Warped Frequency Transform for Damage Detection using Lamb Waves

A novel time-frequency procedure is presented in this paper for guided wave (GW) propagation analysis in structure health monitoring (SHM) applications. The proposed approach combines the Warped Frequency Transform (WFT) with a Basis Pursuit algorithm to generate a sparse yet accurate time-frequency representation of the acquired signals even in the case of multi-modal dispersive propagation associated to broadband excitation of the waveguide. This is obtained through over complete dictionaries composed by optimized atoms which are designed to match the spectro-temporal structure of the various propagating modes. The Warped Basis Pursuit (W-BP) decomposition of several acquired waveforms results in distance signals that can be combined through classical beamforming techniques for imaging purposes. This approach is tested on experimental data obtained by broadband GW excitation in a 1 mm thick aluminum plate with an artificially introduced through crack, followed by multiple waveguide displacement recording through a scanning laser Doppler vibrometer. Dispersion compensation and high-resolution source as well as defect imaging is demonstrated even in domain regions that are not directly accessible for measurement.