## 4 Discussion
In order to determine the optimal configuration for each load case, a sensibility study was conducted in Figures 11-16. For von Mises stress, the effect slope was descending. Figures 11a and 12a illustrate for CL-1 and CL-2 configurations the effect of the thickness of the first and second regions on the von Mises stress. The effect of the thickness of both regions was high, but it was noticed that the thickness of the first region was more important than that of the second region. Figure 13a shows the opposite behavior than that mentioned before: for the SL-1 configuration, the effect of the thickness of the second region was higher than that of the first region. For SL-2, CSL-1, and CSL-2 configurations, Figure 14a, Figure 15a and Figure 16a show that the effect of the thickness of the first region was much more important than that of the second region. As for the weight, Figure 11b to Figure 16b show that the effect of the thicknesses of the first and second regions was merely the same with more importance on the effect of the second region's thickness. For the compressive load case, both CL-1 and CLN-2 showed the effectiveness of the method of integrating L-systems along PSLs' directions in a structure in order to reduce its weight while keeping a reasonable stiffness and strength. However, it was noticed that the CL-1 structures presented higher von Mises values than CL-2 did. For a load of 60 N, the CL-1 configuration presented high von Mises values while for the CL-2 configuration, the von Mises stress stayed lower than the yield stress of the VeroWhite material. Even if the CL-1 structure presented lower weight values than that of the CL-2 structure for each experience in the DOE, the CL-2 structure fulfilled the objective of this study in a better way. The latter statement can be justified by the growth direction of the L-systems in the CL-2 configuration. Indeed, the direction of the branches was colinear to the compressive load direction, thus, the material distribution was closer to the zone where the load was applied, as shown in Figure 6b. Meanwhile for the CL-1 structure, the L-systems branches' growth direction was orthogonal to the load direction, which explained the higher von  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**CL  
Mises values (Figure 6a). That means that the structure CL-2 was better adapted to the compressive load case. For the shear load case for both SL-1 and SL-2 structures, the effectiveness of the method of integrating L-systems along PSLs' directions was again validated. SL-1 and SL-2 structures presented approximatively the same weight values, but the SL-1 structure had remarkably lower von Mises stress values. This difference could be justified by the L-systems' growth along the shear load direction (Figure 6a). SL-1 is then the structure that was better adapted to the shear loads. Hence, the effectiveness of this method was also proved for the combined compressive/shear load cases. The same interpretation done for the CL configurations could be used for the CSL configurations: CSL-2 was better adapted for the combined compressive/shear load. To summarize the analysis of the study of the effect of the first and second region thicknesses on the von Mises stress and the weight for the six structures, the following observations can be stated:  
* A thickness of 2 mm (level 1) for the first region and a thickness of 1 mm (level 1) for the second region gave a maximal von Mises stress value and a minimal weight (Exp. 1 in Tables 5 and 6). * A thickness of 3 mm (level 3) for the first region and a thickness of 2 mm (level 3) for the second region gave a minimal von Mises stress value and a maximal weight (Exp. 9 in Tables 5 and 6). Finally, it was concluded that integrating L-systems generated along the PSLs' directions in a structure reduced its weight and maintained a reasonable stiffness. The latter statement validated the hypothesis raised at the beginning of the discussion section.