A Study On Dynamic Response And Injury Mechanism Of Chest And Lower Extremities In Vehicle To Pedestrian Collisions

A number of pedestrian fatalities occur in traffic accidents, and pedestrian protection is a major issue in vehicle designs. Although vehicle designs incorporate head protection, the percentage of chest injuries is increasing as few studies have examined pedestrian chest injuries. This study aimed to investigate the chest and lower extremity injury mechanism in various types of vehicle-to-pedestrian collisions. The injury risk of the pedestrian was also analyzed in collisions with vehicles at different impact velocities. Furthermore, an attempt was made to evaluate the effect of the subsystem impact test for chest protection.Four vehicle FE models (medium car, minicar one-box vehicle, and sport utility vehicles) were developed for vehicles used in vehicle-pedestrian simulations. The medium car and SUV FE models were modified from the original ones available in the NCAC database. The minicar and one-box vehicle FE models were developed by measuring the geometry of the real cars, and the material properties of each vehicle part were determined based on results of the tensile tests of the specimens. The vehicle FE models were validated by comparing the results of the headform and legform impact tests.The Total HUman Model for Safety (THUMS) pedestrian model was used to investigate the pedestrian globe kinematic behavior in collision with these types of vehicles. The pedestrian trajectories were analyzed by different vehicle shapes, pedestrian size, and impact velocities. Furthermore, the pedestrian head impact dynamics and impact conditions were investigated. The injury risk of the head was evaluated and compared at impact velocities of20,30,40, and50km/h.The pedestrian chest kinematic behaviors, impact conditions and loadings were investigated in collisions involving these types of vehicles. The injury parameters of the chest were evaluated in terms of rib deflections and von Mises distributions. In addition, the ribcage fracture risk in each type of vehicle impact was compared at different impact velocities, ranging from20to50km/h.In order to investigate the effectiveness of the subsystem impact test for chest protection, the chest impact effective mass was calculated using MBS and FE analyses. In the MBS approach, the CPMi50th and JARI simple pedestrian models were used. The THUMS pedestrian models were used in the FE simulations. According to the chest impact conditions in vehicle impacts, the upper legform impact test and simulations were carried out to evaluate the possibilities in chest protections. The simulations of the headform impact test were carried out at the rib contact area in vehicle-to-pedestrian impacts. The correlations between the accelerations of the headform impactor and rib deflection were analyzed in each type of vehicle collision.The results from the subsystem impact tests indicated that a relationship exist between headform impact test and ribcage deflections for the medium car and one-box vehicle, demonstrating that the headform impact test is effective for evaluating the stiffness of the structure that may cause chest injuries. However, the upper legform impact test was not appropriate due to the limitation of its biofidelity and dimensions.The lower extremity injury mechanism was investigated by using a bending moment diagram. The injury risk of the lower extremity was also evaluated and compared according to vehicle types. In addition, the pelvis loadings and fracture risk were analyzed. The results from the simulation illustrated that the bending moment diagram is useful for understanding the kinematic behavior and injury risk of the lower extremities. The kinematic behavior of the lower extremity depends on the front shape of the bumper. The lower extremity is at a high injury risk if struck by the medium car and SUV. The loading and injury types of the pelvis depend on the structure shapes and stiffness as well as the impact velocity. The pelvis is a high-injury risk region in one-box vehicle and SUV impacts.The effect of the vehicle impact velocity on pedestrians’injury risk demonstrated that a significant decrease in injury risk could be achieved if the impact velocity is decreased to30km/h or less, particularly for the head injury risk. However, the injury risk of the ribcage was still high when struck by a one-box vehicle even at low impact velocities.Analyzing the simulations of vehicle-to-pedestrian impacts clearly demonstrates that the pedestrian kinetic behavior depends on the front shape of the vehicles. The loadings to the chest are influenced by the vehicle’s shapes and structure. The stiffness of the structure obviously influences chest injuries. The loadings’mode of the pelvis depends on the vehicle shapes. The bending moment diagram of the lower extremities also depends of the bumper shapes and its structures. The injury risk to pedestrian can be significantly reduced by lower velocities; indeed, reduced vehicle velocity and vehicle structure modifications are effective for pedestrian protection.