Repeated Impacts on a Motorcycle Helmet: What Happens After a Significant Impact?

It is widely accepted that a motorcycle helmet will reduce the risk of a serious brain injury during an accident through energy dissipation. Currently, there is no literature on what happens to a motorcycle helmet after repeated significant impacts or why it cannot be re-used according to the DOT label. It is also unclear experimentally if the foam liner is permanently affected after repeated impacts. In this study, we repetitively dropped one style of DOT-approved motorcycle helmet using a drop tower system in accordance with FMVSS 218. Helmeted Hybrid III and magnesium headforms were dropped onto a flat anvil with contact to the apical region of the helmets. Strips of pressure-indicating film were placed in the mid-sagittal plane between the foam liner and the headform. Headform accelerations and head injury criterion (HIC) for the Hybrid-III headform were calculated for each drop test. There was a trend for maximum headform acceleration to increase with the number of impacts. The results suggest that a significant impact can degrade the protective ability of the helmet. This was consistently shown in both types of headforms. During the impacts, stress concentrations exceeding the plastic yield of EPS foam were consistently seen, indicating pockets of permanent deformation in the EPS foam. Surprisingly, even after the tenth impact, the helmets still retained some protective capability. This suggests that using a helmet that has sustained prior loading is still better than not wearing a helmet at all.