Helmet Optimisation Based On Head-helmet Modelling

Abstract

The aim of this work is to optimise a full face helmet finite element model based on the dynamic behaviour of its components against biomechanical criteria. It is well known that helmets substantially reduce head injury, although the mechanism of this protection is neither well understood nor controlled. Moreover, today helmets are designed to reduce headform deceleration and not optimised to reduce head injury. The helmet used in this study is a full face helmet with a polycarbonate thermoplastic shell and an expanded polystyrene foam, certified to BS6658A [l]. The validation of the helmet FEM corresponds to the impact test stipulated by the British Standard BS 6658A, and the ECER022104 [2]. After a validation with a headform FE model as used in the experimental normative tests, the helmet model was coupled with a previously developed finite element model of the human head . This approach consists to couple the human head with the helmet FE models in order to predict inkacranial field parameters such as acceleration, pressure and Von Mises stress. Concerning the coupling with the human head, a frontal impact has been simulated with the same boundary conditions as for the normative test with standard helmet mechanical properties. The brain pressure varied fiom 10 1 kPa to 267 kPa. These values were higher than the brain tolerance limits for visible injuries proposed in the literature, which are -180 H a in tension and 234 kPa in compression. The highest Von Mises shear stress values in the brain were about 47 @a which is close to the 20 kPa limit proposed in the literature for neurological injuries. The final step of the study then consists to optimise numerically the helmet mechanical parameters against biomechanical criteria such as intra-cerebral stress levels. In order to define the influence of the helmet shell and foam properties on the human head, a parametric study of the model was undertaken and all results were analyse with a PCA method. This study led then to the conclusion that the foam elastic limit has the most important influence on biomechanical head response. Transactions on the Built Environment vol 67, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509