## 4 Discussion  
A helmet with auxetic foam inserts significantly (\(p<0.05\)) reduced the acceleration recorded by the headform accelerometer at the highest impact energy in both frontal and side impacts. In the frontal drop test at an impact energy of 38.9 J the auxetic foam reduced the GSI by 11.4%. In the side drop test at an impact energy of 24.3 J the difference in GSI was 44.2% between the foam types. At lower impact energies it appears that the difference between foam types is less, and only when the impact energy was above 18.3 J for the frontal drops did the auxetic foam reduce the GSI in comparison to conventional foam. For side drops at all impact energies tested, the auxetic foam performed better, reducing all GSI values. For the frontal drop test at the second highest impact energy (34.1 J), the auxetic foam inserts did seem to perform better than the conventional counterpart but this difference was not deemed significant at the 95% confidence level. The reason for this difference not being significant at 34.1 J is likely due to measurements errors, although there could be an underlying interaction effect of the foams, helmet, and headform at this impact energy which renders the auxetic foam less effective. In-depth further investigation, with more samples and tests scenarios, is required to fully understand the effectiveness of auxetic foams in comparison to their conventional counterpart across a range of impact energies. In general, the findings are in line with previous work where auxetic foam tested in isolation or with thin polypropylene sheets attenuated linear impact acceleration to a greater extent at higher impact energies [34; 38; 44]. [PERSON] et al. [38] stated that this behavior could be attributed to the  
Figure 6: The difference in Gadd Severity Index between conventional foam inserts and auxetic foam inserts for the front and side impacts, labeled where these differences were not significant based on \(t\)-test \(p>0.05\) (open markers). auxetic foam's resistance to \"bottoming out\" during impact. 'Bottoming out' is the point at which an impact attenuating system becomes saturated due to high loading and the effectiveness of the system diminishes significantly. In the case of [PERSON] et al. [38], this would be when the foam compresses to such an extent that it was no longer effective, and the impactor is essentially striking the anvil. At this point, the acceleration trace will show a'spike' (short duration high peak). In this study, any 'bottoming out' of the helmet system is hard to identify and assess due to the multi-layered foam construction of the helmet. The 'bottoming out' of the conventional open cell foam is likely to occur when the performance of the auxetic and conventional foam diverges. For the frontal impacts, deviation in impact severities occurs at an impact energy around 20.9 J, but for the side impacts this occurs at the lowest energy tested at 12.2 J. The reason why a conventional foam 'bottoms out' when compared to an auxetic foam is still unknown but could be due to the auxetic foams' deformation process and densification of the material occurring under the impact location [38]. Figure 6 illustrates that during the side impact tests the auxetic foam system appeared to produce significantly lower severity index over the range of energies tested. This significant improvement in the auxetic foam during the side drop test could be explained by the variation in the amount of foam within the helmet. The side conformable foam insert had approximately twice the amount of foam when compared to the front insert (Table 1). Therefore, during the side drop tests the increased amount of foam in the side insert appeared to exaggerate the energy attenuation ability of the auxetic foam. The conventional foam in the side impact test performed in a similar manner to the frontal drop test with similar GSI values. It appears that as the surface area and hence volume of the foam increases this also increases the ability of the auxetic foam to reduce GSI.