## 4 Discussion
### Do Helmets Protect Against the Blast? Early work on the effects of the blast waves using computational methods demonstrated that the impulsive loading of the head could lead to tissue level deformation (strain and stress), and by extension, lead to neurological effects [19; 20; 45; 46; 47]. As mentioned in the previous section, the effects of helmets on the brain loading were extensively investigated using computational approaches (see Table 2 for details). These studies replicated conditions associated with the detonation of high-explosive materials (e.g., TNT or C4) commonly used in improvised explosive devices (IEDs). Thus, the peak overpressures applied to study the brain response are relatively high, and in the range identified as responsible for mild traumatic brain injury (TBI) in service members during recent conflicts [48]. For an unhelmeted head, previous research demonstrated that on the surface of the coup side, the pressure is amplified significantly, and this amplification is also observed on the counter-coup side [5; 11; 19]. This phenomenon is caused by the blast wave separation as it flows around the head and re-joins at the back, which results in the pressure increase. It was also noted that local geometry is an important parameter in the surface pressure amplification and that anatomical regions with concave geometry like eyes, ears, or nose [5; 12; 13], experience higher pressure levels as compared to flat surfaces [13; 24]. The existing work on the effects of the helmet supports the notion that helmets offer protection at the front of the head (H1 and H2 locations), but not in the back due to the under-wash effect [5; 13; 16; 25]. Our experimental results are in stark contrast with these findings; there is no evidence that the helmets improve the protection in the forehead, where typically, the highest pressures were observed (Figures 3A and 6A). It is possible that the discrepancies in the geometry between experimental and computational models: (1) the gap size between the helmet and the head, and (2) material models used for pad suspension systems could account for these differences. The magnitude of the incident blast wave might also be an important factor: 200 kPa [5], 450 kPa [16], 520 kPa [25], and 1600 kPa [13] incident peak overpressure values were used in these studies, whereas the incident pressure value was merely 70 kPa in our experimental design. Some of these limitations were demonstrated in the published \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Helmet Type** & **Peak** & **Charge NEW, kg** & **Standoff** & **Reference** \\  & **Overpressure, kPa** & **TNT** & **Distance, m** & **Reference** \\ \hline ACH & 55, 230, 450 1 & 0.075, 0.095 & 0.75, 1.0, 2.0 & [8] \\ ACH & 260, 290 1 & 7.14 & 3.8, 4.0 & [9] \\ ACH & 450 2 & - & - & [16] \\ PASGT 3 & 170, 326 & 6.85 & 3.0, 4.0 & [17] \\ ACH & 70, 140, 200 4 & - & - & [6] \\ ACH & 520 & 0.07 & 0.58 & [23; 25] \\ ACH & 4071 & 0.20 & 1.0 & [14] \\ PASGT & 103 & 0.0284 & 0.81 & [40] \\ ECH & 180 & 3.2 & 3.0 & [41] \\ CIPHER 5 & 120 & 2.1 & 3.0 & [12] \\ ACH & 270–660 & 0.85–5.4 & 1.0–2.8 & [18] \\ ACH & 70, 150 4 & - & - & [42] \\ ACH, LWH 6 & 1600 & 20 1 & 2.5 & [13] \\ ACH & 10,000 & 3.16 & 0.12 & [43] \\ ACH & 520, 1886 & 0.07, 0.32 & 0.6 & [7] \\ ACH & 195 1 & 2.055 & 3.0 & [11] \\ \hline \hline \end{tabular} 1
[FOOTNOTE:1]Footnote 1: footnotetext: calculated using Kingery–Bulmash equations [44]: incident peak overpressure not specified in the reference; 2 the pressure on the top of a head model used as an equivalent of incident peak overpressure; incident overpressure not specified in the reference; 3 Personnel Armor System for Ground Troops, 4 incident overpressure waveform taken from the shock tube experiments, 5 Conformal Integrated Protective Headgear System, 6 Lightweight Helmet.  
\end{table} 
Table 2: The summary of the computational FEM studies which evaluated blast mitigation performance of various combat helmets using high-fidelity human head models.