A baseline-free damage localization method based on the mode-converted Lamb waves

Lamb waves are widely used for structural damage detection, but most existing methods rely on baseline signals for comparison. In practice, such baseline signals are often unreliable or unavailable, limiting the robustness of these approaches. To overcome this challenge, a baseline-free method is proposed for damage detection and localization, which exploits the mode conversion behavior of Lamb waves. When Lamb waves interact with damage, part of their energy is converted into other modes, providing a distinctive phenomenon of structural defects. However, these converted components are usually weak and difficult to isolate. To address this, the reciprocity principle of Lamb waves is introduced and integrated. Specifically, subtracting response signals recorded before and after exchanging the roles of transmitter and receiver, direct wave components are removed while damage-induced converted modes are retained. Based on this, a mode conversion damage index is constructed using the energy of mode-converted components in the residual signal. Finally, a probabilistic imaging algorithm is employed to visualize and localize the damage. Both numerical simulations and experimental results confirm the effectiveness of the proposed method, demonstrating its potential for reliable and accurate structural health monitoring.