Analysis method for factors influencing pelvis rotation of Hybrid III and THOR dummies during frontal impact

In vehicle frontal impacts, pelvis rotation is a crucial factor in submarining, where the lap belt slips off the pelvis and intrudes into the abdomen. Submarining can occur even when the lap belt engages the pelvis due to pelvis rotation. This study aims to establish an analytical method to evaluate the forces and moments acting on the pelvis of dummies during frontal impacts and to identify factors influencing pelvis rotation. Finite element simulations were conducted using dummies (Hybrid III 50 M, 5 F, and THOR 50 M, 5 F) restrained with a 3-point standard seat belt in the rear seat during a frontal impact at 50 km/h. The pelvis of the dummy was divided into anatomical regions. Based on Euler’s equation, moments around the pelvis’s center of gravity (COG) were calculated based on contact forces in each pelvis region and joint force (lumbar spine and hip joint). The angular acceleration and moment of the pelvis were consistent with Euler’s equations, confirming the accuracy of this moment calculation method. Factors promoting rearward pelvis rotation included femur force, lap belt force, and lumbar spine force, while factors reducing pelvis rearward rotation included lumbar spine bending moment and seat cushion force. These factors varied among dummies due to differences in pelvis shape and lumbar spine stiffness. The rearward rotation of the pelvis in the Hybrid III was small because the lap belt path was close to the pelvis COG; in the THOR, however, rearward rotation was greater because the lap belt path was further from the pelvis COG, and the ischium force reducing the rearward rotation was smaller. This study proposes an analytical approach to understanding pelvis rotation in dummies. This method allows for evaluating various factors influencing pelvis rotation over time, including dummy design and restraint systems, to prevent pelvis rearward rotation and submarining.