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Technical Discussions
Todd Torrence
Todd Torrence
03:15 Oct-04-2000
Snells Law

My co-workers and I were talking about angle beam ultrasonics during break one day. Im not sure how, but started talking about what type of ultrasonic wave would be produced, using contact UT, if the wedge was made out of the same material as the test peice. For example, a steel wedge and a steel plate. There were two opinions, one was the sound wave would refract just as it would if it was a plastic wedge. The second opinion was that the sound wave would not change since the material velocity was the same in both materials. Any thoughts, or votes, on this subject would help settle the bet.



 
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Massimo Carminati
Consultant
procontrol, Italy, Joined Jul 2000, 6

Massimo Carminati

Consultant
procontrol,
Italy,
Joined Jul 2000
6
06:31 Oct-04-2000
Re: Snells Law
Snell's Law clearly states that same sound velocity means same angle. In other words, a 45° steel wedge would generate a 45° Long beam in steel. Now I have another question for you: what would be the energy transmission coefficient?




 
 Reply 
 
Paul A. Meyer
R & D,
GE Inspection Technologies, USA, Joined Nov 1998, 47

Paul A. Meyer

R & D,
GE Inspection Technologies,
USA,
Joined Nov 1998
47
02:27 Oct-04-2000
Re: Snells Law
: Snell's Law clearly states that same sound velocity means same angle. In other words, a 45° steel wedge would generate a 45° Long beam in steel. Now I have another question for you: what would be the energy transmission coefficient?

The equations for calculating energy partitioning at an interface can be found in textbooks. "Ultrasonic Testing of Materials" by Krautkramer and Krautkramer demonstrates results of solid also "sliding" boundary contact between two solid materials. "Ultrasonic Waves in Solid Media" by Rose gives a more detailed description of the derivation.
Paul


 
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Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1307

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1307
03:06 Oct-04-2000
Re: Snells Law
: : Snell's Law clearly states that same sound velocity means same angle. In other words, a 45° steel wedge would generate a 45° Long beam in steel. Now I have another question for you: what would be the energy transmission coefficient?

: The equations for calculating energy partitioning at an interface can be found in textbooks. "Ultrasonic Testing of Materials" by Krautkramer and Krautkramer demonstrates results of solid also "sliding" boundary contact between two solid materials. "Ultrasonic Waves in Solid Media" by Rose gives a more detailed description of the derivation.
: Paul

The idea for a metal wedge is, on the surface, quite simple; however, most elements used in NDT are operated in the dilational mode. That would mean that a compression mode would be impinging on the metal to metal interface. If you made the wedge 45 degrees you would introduce a bimodal effect. The compression mode would pass into the test piece at 45 degress but for steel there would also be an SV shear mode at about23 degrees.
For steel to steel with an incident compression mode at 45 degrees, the Transmission coefficient for the Long wave would be 0.77 and for the transverse it would be 0.32. These are determined from the equations in Krautkramer as noted by Paul Meyer.



 
 Reply 
 
Rainer Meier
R & D
retired from intelligeNDT Systems & Services, Germany, Joined Nov 1998, 15

Rainer Meier

R & D
retired from intelligeNDT Systems & Services,
Germany,
Joined Nov 1998
15
07:37 Oct-04-2000
Re: Snells Law
: : : Snell's Law clearly states that same sound velocity means same angle. In other words, a 45° steel wedge would generate a 45° Long beam in steel. Now I have another question for you: what would be the energy transmission coefficient?

: : The equations for calculating energy partitioning at an interface can be found in textbooks. "Ultrasonic Testing of Materials" by Krautkramer and Krautkramer demonstrates results of solid also "sliding" boundary contact between two solid materials. "Ultrasonic Waves in Solid Media" by Rose gives a more detailed description of the derivation.
: : Paul

: The idea for a metal wedge is, on the surface, quite simple; however, most elements used in NDT are operated in the dilational mode. That would mean that a compression mode would be impinging on the metal to metal interface. If you made the wedge 45 degrees you would introduce a bimodal effect. The compression mode would pass into the test piece at 45 degress but for steel there would also be an SV shear mode at about 23 degrees.
: For steel to steel with an incident compression mode at 45 degrees, the Transmission coefficient for the Long wave would be 0.77 and for the transverse it would be 0.32. These are determined from the equations in Krautkramer as noted by Paul Meyer.

I agree with Ed: Using a steel wedge you would also normally get two wave modes in your steel-testpiece: a compression mode and a shear mode. (But I didn't douplecheck the transmission coefficients, Ed mentioned).
The reason, because nobody uses steel wedges is the big impedance missmatch bedween steel and the coupling media. This whould cause a strong dependence of the transmitted energy on the width of the coupling gap!
The echo of a reflector could change from 100% (o dB) at a coupling gap of zero to appr. - 14 dB at a coupling gap of a quarter of the wavelength!

Rainer


 
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