## 3 Results
### Comparison between Agglomerated Cork and EPS  
The best solution found in the previous simulations is now compared against an EPS liner tested in the same conditions, to compare it against the best agglomerate solution. In other words, this new analysis is performed to verify if a helmet composed of AC liners is truly an alternative to the ones made of EPS. Figure 11 compares the acceleration-time histories measured at the headform's COG during double impacts performed for the same 40 mm thick helmet, between helmet with liners made of AC216 cork agglomerate and EPS90. Regarding the headform acceleration response, the helmet with an AC liner performed better than the EPS one. For the first impact, the acceleration curves are similar. Nevertheless, the helmet with the AC liner managed to induce a slightly wider acceleration curve with a lower peak acceleration. The difference between peak accelerations was also very small, reaching 314.1 g and 328.3 g for AC216 and EPS90, respectively. However, this difference increased considerably for the second impact. Acceleration peaks of 377.7 g and 483.1 g were measured for AC216 and EPS90, respectively. Actually, by analyzing the curves in Figures 8 and 11, it is possible to conclude that the results with a 40 mm thick EPS liner and a 35 mm thick AC216 agglomerate liner are very similar (regarding both the curves peak and width). Table 8 compares the peak accelerations reached by each liner during both impacts. These results show a clear worst response with an EPS liner regarding the second impact. Additionally, the results with AC216 agglomerate were better for the first impact. This was not the case for the first impact with the commercially available helmet modelled previously. By comparing Figures 4 and 11, it is possible to conclude that the results for AC216 improved slightly and the ones for EPS become worst. This difference was caused by two main factors: the thickness of the liner at the impact points and the overall geometry of the liners. The liner of the certified helmet tested previously has an average thickness of 50 mm at the impact point. In addition, the thickness is not constant and, in some regions, there are channels, holes, and ribbed protrusions. Some of these were designed together with the deformation mechanism of EPS, which makes it easier to absorb more energy by deforming permanently. On the other hand, the new helmet has a constant thickness of 40 mm, being at least 10 mm thinner at the impact points. This liner is also completely solid, a 40 mm thick layer covering the head. These are the reasons why the EPS behavior was worst in the first impact. Regarding the AC216 agglomerate, the results indicate the 40 mm thick solution as the best in the range of tested thicknesses (20, 25, 30, 35, 40, and 50 mm). Additionally, in the same conditions with the same helmet geometry, this solution performed better than the EPS alternative. Although some conclusions were already drawn from the comparison of the headform acceleration responses, an additional analysis was performed. These acceleration-time curves were used to drive YEAHM to compare both solutions regarding head injury criteria. The same parameters used to compare the variable thick solutions for the AC216 liner and presented in Table 7 were again computed to compare the 40 mm thick liners made of AC216 and EPS90. Table 9 summarizes the maximum values computed for each criterion. The results presented in Table 9 show a better performance by the AC helmet. In the analysis performed with YEAHM, lower values for all the criteria were computed with the helmet composed with a liner made of AC216 agglomerate. There is a considerable difference between some of the results computed for this helmet and the one made of EPS. Considering the Tables 7 and 9, the results for the AC helmet with a 35 mm thick liner were similar to the ones computed with the 40 mm thick EPS liner. This may be considered normal since the input was also similar, especially considering the peak of the acceleration curves.