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Technical Discussions
Pratheesh
NDT Inspector,
India, Joined Aug 2013, 1

Pratheesh

NDT Inspector,
India,
Joined Aug 2013
1
09:04 Aug-21-2013
UT-SURFACE INTERFERENCE

Sound waves refracts when there is transfer of medium.When a material is scanned, couplant is applied B/w the transducer and the material.When the sound travels from the transducer to material first it crosses the couplant and then to the material. I have a doubt that whether is there any refraction happening wen the sound travel from the couplant to material since its a change of medium while using longitudinal waves Or is it negligible.If so wats the reason.

 
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Gerald R. Reams
Engineering,
Industry, USA, Joined Aug 2012, 182

Gerald R. Reams

Engineering,
Industry,
USA,
Joined Aug 2012
182
16:27 Aug-23-2013
Re: UT-SURFACE INTERFERENCE
In Reply to Pratheesh at 09:04 Aug-21-2013 (Opening).

It generally is not noticeable. I suppose that you may notice changes caused by very thick, viscous, and uneven couplant on a surface that has certain surface discontinuities.

Use Snell's Law to calculate the refracted angle. Anything that causes the sound propagation direction to change from normal incidence will cause it to refract at the interfaces.

Gerald

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
17:27 Aug-23-2013
Re: UT-SURFACE INTERFERENCE
In Reply to Pratheesh at 09:04 Aug-21-2013 (Opening).

As a practical matter, normal couplant layers are a very small fraction of a wavelength in thickness and have no effect, but you ask for the theory, so here's the quick explanation.

In theory, since Snell's Law works to bend beams in both directions, you would have refraction at the wedge/couplant interface (bending the incident beam downward since the couplant is slower than plastic) and then at the couplant/metal interface (now bending the beam upward since the metal is faster than couplant). If you do that math based on the longitudinal velocity of your couplant, and then recalculate for plastic (L-wave velocity) to steel (shear wave velocity) you will find that the refracted angle in the metal is the same in either case. In short, the theoretical refraction at the first boundary (plastic/couplant) is cancelled by an opposite effect at the second boundary (couplant/steel) and the net effect is the same as that of a plastic/steel refraction.

1
 
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Gerald R. Reams
Engineering,
Industry, USA, Joined Aug 2012, 182

Gerald R. Reams

Engineering,
Industry,
USA,
Joined Aug 2012
182
17:45 Aug-23-2013
Re: UT-SURFACE INTERFERENCE
In Reply to Tom Nelligan at 17:27 Aug-23-2013 .

Tom is quite right. His observations hold very true for "normal" conditions. Unfortunately, sometimes the metals we use don't hold up forever to their service or environment.

We all would love to have machined or even ground smooth surfaces to examine but, many times these surfaces are in very poor conditions. That's probably why the customer is having us look. In-service, exposed metals are just difficult to examine sometimes. At least with new fabrication the surface conditions are much better.

 
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