# Radial and Oblique Impact Testing of Alpine Helnets onto Snow Surfaces
## 4 Discussion
For the radial impacts, snow sample collection time and ambient temperature were found to influence mean peak linear headform acceleration, which was significantly (\(p\) = 0.005) lower for snow samples collected at noon compared to samples collected in the morning. The relatively higher ambient temperature during the noon collection likely softened the snow. For tests of the large-size helmet, the frozen snow sample resulted in a higher mean peak linear headform acceleration when compared to the non-frozen snow sample, but the result was non-significant (\(p\) = 0.120). These findings can be explained by considering snow as a foam, which can absorb energy more than a rigid or semi-rigid surface, and support results from previous experimental [18; 19; 20; 21] and computational [24; 25] studies. Recent studies have validated computational models of snow as a foam using a rigid body [24; 25] and finite element techniques [23]. For the snow samples collected in the morning with no freezing, all tests resulted in similar peak linear headform accelerations regardless of headform condition. A drop height of 0.85 m is likely not high enough to cause the snow to \"bottom out\", similar to a foam material [38], and engage the energy-absorbing inner liner of the helmet. It is hypothesised that higher impact speeds increase the reduction in peak linear headform acceleration for helmeted tests relative to unhelmeted tests. However, when a drop test from 1.5 m was performed in the current study, which corresponds to an impact speed of 5.4 m/s, a large amount of snow was displaced from a sample box. The displacement of snow resulted in the headform impacting the base of the sample box. It is unknown if this test is representative of an impact on the soft snow layer that is covering an icy base layer. No further tests were performed from this height; therefore, field tests on actual snow slopes or laboratory tests using deeper snow samples are required to investigate higher drop heights, such as 1.5 m as per the European [14] and Canadian [15] alpine helmet standards. Although [PERSON] et al. [40] previously tested \"beanie helmets\", which are non-approved motorcycle helmets with hard shells, the traditional beanie in the current study was an acrylic knit cap with no shell. Recent advancements in material science have led to the development of a helmet without an outer shell that incorporates a layer of non Newtonian shear thickening fluid (STF) as part of the inner liner [41]. The viscosity of an STF increases when the shear rate increases; therefore, during an impact, the STF layer is flexible during general wear but hardens when the helmet is impacted. The helmet is covered with a knitted layer, which makes it look like a beanie; however, it incorporates several features of a helmet, such as a retention system and an inner liner of energy-absorbing foam. To the authors' knowledge, no previous studies have assessed the impact attention potential of traditional beanies. Interestingly, [PERSON] et al. [42; 43] found that beanie use was significantly associated with reduced head injuries in snowboarding; however, the odds ratio for the effect of beanie use on serious head injuries was non-significant after adjusting for jumping. The current study has several limitations, mainly the quality and repeatability of the snow samples. Although the snow samples were collected from the same location, there may have been differences in structure for various reasons. For example, a vehicle may have driven over one section of the collection site. Efforts were also made to replicate the timing between collection and testing, but slight variations may have resulted in snow samples being exposed to different temperatures. Similar limitations were identified in a previous study by [PERSON] et al. [21]. In addition, there was concern regarding moisture from the melted snow in the laboratory, where much of the equipment is susceptible to water damage.