Mechanical characterisation of micro-structured polymeric foams for shock absorption in sport equipment
Luca ANDENA (Politecnico di Milano)
Sports helmets are made up of two layers: the external shell and the foam liner. The foam liner, typically made of expanded polystyrene (EPS) or polypropylene (EPP), is mainly responsible for impact energy absorption.
Standard helmet design requires the foam to maximize this energy absorption, thus achieving large deformations (up to 25% in compression) while maintaining the stress level below a threshold value. To optimize the helmet construction in terms of foams composition, structure and density, reliable numerical models are required, which in turn need to be fed with experimental data.
A characterisation of several foams was performed, including EPS and EPP having varying densities. In particular here we focus on EPP, whose mechanical properties were tested in tension and compression at varying strain rates for eight different densities. Typical mechanical parameters (elastic moduli and plateau stress in compression, Poisson’s ratio) were compared with existing models and data in the literature.
A marked strain rate dependence – very important for impact applications – was accurately described using the Nagy phenomenological model. The foam microstructure was investigated using scanning electron microscopy (SEM) and relevant 2D parameters such as mean cell area, mean Feret diameter and mean cell wall thickness were obtained. Energy absorption characteristics were analysed with the aid of Maiti graphs.
