# The Impact of Internal Structure Changes on the Damping Properties of 3D-Printed Composite Material
## 4 Experimental Procedure
### Analysis of Specimens  
Experimental measurements were performed at two workplaces using different equipment. The first experiment was processed in SignalExpress 2008 software, using a type 4507-B accelerometer supplied by Bruel & Kjaer located in Darmstadt, Germany and a type IH101-500 dynamic hammer supplied by Omega Engineering located in Norwalk, Connecticut. The second experiment was processed in BK Connect 2020 software using an accelerometer type 4393 and dynamic hammer type 8206, both from Bruel & Kjaer. The measurement procedure was the same in both cases. The specimens were fixed against a rigid structure using elastic straps at the point of occurrence of the nodes of oscillation of the first eigenmode (70 mm from the edges of the specimen). In the first case, the specimen was oriented horizontally in the more flexible straps (Figure 2) and in the second case the specimen was rotated 90\({}^{\circ}\) around the longitudinal axis in the more rigid straps. The remainder of the experiment proceeded identically for both cases. We tapped the specimen with a dynamic hammer and monitored the frequency response with an accelerometer placed in the center of the specimen. We processed the collected data in MATLAB R2023a. There we plotted the frequency response functions for each specimen, which was the basis for further analysis. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Layer** & **Configuration 1** & **Configuration 2** & **Configuration 3** & **Configuration 4** & **Configuration 5** \\ \hline Matrix & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) \\ \hline Reinforcement & 0\({}^{\circ}\) & 45\({}^{\circ}\) & 90\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) \\ \hline Matrix & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) & \(\pm\)45\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Orientation of reinforcement. Figure 1: Fiber layout variants: (**a**) variant 1, a specimen without circumferential fibers; (**b**) variant 2, a specimen with circumferential fibers. Figure 2: Measuring workplace. Based on the processed data, we identified the natural frequencies of the structure and calculated the damping ratio \(\zeta\) from the obtained values. The calculation was based on data in the region of the first and second natural frequencies, corresponding to a frequency range of 20 Hz to 150 Hz for the first natural frequencies and 150 Hz to 1000 Hz for the second natural frequencies (Figure 3). By plotting the frequency response function from the data obtained from the measurements at the second workplace, we observed only one natural frequency for each specimen, with the frequencies occurring in the range from 28 Hz to 82 Hz (Figure 4). Figure 3: Frequency response from workplace 1: (**a**) carbon fiber variant 1; (**b**) carbon fiber variant 2, (**c**) fiberglass variant 1; (**d**) fiberglass variant 2; (**e**) Kevlar fiber variant 1; (**f**) Kevlar fiber variant 2. For the calculation of the damping ratio, we used the half-power bandwidth method. First, we found the maximum amplitude value of the first natural frequency and, using Equation (3), determined the amplitude values at half the peak. Based on these values, we expressed the frequency difference \(\Delta f\) (Equation (4)), which is necessary for the calculation of the amplification factor Q (Equation (5)). Finally, using Equation (6), we calculated the damping ratio for the given structure.