Which wave type would be most appropriate for detecting laminar type defects near the surface of a thin material?

• A. Lamb waves
• B. Rayleigh waves
• C. Surface waves
• D. Longitudinal waves

Answer: (A)

When inspecting a thin plate for planar, layer-bound flaws close to the surface, guided plate waves called Lamb waves are the most effective choice. These waves propagate within the thickness of the plate and come in symmetric and antisymmetric modes, with their velocities and displacement patterns sensitive to the plate’s thickness and to any disruptions inside the thickness, such as delaminations or laminar defects.

Because the energy of Lamb waves is distributed through the plate, they interact strongly with defects that lie near or across the thickness. A laminar flaw near the surface disrupts the regular propagation, causing reflections, mode conversion, and changes in amplitude that are detectable by transducers. This makes Lamb waves especially capable of revealing such defects in thin materials.

Rayleigh waves are surface-bound and decay with depth, so while they can sense near-surface flaws on a bulk half-space, they don’t interrogate the through-thickness region as effectively in a thin plate with laminar defects. Longitudinal (bulk) waves travel through the material but aren’t guided by the plate; they don’t exploit the near-surface, through-thickness interaction as efficiently for delamination-type flaws in thin structures.

So, for detecting laminar type defects near the surface of a thin material, Lamb waves provide the strongest and most interpretable interaction, making them the best choice.