The use of visco-elastic materials for the design of helmets and packaging

The design of packaging to protect delicate objects, such as a head and brain, against impact requires consideration of the duration of the impact compared to a characteristic time associated with the dynamic response of the object. Relatively short impacts cause damage as a result of an excessive change in velocity. Relatively long impacts cause damage as a result of an excessive acceleration. The design strategy of packaging must either mitigate the transmitted force (and increase the duration of the impact) through a compliant cushion, or dissipate energy. In this paper we show that both of these strategies can be implemented using visco-elastic materials. In particular, very efficient energy dissipation, with complete structural recovery, can be achieved by using a visco-elastic cushion with properties matched to the requirements of the design. Specifically, energy dissipation can be maximized by ensuring that the time-scale of the accumulation of stress within the system matches the optimal time-scale for dissipation within the cushion. A procedure is described by which an optimal design for the minimum thickness of packaging can be achieved by considering both strategies of velocity and acceleration control.