## 2 Materials and Methods
Such an approach could lead to inaccurate measurements of the specimen's true dynamic behaviour, causing mode shape distortion or inaccurate damping measurement. For this reason, the measurement of responses in the form of velocities was realized using a non-contact PDV-100 (Polytec GmbH, Wallbronn, Germany) portable digital vibrometer (Figure 2f) based on the laser Doppler principle. This device is designed to remotely acquire a sample's vibrational velocities in a frequency range from 0.5 Hz up to 22 kHz and provides three measurement ranges for full-scale outputs: 20, 125, and 500 mm/s. Frequency response functions (FRFs), i.e., mobilities, were acquired using the Bruel & Kjaer Pulse(r) system with a LAN-XI module type 3050 (Hottinger Bruel & Kjaer A/S, Virum, Denmark), containing six high-precision BNC input channels with an input range up to 51.2 kHz. To carry out the measurements, the Pulse MT Hammer V.21.0.0 software (Hottinger Bruel & Kjaer A/S, Virum, Denmark) was used. The process of modal parameter estimation for the manufactured specimens was carried out in two phases. The first, i.e., the pre-test phase, involved 18 measurements (see Table 3) realized on the specimen with a honeycomb infill pattern. Each of the measurements belonged to a specific stage of the pre-test phase, indicated in Table 3 by the Roman letters Iref to IVF. The experimental process realized herein will be explained for the reference measurements (Iref), the results of which were then compared with those obtained in the other three stages of this phase. Each experimental estimation of modal parameters started with the clamping of the specimen to the modal shaker armature. As in modal analysis, the tightening of the constraint plays a significant role in determining the stiffness of the system, influencing all the modal parameters analysed (i.e., mode shapes, natural frequencies, and damping ratios); therefore, clamping of the specimen to the armature was ensured by  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Stage** & **Torque (Nm)** & **DOFs** & **REFs** & **Sampling Frequency (Hz)** & **Analysis Lines** & **NoM** & **NoE** \\ \hline Iref & 6 & 1 to 48 & 49 & 1600 & 400 & 3 & 3 \\ \hline IIA & 6 & 1 to 24 & 25 & 1600 & 400 & 3 & 3 \\ IIB & 6 & 1 to 96 & 97 & 1600 & 400 & 3 & 3 \\ \hline IIIA & 6 & 1 to 48 & 49 & 1600 & 800 & 3 & 3 \\ IIIB & 6 & 1 to 48 & 49 & 1600 & 1600 & 3 & 3 \\ \hline IVA & & & 49, 50, 51 & &fisturing with four bolts tightened to the same torque of 6 Nm using a torque wrench with a DMA 40/200D (Dema Vertriebs GmbH, Ubrigshausen, Germany) applied digital torque adapter. It should be mentioned that the shaker itself was used in this case just to create the constraint (clamped joint occurring on a small annular area at the top and bottom surfaces of the specimen). However, in the future, it will provide the authors with the possibility to implement modal testing with a different type of excitation. Subsequently, the necessary analysis settings were defined in MTC Hammer. The settings used in the I\({}^{\text{ref}}\) stage were as follows:  
* Sensor types definition: definition of the exciter (Bruel & Kjaer 8206) and response sensor (PDV-100) types and their assignment to individual channels of the LAN-XI module type 3050 was done;
* Geometric model creation: the planar geometric model of the analysed specimen was created. It consisted of the 48 points of defined coordinates, mutually connected by triangular surfaces (Figure 3);
* Measurement sequence definition: the measurement was planned in the form of the roving hammer-impact method and carried out using the fixed embedded vibrometer, i.e., 48 degrees of freedom (DOFs) representing the excitation points and 1 reference degree of freedom (REF): point 49 for response measurement. Excitations and response measurements were carried out perpendicularly to the specimen's top surface, i.e., in the Z-direction, in all stages of the experimental texting.