UT Testing-Section 2 Physics of Ultrasound

Refraction and mode conversion occur
because of the change in L-wave
velocity as it passes the boundary from
one medium to another. The higher the
difference in the velocity of sound
between two materials, the larger the
resulting angle of refraction. L-waves
and S-waves have different angles of
refraction because they have dissimilar
velocities within the same material.
s the angle of the ultrasonic transducer
continues to increase, L-waves move
closer to the surface of the UUT.
The angle at which the L-wave is parallel with the surface of the UUT is
referred to as the first critical angle. This angle is useful for two reasons. Only
one wave mode is echoed back to the transducer, making it easy to interpret
the data. Also, this angle gives the test system the ability to look at surfaces
that are not parallel to the front surface, such as welds.