Predicting Motorcycle Helmet Impact Severity from Residual Crush Damage

Motorcycle helmets reduce the frequency and severity of head and brain injuries by attenuating head acceleration and distributing the impact force. This is achieved primarily through permanent crushing of the interior energy absorbing liner. Our goal is to characterize the relationship between residual helmet damage and impact energy. Working toward this goal, the current study measures the changes in energy absorbing liner residual thicknesses for helmets subjected to drop tests of various severities. We conducted 32 single drop tests of five different helmets on a flat anvil at impact energies of 30 to 259 J (impact speeds = 3.4 to 10 m/s). Percent maximum crush, percent crush volume and stiffness were determined for each helmet impact. For the full-face style helmets tested, linear correlations (r > 0.9) were found showing an increase in the change in thickness measures with increasing impact severities. The shorty helmet showed this trend as well, but also showed a steep increase in percent maximum crush above about 180 J. Of the helmets tested over the range of severities, the stiffness values were found highest at the lowest severity impacts (30 J).