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Teletest
Teletest Focus+ was the first system to use long-range guided wave ultrasonic testing to detect corrosion in pipelines.
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Technical Discussions
Pelly
Pelly
07:07 Aug-11-1998
questions

Hi folks,

I am developing a project about guided waves in very thin plates. As I am (almost) new in this subject I have not enough clear all, and I have some dark points. I would be grateful to you if you help me with any indication, reference or commentary about these questions:

1. Is possible to generate shear waves in a plate with air-coupled transducers?? (By example setting the incidence angle, so that in accordance with Snell relation the shear wave forms 90 degrees respect the perpendicular, hence travels in the plate. In this case the longitudinal wave would be removed) Is it possible?? Is there other chance???
And pure longitudinal waves??

2. The theory that I have know about Lamb waves handle displacements in a plane perpendicular to the plate, in other words plane strain. However a contact transducer creates wave travelling in all directions all the plate. How should I modify my theory to explain the shear and longitudinal Lamb waves???
How a contact transducer excite the signal in the plate???
I suppose that the contact transducer creates Lamb waves. What is the difference between that the crystal of the contact tranducer works in shear or longitudinal mode for the created Lamb waves???

No more questions for the moment. I apologise in some of the questions is too much evident or elemental but I am a few worried about them. Thanks a lot in advance to any kind of help, and sorry for my English.

My e-mail is pelly69@hotmail.com




    
 
 Reply 
 
Yoseph Bar-Cohen
R & D,
Jet Propulsion Lab (JPL), USA, Joined Nov 1998, 26

Yoseph Bar-Cohen

R & D,
Jet Propulsion Lab (JPL),
USA,
Joined Nov 1998
26
03:00 Aug-12-1998
Re: questions (shear waves, air-coupled, lamb waves)
contact transducer excite the signal in the plate???
: I suppose that the contact transducer creates Lamb waves. What is the difference between that the crystal of the contact tranducer works in shear or longitudinal mode for the created Lamb waves???

: No more questions for the moment. I apologise in some of the questions is too much evident or elemental but I am a few worried about them. Thanks a lot in advance to any kind of help, and sorry for my English.

: My e-mail is pelly69@hotmail.com

----------------
------The following are my answers:

1. Is possible to generate shear waves in a plate with air-coupled
transducers??

The term Aircoupled transducer usually is reserved to a piezoelectric
crystal which transmits ultrasound through an impedance matching layer(s).
Since the transmission can be made in an angle, therefore shear wave can be
induced. However, any transmission through an interface can induce both
longitudinal and shear modes. Angulation issues needs to be taken into
account if you try to excite the shear mode only.

And pure longitudinal waves??

Pure longitudinal mode will produced when transmitting normal to a
structure (thru liquid or air coupling).


2. The theory that I have know about Lamb waves handle displacements in a
plane perpendicular to the plate, in other words plane strain. However a
contact transducer creates wave travelling in all directions all the plate.
How should I modify my theory to explain the shear and longitudinal Lamb
waves???

You need to include a geometric-spread factor to account for the contact
source.

How a contact transducer excite the signal in the plate???

If a contact transducer is coupled to a plate and induces wave in an angle
there are frequencies which support the excitation of plate wave modes.
By theoretically driven the dispersion curves you can predict the necessary
angles and frequencies.

I suppose that the contact transducer creates Lamb waves. What is the
differencebetween that the crystal of the contact transducer works in
shear or longitudinal mode for the created Lamb waves???

Contract transducers can induce normal to the surface both pure shear and
pure longitudinal. The specific mode depends on the type of crystal that
excites the wave as well as the coupling material that you are using
(burned honey and other viscous pastes support shear mode). To induce
plate waves, the material thickness, the wave incidence angle and frequency
need to support plate waves as determined from the dispersion characteristics.

Yosi



    
 
 Reply 
 
Pelly
Pelly
08:22 Aug-13-1998
Lamb waves

Thanks for the help. I agree with the answers regarding air-coupled excitation.

However, some aspects about contact transducers are not enough clear for me.

You advice to me to regard a geometric-spread factor to explain why a contact transducer excites the wave in all the direction and no only in one. Do you know some reference to get information about this spread factor or about how a contact transducer excites waves in a very thin plate (which is different, I think, that excitation in a semi-infinite body). I have read something about headwaves... but I think that this is more related with semi-infinites bodies..?

I believe (I would grateful a confirmation) that in order to excite a Lamb wave, the wave created by the contact transducer can be BOTH SHEAR AND LONGITUDINAL, (each type should incide with different angle over the plate) since both launch the two types when incides in the edges. The coupling of these longitudinal and shear waves generated by the reflection in the edges of the initial wave (longitudinal or shear) creates the plate wave.

On the other hand, I believe to understand to you that Lamb waves can be ONLY excited in a very thin plate by setting the right angle (the called coincidence angle I suppose).

However, I have seen in some articles create Lamb waves in sheets of about 0.1 mm of thickness by using a straight beam transducers, or in other words normal incidence of the beam over the plate. They says that the crystal vibration is parallel to the plate. When the receiver is aligned with crystal oscillation of the transmitter a longitudinal Lamb wave is created, and when is perpendicular, a shear Lamb wave is created (which is also non-dispersive as SHand with its msae velocity). Are these waves really Lamb waves??? The concept of longitudinal and transverse Lamb wave, is related with this spread of the Lamb waves in all direction???

And the last related question. Since the possible waves existing in a plate are SV and L (which couple in Lamb waves) and SH (shear horizontal), why is not there LH(longitudinal horizontal)????? (Maybe for the proper nature of the motion??))

I hope do not overwhelm with these questions but it is difficult to understand and to obtain suitable information about this. Thanks in advance and sorry my English.

Pelly




    
 
 Reply 
 

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