Human Body-car Seat Coupling under Rear Impact

The development of new protective systems must be performed on reliable tools and representative of alive human. In an earlier study a simplified and realistic model of the head-neck system under moderate rear impact was performed. It is clear and often addressed in the literature that under such an impact configuration, the deformation of the torso and the car seat, is of extreme importance and defines the initial conditions of the head-neck system. In order to address this issue, an original lumped model of the human torso was developed in the present study and coupled to a car seat-head rest complex. The hypothesis of linear behavior was used for the torso being subjected to small deformations . The modal analysis of the human torso in a seating position conduced by Kitazaki and Griffin in 1992 was used in this study for both masses and mechanical properties identification. In order to reproduce the four mode shapes identified experimentally the torso was divided in six segments to obtain the five degrees of freedom with the head neck system. This model of minimum complexity but able to reproduce the 5 first experimental vibration modes was validated in the frequency domain in terms of natural frequencies and damping as well as mode shapes. In addition to the lumped approach, an external geometry was implemented in order to couple the human body model to a finite element model of the car seat also developed in the present studies. Rear impact simulations for the two different configurations (flexible and rigid torso) showed an increase of about 35% for the maximum T1 acceleration and an increase of about 65% for the acceleration slope when a rigid torso is considered. Realistic body behavior and accurate T1 acceleration are essential aspects in real world accident reconstruction as well as for seat-head rest evaluation and optimization against neck loading.