Previous rear-facing post-mortem human subject (PMHS) studies utilizing a
reinforced seat have prompted questions as to whether the seat could have been a
contributing factor to the severe rib and pelvis injuries observed in those
experiments. In response, a recent PMHS study used an unreinforced seat in a
similar experiment, which was expected to mitigate severe injuries by
dissipating energy from seatback deformations. However, the PMHS tested in the
unreinforced seat sustained even more severe rib fracture numbers than in the
reinforced seat. No studies have investigated how additional variables (i.e.,
countermeasures) may influence rib fractures in high-speed rear-facing frontal
impacts (HSRFFI). Therefore, this study aimed to explore the effect of an
airbag-equipped seat (AES) on male PMHS responses and injuries. Rear-facing sled
tests were conducted using five mid-size male PMHS seated in the AES at ΔV of 56
km/h: PMHS1 with no airbag as a baseline, PMHS2 with a seatback airbag (SA),
PMHS3 with an extended seatback airbag (ESA), and PMHS4 and 5 with ESA and a
wedge airbag (ESA+WA). An instrument panel (IP) and windshield were installed
behind the seat to mimic realistic interior vehicle compartments. A chestband at
mid-sternum, 6-degree motion blocks at the head, T1, T4, T8, T12, pelvis, and
extremities, as well as rib strain gages and rosettes were installed on PMHS to
understand potential mechanisms of injuries. A motion capture system was used to
quantify whole-body PMHS and seatback kinematics. Maximum seatback rotation was
38.1° in the baseline test and 20.3°–25.1° with AES. Peak chest A-P compression
in the anterior-posterior (A-P) direction was 25.7 mm for baseline and 7.3
mm–35.2 mm with AES (23.7 mm for SA, 7.3 mm for ESA, 35.2 and 8.7 mm for
ESA+WA). The number of rib fractures (NRF) was high in baseline (32), SA (25),
and ESA (27) conditions, but was reduced in ESA+WA (6 and 13). Strain rosette
data indicated upward directions of principal strains on the posterior ribs,
likely due to I-S deformation of the PMHS thoraces. Responses from thorax
instrumentation showed that peak chest deflection (A-P) alone did not fully
explain NRF, especially as rib fractures in all tests occurred after peak
deflection in this direction. Instead, maximum principal strains in the I-S
direction (shear), confirmed by strain rosette data, likely influenced rib
fractures. ESA+WA effectively supported PMHS, maintaining upright postures and
minimizing I-S chest shear, which reduced NRF. Limitations include a small
sample size, possible age-related injury effects, and seat designs intended for
low-speed rear impacts, not HSRFFI. Compression and shear loading to the PMHS
thoraces were observed in HSRFFI. The shear loading was likely due to the large
upward thorax deflection induced by the ramping motion and seatback rotation.
One of the AES, ESA+WA, effectively maintained an upright spine and reduced NRF.
This study offers important information for improving current safety tools and
designing rear-facing countermeasures for automated driving systems.
