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Technical Discussions
Bill Prosser
Bill Prosser
00:44 Aug-08-2002
Generation of Rayleigh Waves

I am totally new to the field of Ultrasonics so please excuse this question if it seems extremely trivial.
I want to conduct an experiment whereby I induce Rayleigh waves into a piece of Aluminium / Mild Steel with the aid of wedge transducers. Can anybody suggest a make and model of suitable transducers ? I believe the angle of the wedge should be about 65 degrees, how is this angle calculated ? I am able to calculate the refracted Longitudinal and Shear angles (Snells Law) but fail to see how one can calculate the wedge angle for generation of Rayleigh waves only. Can anyone suggest a suitable experiment with set up instructions ?

Also, can Rayleigh waves travel around corners ???

Any help anyone can give will be much appreciated.

THANKS

Bill



 
 Reply 
 
Jim Knowles
Jim Knowles
00:59 Aug-08-2002
Re: Generation of Rayleigh Waves
: I am totally new to the field of Ultrasonics so please excuse this question if it seems extremely trivial.
: I want to conduct an experiment whereby I induce Rayleigh waves into a piece of Aluminium / Mild Steel with the aid of wedge transducers. Can anybody suggest a make and model of suitable transducers ? I believe the angle of the wedge should be about 65 degrees, how is this angle calculated ? I am able to calculate the refracted Longitudinal and Shear angles (Snells Law) but fail to see how one can calculate the wedge angle for generation of Rayleigh waves only. Can anyone suggest a suitable experiment with set up instructions ?
.
: Also, can Rayleigh waves travel around corners ???
.
: Any help anyone can give will be much appreciated.
.
: THANKS
.
: Bill
.Bill
Try Phoenix Inspection Systems Ltd
for all your needs in transducers, wedges etc.



 
 Reply 
 
William Blum
Consultant, Training, Level III Services
NDT Consulting Group Inc., USA, Joined Nov 2000, 89

William Blum

Consultant, Training, Level III Services
NDT Consulting Group Inc.,
USA,
Joined Nov 2000
89
01:01 Aug-08-2002
Re: Generation of Rayleigh Waves
I recommend you visit transducer manufacturer sites and download their catalogs in which you will find transducer to meet you needs. Here are a couple of links:

http://www.krautkramer.com/company_publications/new_transducer_catalog/new_transducer_catalog.htm

http://www.panametrics.com/div_ndt/pages/products/transducers/index.shtml

Bill Blum

: I am totally new to the field of Ultrasonics so please excuse this question if it seems extremely trivial.
: I want to conduct an experiment whereby I induce Rayleigh waves into a piece of Aluminium / Mild Steel with the aid of wedge transducers. Can anybody suggest a make and model of suitable transducers ? I believe the angle of the wedge should be about 65 degrees, how is this angle calculated ? I am able to calculate the refracted Longitudinal and Shear angles (Snells Law) but fail to see how one can calculate the wedge angle for generation of Rayleigh waves only. Can anyone suggest a suitable experiment with set up instructions ?
.
: Also, can Rayleigh waves travel around corners ???
.
: Any help anyone can give will be much appreciated.
.
: THANKS
.
: Bill
.



 
 Reply 
 
Christophe Mattei
Consultant
Exova AB, NDT Dpt., Sweden, Joined Jan 2001, 8

Christophe Mattei

Consultant
Exova AB, NDT Dpt.,
Sweden,
Joined Jan 2001
8
05:41 Aug-08-2002
Re: Generation of Rayleigh Waves

The same good old Snells law is used to calculate the incidence angle for the generation of Rayleigh wave. You have to know the rayleigh wave velocity Vr in the material of interest and longitudinal wave velocity Vl of the material used for the wedge. Refracted angle is taken as 90deg. You get:
Sin (Incidence angle) = Vl /Vr

If you don't know Vr and your material has a Poisson ration around 0.25, take Vr=0.92Vs (Shear wave velocity in the material of interest). You shouldn't be very far...

Note that this means that you need a wedge with a longitudinal velocity lower than the Rayleigh wave velocity.

When a Rayleigh wave is incident on a corner, part of the energy is reflected as a Rayleigh wave, part is transmitted and part is converted in bulk waves. The amount of energy "travelling around" is function of the corner geometry and the wavelength.

The bible on Rayleigh waves is Viktorov book: Rayleigh and Lamb waves (1967, Plenum Press).

Christophe Mattei



1
 
 Reply 
 
Grant Reig
Grant Reig
06:47 Aug-08-2002
Re: Generation of Rayleigh Waves
: I am totally new to the field of Ultrasonics so please excuse this question if it seems extremely trivial.
: I want to conduct an experiment whereby I induce Rayleigh waves into a piece of Aluminium / Mild Steel with the aid of wedge transducers. Can anybody suggest a make and model of suitable transducers ? I believe the angle of the wedge should be about 65 degrees, how is this angle calculated ? I am able to calculate the refracted Longitudinal and Shear angles (Snells Law) but fail to see how one can calculate the wedge angle for generation of Rayleigh waves only. Can anyone suggest a suitable experiment with set up instructions ?
.
: Also, can Rayleigh waves travel around corners ???
.
: Any help anyone can give will be much appreciated.
.
: THANKS
.
: Bill

.Panametrics makes a few different types
of probes which will produce surface waves in Aluminum that may be well
suited for your application.

One option would be the used of our screw or snap in transducers aswell as
a 90 degree surface wave wedge. An advantage to these probes is if you wish
to change the angle or produce a surface wave in a very different material
you can get a variety of wedges that can all work with the same transducer.
Also, if you foresee heavy usage of the probe, replacement of the wedge will
be far less expensive than replacing the entire transducer. Examples of
these can be found here:
http://www.panametrics.com/div_ndt/pages/products/transducers/angle_1.shtml

Another option which is helpful if you are working in tight spaces would be
our integral angle beam probes. These are considerably smaller than the
screw or snap in wedges and have a fixed angle for each transducer. We offer
a few different sizes and frequencies for surface wave integral angle beam
transducers which can be found here:
http://www.panametrics.com/div_ndt/pages/products/transducers/angle_5.shtml

In reference to your question on snells law and calculating angles, surface
wavesare created at the 2nd critical angle or when the refracted shear wave
approaches 90 degrees. If you would care to experiment with this a popular
product we offer for this application would be our variable angle beam
wedge. Although it may lack in overall performance compared to the above
angle beam transducers, it makes up for it in flexibility in this type of
experimental application. This wedge will allow you to vary the angle back
and forth producing longitudinal, shear and surface waves at different
incident angles. The variable angle beam wedge which can be found here.
http://www.panametrics.com/div_ndt/pages/products/transducers/special011.sht
ml

Last but not least, surface waves can travel around curved surfaces but not
around sharp corners. The smallest radius of curvature they will travel
around depends on the material as well as the quality or roughness of the
surface.

If you have questions or need any further assistance please feel free to
contact me directly either by email or at 800 225 8330 x1383 and I would be
happy to help you out.




 
 Reply 
 
Vikram Thyagarajan
Vikram Thyagarajan
01:17 Oct-08-2002
Application of Rayleigh Waves
Hi,
I am totally new to this field of Rayleigh waves. How does one go about sending a Rayleigh wave onto the surface of the material and if one can send such a wave, how does one go about exciting particular modes that are useful for damage detection on the surface of the material? I will be using Aluminum for my experiments.
What are the specifications of the transducer that have to be used for the same?
Sincerely,
Vikram Thyagarajan.


 
 Reply 
 
Kris Van de Rostyne
Kris Van de Rostyne
00:07 Oct-09-2002
Re: Application of Rayleigh Waves
Hello,
Rayleigh waves are elastic waves that remain confined to the surface during propagation; their penetration depth is around one wavelength. The most common method for Rayleigh wave excitation is using a wedge, although other methods are possible (optical, EMAT).
For example, on Aluminium (vL=6320m/s, vT=3130m/s) the Rayleigh wave velocity can be calculated to be vR= 2920m/s. The opening angle of the wedge can then be calcualted from arcsin(vWEDGE/vR) (Snell's law).
There is no such thing as Rayleigh 'modes' (maybe you are considering Lamb waves?). Rayleigh waves travel without dispersion over an isotropic medium. The frequency should be chosen dependent on the expected flaw size and sample thickness.

More details can be found in e.g.
*Cook, D.A., Berthelot, Y.H., Detection of small surface-breaking fatigue cracks in steel using scattering of Rayleigh waves, NDT&E International 34, 483 (2001),
And the references therein.
*Vu, B.Q., Kinra, V.K., Diffraction of Rayleigh waves in a half-space. I. Normal edge crack, J. Acoust. Soc. Am. 77(4), 1425 (1985)
*Viktorov IA, Rayleigh and Lamb waves, New York, Plenum 1967

Hope this helps,
Kris

: Hi,
: I am totally new to this field of Rayleigh waves. How does one go about sending a Rayleigh wave onto the surface of the material and if one can send such a wave, how does one go about exciting particular modes that are useful for damage detection on the surface of the material? I will be using Aluminum for my experiments.
: What are the specifications of the transducer that have to be used for the same?
: Sincerely,
: Vikram Thyagarajan.
.



 
 Reply 
 
Michael Conry
Student
University College Dublin, Ireland, Joined May 2002, 3

Michael Conry

Student
University College Dublin,
Ireland,
Joined May 2002
3
04:25 Oct-14-2002
Re: Application of Rayleigh Waves
: Hi,
: I am totally new to this field of Rayleigh waves. How does one go about
: sending a Rayleigh wave onto the surface of the material and if one can
: send such a wave, how does one go about exciting particular modes that are
: useful for damage detection on the surface of the material? I will be using
: Aluminum for my experiments.
: What are the specifications of the transducer that have to be used for
: the same?
: Sincerely,
: Vikram Thyagarajan.

Any general excitation of the surface should give rise to some Rayleigh
Waves. To produce any particular mode, you use more coherent excitation.
If you use, for example, a piezoelectric transducer, then that allows you
immediately to select a frequency.
To more efficiently produce (or detect) the waves you need the wavelength
of the desired waves. If you solve the Rayleigh characteristic equation
for your material to work out what phase-velocity the Rayleigh waves will
have, you can calculate the wavelength they will have in the direction of
propagation. Then you could use an angled perspex wedge so that by Snell's
law the waves generated in the perspex will efficiently refract into the
test-piece as Rayleigh waves (you will also have to know the velocity of
the waves in the perspex).

This was discussed before, you can find the thread here:
http://www.ndt.net/wshop/forum/messages/4479.htm
Some of the follow ups give formulae for calculating the required incident
angle. Effectively what you are doing is ensuring continuity at the
interface of the perspex and test-piece. Frequency has to be the same, and
wavelength as projected onto the interface also has to equate.

Michael



 
 Reply 
 
Mark Miller
Mark Miller
00:56 Oct-14-2011
Re: Generation of Rayleigh Waves
In Reply to Bill Prosser at 00:44 Aug-08-2002 (Opening).

I am actually in sort of the same boat as Mr. Prosser here. I have a pitch-catch arrangement going on with two 10 MHz transducers, both of which are attached to adjustable Olympus angle wedges. For now I'd just like them to transmit a Rayleigh wave through an aluminum slab.

I've used the appropriate velocities for my wedge and aluminum to calculate an angle, and I do find that I can get a pretty good output signal. The problem is that it isn't touch-sensitive. I can press down as hard as I can and the signal won't change at all. What's even more frustrating is that I've had just about the same arrangement before where the signal was touch-sensitive. Could someone please shed some light on what's going on here?

For reference, I've been taking the manufacturer's value of 2720 m/s for the longitudinal velocity and anywhere between 2900 to 3100 m/s for the Rayleigh wave velocity in aluminum to calculate my angle. I get a range of about 60 to 70 degrees for the incident angle from this.

Thanks,

Mark

 
 Reply 
 
madrid
madrid
05:33 Oct-14-2011
Re: Generation of Rayleigh Waves
In Reply to Bill Prosser at 00:44 Aug-08-2002 (Opening).

well surface waves travel at a wavelength deep...

there is a little something i have been thinking of for the past little while. say your shooting a weld at a 70 degree angle.. or 60, dont matter. if you take your finger and you start moving it in front of your probe back and forth you can see the noise on the screen close to your standing wave pip moving as you move your finger back and forth in front of the probe. thats my way of looking at surface waves... unless im mistaken. surface waves get dampened really easy by dirt, surface imperfections etc so they are really easy to see in your screen with this techinic...

can somebody correct me if im wrong please.

 
 Reply 
 
Norm Woodward
Norm Woodward
15:53 Oct-14-2011
Re: Generation of Rayleigh Waves
In Reply to Mark Miller at 00:56 Oct-14-2011 .

Mark-

I am not an expert on Rayleigh waves, per se, but I think I can help here.

1. You say that you would like ".. to transmit a Rayleigh wave through an aluminum slab. " Rayleigh waves are surface waves, extending only about a wavelength below the surface. You have some representative velocities at hand so you can see how thin that "slab" would have to be to reach the other side, especially at 10 MHz. Besides, if this wave were to reach both sides, then it would be a "Lamb wave," which might give new challenges.
2. To determine the incident angle for a surface wave, you usually don’t use, or know, the velocity of the resulting wave; in this situation, the angle should be arcsine of the longitudinal velocity of the wedge material divided by the transverse velocity of aluminum (~3130m/s), or about 60 degrees. Increasing this angle shouldn’t have much effect, since the sound can’t rise much further than the surface of the part.

3. I don’t have much hands-on experience with the method so I don’t know anything about touch sensitivity: perhaps some “adjustable wedges” have better internal coupling, perhaps some have flatter bottoms, so you have better contact and less rocking (changing incident angle.) Since dirt and excess couplant gives us so much hassle in surface wave, we normally use eddy current, especially on aluminum surfaces. One technology that seems to get around a lot of these problems is EMAT, which uses eddy current to generate the surface wave, without couplant, which, in your case, could then be picked up with either a regular transducer or another EMAT devise. I don’t know if you have deep enough pockets to consider this, but it looked interesting at the ATA forum I attended a couple of weeks ago.

I hope this helps.

Norm

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

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
15:57 Oct-14-2011
Re: Generation of Rayleigh Waves
In Reply to Mark Miller at 00:56 Oct-14-2011 .

Mr Miller --

The nominal incident angle that we use when manufacturing surface wave wedges for aluminum is 68 degrees, using the same insert material as in our adjustable wedges. This value has been optimized over many years of manufacturing experience. I would expect that an incident angle approaching 70 degrees in your test with the adjustable wedge would give you a healthy surface wave in smooth aluminum which, as you know, would normally be dampened by touching the metal surface with a wet fingertip. At such a high incident angle coupling is inefficient and a lot of the sound energy bounces around inside the wedge, but that in itself wouldn't launch a wave of some other mode in the test piece. It sounds like something is wrong, but I'm afraid that at the moment I don't know what it is.

 
 Reply 
 
Mark Miller
Mark Miller
20:13 Oct-14-2011
Re: Generation of Rayleigh Waves
In Reply to Norm Woodward at 15:53 Oct-14-2011 .

Thanks for the prompt replies and suggestions!

To Mr. Woodward: You may have misunderstood me when I said "through" the aluminum. What I meant was simply that I want the wave to travel along the surface, not through the bulk of the material. I will look into the EMAT you spoke of, though.

To Mr. Nelligan: I have seen spikes around 68 degrees, but nowhere near as great as those around 60 degrees, so I've usually set my transducers to the latter angle. If the Rayleigh wave really does pop up at around 68, though, I suspect I'm actually seeing some sort of surface transverse mode that isn't as sensitive as a Rayleigh wave. Would you say that's possible? If so, do you have any suggestion on how to counter the coupling problem at 68 degrees?

Thanks,

Mark

 
 Reply 
 
Norm Woodward
Norm Woodward
17:34 Oct-17-2011
Re: Generation of Rayleigh Waves
In Reply to Mark Miller at 20:13 Oct-14-2011 .

Like I said, I am not an expert on surface waves, so I would like to make some corrections on my previous post:

1. Of course, once you reach your last critical angle, you can't keep increasing the angle and expect to create more surface waves; eventually nearly all the sound just bounces around in the wedge. I wasn't thinking correctly when I said this, but perhaps I thought this because somewhere I was taught that figuring Rayleigh velocities are usually too difficult to determine, so using the shear wave velocity was "close enough." Of course, if you increase the incident angle a little, more rayleigh waves appear, thus causing me to think that there was great latitude in how much one could add. I sit corrected.

2. Determining the Rayleigh velocity is really not that difficult, if you have a enough elasticity data about the material in question. For aluminum, the shear velicity is appromately 0.93 times the shear velocity, or 2920 m/s. Thus, in this case, the best incident angle should be about 68 to 69 degrees, as suggested above.

If you are getting stronger indications at 60 degrees, you are probably using transverse waves that are running along the surface.

I apologize for any confusion I have created, then or now.

 
 Reply 
 

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