## 3 Results
### Comparison between Agglomerated Cork and EPS
Nevertheless, a significant difference was observed for the maximum CSF strain energy. Actually, the CSF strain energy computed with the EPS helmet was very similar the one computed with the 40 mm AC version. This can be explained by the acceleration curve obtained with the EPS helmet, which is narrower than the one computed for the 35 mm cork version. This possibly leads to a lower storage of strain energy by the CSF, reaching a lower maximum value. Figure 12 shows the variation of strain energy in the CSF during the impacts for both 40 mm helmet versions. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Criterion** & **EPS90** & **AC216** \\ \hline Strain & 0.5067 & 0.3833 \\ von Mises stress [kPa] & 191.7 & 134.2 \\ Pressure [kPa] & 840.9 & 691.7 \\ CSF pressure [kPa] & 459.4 & 291.7 \\ CSF strain energy [mJ] & 12611.5 & 12481.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comparison of head injury criteria computed for helmets with AC216 and EPS90 liners. Figure 13 compares the strain distribution across the brain at the moment of maximum strain in the occipital lobes. Other brain regions (dark regions in Figure 13) exceeded the strain of 0.15 during the impacts and for both solutions, but with values lower to the maximum. In conclusion, the helmet composed by a 40 mm thick liner made of the AC216 cork agglomerate performed better than its EPS version. Thus, AC can be considered a good alternative to EPS, especially for helmets typically subjected to multi-impact scenarios. In addition, the AC helmet performed better for both impacts, although the difference was more significant in the second impact. Even so, it is considered that there is still a margin for improvement considering what was seen in this study regarding some injury predictions. However, this is a good starting point for a further optimization of helmets with liners made of cork. Here it was seen that with the right design, AC can be used as a helmet liner with a better performance than other materials currently used on the market. Figure 12: CSF strain energy history during the double impacts with both 40 mm thick AC and EPS solutions. Figure 13: Brain strain distribution obtained with: (**a**) EPS liner. (**b**) AC liner. In addition, it is worth mentioning and recalling that the double impacts performed in this study were severe, since there were two consecutive impacts separated by approximately 5 ms. These are more severe than most of the tests required by the helmet standards. These are usually one impact, with maximum velocities around 7.5 m/s for motorcycle helmets or even lower for other helmet types. There are also standards that require two impacts, but these are lower-energy impacts and the time between impacts is around minutes. In applications where multi-impacts are frequent, such as winter sports such as skiing and snowboarding, contact sports such as American football and ice hockey, and even urban activities such as roller skating and skateboarding, the impacts are spaced by enough time to allow agglomerated to cork fully recover. In addition, in some of these, the impact velocities are much lower than 7.5 m/s causing none or an insignificant amount of permanent damage to cork. The model here developed is not advanced, considering its constant thickness. The thickness in some regions of the helmet can be reduced, for instance where the impacts are less likely to occur, such as the rear sides of the helmet. Thus, by reducing the liner volume, the helmet mass is also reduced, which means lower impact energies. In addition, the liner geometry may be upgraded with an innovative design that allied with the mechanism of cork deformation can optimize the general impact behavior and hopefully improving the safety level of the helmet. This may also allow reducing even further the helmet thickness. To verify these hypotheses, a new liner was modelled by reducing its volume while keeping its constant thickness of 40 mm.