## 2 Materials and Methods
Note that there is a larger area and hence volume of foam used in the side inserts when compared to the front inserts. Figure 1: Foam inserts: (**a**) forehead, (**b**) right side, and (**c**) left side and views of the helmet showing native closed cell EVA foam and the top open cell foam in situ along with markers showing where inserts are placed. Also showing the foam structures for the auxetic (**bottom left**) and conventional (**bottom right**). An Anthropomorphic Test Device (ATD) headform (Hybrid III 50 th Percentile Male, Humanetics) provided a surrogate head inside the helmet. To measure acceleration, a \(\pm\)200 g triaxial accelerometer, fixed to an evaluation board (ADXL377, Analog Devices) was mounted within the headform, Figure 2. The accelerometer was wired to an NI USB-6210 Data Acquisition Device (National Instruments, Austin, TX, USA), sampling at 41,666 Hz. The accelerometer setup was calibrated using the protocols in the Federal Motor Vehicle Safety Standard 572 No. 208-E [47]. Testing was undertaken using a drop rig (Figure 3) with the helmet/headform in two orientations. Impact testing at multiple locations is typical of helmet test standards [10] and in this case was used to examine the severity of impact from a frontal and a side impact. The frontal and side impact tests engaged the forehead and side foams during the impact respectively. Figure 3 shows both the frontal and side impact orientations, with the helmet landing on two thin (3 mm) polyurethane sheets, as used in the NOCSAE 001-13m15 standard [10]. These polyurethane sheets are used to prevent surface damage to the helmet. A range of drop heights (17, 25, 30, 37.6, 43, 50, 60, 70, and 80 cm) were tested, spanning impact energies of 8.3 to 38.9 J. These impact energies are below the 45 J maximum specified in the NOCSAE 022-10m12 [48] standard for the performance of baseball helmets (test helmet conforms to NOCSAE baseball standard). Impact energies were calculated based on the potential energy of the helmet/headform (mass \(\sim\)5 kg). Five drop tests were completed at each drop height, for each foam type. The Gadd Severity Index was calculated for each impact. For frontal impacts, drop heights corresponding to impact energies of 8.3, 12.2, 14.6, 18.3, 20.9, 24.3, 29.2, 34.1, and 38.9 J were used. Side impacts were carried out at 12.2, 18.3, 20.9, and 24.3 J. The measuring range of the accelerometer in the z-axis prevented higher energy testing for the side impacts. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Location** & \multicolumn{2}{c}{**Foam Thickness (mm)**} & \multicolumn{2}{c}{**Replacement PU Foam**} & \multicolumn{2}{c}{**Conformable Foam**} \\
**within** & **Native EVA** & **Native PU** & **Thickness (Conventional and Auxetic)** & **Mass (kg)** & **Area of Largest Face** \\ \hline Top & 11 & 5 & n/a & Headform: & 4.42 & Front: & \(\sim\)63 \\ Side & 4 & 23 & 20 & Helmet and foam: & 0.54 & Side: & \(\sim\)150 \\ Forehead & 6 & 23 & 20 & Cradle: & 0.1 & - & - \\ \hline \end{tabular}
\end{table} 
Table 1: Helmet and foam properties. Figure 2: Schematic of the headform within the helmet showing position of accelerometer within headform and foam layers (z-axis coming out of the page). Steps were taken to ensure consistency of helmet positioning on the ATD, orientation of the magnetic drop cradle, location of landing pads, and drop height. String was wrapped through an existing hole in the helmet, and under the chin of the headform, to secure it to the headform and ensure that the helmet did not slide loose from the headform mid-trial. To limit any effect of foam degradation, each sample was removed from the helmet and allowed to recover for 10 min before the subsequent drop. Resting time was calculated based on pilot testing with samples from the first converted foam sheet. Drops over a range of energies (14.6-29.2 J) were completed with increasing rest times for the auxetic and conventional foam helmets, until five sequential readings of similar 'peak' accelerations (\(\pm\)5%) were measured.