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861 views
08:32 Jun-07-2005

RT JEYAKUMAR

NDT Inspector
Singapore,
Joined Aug 2002
10
DETECTABILITY WAVE LENGTH

Sir,

What is the minimum size of discountinuity in Ultrasonic Testing. In general, Half a wavelength. But some references given that as 1/4 of wave lenth.

Which one is correct. If so, what is the relation or logic behind it. (why dont we take 1/25 or 1/30 which looks better sensitivity)

Thanks

With regards

RT Jeyakumar



 
01:03 Jun-07-2005

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1185
Re: DETECTABILITY WAVE LENGTH This is an old topic of discussion in UT. Paul Meyers and Phillipe Rubbers raised excellent points concerning directivity aspects and the efficiency coefficients relating shear and compression modes.
But there are many other considerations. Is the item that is to be detected an infinite strip or a circular disc reflector? Is it ideal in surface roughness with well defined edges or irregular? Is it vacuum or air backed or silica filled? Is inclined or perpendicular to the beam? there are many more questions to ask.
I think the half wavelength item was just a simple guide to suggest we should not be expecting to "detect" every flaw present.
Another item to consider is "what constitutes "detection"? We can adjust our detection by amplification, but if we are regulated in a COde by a certain sensitivity setting and an evaluation threshold then other factors can limit "detection".

'What is the smallest 'flaw' that can be detected?' is not a very good question from a practical and realistic perspective. More important to ask would be "What is the largest flaw that might be missed?"

Ed


----------- Start Original Message -----------
: Sir,
: What is the minimum size of discountinuity in Ultrasonic Testing. In general, Half a wavelength. But some references given that as 1/4 of wave lenth.
: Which one is correct. If so, what is the relation or logic behind it. (why dont we take 1/25 or 1/30 which looks better sensitivity)
: Thanks
: With regards
: RT Jeyakumar
------------ End Original Message ------------




 
09:32 Jun-09-2005

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
782
Re: DETECTABILITY WAVE LENGTH ----------- Start Original Message -----------
: This is an old topic of discussion in UT. Paul Meyers and Phillipe Rubbers raised excellent points concerning directivity aspects and the efficiency coefficients relating shear and compression modes.
: But there are many other considerations. Is the item that is to be detected an infinite strip or a circular disc reflector? Is it ideal in surface roughness with well defined edges or irregular? Is it vacuum or air backed or silica filled? Is inclined or perpendicular to the beam? there are many more questions to ask.
: I think the half wavelength item was just a simple guide to suggest we should not be expecting to "detect" every flaw present.
: Another item to consider is "what constitutes "detection"? We can adjust our detection by amplification, but if we are regulated in a COde by a certain sensitivity setting and an evaluation threshold then other factors can limit "detection".
: 'What is the smallest 'flaw' that can be detected?' is not a very good question from a practical and realistic perspective. More important to ask would be "What is the largest flaw that might be missed?"
: Ed
:
: : Sir,
: : What is the minimum size of discountinuity in Ultrasonic Testing. In general, Half a wavelength. But some references given that as 1/4 of wave lenth.
: : Which one is correct. If so, what is the relation or logic behind it. (why dont we take 1/25 or 1/30 which looks better sensitivity)
: : Thanks
: : With regards
: : RT Jeyakumar
------------ End Original Message ------------

Dear Jeyakumar,

Ed has covered the field quite nicely. The half wave length formula was given to technicians more as a guide line and to probably encourage them to start calculating the wavelength of a given frequency ultrasound! In very ideal circumstances, we have detected discontinuities as small as one tenth of the wavelength! But those are more exceptions than rules. And as Ed rightly mentioned, the cases of missing fairly big discontinuities exists and that is of more concern to any testing professional.

With best regards,

Swamy



 


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