## 3 Results
### Ops-Core, Airframe and ECH Helmets Blast Testing  
The blast mitigation of the helmets was performed using the headform facing the shock wave (0\({}^{\circ}\) orientation, Figure 1B) at 70 kPa nominal shock wave intensity. The representative pressure traces recorded by sensors mounted flush on the headform surface are presented in Figure 5, and their quantification results are shown in Figure 6. A bare headform was used as a reference, and we evaluated how the geometry of three modern combat helmets affects the surface pressure distribution. The bare headform has the simplest overpressure fingerprint among tested specimens. The highest pressures are observed for the eye sensors (H8 and H9), similar to our previous tests [30; 31]. The pressure traces for most sensors resemble the incident overpressure waveform, except for the two sensors located at the front of the head (H1, H2) and the occipital sensor (H5). Pressure waveforms for all three sensors are more complex (e.g., it would appear the peak overpressure for the H1 sensor is higher for all helmets (\(\sim\)240 kPa range, Figure 6A)) compared to bare headform (190 kPa). However, the peak overpressure for the H2 sensor is reduced under the helmets, from 115 kPa (bare headform) to 70-75 kPa (helmets). The presence of the under-wash effect is evident: the H4 sensor reports the highest peak overpressures for all three helmets. The H3 and H5 sensor peak overpressures are also elevated, exceeding bare headform by 20-40 kPa, except for ECH. The rise times are, in general, longer for all sensor locations when helmets are used. Figure 5: The evaluation of the three helmets under shock wave loading conditions. The representative overpressure waveforms recorded at a nominal shock wave intensity of 10 psi (70 kPa, denoted as T5). The headform with no helmet (**A**) or equipped with the Ops-Core (**B**), Airframe (**C**), or ECH (**D**) helmets were used in these experiments. The data recorded by the midsagittal (H1–H5) and circumferential (H6–H10, insets) sensors embedded in the headform are presented. Additionally, the H6 sensor located at the temple (Figure 2B) is reporting elevated overpressure exceeding the incident values for Airframe and ECH (Figure 6A). The feature which sets the Airframe helmet apart from the set is secondary pressure pulse at 5-10 ms for the H4 and H5 sensors. These signals are not present in the overpressure signatures of the other two helmets and its strong association with the construction of the helmet, and the gap seems extremely likely. The quantification of the impulse reveals that the H7 sensor reports a decrease of 40% for the helmets, and this is the only piece of evidence suggesting the protective role of the evaluated helmets. On the contrary, the impulse values for the H6 sensor are increased by 30-50% (Figure 6C). The under-wash effect is also evident in this dataset by elevated H4 impulse values. The Airframe helmet performs the worst in this classification, considering the impulse for the H2 sensor increases by 60%. The H2 impulse for the Ops-Core and ECH remains approximately the same as for the non-helmeted headform. As a final step of the data analysis, we performed the PCA using the same data reduction algorithm as used in orientation experiments (Figure 7). The two principal components account for 87.6% of the data variability, which is a very high value. It is evident that the data for the headform are clustered separately and on the opposite side of the PC1 axis compared to all three helmet data clusters. There seems to be a degree of overlap between the ECH and Ops-Core helmets. Simultaneously, the Airframe data cluster is also assigned as a separate entity in the evaluated dataset. The Airframe helmet has the largest aggregate risetime, duration, and impulse values (Table 1). Figure 6: The quantification of peak overpressure (**A**), rise time (**B**), impulse (**C**), and duration (**D**) recorded for a nominal BOP of 70 kPa.