Surface Waves and Ultrasonic Testing

Just longitudinal waves can travel through fluids (They have no shear strength). Shear waves should be reflected when travelling through a solid material and find a fluid interface.

Kind regards

I would like to add my 2-cents if I may. Whenever an acoustic wave encounters a change of acoustic impedance, i.e., a net change in sound velocity, a certain amount of the energy is reflected back towards the source. The amount of energy reflected is dependent on the net change in velocity and the angle of reflection will equal the angle of incidence. The remaining energy continues on but is refracted by an angle dependent on the net change in velocity. A dramatic change in the propagation speed would be encountered where an acoustic wave is traveling from a high-speed, solid object into a low-speed, liquid or gaseous medium.

If I read correctly, you are asking :

"Why are Rayleigh waves effectively *stopped* by liquids on the surface?"...

and the answer to that question lies in the fact that you already mentioned. They are surface-waves only. No acoustic energy enters the part to any significant depth, and, because the particle motion (somewhat elliptical in format) is easily dampened, or attenuated, by anything against the surface ... well, you got it already ... liquids provide that attenuation quite effectively.

A Russian fellow delivered a paper at an ASNT Conference a few years back, wherein he described *one* test that was performed quite well using Rayleigh waves under water ... but only a very short distance.

Best Regards ~

Thanks for your answer. I understand what you're saying BUT surface waves to my better knowledge can penetrate up to 1 wavelength. That to me says that they are "also Subsurface". Then why are they stopped by something on the surface?

The waves in question are not necessarily stopped, but attenuated; the wave continues but at a significantly reduced amplitude. These losses can reach the point that even encountering a reflector beyond the dampened area, insufficient signal returns to the transducer to ring it. Since the wave is running on / just under the surface, anything that would dampen the particle motion in the part itself will also dampen (attenuate) the sound wave.

You should be able to produce an indication with surface wave just by pressing on the part in the sound path with your dampened finger. I have done this on titanium turbine blades. If you can't do this, then I would suggest that you are either not providing enough energy to the transducer, or you have your signal display set too low.

Hi,

Rayleigh-waves act by oscillating the energy between kinetic energy of a limited depth of an elastic material and elastic energy pulling/pushing back the material to the undisturbed surface.

This is similar to water waves (also the elliptical movement of any element of the material) or also to capillary waves.

Water waves: potential-energy exchanged with kinetic-energy

capillary waves: kinetic-energy exchanged with surface energy by capillary force.

So fluids will have their own modes similar but not of the same type as Rayleigh-waves.

So if there is a fluid or low shear-strength material is encountered by the Rayleigh wave this is partially reflected, partially converted in some other wave types with different (from the incident wave) frequency.

So if you want to have the Rayleigh waves to run into or through these discontinuities two necessities exist:

A.: smooth surface (compared to wavelength) to prevent the analog to stray-light at reflection from rough surfaces.

B.: selection of a higher mode Rayleigh wave that has near zero amplitude at the surface.

RHABE