OPTIMIZATION OF SINGLE SKIN SURFACES FOR HEAD INJURY PREVENTION  A COMPARISON OF OPTIMA CALCULATED FOR GLOBAL VERSUS LOCAL INJURY THRESHOLDS Svein Kleiven 1 , Rickard Juntikka 2 1 Division of Neuronic Engineering, CTV  Centre for Technology in Health Care, Royal Institute of Technology and Karolinska Institute, Stockholm, Sweden; 2 Division of Lightweight Structures, Dept. of Vehicle and Aeronautical Engineering, Royal Institute of Technology, Stockholm, Sweden. Key Words: Head impact, Euro-NCAP, Finite element method, optimization, HIC INTRODUCTION THE HEAD INJURY CRITERION (HIC) has been used in crash testing for several years, as a measure of the likelihood of serious brain injury. HIC only treats the resultant translational acceleration and the duration of the impulse and no consideration is made for the direction of the impulse or rotational acceleration components (Bellora et al. 2001; Kleiven, 2003). The human head behaves in a more complex way during impact and since the validity of HIC is intensively debated there is reason to believe that the safety development could be made more efficient through use of more delicate tools, such as biomechanically representative FE models of the human head, together with local tissue strain thresholds. It is hypothesized that designs based on either of the two criteria would come out differently. METHODS A detailed and parameterized finite element (FE) model of the adult human head, including the scalp, skull, brain, meninges, cerebrospinal fluid (CSF), and neck with the extension of the spinal cord and the dura matter was used (Kleiven, 2003). This model has been experimentally validated in previous studies (i.e. Kleiven and Hardy, 2002). For comparison with the FE human head calculations, two head impact dummies were utilized; an FE-model of the Euro-NCAP pedestrian head dummy, modeled and verified according to the Euro-NCAP verification test; and an FE-model of the Hybrid III head developed by Fredriksson (1996), see Fig. 1. The FE head model had a mass of 4.64 kg, while the Euro-NCAP and Hybrid III dummies weighed 4.80 and 4.55 kg, respectively. Fig. 1 - Impact simulations with (going upwards), Euro-NCAP dummy; Hybrid III dummy; FE- model of human head. Fig. 2 - Material properties for the optimization of the plate. The head models were impacted according to the Euro-NCAP pedestrian head impact test (40 km/h, horizontal angle 65°) towards a 900*900 mm quadratic plate with clamped boundary conditions at the edges with the plate rotated 10° to the horizontal to simulate the slope of an actual bonnet. The material properties of the plate were optimized for a minimum displacement of the plate in the vertical IRCOBI Conference  Lisbon (Portugal), September 2003 345