## 3 Results and Discussion
### Case II: Experimental Work  
#### 3.2.1 Experimental Setup  
As part of the presented study, the experimental measurement of the wind turbine blade in laboratory conditions was also conducted. The main purpose was verifying the results of the numerical simulation, as well as testing the damage detection method proposed by the authors on the real object. The measured blade was manufactured using a custom lay-up moulding process, with glass fibres reinforced epoxy resin as the material of the laminate skin. Figure 9 shows the scheme of the laboratory stand, which was used to measure the modal parameter of the blade. The laboratory stand included the tested object (1), an electromechanical shaker (2) and a scanning laser vibrometer with equipment (3-7). The experimental identification process of modal parameters of the blade was carried out using a PSV-400 Laser Scanning Vibrometer by Polytec Ltd. [27]. The primary advantage of using this type of instrument is being fast, highly accurate and taking non-contact vibration measurements. The pictures of the measurement stand components are shown in Figure 10a-c. The most important element of the vibrometer set was the scanning head with a precise optical sensor (Figure 10b), which measured changes in the frequency of light reflected from the moving surface. Based on the principle of the Doppler effect, the vibrometer calculated velocities of vibrating objects. The significant feature of this type vibrometer was the possibility for the user to predefine the scanning grid. During the measurement, the unit automatically moved a laser beam from point to point on the surface of vibrating objects, enabling the full-field vibration analysis. Further to the scanning head, the vibrometer set also included a signal generator, a junction box and a control unit. During the tests, the appropriate measurement conditions were provided to obtain experimental data with the lowest possible noise level. The problem of the measurement noise is crucial in terms of the interpretation of the experimental data in the damage detection process. The high level of noise values can mask information about the damage and consequently can prevent its detection. Therefore, the test object was isolated from any external vibration sources by fixing it to a special table. At the same time the maximum level of the signal received by the vibrometer was achieved by wrapping the tested blade surface with a special retro-reflective foil. Figure 9: Scheme of the laboratory stand: (**1**)—tested blade, (**2**)—shaker, (**3**)—head of laser scanning vibrometer, (**4**)—acquisition unit (**5**)—signal generator, (**6**)—signal amplifier, (**7**)—computer. The experimental part of the study included measuring the intact state of the object and three scenarios of damage location of the blade coating. A delamination zone was simulated using high stiffness steel element fixed to the blade skin on the measured side, as shown in Figure 10c. This element changed the local stiffness of the composite coating. The blade damage pattern for the experiment is presented in Figure 4--a, b and c locations. The first scenario relates to damage location, which is nearest to the blade fixing (Figure 4--element a). In order to minimize the possible impact of the edge effect, that could arise in the event of overlapping damage and the beginning of the analyzed signal, the position of the damage was shifted towards the centre of the blade. The same procedure was used for damage located on the opposite end of the midspan (Figure 4 element c). The last damage position was determined halfway between the extreme locations (Figure 4 element b). For all scenarios, the basic length of the damage was 6% of the total length measurement line, corresponding to 12 measurement points. Additionally, for the damage located in the blade centre, the data were supplemented with measurements for damage spanning over 5% and 4% of the total length measurement line (10 and 8 samples, respectively).