Simultaneous Influence of Static Load and Temperature on the Electromechanical Signature of Piezoelectric Elements Bonded to Composite Aeronautic Structures

Abstract

Electromechanical (EM) signature techniques have raised a huge interest in the structural health monitoring community. These methods aim at assessing structural damages and sensors degradation by analyzing the EM responses of piezoelectric components bonded to aeronautic structures. These structures are subjected simultaneously to static loads and temperature variations that affect the metrics commonly used for damage detection and sensor diagnostics. However, the effects of load and temperature on these metrics have mostly been addressed separately. This paper presents experimentations conducted to investigate the simultaneous influence of static load and temperature on these metrics for two kinds of piezoelectric elements (PZT: lead zirconate titanate, and MFC: macro fiber composite) bonded on sandwich composite materials, for the full range of real-life conditions encountered in aeronautics. Results obtained indicate that both factors affect the metrics in a coupled manner in particular due to the variations of the mechanical properties of the bonding layer when crossing its glass transition temperature. Furthermore, both piezoelectric elements globally behave similarly when subjected to temperature variations and static loads. Simultaneous accounting of both temperature and static load is thus needed in practice in order to design reliable structural health monitoring systems based on these metrics. IntroductionIn the past decades, electromechanical (EM) signature techniques have gained importance in the structural health monitoring community [1][2][3][4]. The EM signature technique consists of measuring the EM signature (real or imaginary part of the impedance or admittance as a function of frequency) of a piezoelectric sensor which is mounted on a host structure in order to pinpoint incipient damages that may appear on the structure (damage detection) or to detect any defect on the sensor itself (sensor diagnostics). In an aeronautic context, temperature variations as well as static loads induced on the structure during a flight can both be very important. Experiments have been carried out that focused on temperature [5][6][7][8][9][10][11][12][13][14][15][16][17] or static loads [18][19][20][21][22], but rarely simultaneously [23][24][25][26]. Regarding temperature effects, it has been observed that the effect of temperature changes consists of a shift of the piezoelectric resonance frequencies. It has also been shown that higher temperature increases the apparent capacitive value of the bonded piezoelectric patch. Regarding the effect of static loads, it has been shown that the magnitude of the peaks and valleys of the EM signature decreases with an increase of applied loads and that natural frequencies shift proportionally with the applied load. Regarding the influence of both temperature and static loads, a strategy has been designed by Lim et al. [23] to compensate for their effects and Zhu et al. [24,25] and Xu et al. [26] analyzed experimentally both effects, but in a separate manner.This...

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