## 2 Methodology
### New Helmet and Its Optimization
Additionally, special attention was given to the time increment, avoiding very small elements to have a reasonable computation time but at the same time a mesh refined enough to obtain precise results. Figure 6 shows the FE helmet mesh. The helmets were tested according to the conditions required by the ECE 22.05 standard. Additionally, two impacts with the same conditions were performed. The impact point B was chosen because it is the most common impact. Statistics show that independently of the helmet type, this is the most impacted area [57; 58; 59; 60; 61; 62; 63; 64]. In addition, in the previous section, this was the impact point where the EPS helmet had lower accelerations (Figure 4). On the other hand, at this impact location, the AC216 agglomerate version presented 50% of deformation for a thickness of 50 mm, not reaching densification (Figures 3 and 5). Two consecutive impacts were performed at an initial impact velocity of 7.5 m/s. Generally, cork agglomerates can recover and withstand multi-impacts whereas synthetic foams absorb impact energy by deforming permanently [4]. The second impacts were performed immediately after the first, usually separated by 5 ms. This is challenging for AC since its recovery is time dependent. A material with a good performance in such conditions is an excellent alternative to the ones used in contact sports such as American football and ice hockey. The impact tests required by the standards for such helmet types are characterized by low multi-impact energies and with long recovering times between impacts. For instance, the standards that regulate American football and ice hockey helmets, NOCSAE ND002 and NOCSAE ND030 respectively, define that at least two impacts of 5.46 and 3.46 m/s, must be performed at the same area. To be approved the peak severity index (introduced by [PERSON] [65]) of any impact shall not exceed 1200. There are also other standards used in North America regulating American football and ice hockey helmets, ASTM F717 and F1045 respectively, using only one lower-energy impact of 75 J (approximately 5.47 m/s) and the maximum acceleration shall not exceed 275 g or 300 g depending on the impact site. Currently, these specific helmets designed for multi-impacts with low energies are usually made of foam materials other than EPS, for instance EPP. However, as seen in [4], for more demanding tests, EPP is not an option, performing worse than the other materials. The interactions between the different parts were the same ones defined in the previous helmet model. Additionally, the same material models and properties used in the previous chapter to simulate ABS, EPS, and the AC216 agglomerate were also used. Figure 6 shows the impact at point B and a sagittal cut view of the helmet, showing its constant thickness. To evaluate the applicability of AC as helmet liner, the performance of each helmet will be analyzed with YEAHM, by comparing its predictions with the injury criteria [56]. In other words, optimizing the helmet thickness based on YEAHM response. The best one will be compared against the EPS version, changing only the material. Figure 6: New jet helmet with constant thick liners: (**left**) render with 40 mm thick agglomerated cork liner; (**center**) Sagittal cut view of the FE model positioned for an impact at point B; (**right**) FE model. With the aim of reducing the computational resources needed for a singular simulation, two different steps were performed to assess the helmet models with YEAHM. First, the headform was coupled to the helmet model and the impacts were simulated. Then, the measured kinematics were induced to YEAHM. This is also the procedure usually carried out in other studies. For instance, a similar analysis was performed by [PERSON] et al. [22], simulating two simpler simulations. The modelling of human head models using FEM provides a strong basis for helmet design improvements, allowing a further accurate computational-based prediction of brain injuries.