## 4 Discussion  
The experimental and simulation results show two main peaks in the headform acceleration profile for the full-face helmet: an initial peak with a value of 204 G/197 G (experimental/simulation) after nearly 3 ms, and a second peak with a value of 196 G/186 G (experimental/simulation) after approximately 5 ms (Figures 6 and 7). The first peak corresponded to the initial moment of contact between the helmet and the anvil block, when the helmet first started to slow down and undergo rapid deceleration. In contrast, the second peak corresponded to the point of full contact between the helmet and the block. In particular, the second peak could be attributed to the high-density rebound of the liner material (EPS foam), which limited the amount of energy that could be absorbed by the liner, thus increasing the energy transferred to the shell upon impact with the anvil [18]. The simulation results presented in Figure 8 show that the full-face helmet resulted in a lower peak headform acceleration (197 G) than either the open-face helmet (235 G) or half-face helmet (303 G). Thus, of the three helmets, the full-face design provides the best user protection. Notably, however, both the full-face and open-face helmets were observed to satisfy the ECE standard (275 G), whereas all three designs were determined to conform to the Snell standard (300 G) and DOT standard (400 G). In general, half-face helmets are more easily fabricated than open-face (or full-face) helmets, because the need for additional vents and cooling structures is removed in such helmets. However, according to the results presented in Figure 8, the peak headform acceleration associated with the use of the half-face helmet (303 G) was approximately 54% higher than that associated with the use of the full-face helmet (197 G) and 29% higher than that associated with the use of the open-face helmet (235 G). Hence, from a safety perspective, the use of a half-face helmet should be avoided. In other words, the open-face design provides a better compromise between rider safety and rider comfort under warm conditions. Helmets protect the head in two ways. First, the hard outer shell resists penetration and absorbs the initial shock. Subsequently, the softer foam liner progressively absorbs the impact energy by gradually collapsing. Previous studies have shown that the stress-strain response of polystyrene foam contains three characteristic regions under compression loading, namely linear elasticity (because of cell wall bending), plastic deformation (because of cell wall collapse), and densification (because of  
Figure 10: Stress contours of (**a**) the full; (**b**) open-face; and (**c**) 8-mm ventilation slot helmets; and the displacement contours of (**d**) the full; (**e**) open-face; and (**f**) 8-mm ventilation slot helmets during simulated impacts. contact and crushing between opposing cell walls). Thus, a study reported that the force rises rapidly at strains greater than approximately 80% [19]. The results presented in Figure 9 show that the prototype open-face helmet with ventilation slots resulted in a lower peak headform acceleration than the original open-face design, irrespective of the ventilation slot width. This finding is reasonable because the ventilation slots in the shell/liner increase the space available for the foam liner to freely deform under impact, thus increasing the amount of impact energy that can be absorbed prior to densification. Notably, however, the ability of the liner to absorb the impact energy is not directly correlated with the ventilation slot width. For example, the maximum headform acceleration associated with a slot width of 14 mm (175 G) was greater than that associated with a smaller slot width of 8 mm (160 G). This result is reasonable because the absorbed energy is greater at a smaller width, thus enabling the helmet to withstand more impact. The minimum headform acceleration (150 G) was obtained using the maximum slot width of 18 mm.