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

Second Critical Angle
The second critical angle occurs when the angle of incidence is at such an
angle that the remaining shear wave within the part is refracted out of the part.
At this angle, when the refracted shear wave is at 90° a surface wave is
created on the part surface
Beam angles should always be plotted using the appropriate industry
standard, however, knowing the effect of velocity and angle on refraction will
always benefit an NDT technician when working with angle inspection or the
immersion technique.
The above calculator uses the following equation:
ultrasonic snells law formula
Where:
A1 = The angle of incidence.
V1 = The incident material velocity
A2 = The angle of refraction
V2 = The refracted material velocity