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

More on Snell Law
Like light, when an incident ultrasonic wave encounters an interface to an
adjacent material of a different velocity, at an angle other than normal to the
surface, then both reflected and refracted waves are produced.
Understanding refraction and how ultrasonic energy is refracted is especially
important when using angle probes or the immersion technique. It is also the
foundation formula behind the calculations used to determine a materials first
and second critical angles.
First Critical Angle
Before the angle of incidence reaches the first critical angle, both longitudinal
and shear waves exist in the part being inspected. The first critical angle is
said to have been reached when the longitudinal wave no longer exists within
the part, that is, when the longitudinal wave is refracted to greater or equal
than 90°, leaving only a shear wave remaining in the part.