Generally a helmet comprises two main components: the shell and the fitting system. Despite the variations in designs due to the different usage requirements, typically helmets are intended to protect the user’s head through an energy absorption mechanism. The weight and volume are important factors in helmet design since both may alter the injury risk to the head and neck. The helmet outer shell is usually made of hard material that will deform when it is hit by hard objects. This action disperses energy from the impact to lessen the force before it reaches the head. The fitting system frequently includes a dense layer that cushions and absorbs the energy as a result of relative motion between the helmet and the head. A balance needs to be achieved on how strong and how stiff a helmet should be to provide the best possible protection. If a helmet is too stiff it can be less able to prevent brain injury in the kinds of impacts that may occur. If it is too flexible or soft, it might not protect the user in a violent, high-energy crash. For military applications, the requirements for helmet performance may be even more demanding. Not only do helmets have to protect a Soldier’s head from blunt impacts, but helmets also are expected to provide mounting platforms for ancillary devices and to function in ballistic and blast events as well.

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