Dispersion compensation of Lamb waves with the warped frequency transform in passive monitoring techniques

Guided Wave (GW) testing for Structural Health Monitoring (SHM) is complicated by dispersion in wave mode propagation. Compensation approaches available in literature are often limited to active monitoring technique as they require the knowledge of the time instant in which the acoustic emission starts. In this work we present a novel procedure for passive sensors which overcomes this limitation and precisely locates the acoustic source. The proposed procedure is based on the Warped Frequency Transform (WFT). Such transformation causes a resampling of the time-frequency domain which matches the spectro-temporal behavior of guided waves. In the resampled domain a given guided wave mode have an unique pattern which is independent from the distance of propagation thus allowing a precise time of flight measurement. This work demonstrates how this property can be effectively combined with an hyperbolic positioning algorithm to accurately image the source location from the signal captured by passive sensors.