At a water-to-steel interface with a 7-degree incidence in water, what is the principal mode of vibration in the steel?

• A. Longitudinal
• B. Shear
• C. Both A and B
• D. Surface

Answer: (A)

When a compressional wave in water hits a steel interface at a shallow angle, the energy that enters the steel is mainly in the longitudinal (P) mode. The particle motion of the incident wave is along the direction of travel, and this couples most efficiently into a longitudinal wave in the solid. While shear-wave (S) energy can be generated at a boundary, its amplitude is much smaller at this small incidence angle, so it is not the dominant transmitted mode. The result is that the principal vibration you detect in the steel is a longitudinal wave.