When the incident angle is between the first and second critical angles, the ultrasonic wave generated within the part will be which mode?

• A. Longitudinal
• B. Shear
• C. Surface
• D. Lamb

Answer: (B)

When an ultrasonic wave in the couplant hits the solid, the energy can couple into different wave modes inside the part depending on the incidence angle. This is governed by Snell’s law and the different wave speeds for longitudinal (P) and shear (S) waves in the solid.

Two critical angles exist: one establishes whether a longitudinal wave can propagate in the solid, and the other does so for a shear wave. If the incident angle is between these two, the longitudinal component in the solid would be evanescent (not propagating), while the shear component can propagate. Since only the shear wave has a propagating solution in that angular range, the wave generated inside the part is a shear wave.

For typical steel in water, the first critical angle (for P-wave propagation) is around a small angle, and the second critical angle (for S-wave propagation) is larger. Therefore, angles between them selectively produce a shear wave in the part.