11:19 Dec-07-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
Good matter. This has been identified by the company who I worked for years, and as a result their internal and additional requirements in specs/standards states that the sizing shall be made at the extremities of the flaws, implying that the inspector shall search the whole lenght of indications.
14:17 Dec-07-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 13:46 Dec-07-2015 .
DNV OS-F101, App D, B 337 includes the fixed level method
You have provided some very good information. Technicians should know the limitations of the 6dB drop method and understand how the shape and size (when flaws are smaller than beam width) affect the measured size.
Vadim Kniazev R & D, Director Wave Process Simulation System Laboratory, Russia, Joined Jun 2010, 3
Vadim Kniazev R & D, Director Wave Process Simulation System Laboratory, Russia, Joined Jun 2010 3
15:16 Dec-07-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
I hope that this article will be interesting for you:
Opposite Phased Transducer – OPT for Sizing of Defects
It uses a fundamentally different technology for length sizing.
18:25 Dec-07-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
Early on in my inspection career, before I had much accumulated knowledge and went by the letter of the codes, I found myself making these same mistakes.
I remember one such case where after UT examination to CSA W59, I had a piling weld marked up over 50% of the circumference in 2" to 6" lengths using the 6 db sizing method required by the code. The welder brought me over after he had excavated 25% of the circumference and instructed me to put a welding helmet on. He then proceeded to show me the 100% circumferential incomplete penetration as he gouged the remaining weld out.
It really opened my eyes to sizing defects with UT.
19:11 Dec-07-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
I applaud you that you have learnt 6dB is not an accurate method and you will to speak out. 6dB drop is at a “that will do” lazy way of sizing discontinuities. It is a good reproducible sizing method for repeatable results. It in no-way could be classed as accurate. I defy anyone to size the 50mm SDH in the IIW V1 with 6dB drop method. When a technician gets his/her first qualification to practice he/she knows nothing but the bare minimum to pass an exam. (You would not class a person who has just passed a driving test as skilled). Knowing that amplitude is governed by orientation before size, I one wonder how much practical ultrasonic experience the writers of procedures and codes have? It is up to the technician to learn other sizing methods so he/she knows what sizing method is best suited to the discontinuity found.
05:23 Dec-08-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 19:23 Dec-07-2015 .
Please refer to AWS D1.1 ,section 22.214.171.124.Indications of discontinuities that remain on the display as the search unit is moved towards and away from the discontinuity (scanning movement “b”) may
be indicative of planar discontinuities, relative to the sound beam, deviates from
the perpendicular dB rating which do not permit direct, reliable evaluation of the
welded joint integrity may result. When indications that exhibit these planer
characteristics are present at scanning sensitivity, a more detailed evaluation of
the discontinuity by other means may be required (e.g., alternate ultrasonic
techniques, radiography, grinding or gouging for visual inspection etc.,)
12:30 Dec-08-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
Guys despite which spec you use you have to apply the basics you should have learned during training. Find the flaw which will have many reflection faces and therefore variable amplitudes along its length then find the last facet of the flaw and apply your sizing technique - do not get the biggest echo and just drop it by half. By finding the last facet and applying all the technques correctly to a flaw bigger than the beam - 6db - 20dB - Max amp you should have hardly any difference.
We can all get wrapped up in specs but dont forget just apply the basics first -
and yes Ive applied AWS - ASME and the many varied BS specs for over 40 years -
Paul Holloway Consultant, UT Level 3 Holloway NDT & Engineering Inc , Canada, Joined Apr 2010, 169
Paul Holloway Consultant, UT Level 3 Holloway NDT & Engineering Inc , Canada, Joined Apr 2010 169
15:21 Dec-11-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Charlie Jackson at 12:30 Dec-08-2015 .
6dB drop from AWS site
"...apply the basics you should have learned during training"
Charlie, this is the main problem. The methods are often taught at a bare minimum without having to bother the trainees with pesky real-life complications. Manufactured flaw specimens often contain completely artificial defects like completely uniform lack of penetration with easily identifiable starts and stops. These specimens are used for both training and examination, they're easy to teach with and they're even easier to mark. But they're not indicative of real world defects, and students leave with a false sense of security: "I nailed that lack of pen, bro! I passed first shot, I'm the best they've ever seen!".
I'm sure it's not this way all over the world, but I think many can agree that there are some basic UT principles which are poorly understood by trainees and experienced technicians alike. Everyone *should* have learned the right way during training, but most don't.
By way of example, I've attached an image from the AWS site (https://app.aws.org/itrends/2005/01/021/). What is shown is the classic 6dB drop method, basing the method on the peak (incorrect), yet its straight off the AWS website. The text which accompanies the original image references 6.31.2 in D1.1 (2000), which *does* instruct to "drop 6dB below the rating for the applicable flaw classification", but how many NDT technicians are running around out there with a copy of D1.1, or at least have read and memorized that requirement? I'm going to hazard a guess and say less than 5%. The rest of them are going to go on what they were taught in a classroom years before, and the simple, easy to mark method(s) they used to pass their qualification exams.
17:12 Dec-11-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 15:21 Dec-11-2015 .
Thanks for the reply and the link and whilst I am glad I no longer have to wrestle with the AWS D11 spec I proffer the following reasoning for the AWS approach. I can only assume their intent was to size defective areas and not the extent of the flaw. By dropping 6dB below the relevant unacceptable rating they will remove the ‘bad’ bit of the flaw which can be classified as a defect. This could leave a continuation of the flaw either side but this area would be classified as acceptable once the defective part is removed. It’s an approach which can be reasoned in a production environment.
I guess my memory isnt as good these days and I forget those years spent in construction shops tangling with the various specs and 'not so good' welding.
Apologies to all for my previous over simplification of the problem
21:11 Dec-15-2015 Re: Error inherent when using 6dB drop for length sizing (AWS/CSA)In Reply to Paul Holloway at 04:57 Dec-07-2015 (Opening).
Very interesting topic and very well detailed. I also try to teach people to "look" into the weld despite to what sizing methodology is written in the paper. In other words, to deeply investigate when very long indications with mostly low amplitude has few points with "recordable" amplitude. Real defects are not SDH and do not behave the same along its whole length.
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