Impact of cryogenic environment on piezoelectric transducers used in SHM applications

Abstract

A high number of actors of the space industry, including ESA (European Space Agency) and CNES (French national space agency), are engaged in a program aiming at developing future European reusable space launchers. In this context, PYTHEAS Technology has developed an embedded Non-Destructive Testing (eNDT) solution to ensure that strategic structural elements of the launcher are free of damage before a new launch, performing Structural Health Monitoring (SHM). The sensors used for this application are subject to severe temperature variations, including cryogenic temperatures. Cryogenic conditions are present in other fields requiring SHM solutions, such as liquid hydrogen reservoirs, increasing the importance of this study. Results of experimental tests are presented, where piezoelectric transducers are glued with two types of glue on samples of aluminum plates and carbon composite plates. A reference measurement is carried out before plunging the plates in a climatic chamber, down to -196°C. Pulse-echo and admittance measurements are performed during the temperature decrease and after the temperature rise. The admittance measurements show no deterioration of the samples after being immersed in liquid nitrogen: the transducers, wires, glue, and samples themselves have resisted to the temperature cycle. The pulse-echo signals’ time and frequency analysis show a linear attenuation of the signals relative to temperature. A difference in signal intensity and wave behavior has also been observed relative to the sample’s material: composite or aluminum. No difference regarding the type of glue is witnessed. These results pave the way for damage detection in harsh environments, since the system’s performance is maintained through a whole temperature cycle in cryogenic conditions.