Characterization of Piezo Electric Wafer Active Sensors after Exposure to High Temperature

Piezoelectric wafer active sensors (PWAS) at high temperature with reliable operation are desired for structural health monitoring. The PWAS may be generalized as an electromechanical system because of the incorporation of electromechanical coupling. The basic principle of SHM method by PWAS is to monitor variation in the electro-mechanical (E/M) impedance/ admittance signature. The operational temperature range of PWAS can be limited by the sensing capability of the piezoelectric material at elevated temperatures. Therefore stability of PWAS at high-temperature environments is of great interest for SHM. In such cases SHM can be done at room temperature or at relatively lower temperature. However, during service permanently bonded PWAS can be exposed to very high temperature. The traditional PWAS use piezoelectric materials Lead Zirconate Titanate (PZT) that have been attracted by researchers due to its enhanced sensing, actuation or both capabilities. This paper discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available. For temperature dependence study PWAS were exposed to 50 0 C to 250 0 C with 50 0 C interval at around 2 0 C/min heating rate. E/M impedance/ admittance and different material properties such as, dielectric constant, dielectric loss, mechanical quality factor, P-E hysteresis loop, in plane piezoelectric constant were determined experimentally at room temperature after exposure to high temperature. The variation in E/M impedance and admittance signature and different material properties were obtained at each temperature. The piezoelectric material degradation was also investigated by microstructural and crystallographic study. INTRODUCTION Piezoelectric wafer active sensors (PWAS) have been used extensively for detecting damages and flaws in the structure in SHM [1], [2], [3], [4]. PWAS can be used for electromechanical (E/M) impedance/ admittance method for detecting damages. The (E/M) impedance/ admittance method has been utilized to determine the local dynamic characteristics of a PWAS bonded on a host structure for in situ ultrasonic inspection [3]. The basic principle of this non-destructive monitoring method is to monitor variation in the impedance/ admittance signature measured from the permanently attached PWAS to the host structure. PWAS use transduction of ultrasonic elastic waves into voltage and vice versa. Since a relationship exist between the mechanical impedance/ admittance of the host structure and the electrical impedance/ admittance of the PWAS, any change in the material state can be attained by measuring the coupled electro-mechanical impedance/ admittance. However there is a major problem when using (E/M) impedance/ admittance method by PWAS on host structures after exposure to high temperature. After exposure to high temperature the E/M impedance/ admittance method can lead to unsuccessful damage detection due to ambiguous change in the impedance/ admittance signature. Any change in the material state of PWAS due to crossing characteristic temperature limit can lead significant change in (E/M) impedance/ admittance. As a result, (E/M) impedance/ admittance of free PWAS after exposure to high temperature are important dynamic descriptor for characterizing the sensor prior to its installation on a structure. The thermal effect of PWAS after exposure to hightemperature environments such as dry cask storage canister, Proceedings of the ASME 2016 Pressure Vessels and Piping Conference PVP2016 July 17-21, 2016, Vancouver, British Columbia, Canada