Influence of Seatback Repositioning Precrash Timing and Rotational Speed on Occupant Protection in Zero-Gravity Seats Under High-Speed Frontal Impact

Under high-speed frontal impact, occupants in zero-gravity seats restrained by a three-point seatbelt are at risk of head injury, lumbar spine injury, and submarining. To reduce occupant injury, this study systematically investigated the protective effects of active seatback repositioning strategies on occupants under 56 km/h frontal impact. First, the THUMS (Total Human Model for Safety)-based finite element assembly consisting of a regular seat model and a 3-point seat belt system was verified by comparing thoracic acceleration, head acceleration, lap belt force, and shoulder belt force with PMHS test data. On this basis, a zero-gravity seat model with an active seatback repositioning function was established. Using this model, the effects of seatback repositioning timing and seatback rotational speed on occupant kinematic responses and injury protection performance were investigated. The results showed that with the introduction of active seatback repositioning before the crash, occupant head and lumbar spine injuries, as well as submarining tendency, can be significantly reduced. Particularly, with respect to the repositioning time analysis, the seatback repositioning with 100 ms precrash setting provided the best protective performance and compared with the fixed-seatback condition, the occupant head injuries of HIC15, BrIC, CSDM(0.25), injury probability of lumbar spine and pelvic forward excursion was reduced by 66.4%, 65%, 99.8%, 34.1%, and 48%, respectively. Furthermore, the influence of seatback repositioning speed on occupant injury protection was examined. The results indicated that an excessively high seatback rotational speed increased head and neck injury, thoracolumbar flexion, as well as risk of rib injury.