## 3 Results
### Realization and Evaluation of the Pre-Test Phase
#### 3.1.4 Results Obtained in the IV Stage of the Pre-Test Phase  
In structures with geometric or material symmetry, so-called repeated modes commonly arise as a consequence of the symmetry in the structure's stiffness and mass distribution. It is characteristic of repeated modes that they occur at the same or nearly the same natural frequency, but correspond to distinct (mathematically orthogonal) mode shapes. As these modes share the same frequency, identifying repeated modes can be difficult. The CMIF is one of the functions used for the identification of repeated modes. In the previously mentioned measurements, repeated modes were not identified. The reason for this is that only one reference degree of freedom was used to capture the responses of the specimen and, thus, the CMIF plot consists of only one singular curve. However, modes that might appear as a single peak in a single-reference CMIF plot may split into distinct peaks in a multi-reference setup. If the measurement is carried out in a multiple input, multiple output (MIMO) form, the result is a graphical plot containing the same number of singular curves as the number of reference degrees of freedom used to capture the response. To analyse the effect of the chosen position of the reference degree of freedom, three multi-reference measurements were realized. Since the authors' workplace possesses only one Polytec PDV-100 vibrometer, the implementation of the MIMO measurements was carried out in the following way. The measurement sequence consisted of three parts. Firstly, the excitation of the specimen was realized using the roving hammer-impact method in 48 DOFs with the responses being captured by a fixed embedded vibrometer located in reference degree of freedom 49 (the same process as in the I\({}^{\text{ref}}\) stage). Subsequently, the position of the vibrometer was changed to the location of reference degree of freedom 50 and the same 48 DOFs were excited by the roving hammer. Finally, the same process was implemented after changing the vibrometer position to the location of reference degree of freedom 51 (see the locations of reference degrees of freedom in Figure 5). However, these measurements were evaluated separately with different amounts of obtained FRFs depending on the number and location of the selected reference degree(s) of freedom (according to column 4 of Table 3 and Figure 5d-i, respectively). Such measurements and evaluations were aimed to highlight the fact that inappropriately chosen positions of the reference degrees of freedom can affect the results of the analyses in such a way that not every mode will be detected. On the other hand, an appropriately chosen larger number of reference degrees of freedom allows one to obtain information not only about repeated modes but also about closely spaced modes that may not have been identified in the previous three stages. As the number of singular curves obtained from the SVD of the FRFs corresponds to the number of reference degrees of freedom, three singular curves were obtained in each measurement evaluated in the IV\({}^{\text{A}}\) stage. Similarly, two singular curves represented the output from the SVD realized in stages IV\({}^{\text{B}}\) to IV\({}^{\text{D}}\) and only one singular curve corresponded to each measurement of the IV\({}^{\text{E}}\) and IV\({}^{\text{F}}\) stages of the singular curves obtained from the three measurements realized in stages IV\({}^{\text{A}}\) to IV\({}^{\text{D}}\) can be seen in Figure 13, whereby the same representation of y-axis limits was used for better visual comparison of the singular values' magnitudes. Each of the singular curves captures different aspects of the specimens' modal behaviour, i.e., the modes that are poorly captured by one reference degree of freedom can be better captured by another. Comparing the CMIF plots, it is obvious that each combination of reference degrees of freedom used in the MIMO measurements contributes to the magnitudes of singular values differently.