Deformation mechanisms and energy absorption of polystyrene foams for protective helmets

The deformation mechanisms and energy absorption capability of polystyrene foams and polycarbonate shells for protective helmets are experimentally studied with the aim of developing a comprehensive constitutive law to be implemented into FEM codes for impact analysis. Expanded polystyrene (EPS) of different densities are considered. Tensile and bending tests on both EPS and PC are performed. Static and dynamic compression tests on EPS are performed as well, according to both variable- and fixed-volume methods. Falling weight tests are performed on both plane PC and sandwich PC/EPS at different energy contents to investigate possible couplings. EPS dynamic-mechanical tests are also carried out at different frequencies to evaluate temperature and strain rate influence on material stiffness. The extensive scanning electron microscopy analysis allows the investigation of strain mechanisms responsible for energy absorption as well as the validation of existing theoretical models. It is demonstrated that the energy absorption capability of these materials can be controlled at two different stages: at the macroscopic scale, by choosing the foam density able to minimise the transferred load and the acceleration value in relation to the available absorbing volume; at the microscopic scale, by modifying EPS internal structure in terms of hollow bead dimensions and walls thickness.