Lamb waves can detect laminar type defects near the surface of a thin material. Which statement best describes this capability?

• A. Laminar type defects near the surface of a thin material
• B. Lack of fusion in the center of a thick weldment
• C. Internal voids in diffusion bonds
• D. Thickness changes in heavy plate

Answer: (A)

Lamb waves are guided waves that travel in thin plate-like structures, and they interact strongly with discontinuities that lie close to the surface. Because the plate is thin, the wave modes have fields that extend to and near the surface, so a flat, laminar defect that sits near there perturbs the wave in a way that is detectable—via changes in amplitude, phase, or arrival time—more readily than defects that are deeper or of different types. This makes Lamb waves especially suited to spotting laminar-type separations near the surface of a thin material.

Other scenarios describe defects located deeper in a thick weld, internal voids in diffusion bonds, or thickness changes in heavy plate. Those conditions are either not near the surface or not the laminar, surface-adjacent type that Lamb waves are most sensitive to, so they aren’t the best description of what Lamb waves detect best.