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Technical Discussions
Naeem Anjum
Naeem Anjum
06:36 Jun-25-2009
Sound velocity for two dissimilar matel

A carbon steel vessel has Monel lining inside. What we select sound velocity for thickness measurement of shell from out side or inside? Actual thickness of shell is 25mm & lining thickness is 3.5mm.

    
 
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S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
07:52 Jun-25-2009
Re: Sound velocity for two dissimilar matel
In Reply to Naeem Anjum at 06:36 Jun-25-2009 (Opening).

Dear Naeem,

It depends on which thickness you are trying to measure. And if you are trying to measure both, you need to calibrate using a block of known thickness. The interface also gives an echo and you can get additional information about any gap in lining from that. Feel free to ask for more details if needed.

    
 
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Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
15:16 Jun-25-2009
Re: Sound velocity for two dissimilar matel
In Reply to Naeem Anjum at 06:36 Jun-25-2009 (Opening).

In any situation where where you are measuring the total thickness of a two-material test piece, the effective sound velocity through the total thickness will be the weighted average of the velocities of each of the two materials. Monel is somewhat slower than carbon steel (typically about 5400 m/s for longitudinal waves), but of course the velocity of carbon steel varies through the 5800-5900 m/s range depending on the specific alloy. Since the Monel is only about 12% of the total thickness, you could use an estimated velocity around 5800 m/s and probably be fairly close. Unfortunately, the exact velocity varies as the radio of steel to Monel changes, which is likely to be the case when the total thickness changes. To calculate the EXACT velocity of the multilayer test piece, you would need to know both the velocity of each component and their thickness, and if you knew that, further ultrasonic thickness measurement would be unnecessary! As a practical matter, you could use a two-material calibration block of known thickness as Mr. Swamy suggests, and just be aware that there will be a few percent uncertainty in your results for other thicknesses.

Separate measurements of each layer would be another possibility, but the acoustic impedances of Monel and carbon steel are so close that there will be very little reflection from the boundary between the two metals, far smaller than the backwall echo. You may have trouble seeing it.

    
 
 Reply 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
03:56 Jun-27-2009
Re: Sound velocity for two dissimilar matel
In Reply to Tom Nelligan at 15:16 Jun-25-2009 .

Dear Tom,

Thanks for a detailed explanation and many more thanks for correcting me w.r.t. the interface echo.

Best regards.

Swamy

    
 
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