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The highly conservative approach currently used to ensure the safety of key aerospace components has led to parts, such as turbine blades, being among the most commonly rejected or replaced components (Carter, 2005). Therefore, current efforts primarily focus on detecting potential damages before they reach a critical size for rapid growth and component failure. However, most previous literature reports have focused on crack size and location, without accounting for the ensuing local hot spot and overall increase in blade substrate temperature. Moreover, little is reported on the effect of TBC coating loss or spallation on the resonance behavior of turbine blades. Operating in an extreme environment, turbine blades can be subject to a variety of damage mechanisms including: chattering, high cycle fatigue (HCF) from high frequency engine vibrations, low cycle fatigue (LCF) from extreme cold and hot phases, creep, corrosion, and foreign object damage (FOD), among others. To improve the performance and durability of turbine blades, advanced coating technologies have been developed and used for decades. Thermal barrier coatings (TBCs) are among the most performant protective systems (Padture et al., 2002; Sankar et al., 2019, Dhomne & Mahalle, 2016). Current turbine blade substrates are generally metal base, such as nickel base superalloys, and have lower temperature and corrosion resistance than TBCs. Therefore, substantial substrate temperature increases resulting from potential TBC loss can cause failure of the component within relatively few operational cycles. TBC damage mechanisms are complex and ultimate failure can result in the spallation of the top coat (Ali et al., 2018; Evans et al., 2001; Schlichting et al., 2003).
To further improve safety and economy efficiency while reducing downtimes, a great deal of efforts are currently being deployed to develop more robust, compact and efficient onboard monitoring systems (Yildirim & Kurt, 2018; Roemer & Kacprzynski, 2000; Mevissen & Meo, 2019). The goal is to continuously evaluate residual strength or health and to estimate the remaining service life of components (Boyd-Lee et al., 2001). Ongoing research efforts are making critical advances in monitoring approaches and sensing technology (Borovik & Sekisov, 2020; Sunar & Al-Bedoor, 2008; Ranjan, 2016). Non-contact sensors are favoured for the monitoring of turbine blade parameters to avoid interference with aerodynamic and structural performance (Procházka & Vank, 2011; Devi et al., 2021). Resonance vibrations are highly dependent on the design, microstructure, materials properties and damage state of blades (Efe-Ononeme et al., 2018; Pridorozhnyi et al., 2019; Prasad et al., 2017). As such, they are among the most reliable indicators of blade health. Particularly, shifts in resonance frequencies can be readily measured for the detection of potential blade damages (Djaidir et al.,2017; Madhavan et al., 2014; Atiyah & Falih, 2019; Rani et al., 2019). Therefore, the current study is based on a solid foundation of achievements realized so far. However, the work goes beyond blade cracks. It expands to not yet sufficiently studied aspects such as the effect of TBC damage on the dynamic behavior of turbine blades and how the impact of defects relates to specific mode shapes.