Baseline-free damage detection and sizing under varying temperatures using Lamb waves without temperature compensation

The Lamb wave time-reversal method (TRM) has been widely proposed as a baseline-free technique for Structural health monitoring (SHM) of plate/shell-type structures. Still, its efficacy has never been tested under a variable environment. The present study examines the conventional TRM and establishes that it is unsuitable for baseline-free detection and sizing of damage under varying temperatures since the damage index (DI) is prone to change with temperature rise and generally has low sensitivity to damage. The investigation is performed for block mass-type and notch-type defects using experiments and numerical simulations. The study further tests the refined time-reversal method (RTRM), presented recently, against the same objective. This technique uses Lamb waves of the best reconstruction frequency to probe the structure, at which the reconstructed signal in a healthy plate undergoes minimum deviation from the input signal. It considers an extended wave packet for computing the DI instead of using the main mode alone. We show that this process leads to the lowest DI for undamaged plates and the highest sensitivity to damage. Also, the DI remains almost unchanged during temperature change, irrespective of the damage type, size, and plate thickness. It makes the RTRM very effective for baseline-free identification and sizing of damage under varying temperatures.