In this work, we critically examine the effectiveness of the time-reversed Lamb wave based baseline-free damage detection technique for notch-type damage in isotropic plates, through finite element (FE) simulation of an integrated actuator–plate–sensor system. The FE simulation has been verified with experiments. We show that the single-mode tuning, hitherto recommended for improving performance of the time-reversal process (TRP) based technique, does not generally lead to the best reconstruction of the original input signal. The results of the TRP in the presence of notch-type damage show that the damage indices (DIs) computed using the conventional main wave packet of the reconstructed signal do not show any significant change with an increase in damage size, which is consistent with some recently reported experimental results by other groups. A new method of computing the DIs with extended signal length is proposed, which shows excellent sensitivity to damage, and also ensures a low threshold for the undamaged case, when used at the best reconstruction frequency. The same refined DIs, however, are not effective, when used at the so called sweet spot frequency exciting a single mode. Refined DIs based on correlation and similarity of the reconstructed signal reflect the true severity of damage.

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Summary A recently proposed refined time‐reversed Lamb wave method for baseline‐free damage detection is tested experimentally for detecting block mass and notch‐type damages in isotropic plates. The experimental results were compared with finite element simulations. The frequency of best reconstruction has been determined experimentally for the actuator–plate–sensor system by performing the time reversal process for a range of frequency, which is found to be very different from the sweet spot frequency exciting a single mode, hitherto recommended for improving the performance of the time reversal process‐based techniques. It is shown that the damage indices (DIs) computed by using the conventional main wave packet of the reconstructed signal are less sensitive to the presence of damage, which is consistent with some recently reported experimental results by other groups. The present method with extended wave packet shows excellent sensitivity to damage for both block mass and notch‐type damages and also ensures a low threshold for the undamaged case when used at the best reconstruction frequency. The refined DIs reflect the true severity of damage. It was observed that a putty on the plate has no significant change in the DIs in the present method, whereas a baseline method would identify it as a damage due to very significant scattering by the putty.

An analytical solution is presented for excitation, forward propagation, sensing, and time reversal of Lamb waves, considering the shear-lag effect of the bonding layer between the piezoelectric patch transducers and the plate. Consistent shear-lag solutions for actuator–plate and plate–sensor interactions are developed, considering plane strain condition. The Lamb wave solution is obtained by transforming the interfacial shear stress distribution and wave equations into the wavenumber domain by applying the spatial Fourier transform and converting the same to the physical domain using the inverse Fourier transform and residue theorem. The time-domain response for an arbitrary excitation is obtained by applying the temporal Fourier transform to the excitation and using convolution integral. The solution is compared with the experimental and numerical results for both forward and time-reversal responses and also for the best reconstruction frequency. The effect of transducer mass on the accuracy of the response is investigated. Finally, the shear-lag effect on the time reversibility of the reconstructed signal is studied for varying adhesive layer thickness, transducer thickness, host plate thickness, and the tone burst count. The solution will be useful for designing structural health monitoring systems using both baseline-based and baseline-free methods.

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