Higher harmonics induced in lamb wave due to partial debonding of piezoelectric wafer transducers

Piezoelectric wafer (PW) transducers used for Lamb wave actuation may get partially debonded from the host structure, because of their prolonged use, excessive voltage supply, or improper bonding onto the host structure. In this paper, higher harmonics induced in Lamb wave because of such debonding of the PW actuator are studied both experimentally and through finite element simulation. In experiments, an artificial partial debond is created while bonding the actuator patch onto a pristine aluminium plate. Lamb wave transduced by this actuator in the plate is picked up by a PW sensor which does not have any debonding. In FE simulation, Augmented Lagrangian algorithm is used to solve the contact problem at the breathing debond. Three higher harmonics are observed in the experiments and also in the FE simulation. To ensure that the generated higher harmonics correspond to Lamb wave, time-frequency analysis is carried out using Morlet wavelet transform, and the results are reported in the paper. Spectral damage index (SDI), obtained from spectral attributes of first four harmonics in experiments and simulation, is found to be decreasing with an increase in debonding area. This shows that actuator debonding introduces contact nonlinearity which induces higher harmonics in Lamb wave. Therefore, in damage detection using Lamb wave based nonlinear techniques, the higher harmonics produced may get influenced by the false higher harmonics produced by actuator debonding, leading to incorrect results, if bonding of the actuator is not taken care of properly. (C) 2014 Elsevier Ltd. All rights reserved.