Nij, a function of upper neck forces and moment, plays a dominant role in the vehicle's star rating under the new NHTSA NCAP front impact program. This is mainly due to an artifact in the mapping of the Nij into the "risk" value used in the star rating, and the fact that the neck region is not weighted appropriately to reflect its real world significance relative to the other body regions in the NCAP rating. New test data also show that compared with the 50th male driver Nij, the 5th female passenger Nij is significantly more challenging to contain and therefore it is more dominant in the star rating. This paper describes the Hybrid Ⅲ dummy head and neck impact response and provides a method to determine the external force acting on the head. The force and its acting point on the head are determined from head acceleration, angular acceleration, and the upper neck forces. This paper also analyzes the neck bending mechanism and the associated forces, and how the thorax rotation and translation can affect the upper neck forces. It discusses how the airbag shape and stiffness can affect the external force acting point and direction, and how the force affects the neck internal forces. The analyses have focused on the response of the 5th female dummy neck due to the aforementioned practical difficulty with its Nij. The analytical methods presented should aid in the understanding of vehicle restraint systems.
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