A new Warped Frequency Transformation (WFT) for Guided Waves characterization

The characterization of the dispersive behaviour of stress guided waves (GWs) from a time transient measurement is generally attempted by means of time-frequency representations (TFRs). Unfortunately, any TFR is subjected to the time-frequency uncertainty principle that limits the capability of the TFR to distinguish multiple, closely spaced guided modes, over a wide frequency range. To this aim we implemented a new Warped Frequency Transform (WFT) that in force of a more flexible tiling of the time-frequency domain presents enhanced modes extraction capabilities. Such tiling, composed by non linearly modulated atoms, is built on the dispersive group velocity curve of a particular propagating mode. The resulting TFR thus emphasizes the energy content associated to that particular guided mode within the recorded time waveform. Here we propose an application of the WFT to numerically simulated Lamb Waves propagating in an aluminum plate. The results show that the proposed WFT limits interference patterns which appears with others TFRs and produces a sparse representation of the dispersive Lamb wave pattern that can be suitable for identification and characterization purposes.