Which type of wave travels along the surface and is commonly used for surface inspection in metals?

• A. P-wave
• B. S-wave
• C. Rayleigh wave
• D. Lamb wave

Answer: (C)

Rayleigh waves are surface waves that travel along the surface of a solid. Their particle motion is elliptical and stays near the surface, so most of the energy is confined to a shallow region (roughly within one wavelength). This makes them exceptionally sensitive to defects at or very close to the surface, which is why they’re commonly used for surface inspection in metals.

P- and S-waves are body waves that move through the material’s interior, not confined to the surface, so they’re not as well suited for surface-specific flaw detection. Lamb waves are plate waves that propagate in thin sections of material and involve multiple modes with energy spread through thickness; they’re used more for assessing plate thickness or through-thickness flaws rather than surface defects.