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Technical Discussions
Yung-how wu
Yung-how wu
09:42 Jul-13-2007
CHIME technique

Recently, when I reviewed the paper of CHIME published at http://www.ndt.net/article/ecndt98/chemical/103/103.htm, I was confused by some description of the technique .

According to the paper, the second part of wave train, which signal amplitudes are higher than the first part, came from direct reflections of bulk wave in different multiple skips. The paper didn't clearly describe what the bulk wave is but just mentioned that there were waves "generated below or above the critical angle due to the beam divergence of a finite width transducer." I assumed it the shear wave, but not Head wave, accompanied with the 90 degree Longitudinal wave of the Creeping wave probe.

Since the latter part of wave train are higher in amplitude, it seemed more useful and reliable than the first part of wave train come from Creeping/Head wave combinations. In this case, I will have wave train similar with the second part if I used a conventional shear wave probe of same incident angle.

Then, my question is:
What isthe benifit of CHIME technique though the paper did also mention that "the Head waves are non-divergent plane waves therefore they have little decay and show strong peaks in the latter half of the signal."

Can anyone answer and confirm if the second part of the wavetrain in CHIME are generated by the shear wave generated by creeping probe and, is similar with conventional shear probe.



 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1285

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1285
05:33 Jul-13-2007
Re: CHIME technique

Mr. Wu, you asked if anyone can confirm if the second part of the wavetrain in CHIME are generated by the shear wave generated by creeping probe and, is similar with conventional shear probe.
The image in Figure 1 of the referenced paper shows the 2 arcs that are the main features of the bulk shear wave and how they are connected to the surface-glancing compression wave by the shear headwave. However, the image in that Figure 1 seems to give the impression that the headwave has a very strong amplitude. I have attached a photo from a photoelastic imaging of a so-called creeping wave probe. This illustrates the same modes as predicted by the finite element modelling that was used to image the Figure 1 in the referenced article. But this photo shows the headwave to be weaker than the bulk shear wave. This seems to make sense when we follow the transmission coeeficient curves for the shear mode. These indicate a higher transmission just after the critical angle.
However, the shear bulk wave is divergent (follows Snell's Law and spreads spherically) whereas the headwave from a flat plane surface is forming continuous plane wavefront. This ensures a full volume coverage in the early part of the soundpath whereas the spherical bulk shearwave will have reduced volume coverage in the same region.
The effect of a multiskip shear mode will be "similar" in that multiple arrivals will be seen (some of these from mode conversions).

Regards
Ed

----------- Start Original Message -----------
: Recently, when I reviewed the paper of CHIME published at http://www.ndt.net/article/ecndt98/chemical/103/103.htm, I was confused by some description of the technique .
: According to the paper, the second part of wave train, which signal amplitudes are higher than the first part, came from direct reflections of bulk wave in different multiple skips. The paper didn't clearly describe what the bulk wave is but just mentioned that there were waves "generated below or above the critical angle due to the beam divergence of a finite width transducer." I assumed it the shear wave, but not Head wave, accompanied with the 90 degree Longitudinal wave of the Creeping wave probe.
: Since the latter part of wave train are higher in amplitude, it seemed more useful and reliable than the first part of wave train come from Creeping/Head wave combinations. In this case, I will have wave train similar with the second part if I used a conventional shear wave probe of same incident angle.
: Then, my question is:
: What is the benifit of CHIME technique though the paper did also mention that "the Head waves are non-divergent plane waves therefore they have little decay and show strong peaks in the latter half of the signal."
: Can anyone answer and confirm if the second part of the wavetrain in CHIME are generated by the shear wave generated by creeping probe and, is similar with conventional shear probe.
:
------------ End Original Message ------------




 
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