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1094 views
02:03 Jan-22-2010

Deston Henson

NDT Inspector,
Canada,
Joined Nov 2009
20
Total Drop Method of Sizing Indications

Has anyone heard of &/or used the 'total drop' method of sizing defects / indications ultrasonically? If so, please give me your thoughts.


    
 
13:43 Jan-22-2010

J. Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
Re: Total Drop Method of Sizing Indications In Reply to Deston Henson at 02:03 Jan-22-2010 (Opening).

You did not mention whether this is length sizing or through wall height sizing.

Total Drop is a variation of the 6 and 12 dB drop methods. Total loss of sound pressure is when the signal is peaked and the transducer is moved left and right (length sizing) , or forward or backwards (height sizing) until the signal drops into the noise.

At EPRI back in the 80s it was determined this was the best method for length sizing of IGSCC was the total loss of the signal into the noise.

As for through wall height sizing, experience, and research at the EPRI NDE Center shows that a dB drop sizing method is not very effective. When the flaw is smaller than the beam spread, you tend to size the beam spread and not the flaw. Hence the reason for using Time of Flight methods or Creeping Wave methods for height sizing.

A simple exercise to demonstrate the effects for through wall height sizing error using dB drop is to obtain a block of steel with a variety of notches at 20, 40, 60 and 80 % deep. Using a 6 dB drop or a 12 dB drop method, you will see that all notches end up with the approxiamtely the same through wall height.

For length sizing the 6 and 12 dB drop methods, it works better because generally the flaw length is greater that the beam spread. Total Drop or Total Loss of signal is a conservative method to length size an IGSCC Crack.

One note; Phased Array has solved some of these issues using an angular dB drop methodolgy and focusing for through wall height siizng.

I hope this helps.

Sincerely,

J. Mark Davis
University of Ultrasonics


    
 
23:25 Jan-26-2010

Andrew Cunningham

NDT Inspector
Canada,
Joined Jun 2008
238
Re: Total Drop Method of Sizing Indications In Reply to Deston Henson at 02:03 Jan-22-2010 (Opening).

Hi Deston,
I have been using and demonstrating Total Drop since the late 70s. I have found Total Drop sizing for length the most reliable method of all the many techniques. I would never use it for cross sectional sizing, especially on an ID connected defect ( the corner trap will grossly over size the reflector ). I must emphasize that sizing with just one method, is like scanning with just one probe angle. The more methods you know and USE the less likely you are of miss-sizing. I cannot emphasize enough to practice Max Amp, 6dB,14dB, 20dB and Total Drop on all the reflectors you find. Each type of defect may be better sized with different methods. So its up to you to practice them all and learn what is best for each type of defect. As for cross-sectional sizing, I personally would recommend Max Amp.

Andrew Cunningham.
www.practical-ndt.com


    
 


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