## 2 Related Work
### Conventional Approaches  
Traditional damage identification techniques primarily include vibration analysis, acoustic emission, ultrasonic testing, and thermography. Vibration-based detection methods monitor the vibrations of WTBs using vibration sensors. When a blade becomes damaged, its vibrational characteristics can change [15]. [PERSON] et al. [16] tested four vibration-based techniques to detect partial damage to the blades. These approaches rely on assessing the vibrational behavior of the blades upon stimulation, utilizing patches of piezoelectric ceramic actuators that are attached to the blades. By examining vibration data, [PERSON] et al. [17] used an empirical wavelet thresholding approach to identify the kind of damage in the blade bearings. This method aims to enhance the processing capability of vibration data, thereby increasing the accuracy and efficiency of fault detection. Employing vibration-based techniques for blade damage detection presents a significant challenge: differentiating between vibrations resulting from damage and those arising from environmental and operational factors. Acoustic emission (AE) technology is a non-destructive testing technique utilized to monitor internal damages, cracks, micro-defects, or other anomalies within materials or structures; it is extensively applied within engineering and structural health monitoring [18]. In the monitoring of WTBs, acoustic emission-based detection techniques not only allow for the assessment of the structural health status but also enable the evaluation of the severity of damage by analyzing the acoustic emission signals. [PERSON] et al. [19] confirmed the practicality of AE technology for the real-time monitoring of WTBs structural health. Cyclic loading was conducted using a compact resonant mass to accurately replicate the operational loading conditions. [PERSON] et al. [20] monitored the condition of a 59.5-m composite WTB under fatigue stress using AE technology, identifying and classifying different types of damage on the WTBs. Nonetheless, a significant quantity of acoustic emission sensors must be installed on the WTBs, and there is not any physical connection between the acoustic emission signals and the corresponding damage. Concurrently, a principal challenge of this technology lies in differentiating the acoustic emission signals attributable to damage from those caused by noise, which renders the data processing task complex and significantly more costly. It is possible to use ultrasonic testing technologies to find exterior and internal damage in WTBs. By transmitting ultrasonic waves and receiving the reflected signals within or on the surface of the blades, the structural health of the blades can be assessed [21]. Ultrasonic technology is capable of detecting a variety of damage types, such as delamination and cracks, which alter the propagation path of ultrasound waves. Because of this, ultrasonic technology is an essential tool for the inspection and maintenance of WTBs since it can be used to assess the position and severity of damage by analyzing the received signals. [PERSON] et al. (2012) utilized ultrasonic non-destructive testing (NDT) and structural health monitoring (SHM) techniques to detect damage in an actual WTB within a laboratory setting. [PERSON] et al. (2013) suggested employing a novel detection technique alongside non-destructive ultrasonic examination to pinpoint structural defects in WTBs. Ultrasonic inspection technology typically presents a limitation in that it necessitates direct placement of an ultrasonic transducer on the surface or within the interior of a blade. This may require a shutdown and contact with the blade, leading to interruptions in production and additional maintenance costs. Thermographic inspection techniques aim to detect variations in the thermodynamic properties of blades by capturing thermal images of the blade surface using a thermal imager or infrared camera.