Assessment of Seat Belt Anchor Position on Rear Seat Occupant Kinematics in Frontal Impact Using a Validated THOR-5F Finite Element Model

Abstract

Rear seat occupant safety has been identified as an area of concern in frontal impacts, with small stature females experiencing severe injuries more frequently, including a higher rate of thoracic injuries. Seat belts, the primary restraint for rear seat occupants, have been identified as contributors to thoracic injury, with studies finding that belt geometry and belt anchor locations influence occupant response. A new small stature female Anthropometric Testing Device (ATD) to be used in vehicle testing is the THOR-5F ATD, designed with improved anthropometry and updated measurement capabilities, including four chest deflection sensors. In this study, a Finite Element (FE) model of the THOR-5F was validated against physical testing of the THOR-5F ATD in a Rigid Bench test configuration with a 23 g frontal crash pulse, by comparing belt interaction, iliac forces, pelvis accelerations, chest deflections, and head accelerations. The THOR-5F FE model was also assessed on a simplified version of an existing bench with a pivoting seat (Semi-Rigid Bench) and a 4-Point 2-Belt system, and the results were compared with those from a 9 g sled test reported in the literature. In both bench configurations, the overall THOR-5F FE response matched the reported physical response well, with an average ISO 18571 rating of 0.71, corresponding to a “Fair” grade. Physical testing of the THOR-5F ATD also showed bottoming out of the lower-right chest deflection sensor, which is currently under investigation as the THOR-5F design is being refined. Parametric studies were conducted to explore the effects of shoulder and lap belt anchor positioning in the fore/aft (X), lateral (Y), and vertical (Z) directions on THOR-5F FE peak chest deflections and peak resultant pelvis accelerations. The shoulder belt anchor X and Y-positions had a large effect on the overall peak chest deflection in both bench configurations, with rearward and inboard anchor locations reducing deflection, and the lowest deflections approximately 16% lower than the average. The lap belt anchor position had a smaller effect than the shoulder belt anchor on chest deflections. Based on preliminary chest deflection injury risk curves, rearward and inboard shoulder belt anchor positions resulted in lower corresponding AIS 3+ injury risks. However, the risks remained very high across all anchor configurations in both benches, indicating a need to implement restraint technologies, such as load limiters, in addition to favorable anchor positions. The lap belt anchor position had a larger effect on peak pelvis acceleration than the shoulder belt anchor position. The X, Y, and Z-positions of the lap belt anchor influenced the peak resultant pelvis acceleration in the Rigid Bench, with rearward, upward, and inboard locations reducing pelvis accelerations. The Y-position of the lap belt anchors and the YZ interaction had the largest effect on pelvis acceleration, indicating a complex interaction between the anchors and the Semi-Rigid Bench components. This study demonstrated that the Rigid and Semi-Rigid Bench FE models could replicate physical occupant and restraint responses, and that anchor position affected the chest deflection and pelvis acceleration of the THOR-5F FE. Future research should focus on belt material properties, vehicle fleet-specific anchor positions, the implementation of load limiters, and the combined interactions between shoulder and lap belt anchor positions.