## 1 Introduction
The prevention maintenance should be realised on a regular basis for a reliability-and-risk inspection strategy [17]. Thus, the long-term goal is to develop reliable methods for testing, verification, and monitoring, which could estimate the severity of damage to the residual life of WTBs [11]. Figure 1: The waste hierarchy according to ‘the Waste Framework Directive’ of the European Union [7]. However, due to the very complex WTB construction, which consists of different layers with different thicknesses and the anisotropic materials used in their manufacturing, the WTBs are difficult to monitor. Generally, the research in WTB is based on nondestructive testing (NDT), which is used for fault detection and diagnosis. Because NDTs can detect both surface and internal faults in WTBs, it is applied in structural health monitoring (SHM) systems for solving the preventive maintenance tasks for safety and failures [22]. Therefore, numerous different techniques, algorithms, and structural health monitoring devices for damage detection and identification of potential failure of WTBs have been developed over the last decades. The SHM of blades can be performed applying strain measurements (strain gauges, fibre optic cables, photogrammetry with full-field strain measurement), fatigue and modal properties monitoring, ultrasonic methods, acoustic emissions, thermography, impedance techniques, embedded conductive nanoscale particles, etc. [23; 24; 25]. Acoustic emission (AE) can be a very informative diagnostic tool for detecting early damage events but requires a high number of distributed sensors and has a high sensitivity to background noise. However, embedded at the root of the blades can be an effective tool to monitor the overall structural integrity of the blades [26]. Even though active thermography is fast and cost effective, using it for large structures is impractical due to active external heating. The passive thermography under lab conditions under fatigue testing can be very effective to detect various types of progressive damage remotely, but incorporating such external factors as solar radiation, variation of environment temperatures, wind, etc., in real conditions [27] is still challenging. Strain measurements can reflect the overall accumulated damage of the blade due to the stiffness loss, but it limits the possibility to indicate critical structural failure [28]. The ultrasonic technique is mostly used in NDT, and one of the promising tools of the ultrasonic technology is based on Lamb waves and applied in both NDT and SHM task solutions [11; 22]. Lamb waves are used for detecting structural changes during WTB production and applied in monitoring structural changes during the service life of WTB [22; 29]. Such a wide range of Lamb wave applications lies in their unusual properties and behaviour. Properties such as an infinite number of modes and the ability to propagate long distances extend the application areas of such waves. One of the key points of the proposed approach is the property of Lamb waves to propagate large distances with a small attenuation. Leakage losses of Lamb waves in WTB structures are very low as well; therefore, it is possible to investigate large areas of structures in a quick and reliable way. On the other hand, such properties as dispersion, infinite number of modes, the convergence of modes, and interlaced modes affect the changes in propagating signal and complicate the use of such waves. As a result, the common signal processing methods (SPMs) are not suitable for the analysis of Lamb wave signals. In order to acquire the required information, specific algorithms should be developed [22], which could recognise and separate different modes of Lamb waves in the whole wave train of the signals. That is why developing new signal processing methods is an important and necessary task.