Restraint Biomechanics in Frontal Impacts with Inboard-Leaning Occupants

Up to one half of drivers swerve before a crash. Pre-crash swerving moves occupants into lateral out-of-position postures, which can affect the interaction of the occupant with the restraint and subsequently the risk of injury. The influence of lateral out-of-position postures on the performance of restraint systems remains poorly understood. Because of this, design and evaluation of restraint systems do not yet account for lateral out-of-position postures. Therefore, the goal of this thesis was to evaluate the effect of a swerve-induced inboard lateral lean on the kinematics and kinetics of an occupant in a frontal crash.

A realistic inboard-leaning posture was quantified from simulated swerving with nineteen human volunteers. Repeated frontal impact tests were then performed with three post-mortem human subjects (PMHS) seated in a neutral, in-position posture and in the inboard-leaning, out-of-position posture obtained from the volunteer tests. The PMHS were restrained by a contemporary three-point seatbelt with a retractor pre-tensioner and a nominal 2.5-kN retractor force-limiter.

Compared to the neutral posture, the inboard-leaning posture increased the initial length of seatbelt webbing from the D-ring to the acromion by 130 mm ± 25 mm (a 32% increase). The increased initial length permitted the head to displace farther forward during the impact by 69 mm ± 13 mm (a 27% increase). Compared to the neutral posture, the inboard-leaning posture also increased the initial angle between the shoulder-to-D-ring seatbelt segment and the impact acceleration vector (the forward-rearward axis) by 16° ± 2° (a 48% increase). The increased initial angle caused the occupant to swing forward and outboard, tracing out a greater displacement than in the neutral tests but without delaying the time of maximum forward displacement. This increased the maximum resultant velocity of the head by 1.5 m/s ± 0.1 m/s (a 40% increase), which may significantly increase the risk of head injury if the head were to contact another occupant or the vehicle interior. Compared to the neutral-posture tests, the inboard-leaning tests did not significantly change the timing and magnitude of forces in the seatbelt.

This study's results suggest the need for pre-crash safety systems. The postural dependence of occupant restraint biomechanics implies that systems that keep an occupant in a neutral posture before the crash may be more effective than systems that attempt to restrain the occupant during the crash. Future work should explore the effectiveness of pre-crash countermeasures and the consequences of such countermeasures for occupant protection.