## 2 Methodology
### New Helmet and Its Optimization  
A new helmet model is developed, and its impact performance is assessed based on the YEAHM predictions. In the previous section, the results indicated that AC216 is the most promising agglomerate to be used as helmet liner. Additionally, in a previous study [49], it was verified that the helmet validated in this previous section fails to protect its user in the same impacts in which it was certified. This was inferred by evaluating its impact performance with a FEHM. The same helmet was tested with cork liners. Although lower peak accelerations were measured for EPS, the acceleration-time curves measured for the agglomerate AC216 are promising (Figure 4). The certified helmet is composed by liners with a thickness around 50 mm in the impact points (B, P and R). From what was seen in [4; 14] and the results plotted in Figures 4 and 5, it is believed that a thinner liner of AC216 agglomerate is enough to absorb the impact energy, in the same impact tests, without increasing the stress levels. For instance, Figure 5 shows a 50% deformation for a thickness of 50 mm, not reaching densification as verified in Figure 3. In addition, more material means more mass, which means higher impact energy. For impact point X, the results between cork agglomerates and EPS were closer (Figure 4). In this area, the liner has a thickness of approximately 40 mm. Another factor that may have contributed to the lower accelerations for this impact point is the larger impacted area. Thus, considering the great capacity shown by the AC216 agglomerate and its impact performance in the standard tests, for instance in impact X, a new helmet model is developed. The strategy with this helmet is based on the 40 mm maximum thickness seen for impact X. This is already a significant thickness, which is usually exceeded in the current motorcycle helmets. Considering that the last updates in standards are demanding, with more impacts or higher energies, helmet liners are becoming thicker. In a technical report, [PERSON] and [PERSON] [55] indicated that a helmet made of EPS needs a thickness between 75 and 90 mm to achieve an acceleration of 100 g for a 6.2 m/s impact, and to pass the standard criteria they indicated thicknesses ranging between 50 and 60 mm, which may increase up to 72 mm due to safety margins used by some manufacturers. Therefore, a new helmet model composed of thinner liners made of AC216 cork agglomerate is developed. A maximum thickness of 40 mm was set, and several models were created by varying the thickness. Then, the impact performance of each one of these helmets is analyzed with YEAHM, comparing its predictions with head injury criteria [56] and therefore, evaluating the applicability of AC as helmet liner. In other words, this model is optimized according to YEAHM response to double impacts, computing an optimal thickness. This new helmet model has a generic geometry and a constant thickness. Its style is based on jet helmets, also known as open face helmets. This design was chosen because it is one of the most common helmet types. It is used in a great variety of applications, for instance in contact sports such as American football and ice hockey, in urban and recreational activities, winter sports, among many other activities. The geometry was created in CATIA V5 software and different versions of it were created by varying the thickness. The entire liner is designed with a constant thickness with a maximum value set to 40 mm. Figure 6 shows a render of the thicker model. The thinner versions were created with increments of 5 mm. Four helmet models were created with a thickness of 40, 35, 30, and 25 mm. These models were further imported into Abaqus CAE software to develop the FE versions of these helmets. The helmet shells have a thickness of 3 mm, which is independent of the liner thickness. The same type of elements used to model the previous helmet were used. The meshes were created always avoiding distorted and warped elements.