T.O. 33B-1-1
5-7
5.1.4.3
Surface Waves.
Surface waves are those waves in which the particle motion is elliptical in a plane parallel to the propagation direction
and perpendicular to the surface. Surface waves, or Rayleigh waves, are created at the second critical angle (see
paragraph 5.1.5.2). Surface waves are confined to a thin layer up to one wavelength thick on the free boundary of a
solid. A free boundary is a surface bounded by a gas. Figure 5-8 shows an angle beam search unit containing a steeply
angled wedge. The steep angle causes the longitudinal beam in the wedge to strike the test surface at an angle that
results in conversion to the surface mode of sound travel. Surface waves will travel around curves. Reflections occur
only at sharp corners on the surface. The energy of surface waves decays rapidly below the surface of a test part as
shown in Figure 5-9. Surface waves are therefore most suitable for detecting surface flaws (stress risers), such as
cracks and machining lines. Surface waves may also be used to detect discontinuities lying slightly below the surface
(about one-half wavelength in depth). Surface waves travel at about 90 percent of the velocity of shear waves.
Figure 5-8. Surface Wave Mode
Figure 5-9. Distribution of Surface Wave Energy with Depth
