Rear end collisions account for approximately $9 billion annually in the United States alone. These types of collisions account for nearly 30% of all vehicle impacts making them the most common type. Soft tissue injury to the neck (i.e. “whiplash”) is typically associated with this type of collision due to the occupant dynamics of the passengers in the struck vehicle. At low relative impact velocities, whiplash-type injuries are known to occur but are typically attributed to: 1) improper seat adjustment, 2) an “out-of-position” event, or 3) a low injury threshold due to age, gender, etc. In high impact collisions, both whiplash and occupant ejection can take place, the latter placing far greater risk of injury not only to the front seat occupant, but also to any rear seat passengers as well. The automobile seating system is the predominant safety device employed to protect the occupant during these types of collisions. In current seats, designers have focused predominantly on modifying seat base strength, seat back stiffness/compliance and head restraint size and position as a means of mitigating injury. While all of these aspects are important and have increased the crashworthiness of the seat, the concept of utilizing the seat base as a component to absorb collision energy has remained relatively unexamined, and may offer the potential for reducing injuries in higher severity collisions.
This paper reports work related to the development of a supplemental safety system for the automobile seat where the goal is to further reduce the dynamic loading of the occupant to mitigate injury during a rear end collision. The initial step of this work was to develop and utilize several simulation models (using LS DYNA) to examine the relationship between collision conditions (such as vehicle-to-vehicle collision speeds, initial occupant and seat position, and occupant size) to accepted neck injury criteria for both low and high speed collisions. Utilizing these results, this paper examines the potential for deformable materials (in this study closed-cell foam) to absorb enough transmitted energy in order to reduce the dynamic loading experienced by the occupant. The relationships between energy levels transmitted to the occupant, the occupant dynamics, and how they affect the neck injury criteria assessed values are reported. Several foams, used industry wide, have been investigated in order to determine the best foam characteristics and geometrical shapes for low, high, and severe speed collisions. The results indicate that the foam's stress vs. volumetric strain curve, length, and cross-sectional area can affect the degree to which injury potential is reduced over a broad range of collision speeds. Further discussion of design considerations for such a supplemental system is also provided in this paper.