## 3 Results and Discussion  
### Bending Test at 16 mm, Spring Back, and Shape Recovery  
In Figure 6, the stress-strain curves for tests carried out at a speed of 2 mm/min and with a displacement of 16 mm are shown, taking two replicates for each structure. For the lozenge, double hoop, hoop, and roller structures, there is a certain replicability of the force-displacement curve, while the honeycomb and clepsydra structures show different behavior between the two replicates, with a variation in maximum load of 30% and 40%, respectively. Lozenge and double hoop structures, on the other hand, have the highest mechanical strength with an average load of 41 N and 53 N, respectively. The results obtained in this study are consistent with the findings of recent works on 3D-printed lattice cores in sandwich panels, where the use of structured PLA lattices under flexural loading demonstrated enhanced energy absorption and mechanical performance. As reported by [50], specific cell morphologies play a crucial role in controlling the flexural response, which supports the favorable behavior observed for the lozenge and roller geometries tested in our work. The honeycomb structure shows a marked descending peak, absent in the first sample. This anomaly was caused by internal failure of the structure, which is why it was decided later to exclude this geometry from subsequent tests. Study [48] analyzed the mechanical behavior of Kevlar fiber sandwich panels with a lozenge-shaped core filled with foam. Overall, the lozenge structure shows good bending behavior, and the results indicate that foam filling significantly improves mechanical properties, increasing energy absorption capacity and structural strength. In [51], various lattice structures with relative densities comparable to those of the structures examined in this study are analyzed. The mechanical strength shows values very similar to those of the lozenge grid structure, with a maximum load capacity averaging around 50 N. The spring back was then measured, with the graph shown in Figure 7. For each structure, the maximum, minimum, and mean final values of each test piece are shown, while the hatched horizontal line at the bottom of the graph represents the average value measured at the end of the bending test before the load is removed. Looking from left to right, the lozenge and hoop structures show a lower elastic recovery than the others, while the clepsydra and honeycomb structures show a higher elastic recovery. In the case of the lozenge, the limited elastic recovery is also due to the limited deformation of the structure itself. Figure 6: Force–displacement curve bending test for 16 mm. The trend of the recovery of the angle of bending, for each of the six structures in consideration, is shown in Figure 8.  
It is possible to observe that the recovery of form mainly takes place within 60-90 s from immersion in water; above this time interval, the recovery is almost insignificant. When analyzing the graphs shown in Figure 6 in detail, it is noted that structures which showed a lower elastic recovery, such as lozenge and hoop, have almost completely returned to their initial geometry in a relatively short time. In addition, such structures show a certain replicability in behavior, unlike others, which have marked variability. Therefore, for the purposes of the next study, hoop and lozenge will be the tested structures. This statement is confirmed by the behavior of the double hoop structure, which, in addition to showing a lower angular recovery than the others, shows a similar difficulty in recovering the shape of the midpoint, as illustrated in the following images. Figure 9 shows the geometries before (a) and after shape recovery (b). The lozenge structure, after a single bending-elastic return-shape recovery cycle, has completely restored its initial shape. The double hoop, on the other hand, does not show an obvious angular recovery, unlike the hoop, where the recovery is almost total.