In-situ baseline-free lamb wave imaging method for accurate defect characterization using chirp signal excitation

The in-situ baseline-free Lamb wave imaging method shows significant potential for non-destructive evaluation of plate-like structures, as it mitigates the influence of varying operational conditions that affect baseline-based methods. This work presents a thorough analysis and derivation of the time reversal process along damage paths. The study examines the effects of mode conversion on the time reversal focusing signal, optimizing the virtual time reversal method based on this phenomenon, and validates the correctness of the modal conversion sideband theory through simulation. Traditional virtual time reversal methods for air-coupled Lamb waves often struggle to accurately invert sideband signals due to the reliance on narrowband signals for transfer function recovery. Therefore, a broadband signal is required to accurately calculate the transfer function. Existing broadband techniques are not suitable for air-coupled ultrasound detection. To introduce broadband technology for air-coupled virtual time reversal, this study proposes a method utilizing a chirp signal exciting air-coupled transducers. An extended time window that covers the mode conversion sidebands is proposed. This method captures signals that include modal transition sidebands alongside the main mode, thereby improving the precision of defect characterization. Experimental results indicate that by optimizing the damage factor with the proposed extended time window, micro-defects can be characterized with greater accuracy. The imaging accuracy of this method for a single defect is 2.9 % (in contrast to 16.9 % for traditional methods), and for an 8 mm double hole with a center-to-center distance of 15 mm, the imaging accuracy is 24 % (compared to 42 % for traditional methods). Finally, the robustness of the method is validated under different temperature conditions. This approach demonstrates considerable promise for facilitating rapid, on-site, and precise measurement of aircraft skin defects in the future.