Meta-structure enhanced second harmonic S₀ waves for material microstructural changes monitoring.

Area of Science:

• Materials Science
• Acoustics
• Nonlinear Wave Phenomena

Background:

• Cumulative second harmonic Lamb waves offer insights into material microstructural changes.
• Excitation of both S0 and A0 Lamb wave modes complicates monitoring due to A0 mode interference.
• Efficient generation of cumulative S0 harmonics is crucial for sensitive structural health monitoring.

Purpose of the Study:

• To develop a metamaterial structure (meta-structure) for enhancing cumulative second harmonic S0 Lamb waves.
• To achieve efficient A0-to-S0 wave mode conversion using topology optimization.
• To improve the sensitivity of nonlinear Lamb wave-based monitoring for material microstructural changes.

Main Methods:

• Topology optimization was employed to design a surface-mounted meta-structure for A0-to-S0 wave mode conversion.
• Numerical simulations were performed to evaluate the meta-structure's performance in enhancing S0 wave amplitudes.
• The designed meta-structure was fabricated using 3D printing and experimentally validated on an aluminum plate.

Main Results:

• The meta-structure increased fundamental S0 mode wave strain amplitudes by up to 60%.
• Significant enhancement of second harmonic S0 mode waves was observed at various propagation distances.
• Experimental validation confirmed increased linear S0 mode wave amplitudes with the meta-structure.

Conclusions:

• The developed meta-structure effectively converts A0 Lamb waves to S0 modes, enhancing cumulative second harmonic generation.
• The system demonstrates improved sensitivity for characterizing material microstructural changes, showing promise for damage detection.
• This approach offers a promising solution for advanced structural health monitoring applications.