Lamb Wave-Mode Tuning of Piezoelectric Wafer Active Sensors for Structural Health Monitoring

An analytical and experimental investigation of the Lamb wave-mode tuning with piezoelectric wafer active sensors (PWASs) is presented. The analytical investigation assumes a PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lag transfer of tractions and strains is assumed, and an analytical solution using the space-wise Fourier transform is reviewed, closed-form solutions are presented for the case of ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary). The analytical solutions are used to derive Lamb wave-mode tuning curves, which indicate that frequencies exist at which the AO mode or the SO mode can be either suppressed or enhanced. Extensive experimental tests that verify these tuning curves are reported. The concept of "effective PWAS dimension" is introduced to account for the discrepancies between the ideal bonding hypothesis and the actual shear-lag load transfer mechanism. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring (SHM) applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar (EUSR)) and the time reversal process (TRP). In PWAS phased array EUSR applications, the basic assumption of the presence of a single low-dispersion Lamb wave mode (SO) is invoked since several Lamb wave modes traveling at different speeds would disturb the damage imaging results. Examples are given of correctly tuned EUSR images versus detuned cases, which illustrate the paramount importance of Lamb wave-mode tuning for the success of the EUSR method. In the TRP study, an input wave packet is reconstructed at a transmission PWAS when the signal recorded at the receiving PWAS is reversed in the time domain and transmitted back to the original PWAS. Ideally, TRP could be used for damage detection without a prior baseline. However, the application of TRP to Lamb waves SHM is impended by the dispersive and multimodal nature of the Lamb waves. The presence of more then one mode usually produces additional wave packets on both sides of the original wave packet due to the coupling of the Lamb wave modes. The PWAS Lamb wave tuning technique described in this paper is used to resolve the side packets problem. Several tuning cases are illustrated. It is found that the 30 kHz tuning of the AO Lamb wave mode with a 16-count smoothed tone burst leads to the complete elimination of the side wave packets. However, the elimination was less perfect for the 290 kHz tuning of the SO mode due to the frequency sidebands present in the tone-burst wave packet.