Response of helmets in direct and oblique impacts

The predictions of a computer model of motorcycle helmet performance in direct impacts were shown to agree with experiments at a range of impact velocities. A rig for testing helmet materials under oblique impact was constructed, and the three degrees-of-freedom of its motion analysed. Oblique impacts are the most common type, and can cause brain injury through rotational head acceleration. The friction coefficient of polystyrene and polypropylene foams were found to be about 0.5 under impact conditions, and a PVC shell reduced this to about 0.3. Polystyrene foam cracks in oblique impacts unless it is protected by a thermoplastic skin, but polypropylene foam does not crack. The peak linear acceleration is slightly reduced by the tangential velocity component, but high levels of rotational head acceleration can occur for sliding velocities of 8 m/s.