# Wideband Vibro-Acoustic Modulation for Crack Detection in Wind Turbine Blades
## 1 Introduction  
WT are among the most vital sources of clean energy production, owing to their low installation costs and the abundance of wind energy. To enhance their energy production capabilities, assessing the health of all WT blades is important. Blades play a significant role in energy generation; they are the most dynamic part of the WT and exposed to harsh environments. As a result, a variety of structural health monitoring (SHM) and non-destructive evaluation (NDE) methods have been developed to assess damage caused to WT blades [1; 2; 3; 4]. The vibro-acoustic structural health monitoring (SHM) technique is one of the most pragmatic methods used to identify damage in blades. Various vibration measurements can be applied to WT blades, such as active vibrations [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] and passive vibrations [1; 20; 21; 22; 23; 24]. In active vibrations, the vibrations are artificially generated using an electrodynamic shaker or actuator. Meanwhile, in passive vibration measurements, the blade vibrations are naturally excited. These excitations can be produced by the aerodynamic force acting on the bladesas they rotate in the wind. A common mechanism is vortex shedding, which occurs when alternating vortices are shed from the blade's trailing edge [25]. Numerous methods have been used to operate turbines passive and active vibration on blades, as reported in various studies [10; 13; 14; 15; 16; 17; 20]. These investigations have primarily focused on the operational conditions of the blades while they are rotating, thus evaluating real-time performance and damage that might occur during the rotation. Meanwhile, laboratory testing methods have also been crucial to assessing the health of turbine blades. Studies such as [5; 6; 7; 8; 9; 12; 18] have conducted thorough active vibration assessments of blades separated from the turbine system. The laboratory investigations of passive vibrations were conducted using windmill models, with electrical motors being used to enable blade rotation [21; 24]. Blade vibration and their variations in the presence of cracks have been examined using finite element software, as shown in [22; 26]. As depicted in Figure 1, various types of vibrations are categorized based on their method and the condition of vibration measurement. Damage and cracks result in the modification of the natural frequencies, damping coefficients, frequency response characteristics, and mode shapes, as well as their derivative functions. These changes can be assessed via various analysis methods; for example, the operational modal analysis (OMA) can be used [22; 23; 27; 28; 29; 30]. This method measures natural blade vibrations in windy environments to identify and predict crack locations based on changes in frequencies, damping ratios, and frequency response functions [28]. Other methodologies, including operational deflection shapes (ODS) [8; 31], as well as statistical methods, aim to characterize WT blades by utilizing vibration measurements and statistical models. Table 1 demonstrates the effect of damage on the resonance frequencies of blades, notably emphasizing the percentage change in frequencies between damaged and undamaged blades. However, the functional applications of vibro-acoustic SHM methods are limited because they rely on comparing the vibrations of damaged and undamaged structures. Cracks and damage inclusions to the initially intact materials change materials' elasticity properties and lead to the non-linear relationship between stress and strain. This non-linearity can be detected based on non-linear effect in the acoustic waves. Non-linear vibro-acoustic SHM techniques have high sensitivity to cracks and do not necessitate a comparison between damaged and undamaged structures for integrity assessment. Figure 1: Types of vibration measurement used to assess WT blades SHM. \begin{table}
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The application of non-linear acoustics (NA) to detect damage within materials began in the 1970s.