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Technical Discussions
Pramod
India, Joined Feb 2019, 4

Pramod

India,
Joined Feb 2019
4
18:40 May-18-2019
Why sizing @ 20% and below FSH is not recommended

Need a technical write up, thought process to confirm
Mathematical approach, to explain.

Secondly in other words DAC/ DGS points below 20%. Are converted
To split DAC/ DGS. Why? And the why its recommended?

Regards
Pramod Myakal
RSM Asia
Sonatest LTD UK.

    
 
 Reply 
 
Paul Holloway
Consultant,
Holloway NDT & Engineering Inc , Canada, Joined Apr 2010, 189

Paul Holloway

Consultant,
Holloway NDT & Engineering Inc ,
Canada,
Joined Apr 2010
189
21:01 May-18-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Pramod at 18:40 May-18-2019 (Opening).

Because it's hard to see things that are only 20% screen height.

    
 
 Reply 
 
Martin Goodman
,
United Kingdom, Joined Apr 2019, 4

Martin Goodman

,
United Kingdom,
Joined Apr 2019
4
07:30 May-19-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Paul Holloway at 21:01 May-18-2019 .

Sizing at 20% FSH is generally for height sizing of linear defects. Length sizing will normally be done at 50% (6db drop). The reason for 20% sizing is that more of the beam is on the defect than for length sizing where half the beam is on the defect. If you use LOG20x100/20 you will see your Db difference between FSH and 20%.

    
 
 Reply 
 
Anmol Birring
Consultant,
Birring NDE Center, Inc., USA, Joined Aug 2011, 746

Anmol Birring

Consultant,
Birring NDE Center, Inc.,
USA,
Joined Aug 2011
746
19:34 May-19-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Pramod at 18:40 May-18-2019 (Opening).

Results in oversizing

    
 
 Reply 
 
Edward Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1268

Edward Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1268
20:31 May-19-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Pramod at 18:40 May-18-2019 (Opening).

Pramod, you have been getting a few different perspectives on this, but that is probably because it is not clear what you are asking about. Your second statement begins with "Secondly in other words..." but you did not make a first question, just a statement that you need to explain "something".
There are 2 aspects in the second paragraph you made. One relates to DAC/DGS. I think the DGS is not correct as we usually see the curves for the various diameter DSRs drawn down to the horizontal baseline. But a "Split DAC" is usually split because, as Paul noted, things are difficult to see at such a low amplitude. A DAC point at 80% screen height allows you to see a significant change in signal amplitude to represent 2dB (e.g. 80% to 63% represents 2dB but a 20% DAC point allows only 4% change in screen height before you have reached 2dB).

The title you placed on the thread seems to ask about sizing (not Split DACs). This SEEMS to suggest you are now asking about the probe movement to assess flaw size (length or height). This is not the same rationale for a Split DAC. Using the -6dB, -14dB or -20dB drop from a maximum or from the DAC level are merely options for the empirical beam-boundary sizing techniques. For sizing laminations that are larger than the beam dimension the planar nature of the flaw makes the -6dB option a reasonable estimate of the extents. But when trying to estimate the length of a nonplanar flaw (e.g. cluster porosity or slag), the tenuous nature of the flaw will provide lower amplitude responses at the ends of the flaw extent. Therefore even though you have moved the probe such that the amplitude has dropped to -6dB, it will probably not be a suitable indication of the end of the flaw. For this reason, it is sometimes recommended to use a -14 or -20dB boundary to indicate the flaw extent.

    
 
 Reply 
 
Paul Holloway
Consultant,
Holloway NDT & Engineering Inc , Canada, Joined Apr 2010, 189

Paul Holloway

Consultant,
Holloway NDT & Engineering Inc ,
Canada,
Joined Apr 2010
189
20:39 May-19-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Martin Goodman at 07:30 May-19-2019 .

I'm not so crazy about the 6dB drop method for length sizing. It has a high probability of undersizing flaws.

Article here: http://www.hollowayndt.com/news/2018/10/22/the-shortcomings-of-the-6db-drop-method-for-ultrasonic-flaw-length-sizing

    
 
 Reply 
 
Martin Goodman
,
United Kingdom, Joined Apr 2019, 4

Martin Goodman

,
United Kingdom,
Joined Apr 2019
4
10:58 May-21-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Paul Holloway at 20:39 May-19-2019 .

I completely agree Paul, and that's a good article explaining the pit-falls of sizing. I started out using 20 / 6 Db drops for sizing, and it is still widely used as a standard technique.

6Db makes much more sense for laminations obviously.

1    
 
 Reply 
 
Ricardo Andreucci
FATEC - Technological University of Sao Paulo, Brazil, Joined May 2018, 23

Ricardo Andreucci

FATEC - Technological University of Sao Paulo,
Brazil,
Joined May 2018
23
17:58 May-27-2019
Re: Why sizing @ 20% and below FSH is not recommended
In Reply to Paul Holloway at 20:39 May-19-2019 .

Hi Paul,
I have read the article "the short comings of the 6db drop method for ultrasonic flaw length sizing", by the way is very interesting, but I believe that most of the time repairing rejected indications in welds detected by ultrasonic testing, are performed in dimensions larger than the technician sized. In addition the repair cavity normally is performed liquid penetrant testing in order to ensure the elimination of the defect.
So, in my opinion I think that sizing by 20dB or 6dB drop for rejected descontinuities whatever for me.

Regards

    
 
 Reply 
 

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