Research On Pedestrian Head-to-ground Impact Injury Mechanism And Risk In Vehicle Collision

In-depth investigation of road traffic shows that pedestrian head-to-ground impact(ground impact)is one of the important sources of pedestrian head injuries.However,the diversity of ground impact mechanisms always results in more unpredictable head injuries as compared to the head-to-vehicle impact(vehicle impact).Based on the injury biomechanics literature research,this dissertation made full use of the advantages of multi-disciplinary by combining the multi-body system analysis,high-accuracy finite element analysis and video information of the real-world vehicle accidents.This article aimed to do some exploration on the pedestrian landing kinematics,vehicle front-end structures and safety countermeasures,in-depth ground impact accident reconstruction and the effectiveness evaluation of head injury criteria.The main research contents and results are as follows:(1)Research on pedestrian-to-ground impact kinematics and head injury risk in vehicle collision under uncertain conditions.The purpose of this study is to investigate the mechanism of the effects of vehicle frontal structure,vehicle impact velocity,and pedestrian size and gait on pedestrian landing kinematics and head injury risk.A total of 600 simulations were performed using the MADYMO(MAthematical DYnamic MOdel)multi-body system and four different sizes of pedestrians and six types initial gait were considered and impacted by five vehicle types at five impact velocities respectively.The pedestrian rotation angle ranges(PRARs)were defined to identify and classify the pedestrian landing kinematics.It found that the pedestrian landing posture shows a periodic change with the increase of the vehicle velocity.The PRAR b has the highest risk of head landing injury,while d has the lowest.There was a significant correlation between vehicle velocity and HIC(Head Injury Criterion)(car),but the HIC(ground)was un-correlation with vehicle velocity.In the low velocity collisions,the pedestrian head injury caused by the secondary was more serious.In the high velocity collisions,pedestrian head injury caused by the primary impact was significantly higher than the secondary impact.Compared with adults,children have a higher risk of headto-ground collision injuries.(2)Effects of vehicle front-end structural parameters and safety countermeasures on pedestrian head injury risk during ground impact.The purpose of this study is to clarify whether the vehicle front-end structural parameters and pedestrian safety countermeasures can reduce or prevent pedestrian injuries caused by ground impact.A total of 252 multi-body simulations of vehicle-to-pedestrian impacts were conducted with considering five vehicle front-end structural parameters and three types of pedestrian safety countermeasures.It was found that,in the vehicle impact,BLEH shown the most significant effect on the head HIC(car),and the lower BLEH had the better protection effect on head.Head injury risk caused by primary impact could be significantly reduced in vehicle equipped with a pop-up hood and an external airbag.The protection effect of the three kinds of pedestrian safety countermeasures on pedestrian head injury are:the vehicle equipped with an external airbag are better than the vehicle equipped with a pop-up bonnet and better than the baseline vehicle.In ground impact,BLEH shown the most significant effect on pedestrian rotation angle.BLEH reduction would lead to dramatic changes in pedestrian rotation angles.The vehicle equipped with a pop-up bonnet and an external airbag could lead to an increase in pedestrian flying height,an increase in potential energy of pedestrian impact on the ground,and an increase in risk of head-ground impact injury.(3)Research on in-depth accident reconstruction of ground impact.The purpose of this study is to explore a method for the in-depth accident reconstruction of ground impact which is based on video information and coupled with various numerical analysis models,so as to provide a more scientific research method to clarify the mechanism of head-to-ground impact injury.Firstly,the multi-body dynamics model CPM(Chalmers Pedestrian model)was used to reconstruct the whole accident process,and then the reconstructed kinematics were compared with the video information to ensure consistency.Secondly,the kinematic posture and dynamic parameters of the multi-body model at the moment before CPM-to-ground impact were extracted and loaded into the THUMS(Total HUman Model for Safety)(Ver.4.0.2)finite element model for head-to-ground impact injury reconstruction.The reconstruction method in this research fully utilized the advantages of video accidents and the strengths of each numerical analysis model,which significantly improved the reliability of the reconstruction results.(4)Effectiveness evaluation of head injury criteria and injury thresholds.The aim of this study was to evaluate the effectiveness of various head injury criteria and injury thresholds in prediction of severe head injuries caused by ground impact.Ten VRUs(Vulnerable Road Users)accidents with video information and severe head injury records were selected and reconstructed.The head kinematics were used to calculate injury risks for seven head kinematics-based criteria and seven brain tissue injury criteria.The injury risks predicted through simulations were compared to the severe head injury thresholds and the results indicated that angular acceleration,linear acceleration,HIC,coup pressure,MPS(Maximal Principal Strain)and CSDM0.15(Cumulative Strain Damage Measure)have a "good" prediction capability for severe head injuries caused by ground impact.The shear stress,contrecoup pressure;and CSDM0.25 show a "moderate" prediction.The angular velocity,HIP(Head Impact Power)and BRIC(Brain Injury Criteria)showed a "poor" prediction and the BrIC(Brain Injury Criteria),VM stress and DDM(Dilatation Damage Measure)showed a"worse" prediction.The injury thresholds for the "good" prediciton are,angular acceleration=10,000 rad/s2,linear acceleration=200 g,HIC=1440,coup pressure-234 kPa,MPS=0.9,CSDM0.15=55%.