Coupled head–neck–torso and seat model for car seat optimization under rear-end impact

The development of new protective systems must be performed on tools reliable and representative of alive human. In an earlier study, a simplified but realistic modelling of the head–neck system under moderate rear impact was performed. In order to address this issue, an original lumped model of the human torso was developed and coupled to a car seat-head rest complex. The experimental modal analysis of the human torso in a seating position performed by Kitazaki in 1992 [Paper presented at the United Kingdom Meeting on Human Response to Vibration held at I.S.V.R. University of Southampton, Southampton, UK, 28–30 September 1992.] was used in the present study for the identification of the mechanical parameters of a lumped human torso model. Despite its low complexity, this model was able to reproduce the five first experimental vibration modes and it was possible to validate it in terms of natural frequencies, damping ratio and mode shapes. In addition to the lumped approach, an external geometry of the human torso was implemented in order to provide a realistic coupling of the human body model to a finite element model of the car seat also developed in the present study. A parametric study was finally carried out in order to evaluate the influence of the torso behaviour and of the different parts of a car seat on the mechanical neck response under rear-end impact. The results of this study allow concluding that the torso behaviour has an important influence on the neck loading and therefore that the quality of a car seat depends on the human body substitute used. For instance, with the proposed torso model, a low-neck injury criterion (NIC) rearward value was obtained with low rigidity of the backrest foam and a stiff backrest net.