The ultrasonic test method in which finger damping is most effective to locate a discontinuity involves which wave type?

• A. Compressional wave
• B. Surface wave
• C. Longitudinal wave
• D. Shear wave

Answer: (B)

Finger damping works best when you’re using waves that stay on and near the surface. Surface waves, like Rayleigh waves, travel along the surface and have most of their energy confined just beneath it. Damping the surface with a finger mainly affects this surface-confined motion, which reduces extraneous energy and noise and makes the response from a near-surface discontinuity more distinct. That clearer, localized signal along the surface helps pinpoint where the flaw is. In contrast, compressional (P) and shear (S) waves travel through the bulk of the material and are not as strongly confined to the surface, so finger damping doesn’t provide the same localization benefit for those wave types.