Laser beam shaping for enhanced Zero-Group Velocity Lamb modes generation

TL;DR

This paper investigates laser beam shaping techniques to optimize the excitation of Zero-Group Velocity Lamb modes in plates, demonstrating theoretical predictions and experimental validation for enhanced resonance amplitudes using tailored Gaussian and annular sources.

Contribution

It introduces a method to optimize laser source parameters for ZGV Lamb mode excitation, combining analytical, simulation, and experimental approaches.

Findings

Annular sources increase amplitude by at least 3 times compared to Gaussian sources.

Optimal Gaussian source radius is approximately λ_{ZGV}/π.

Experimental results confirm theoretical predictions with a resonance amplification factor of 2.1.

Abstract

Optimization of Lamb modes induced by laser can be achieved by adjusting the spatial source distribution to the mode wavelength ($\lambda$). The excitability of Zero-Group Velocity (ZGV) resonances in isotropic plates is investigated both theoretically and experimentally for axially symmetric sources. Optimal parameters and amplitude gains are derived analytically for spot and annular sources of either Gaussian or rectangular energy profiles. For a Gaussian spot source, the optimal radius is found to be $\lambda_{ZGV}/\pi$. Annular sources increase the amplitude by at least a factor of 3 compared to the optimal Gaussian source. Rectangular energy profiles provide higher gain than Gaussian ones. These predictions are confirmed by semi-analytical simulation of the thermoelastic generation of Lamb waves, including the effect of material attenuation. Experimentally, Gaussian ring sources of controlled width and radius are produced with an axicon-lens system. Measured optimal geometric parameters obtained for Gaussian and annular beams are in good agreement with theoretical predictions. A ZGV resonance amplification factor of 2.1 is obtained with the Gaussian ring. Such source should facilitate the inspection of highly attenuating plates made of low ablation threshold materials like composites.