Lamb waves are particularly suitable for inspecting which type of geometry or material?

• A. Forgings
• B. Bar stock
• C. Ingots
• D. Thin sheet

Answer: (D)

Lamb waves are plate waves that exist when an ultrasonic wave is guided between two parallel free surfaces, i.e., in a plate. The energy is confined across the thickness and the wave modes (symmetric and antisymmetric) depend on the thickness relative to the wavelength. This geometry makes them especially effective for inspecting thin sheet-like structures because the plate constraints create well-defined modes that travel along the sheet and are sensitive to flaws that run through or along its thickness.

In bulk shapes such as forgings, bar stock, or ingots, the plate-wave conditions don’t apply—the thickness is not small enough relative to the wavelength to support the same guided plate modes. Other ultrasonic approaches are typically used for those bulk geometries, so Lamb waves are best suited to thin sheets.