Occupant protection in rear impact involves two competing
challenges. On one hand, allowing a deformation of the seat would
act as an energy absorber in low severity impacts and would
consequently decrease the risk of neck injuries. However, on the
other hand, large deformations of the seat may increase the
likelihood of occupant ejection in high severity cases. Green et
al., 1987 analyzed a total of 919 accidents in Great Britain. They
found that occupant ejection resulted in a risk of severe injuries
and fatalities between 3.6 and 4.5 times higher than those cases
where no ejection was observed. The sample included single front,
side and rear impacts as well as multiple impacts and rollover. The
rate of belt use in the sample was 50%. While this analysis
included all forms of impact scenarios, nevertheless, it highlights
the relative injury severity of occupant ejection.
Extensive literature search has found no full-scale rear impact
tests involving Post Mortem Human Subjects (PMHS) conducted in a
laboratory environment and resulting in ejection.
This paper describes a total of 10 sled tests conducted on 3
belted PMHS using a simplified seat design composed of rigid plates
assembled such that the angular and linear stiffness of the
seatback (including the foam) was modeled. The initial angular
position and the range of motion of the seatback, the size of the
PMHS, the slack length of the seatbelt, the angular stiffness of
the seatback, and the use of headrest were varied in the test
matrix while the pulse was kept constant (triangular acceleration
with a peak of 17 G at 30 ms and a duration of 95 ms).
In the test series, the tests were not run randomly but the
likelihood of occupant ejection was increased systematically until
ejection occurred.
PMHS seat ejection was observed only for the 95th percentile,
initially positioned with a seatback angle relative to the vertical
equal to 22°, a range of seatback angular motion equal to 44° and
no headrest. Repeating the test under the same conditions but with
the pretensioner fired did not prevent the ejection. In addition,
the 50th percentile belted specimen was not observed to sustain
rearward seat ejection under realistic conditions including the use
of headrest.