Phase velocity-based criterion for the identification of Lamb wave modes in similar regions of time-frequency distributions

This study establishes a universal physical criterion for identifying the A0 and S0 Lamb wave modes, based on the local relationship between frequency components and their corresponding phase velocities. Numerically simulated A0 and S0 modes signals across different frequency ranges (300 kHz, 800 kHz, and 2.2 MHz), representing different trends of the group and phase velocities, were used in the theoretical analysis. While group velocity governs the temporal redistribution of frequency components, conventional time-frequency representations of the two modes overlap and become indistinguishable in frequency bands where their group velocities coincide. The proposed dual-point criterion resolves this ambiguity by exploiting a fundamental physical regularity: for the A0 mode, higher-frequency components exhibit higher phase velocities, regardless of their position in the waveform, whereas for the S0 mode, lower-frequency ones exhibit higher phase velocities. Tracking these local frequency-phase-velocity trends enables the determination of group-velocity dispersion behavior. The proposed criterion was validated experimentally using multimodal wave packets propagating in an isotropic aluminum plate (300 kHz range) and an anisotropic Glass Fiber Reinforced Polymer (GFRP) composite laminate (130 kHz range). The results demonstrate that the criterion identifies both modes and characterizes group-velocity dispersion trends, completely bypassing computationally intensive algorithms or dense multi-point spatial scans.