Comparative study on the performance of high temperature piezoelectric materials for structural health monitoring using ultrasonic guided waves

Focusing on predictive maintenance for optimised maintenance schedules, energy, aerospace, oil and gas industries are seeking technologies to enable in-service structural health monitoring (SHM) of their critical assets. Many of these critical assets such as turbine engine components and steamlines operate at elevated temperatures. For such high temperature (HT) applications, advanced piezoelectric materials are required for construction of ultrasonic transducers. Ultrasonic guided wave (UGW) technology has been widely used for pipeline inspection but HT ultrasonic transducers are required to enable in-service SHM of steamlines. The main criterion for HTUGW transducer design is an appropriate, temperature-stable ultrasonic response at target temperatures and a stable frequency response (in the range10-150 kHz for pipes of 2-48 inch diameter) to maintain defect sensitivity at HT. These transducers comprise piezoelectric materials of appropriate polarisation and dimensions, which, when excited with an electrical input, transmit the desired displacement patterns to the UGW modes in the structure being monitored. The detection of defects is indicated by changes in the received ultrasonic measurements. With temperature variations and over time, the dielectric, elastic and piezoelectric properties of the active material can diverge, leading to deviations in the ultrasonic response that may lead to false alarms. This comparative study investigates and compares the performance of four commercially available HT piezoelectric materials: PZT-5A, MBT, LiNbO3 and GaPO4. The maximum recommended operating temperatures for long-term use of these selected materials are 200°C, 400°C, 600°C and 720°C, respectively. Elastic, dielectric and material properties representing a figure of merit for piezo transducers are determined at increasing temperatures up to 600°C and over a period of 1000 hours. The findings from this work will enable transducer design to use the most appropriate piezoelectric material for the target temperature range.