Selective generation of Lamb modes by a moving continuous-wave laser

This Letter focuses on the selectively non-contact generation of Lamb wave modes in plates by using a continuous-wave (CW) laser moving along sample surface. Compared with the generated Lamb waves with broadband, multiple modes (the existence of at least two modes at any given frequency) excited by a pulsed laser, the desired single narrowband mode of the Lamb wave can be generated by a moving CW laser, as long as the scanning speed matches the phase velocity of the mode. Moreover, the dispersion curves of the Lamb wave can be obtained directly from the power spectrum of the time-domain signal recorded at each laser's moving speed. Single A₀ mode excitation, coupled resonance phenomenon of A₀ mode and S₀ mode, single S₀ mode excitation, and high-order modes appeared successively as scanning speed increased. Especially, the excitation of the pure single S₀ mode can be realized, which is suitable for propagation in the case of liquid loading. It is proposed, for the first time to the best of our knowledge, to realize the selection of a single Lamb wave mode by using the CW laser scanning method, which provides a brand-new way for laser ultrasonic excitation.