Commonly employed motorbike helmets are generally built up of a thermoplastic shell combined with a foam linear. International security standards require the helmets to be subjected to multiple drop tests executed with a severe combination of testing conditions such as velocity, temperature and impactor shape.
An extensive experimental characterization program has been carried out over a specific ABS blend and polystyrenic foam samples, in order to define strain rate, temperature and anvil shape dependency. Starting from the collected database, numerical simulations were performed by means of the explicit code LS-DYNA in order to evaluate the amount of the difference in results between virtual and experimental measurements. As second step, a finite element model of a complete thermoplastic helmet was developed and parametric analyses were conducted. Data reviews allowed to highlight the most weighting parameters leading the energy adsorption capability of a motorbike helmet. In particular, results confirmed the foam liner to be the leading parameters for the energy absorption, but evidenced the need to locally optimize shell thickness in order to improve the global impact response of the helmet. Indisputable enhancement in the safety properties was observed in the homologation phase in parallel with a decrement of the dispersion in the experimental results, opening a mere scenario in the development of general guidelines for the impact design of personal protective equipments. |