Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

When a longitudinal wave moves from a slower to a faster material, there is
an incident angle that makes the angle of refraction for the wave 90 o . This is
know as the first critical angle. The first critical angle can be found from
Snell's law by putting in an angle of 90° for the angle of the refracted ray. At
the critical angle of incidence, much of the acoustic energy is in the form of an
inhomogeneous compression wave, which travels along the interface and
decays exponentially with depth from the interface. This wave is sometimes
referred to as a "creep wave." Because of their inhomogeneous nature and
the fact that they decay rapidly, creep waves are not used as extensively as
Rayleigh surface waves in NDT. However, creep waves are sometimes more
useful than Rayleigh waves because they suffer less from surface
irregularities and coarse material microstructure due to their longer
wavelengths.