T.O. 33B-1-15-75.1.4.3 SurfaceWaves.Surface waves are those waves in which the particle motion is elliptical in a plane parallel to the propagation directionand perpendicular to the surface. Surface waves, or Rayleigh waves, are created at the second critical angle (seeparagraph 5.1.5.2). Surface waves are confined to a thin layer up to one wavelength thick on the free boundary of asolid. A free boundary is a surface bounded by a gas. Figure 5-8 shows an angle beam search unit containing a steeplyangled wedge. The steep angle causes the longitudinal beam in the wedge to strike the test surface at an angle thatresults in conversion to the surface mode of sound travel. Surface waves will travel around curves. Reflections occuronly at sharp corners on the surface. The energy of surface waves decays rapidly below the surface of a test part asshown in Figure 5-9. Surface waves are therefore most suitable for detecting surface flaws (stress risers), such ascracks 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 ModeFigure 5-9. Distribution of Surface Wave Energy with Depth
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