# Evaluation of Combat Helmet Behavior under Blunt Impact
## 4 Discussion
### Effectiveness of Current Standards to Prevent Head Injuries  
The analysis in Section 3.1 reveals that, although combat helmets may satisfy ballistic standards (STANAG 2920 [5] or NIJ 0106.01 [6]), the helmet's performance in blunt conditions is different. This study is carried out using the same configuration as the combat helmet analyzed in other studies [12; 30; 32; 33], complying with the requirements imposed by ballistic standards. However, standards with low impact conditions such as the ECE R22/05 standard [19] and the NHTSA [43] have not been sufficiently studied and reveal specific considerations. The results obtained with simulations for the surrogate configuration carried out in this paper show a good correlation with data provided by a helmet manufacturer and the results from [23]. The differences observed on frontal and rear impacts with the experimental test can be due to the different possible adjustments of the pads and the support system or also due to the location of impact, which can vary significantly depending on the exact location when striking against the anvil. However, these values are similar to those obtained in simulations with the ACH helmet in front, rear, and side impact, carried out by [PERSON] and [PERSON] [23], as shown in Figure 11.  
Therefore, the results show that the current shell configuration has a high probability of causing mild traumatic brain injury to its wearer. We do not suggest changing the material of the helmet, but we do consider that further research should be focused on the internal system of the helmet (foams pads) since, in specially designed helmets for blunt impact, such as in motorbike and bicycle helmets, the internal system is generally made of materials that are different from those used in the ballistics field.  
The present results highlight the need to consider the use of human head numerical models in current combat helmet testing methodologies to obtain more information about brain damage, as well as other parameters that have not been included in this work. Understanding the behavior of the different parts of the combat helmet and the interaction with a human head model may improve the knowledge of combat helmets' behavior, leading to more efficient developments.  
The ECE R22/05 standard [19] defines a threshold acceleration that helmet manufacturers must achieve, ensuring that helmets have to be at least as safe as the standard deems. Nevertheless, the results obtained suggest that, under standardized impact conditions, there is a high probability of multiple head injury modes occurring, even with the helmet meeting the standard thresholds, particularly,  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Head Model**} & \multicolumn{2}{c}{**Impact Velocity**} & \multirow{2}{*}{**HIC**} & **AIS 1** & **AIS 2** & **AIS 3** & **AIS 4** & **AIS 5** & **AIS 6** \\  & **(m/s)** & & **(\%)** & **(\%)** & **(\%)** & **(\%)** & **(\%)** & **(\%)** \\ \hline \multirow{3}{*}{**Head**} & 3 & 243 & 33 & 11 & 4 & 0 & 0 & 0 \\  & 3.5 & 521 & 82 & 41 & 12 & 4 & 0 & 0 \\
**surrogate** & 3.9 & 842 & 98 & 80 & 38 & 10 & 2 & 0 \\  & 4.4 & 1245 & 100 & 96 & 77 & 33 & 7 & 0 \\ \hline \multirow{3}{*}{**Human head**} & 3 & 446 & 72 & 31 & 10 & 2 & 0 & 0 \\  & 3.5 & 625 & 90 & 55 & 20 & 5 & 0 & 0 \\ \cline{1-1}  & 3.9 & 978 & 99 & 89 & 52 & 16 & 3 & 0 \\ \cline{1-1}  & 4.4 & 2017 & 100 & 100 & 97 & 90 & 70 & 28 \\ \hline \hline \end{tabular}
\end{table}
Table 11: HIC and AIS (%) for different impact velocities.  
in the top helmet location. Thus, it is suggested to assess the safety level on standards based on an AIS curve rather than only measuring the peak linear acceleration.