Abstract
Turbomachine components used in aerospace and power plant applications preferably require continuous structural health monitoring at various temperatures. The structural health of pristine and damaged superalloy compressor blades of a gas turbine engine was monitored using real electro-mechanical impedance of deposited thick film piezoelectric transducers at 20 and 200 °C. IVIUM impedance analyzer was implemented in laboratory conditions for damage detection in superalloy blades, while a custom-architected frequency-domain transceiver circuit was used for semi-field circumstances. Recorded electromechanical impedance signals at 20 and 200 °C acquired from two piezoelectric wafer active sensors bonded to an aluminum plate, near and far from the damage, were initially utilized for accuracy and reliability verification of the transceiver at temperatures >20 °C. Damage formation in both the aluminum plate and blades showed a peak shift in the swept frequency along with an increase in the amplitude and number of impedance peaks. The thermal energy at 200 °C, on the other hand, enforces a further subsequent peak shift in the impedance signal to pristine and damaged parts such that the anti-resonance frequency keeps reducing as the temperature increases. The results obtained from the impedance signals of both piezoelectric wafers and piezo-films, revealed that increasing the temperature somewhat decreased the real impedance amplitude and the number of anti-resonance peaks, which is due to an increase in permittivity and capacitance of piezo-sensors. A trend is also presented for artificial intelligence training purposes to distinguish the effect of the temperature versus damage formation in sample turbine compressor blades. Implementation of such a monitoring system provides a distinct advantage to enhance the safety and functionality of critical aerospace components working at high temperatures subjected to crack, wear, hot-corrosion and erosion.
Highlights
Temperature effects have always been a critical problem for piezoelectric-based monitoring methods such as pitch-catch [1], pulse-echo [1,2] and electromechanical impedance spectroscopySensors 2019, 19, 2805; doi:10.3390/s19122805 www.mdpi.com/journal/sensors (EMIS) [3,4,5,6,7,8], especially in detecting incipient damages
Apart from high cyclic fatigue (HCF), low cycle fatigue (LCF) and very high cycle fatigue (VHCF) of compressor and turbine blades were introduced as serious challenges in structural health monitoring (SHM)
Microcracks initiated at LCF and VHCF conditions inside the material at a normal level of the mechanical stress are difficult to detect by conventional non-destructive methods [41]
Summary
Temperature effects have always been a critical problem for piezoelectric-based monitoring methods such as pitch-catch [1], pulse-echo [1,2] and electromechanical impedance spectroscopySensors 2019, 19, 2805; doi:10.3390/s19122805 www.mdpi.com/journal/sensors (EMIS) [3,4,5,6,7,8], especially in detecting incipient damages. Temperature effects have always been a critical problem for piezoelectric-based monitoring methods such as pitch-catch [1], pulse-echo [1,2] and electromechanical impedance spectroscopy. It has directed several research works towards the development or implementation of high-temperature piezoelectric transducers for structural health monitoring (SHM) of mechanical components, which work in harsh environments, and are subjected to wear, corrosion, erosion or surface cracking [9,10,11]. Li et al [8] recently proposed the concept of a smart corrosion coupon for corrosion monitoring and studied the effect of temperature on the electromechanical impedance (EMI) signatures of PZT (lead zirconate titanate) patches at 10 to 40 ◦ C. Installed high-temperature piezoelectric wafer active sensors (HT-PWAS) have been one of the extensively employed sensor technologies for high-temperature SHM [12,13]. A report published by NASA [15]
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