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Posted on sab dic 01, 2007 05:46
Post Subject: Guided waves in a tube
I placed two normal incidence Shear Wave Transducers (0.5MHz VIDEOSACAN) 10 cm apart axially on a circular aluminium pipe (2mm thickness) and sent a pulse through the transmitter. I received to my surprise (given that this is a normal incidence transducer) pulses on the receiver. These are guided waves but which ones, Rayleigh or Lamb? Is it possible that the wave received be a shear one ? The velocity calculated using the time of flight gives 3120 m/s for aluminium which is the the value for the shear wave velocity. I find that here is some contradiction with the way/angle the transducers are supposed to function.
Here is a simple sketch of the experiment and the supposed propagation direction given for the transducer
TX RCVR
------- ------
------- 10cm -------
---------------------------------------------------------
ooooooo
ooooooo shear wave Pipe
ooooooo
---------------------------------------------------------
Posted on lun dic 03, 2007 09:57
Post Subject: RE: Guided waves in a tube
This is a very common phenomenon that actually occurs in almost all test situations involving smooth metals. Because of the elastic properties and low attenuation of aluminum, any transducer (longitudinal wave or shear wave) will launch multiple wave modes. In your case, most of the ultrasonic energy generated by the transducer will be found in the shear wave that travels straight downward from the transducer, but the shear force applied to the surface of the steel also launches a wave that travels outward. In the case of a thick plate, that would be a Rayleigh wave, but since in this case the thickness of the medium is less than one wavelength you are probably seeing some sort of Lamb wave.
You can often see the same sort of phenomenon with two longitudinal wave transducers coupled side-by-side on a smooth steel or aluminum plate. At high gain, you can observe a Rayleigh wave traveling between them, a wave that can be dampened by touching the surface of the metal with a fingertip that has been wetted with couplant.
Posted on mar dic 04, 2007 11:25
Post Subject: RE: Guided waves in a tube
Dear Sir,
From the experiment you are describing, I guess that you have somehow excited what is similar to a lamb wave, althought the terminology Lamb waves applie stricto-sensus only to plate waves.
The reason you measured a velocity close to the shear wave velocity is because you have excited the pipe assymetrically (on one side), generating what corresponds to a A mode, probably a A0 mode which above a frequency-thickness product above one propagates in the plate at about 0.9x the shear wave velocity, if my memory is right (I did compute this 7years ago).
Because of the curvature of the pipe material between your two transducer the calculation mode is differnet to that inplates but I don't think it would change much to the velocity at all.
Is that responding to your question? don't hesitate to challenge my memories but if you want expert advice I can direct you to the experts in this field.