Section 2: Physics of Ultrasound

Other Reading: Rayleigh Waves
Surface waves (Rayleigh waves) are another type of ultrasonic wave used in
the inspection of materials. These waves travel along the flat or curved
surface of relatively thick solid parts. For the propagation of waves of this type,
the waves must be traveling along an interface bounded on one side by the
strong elastic forces of a solid and on the other side by the practically
negligible elastic forces between gas molecules. Surface waves leak energy
into liquid couplants and do not exist for any significant distance along the
surface of a solid immersed in a liquid, unless the liquid covers the solid
surface only as a very thin film. Surface waves are subject to attenuation in a
given material, as are longitudinal or transverse waves. They have a velocity
approximately 90% of the transverse wave velocity in the same material. The
region within which these waves propagate with effective energy is not much
thicker than about one wavelength beneath the surface of the metal.