonstrate the system accuracy statistically...but that can get costly because you must confirm the TRUE size of what you have estimated and that is done by destructive testing.

 
 Reply 
 
Roberto Otero
Engineering,
OMEGA SERVICIOS SPA, Chile, Joined Aug 2004, 15

Roberto Otero

Engineering,
OMEGA SERVICIOS SPA,
Chile,
Joined Aug 2004
15
21:20 Sep-18-2015
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 14:22 Sep-18-2015 .

I was reviewing the previous discussions on the subject here in ndt.net and found this from 2010 ... It seemed to me that both played common points of discussion and I thought it would be helpful and interesting to introduce it here... especially because it deals with this subject …

Height measurement in sectorial scan image

http://www.ndt.net/forum/thread.php?admin=&forenID=0&msgID=36300&rootID=36140#36300


Roberto

 
 Reply 
 
dan chen
,
China, Joined Mar 2016, 2

dan chen

,
China,
Joined Mar 2016
2
07:17 Mar-05-2016
Re: To Focus or Not to Focus
In Reply to Ed Ginzel at 20:05 Aug-31-2015 .

Hi Ed,
According to your rich experience, if a ball defects of 50 micron will be oversized or not, for the transducer with focal spot of 100 micron?Thank you!

 
 Reply 
 

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