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

Area of Science:

• Physics
• Materials Science
• Acoustics

Background:

• Laser-induced Lamb waves are crucial for non-destructive material testing.
• Zero-Group Velocity (ZGV) resonances offer high sensitivity but require efficient excitation.
• Optimizing source parameters is key to maximizing ZGV resonance amplitude.

Purpose of the Study:

• To investigate the excitability of ZGV resonances in isotropic plates using various axially symmetric laser sources.
• To analytically derive optimal source parameters and predict amplitude gains for different energy profiles.
• To experimentally validate theoretical predictions and demonstrate enhanced ZGV resonance excitation.

Main Methods:

• Theoretical analysis of Lamb wave generation with different spatial source distributions (spot, annular, Gaussian, rectangular).
• Analytical derivation of optimal source parameters and amplitude gains.
• Semi-analytical simulation of thermoelastic generation, including material attenuation.
• Experimental generation of Gaussian ring sources using an axicon-lens system.

Main Results:

• Optimal radius for a Gaussian spot source is λ_ZGV/π.
• Annular sources achieve at least a factor of 3 amplitude increase over optimal Gaussian sources.
• Rectangular energy profiles yield higher amplitude gains than Gaussian profiles.
• Experimental results show good agreement with theoretical predictions, achieving a ZGV resonance amplification factor of 2.1 with a Gaussian ring source.

Conclusions:

• Spatial source distribution significantly impacts Lamb wave excitation and ZGV resonance amplitude.
• Annular and rectangular sources are superior to spot sources for ZGV resonance excitation.
• The findings facilitate the inspection of challenging materials like composites using laser-induced ZGV resonances.