## 3 Setup and Results
From these last tests, we expect that a structure alteration causes longer settling times and higher oscillation frequencies because of the lower structure weight and higher inertia. With regard to this last test, it is interesting to analyze the spectrogram, which gives us direct information about various events. Still taking node 5 as a reference, Figure 10 shows the spectrogram related to the \(X\)-axis, the most stressed one, and reports the main test states. Figure 8: Time response comparison of main (**a**) and backup (**b**) accelerometers in node 5 in case of perturbation performed on the intact structure. Figure 9: Time response comparison of main (**a**) and backup (**b**) accelerometers in node 5 in the case of perturbation performed on the open structure. The sequence of states of the structure emerges clearly: rest, structure alteration, rest, impulsive stimulus, rest. Moreover, we can see how the resolution of the single accelerometer consistently affects the detection of modal components related to the structure: a higher resolution allows us to distinguish the frequency components more clearly. In addition, in Figure 11a, the comparison between the two spectrograms, related to the same test, before and after structural alteration, is reported. As can be seen, the spectrograms, although similar, have some differences. This confirms that in a structure, when changes in attitude occur, the frequencies change immediately, even if the structure itself (apparently) returns to its initial conditions. Firstly, the different resonant frequency in the perturbed structure is noticeable. This confirms the hypothesis made for time domain tests. A second important difference regards modal harmonics alteration in the two cases, as it is visibly different between the two cases. A first point is the drifting of the harmonics in proximity of the resonant frequency towards higher values, between 150 and 225 Hz, after structure alteration. Low-frequency harmonic components experience an apparently negligible drift, stacking into two harmonics around 60 and 90 Hz, respectively. Finally, Figure 12a,b shows another interesting fact; the reported time responses of a node to the aforementioned stimulus, respectively, before and after the box structural alteration. Figure 11: Spectrograms before (**a**) and after (**b**) structural alteration retrieved from the \(Z\) axis results on node 3. Figure 10: Spectrograms of the primary (**a**) and secondary (**b**) sensors from node 5 related to the third test. A first difference in these two cases can be seen in the settling time, which is equal to 100 ms in the first situation and 300 ms in the second one. Another important difference is the different oscillation frequencies, equal to 145 Hz and 152 Hz before and after the structural alteration, respectively. This situation brings us to a slightly different structure main frequency in the two tested cases. From these first results, it is noticeable how the removal of the box's detachable part causes a less damped and faster response due to the structural lightness obtained in the second case, hence, to its least inertia. Frequency domain analysis has been performed in the range 0-250 Hz in order to observe modal harmonic changes caused by the introduced alteration.