The Research On Occupant Neck Injuries Protection During Vehicle Rear Impacts

Vehicle traffic accidents result in a large number of fatalities and enormous social economy loss every year all over the world. Therefore, vehicle traffic accidents attract more and more attention in order to decrease the vehicle crash accidents and minimize the risks of traffic injuries. In vehicle rear impacts, it is reported that neck is the most common injuried human body part. About 90 percent of neck injuries in car rear impacts occurred at low speed (ΔV≤25km/h), and most of them are classified as AIS 1(minor) injury. The high incidence and often long-term consequences of the neck injuries lead to great harm to patients and huge societal costs. This paper aims at developing effective tools, methods and guidelines for improvment of seat restraint system design. For this purpose, the research into the kinematics and injury mechanisms of neck in rear impacts was carried out using the new developed mathematical models.Two approaches are used commonly in crash safety field: experimental tests and mathematical modeling. The present study is conducted using Multi-body system (MBS) models and finite element (FE) models as main approach and sled crash test as assistant means. The neck responses are analyzed at two different levels: first, the global kinematics like neck loads, rotation angles, and accelerations is analyzed with MBS models; secondly, local biomechanical responses and neck injury mechanisms are investigated by using FE models in terms of ligament strain and intervertebral disk stress. The variables of seat design are analyzed for minimizing risks of neck injuries.A sled test of vehicle rear impacts is carried out with a Hybrid III dummy. The responses of the Hybrid III dummy are analyzed. Furthermore the performance of the Hybrid III dummy model is compared with that of the BioRID II dummy model in rear impact condition using MADYMO code.Biofidelity of two MADYMO dummy models is evaluated with the volunteer test results in JARI. BioRID II dummy can simulate the extension and ramping-up procedure of the human spine with better correspondent to the volunteer test results. The results show that BioRID II dummy has good biofidelity on spine structure, while the biofidelity of Hybrid III dummy is poor than that of Hybrid III, which is rather stiff during rear end impacts.A MBS model for occupant neck kinematics analysis is set up on MADYMO platform, and the model is comprised of BioRID II dummy MBS model, vehicle model, seat model and safety seat belt model. The effects of head restraint geometry, seat back cushion stiffness, recliner stiffness, impact velocity and acceleration on neck global kinematics responses are analyzed. The output global parameters from simulations include head rotation, rotation between adjacent vetebras, thorax acceleration and neck tourque. Neck injury criterion such as Nkm and NIC were used for assessment of neck injury risks based on simulation results. It was found that calculated NIC and Nkm values from BioRID II dummy can be used to predict AIS1 neck injury risk and evaluate the performance of vehicle seat for neck injury prevention.A FE model for occupant neck injury biomechanics study is developed, the model consists of the HBM-neck model, BioRID II dummy FE model, occupant seat and safety seat belt model. The HBM-neck FE model describes human neck structure such as vertebrae, disks, ligaments and muscles in detail based on anatomy geometrical. The BioRID II dummy FE model includes a human-like spine, which consists of 24 vertebraes that are connected by joints, and the model is validated with data from Linder's pendulum impact test and rear impact volunteer tests in JARI. A hybrid FE occupant model was created by using the HBM-neck model and the BioRID II dummy FE model. The hybrid FE occupant model can be used to simulate the interaction between spine and seat back with higher biofidelity, and to analyze the neck local biomechanical responses and injury mechanisms.The effects of seat design parameters such as headrest position on neck injury risks in various conditions are investigated by using the developed FE model for occupant neck injury biomechanics study in rear impact. The calculated parameters include ligaments elongation, loads and stresses of disks, as well as stresses of vertebrae. The mechanisms of neck injuries in various impact conditions are analyzed. The horizontal gap between head and headrest, headrest height have large influence on local injury parameters of neck tissues, and local injury risks of neck tissues are increasing with the increasing gap and decreasing height. A suitable softer upper seat back cushion and a stiffer upper seat back cushion results in lower injury risks of neck tissues. Equiped with a deformable recliner of proper stiffness characteristics can decrease all of the local injury risks greatly. It is proved that impact acceleration affects injury parameters of neck tissues more than impact velocity. Decreased impact acceleration especially acceleration before head contacts with headrest results in reduced neck injury risks.From above investigation, it is concluded that neck injury risks can be reduced effectively through following measures:Increase the head rest height and reduce headrest distance;Soften upper seat back cushion and stiffer lower seat back cushion;Equiped with a deformable recliner of proper stiffness characteristics to reduce thorax acceleration;Increase energy absorbing capacity of the seat to minish occupant rebound velocity.