dopamine-2 receptor antagonist Park et al demonstrated both

Park et al. [18] demonstrated both theoretically and experimentally in a composite plate that it is possible to minimize the amplitude dispersion effect and successfully reconstruct the original input waveform by using a narrow-band excitation signal. The frequency was selected so as to excite only the fundamental antisymmetric mode. Xu and Giurgiutiu [19,20] and Santoni et al. [21] also studied single-mode tuning effects on Lamb wave time-reversal in thin metallic plates with surface-bonded PZT patch transducers, and concluded that single-mode Lamb waves are rigorously time reversible under narrow-band excitation, and hence are recommended for time-reversal based damage detection. The time-reversal of a mixed mode () Lamb wave creates two extra wave packets symmetrically placed around the main wave packet. Their experimental results conducted on an aluminum plate using surface mounted PZT transducers, however, showed that a two-mode () Lamb wave yielded a better reconstruction by the TRP than either of the single modes ( or ).
The use of the TRP of Lamb waves for damage detection was illustrated by Sohn et al. [22], examining the deviation of the reconstructed signal from the known input signal in a composite plate containing delamination damage. Poddar et al. [23] used the time-reversal method for experimentally detecting notch, block mass and surface corrosion type defects in metallic plates. At the same time, it has also been reported by several researchers that the similarity index of the reconstructed signal with the original input signal does not show appreciable difference between the undamaged and damaged states for notch and surface damages in metallic plates [24,25]. This aspect was recently examined by the authors [26] through finite dopamine-2 receptor antagonist (FE) simulations verified by some experimental data. Using frequency sweeping with narrow-band modulated tone burst excitations, they showed that (i) the single-mode tuning does not lead to the best reconstruction of the original input signal in the undamaged plate, and (ii) the damage index computed using the conventional main wave packet of the reconstructed signal does not indeed show any appreciable change in presence of notch-type damage. To overcome this drawback, the authors introduced the concept of the best reconstruction frequency at which the similarity index in the undamaged plate is maximum over a given range of excitation frequency. It can be determined for a given structure-transducer-adhesive system in undamaged state by frequency sweeping over a desired frequency range depending on the size of defects to be detected. At this frequency, the wave contains both and modes in general. A refined method of computing the damage index was proposed using an extended wave packet ranging between the two side bands accompanying the main wave packet, which showed excellent sensitivity to damage when used at the best reconstruction frequency. The FE model employed in the aforementioned study, however, assumed a perfect bonding between the transducers and the plate. Apart from the excitation frequency, several parameters such as the bonding between piezoelectric wafer transducers and the host plate, the transducer size, the plate thickness as well as the tone burst count in the excitation signal affect the amplitude dispersion and consequently the time reversibility. These effects must be understood thoroughly for successfully applying the TRP of Lamb waves for damage detection.
The effect of adhesive layer on the performance of piezoelectric transducers for impedance based SHM of metallic plate structures was experimentally studied by Qing et al. [27]. It was shown that an increase in the adhesive thickness alters the electromechanical impedance and the resonant frequency of the piezoelectric elements. Ha and Chang [28] studied the effect of adhesive layers on the sensor signal for Lamb wave propagation through numerical simulations as well as experiments. It was shown that the amplitude of the sensor signal may increase with the decrease in the shear modulus of the adhesive layer or with the increase in its thickness, when the excitation frequency is near the resonant frequency of the surface-mounted sensor, which is opposite to what is expected due the shear lag effect. This happens due to what was termed as the resonant effect. They also studied these effects at elevated temperatures [29]. Lanzara et al. [30] studied the effect of bond degradation on the performance of piezoelectric actuators and sensors. To the best of authors’ knowledge, no study on the effect of adhesive layer on the time reversibility of Lamb waves has been reported so far. The effect of number of cycles in the modulated tone burst excitation on the time reversibility was examined by Poddar et al. [23] and Agrahari and Kapuria [26]. But, these studies were conducted at specific frequencies, and the obtained trend may not be valid at all frequencies. Also, there has been no experimental study reported so far on the effect of the transducer thickness and the plate thickness on the time reversibility, which may have significant effect on the quality of the reconstructed signal after the TRP.