## 2 Materials and Methods
The blue annular surface in the scheme in Figure 4 corresponds to the clamped part of the specimen surface. The positions of the four bolts were, in each measurement, aligned to the 1-4-7-10 DOFs. Figure 4: Measurement sequence of the specimen analysed in I\({}^{\text{ref}}\) analysis containing 48 degrees of freedom and one reference degree of freedom (point 49). Figure 3: A geometric model of the specimen created in the I\({}^{\text{ref}}\) stage, containing 48 points and the corresponding number of triangular surfaces. * Frequency span and resolution definition: the sampling frequency set to 1600 Hz allowed analysis of the dynamic behaviour of the specimen in the 0-800 Hz frequency span. The definition of the analysis lines number as equal to 400 ensured the frequency resolution of 2 Hz (acquisition time 500 ms);
* Triggering and weighting of the signals: after defining the proper level of trigger impact signal, and hammer and response weighting, the measurement of the FRFs was started. Each of the DOFs was excited five times by the impact hammer, and the resulting signals were obtained by averaging using the linear peak hold average method on the fast Fourier transformation (FFT) signals, which should ensure adequate accuracy of the FRFs. The evaluation of FRFs was carried out in Pulse Reflex(Hottinger Briel & Kjaer, A/S, Virum, Denmark). Information on the natural frequencies, damping ratios, and mode shapes of the individual modes of oscillation was analysed in the 0-800 Hz frequency span. Stability diagrams resulting from the magnitude of the complex mode indicator function (CMIF) were used to estimate the particular modes. The CMIF is based on singular value decomposition (SVD), has one function for each reference degree of freedom and can be used to detect closely spaced or repeated modes. To identify the stability of the poles, the rational fraction polynomial-Z method with 40 iterations was used. The selected possible modes were analysed by the automatic modal assurance criterion (auto MAC), which is a method based on orthogonality checking that is used to compare mode shapes and determine the level of similarity between them. It is a very useful tool for the identification of closely spaced modes or repeated modes which have very close or the same frequencies but distinct mode shapes. The so-called MAC value between two mode shape vectors \(\Phi_{\mathrm{i}}\) and \(\Phi_{\mathrm{j}}\) is calculated according to:  
\[\mathrm{MAC}\Big{(}\Phi_{\mathrm{i}},\Phi_{\mathrm{j}}\Big{)}=\frac{\left| \Phi_{\mathrm{i}}^{\mathrm{T}}\Phi_{\mathrm{j}}\right|^{2}}{\left(\Phi_{ \mathrm{i}}^{\mathrm{T}}\Phi_{\mathrm{i}}\right)(\Phi_{\mathrm{j}}^{\mathrm{ T}}\Phi_{\mathrm{j}})} \tag{1}\]  
The resulting matrix, with the rows and columns representing the mode shapes, has diagonal elements equal to 1 and off-diagonal elements which provide information about the correlation between selected mode shapes. A MAC value equal to 1 represents identical mode shapes, while a MAC value equal to 0 corresponds to completely distinct mode shapes. After completion of the measurement, the specimen was unmounted. Subsequently, the specimen was clamped again and the above-described process (without geometry definition, which can be imported) of modal analysis was repeated another two times. The results obtained from each set of three measurements were averaged to ensure the proper graphical and quantitative comparison of the modal parameters. The pre-test phase was designed to analyse the influence of the selected parameters, i.e., the number of degrees of freedom (DOFs), the number of analysis lines, and the number and location of reference degrees of freedom (REFs) on the modal parameters of the specimen with a honeycomb infill pattern. The measurement sequences, DOFs, and REFs considered for the pre-test stages Iref up to VF (according to Table 3) are shown in Figure 5.