Performance and diagnostic of guided wave PZT transducers under reusable launch vehicles thermal flight conditions

This paper examines the impact of brief but intense thermal cycling on surface-bonded guided waves (GW) piezoelectric transducers (PZT). The effects on both electromechanical impedance (EMI) self-diagnostic measurements and Lamb wave emission and reception performance are investigated through experimental and numerical studies. EMI spectra are acquired in the 80–400 kHz range with both the real and imaginary parts being studied. GW pitch-catch measurements are conducted for two frequency ranges (15–25 kHz and 150–250 kHz) to investigate the two fundamental Lamb wave modes. Furthermore, laser Doppler vibrometry (LDV) scans are performed to detect and localize transducer debondings. A finite element model of a surface bonded PZT is used to reproduce qualitatively both pitch-catch measurements and EMI spectrum in order to test the impact of different parameters and to support the interpretation of the test results. Two types of evolution with thermal cycling have been identified, each with distinct EMI and GW signatures. Firstly, the PZTs show a uniform 15%–20% reduction in GW sensing capabilities across the studied frequency range, with unchanged emission capabilities. This behavior is associated with irreversible effects of the thermal cycling on the constitutive properties of the PZT (mainly on the relative permittivity) and, to a lesser extent, to the evolution of the mechanical properties of the bonding layer. Secondly, a unique signature of partial PZT debondings is exhibited on the EMI spectra and LDV scans. The debonding has little impact on the GW signals at low frequency but results in amplitude changes and phase delays of both the received and emitted waveforms in the higher frequency range.