## 4 Discussion
### Do Helmets Protect Against the Blast?
The helmet type, incident peak blast overpressure (reported or calculated) with a corresponding mass of the charge mass (TNT equivalency in kg), and standoff distance are presented. work, that attempted validation of numerical models against the experimental data [9]. The decreased volume under the helmet by the unyielding pad suspension system caused the pressure amplification on the surface of the head not covered by pads [12]. Overall, our analysis indicates that all four tested helmets cause surface pressure increases compared to an unprotected headform. The aggregate (sum) peak overpressure for ten pressure sensors is 1610 kPa for the unprotected headform, while for helmets, it is in the range of 1856-2180 kPa (frontal blast loading, Table 2). It appears that each helmet system has a unique surface pressure signature, which was determined by the quantification of surface pressure waveform characteristics followed by PCA (Figures 4 and 7). All helmets increase the pressure in the eyes (H8 and H9) and front of the headform (H1). Further, we noted the pressure increase: (1) on the temple for the ECH (H6 and H10); and (2) on the back (H4 and H5) for the low-cut helmets (Ops-Core and Airframe). The latter had consistently elevated impulse values for all five midsagittal sensors (H1-H5, Figure 6C). There exists some evidence that increase in surface pressure translates into modulation of the ICP [6; 25], but this problem was not studied in detail. More work is necessary to provide a definite answer to the question whether pressure amplification on the surface of the head would translate into increase of the ICP and resultant brain injury. On the contrary, the effects of the blast direction on the brain loading and helmet protection are relatively well understood [10; 13; 18; 25]. In these works, the results are presented as pressure histories or peak pressures at a few selected locations on the head surface [11; 13; 25] where elevated pressures were likely to occur (forehead, back of the head, eyes, and ears) or as peak intracranial pressure [18; 25]. Previous studies did not attempt to evaluate the surface pressure distribution (head loading) in a collective and integrated way incorporating a larger number of measurement sites (Figures 2 and 6) and blast waveform parameters (Figures 4 and 7), in comparison to the current study. However, even with these limitations, there is a general agreement between these previous studies and results reported herein, i.e., the exposure at the 0\({}^{\circ}\) orientation results in the highest pressure. In contrast, when the blast wave is directed at the back of the head, a significant pressure attenuation was observed. Interestingly, [PERSON] and co-workers noticed that at the 45\({}^{\circ}\) orientation, the pressures reached the highest levels [11]. Considering that the surface pressure changes considerably in response to such factors as helmet shape (design) and blast wave directionality, it is difficult to formulate a simple yes-no answer about helmets' blast protection properties. However, each combination of helmet and blast wave orientation has its own unique \"pressure fingerprint\", which can be used to identify variables behind the efficacy of helmet design. The surface pressure fingerprinting, dimensionality reduction via PCA and aggregate parameters of the surface pressure (Table 1) combined are necessary to answer the question about the blast mitigation efficacy with a high degree of confidence. There are several factors that can be considered as limitations of the current work. The padding type was not standardized in our experiments, and the pads we used are a \"standard issue suspension system\" available with a specific helmet type. The material properties of the pads and their shape and spatial distribution might contribute to the variability of the results. The gap between the headform and the helmet was not controlled and reliant upon the used suspension pad system.