| During a vehicle-to-vehicle impact, the occupants of the vehicles are protected by the use of seat-belts and airbags. But for pedestrians, there is not such type of protection. Therefore, pedestrians are the most vulnerable road users and it could lead to the severe injuries or loss of lives in accidents. With the rapid development in the field of auto industry and safety investigation, investigation of pedestrian safety has been attracting more and more attentions and has become a very important topic. The head is the most commonly injured body region in fatal vehicle-to-pedestrian crashes, while lower extremity injuries often resulted in long term disability. Therefore, based on massive literature review, this paper focused on the study of injury epidemiology of pedestrians, protective measures and evaluation methodology for lower extremity,and injury mechanisms of pedestrian lower extremity using statistic analysis, multi-body analysis, and finite element method. The main contents of this paper are listed as follows:1. Investigation of injury epidemiology of pedestrians. In this study, the NASS PCDS database was used to investigate upright standing pedestrian with an age of 14 and older and a height of 1.5 m and taller sustaining MAIS 3+, AIS 3+ head, AIS 3+ torso, and AIS 2+ lower extremity injury odd ratios for a crash factor (impact speed), pedestrian factors (age, gender, height and weight), and vehicle factors (BCH - Bumper Central Height, BL - bumper lead, FTTPH - ground to Front/Top Transition Point Height, RHOD - Rear Hood Opening Distance) in vehicle-to-pedestrian frontal crashes using a logistic regression method. The results show that impact speed is a statistically significant predictor for AIS 3+ head, AIS 3+ torso, and AIS 2+ lower extremity injury odds. Pedestrian age is also significant for AIS 3+ torso and AIS 2+ lower extremity injury odds, but not for AIS 3+ head injury odds. Pedestrian weight is a significant predictor for MAIS3+ and AIS 2+ lower extremity injury odds. FTTPH is a statistically significant predictor for pedestrian AIS 3+ torso injury odds. Vehicles with higher FTTPH and more vertical frontal structures are aggressive to pedestrians, especially regarding injuries to the torso. RHOD is a statistically significant predictor for pedestrian AIS 3+ head injury odds. A very short RHOD would be more likely to lead the pedestrian to impact the windshield and windshield frame, thus increasing the head injury risk. The study findings will give general directions for designing experiments investigating injury biomechanics and designing the pedestrian friendly vehicles.2. Investigation of protective measures and evaluation methodology for lower extremity. A vehicle-pedestrian impact simulation model was developed based on the multi-body pedestrian model with advanced knee structure by Dr. Yang J.K., which has a good biofidelity. The influence of vehicular front structure parameters (such as bumper height, bumper width and bumper lead) on shearing displacement of the knee, bending angle of the knee and tibia acceleration were investigated. Some measures, such as lowering of the height of the bumper, increasing of the bumper lead, and increasing of the vertical width of the bumper were proposed to improve pedestrian-friendly vehicular front structures. Furthermore, a series of simulation models of vehicles with different frontal structures and legform impactor proposed by EEVC had been developed. And an evaluation method with coefficients of injury risks for pedestrian safety was proposed, for the shortcoming of evaluating protective performance of vehicles with different frontal structures. The simulation results were divided by the technical indexes respectively to get the coefficients of injury risks. According to the sum up values of the coefficients, the ranks of the protective performance of different vehicle-front structures were obtained. The rank results indicated that the simulation of legform impactor tests can match the results of the simulation of vehicle-pedestrian impact well. Therefore, the ranking technologies based on the injury risk coefficients can be used to assess the protective performance of the vehicle-front structures. Meanwhile, the tibia acceleration did not reflect tibia impact force well, and the shearing displacement of the knee was bigger than that obtained from the legform impactor tests. The EEVC legform impactor needs to be improved in the future.3. Improvement and validation of finite element model of pedestrian lower extremity and coupling it with pedestrian finite element model. The previous finite element models of pedestrian lower extremity have some disadvantages, such as lack of experimental data, overall validations, and coupling with other parts of human body. According to the typical and present comparatively new biomechanics experimental data, material parameters of bones and ligaments of lower extremity were improved and contact type and parameters among tissues were revised on the basis of THUMS model. Thereafter, the improved model of pedestrian lower extremity was validated exhaustively against experimental results: i) quasi-static and dynamic 3-point bending tests of femur, tibia, and fibula with different impact locations and orientations, ii) dynamic 3-point bending tests of thigh and leg with different impact locations, iii) tension tests of knee ligaments (MCL, LCL, ACL, and PCL) at different strain rate, iv) 4-point bending tests of the knee and 3-point combined tests of the knee, v) lower extremity bending tests at low and high impact velocities. The analysis demonstrates that the simulation results of the improved model can match the corresponding experimental data well, and it can also simulate the biomechanics characteristic of pedestrian's lower extremity thoroughly. Simultaneously, it can be used to assess the protective performance of the vehicle-front structures. At last, the verified lower extremity model was coupled with the other parts of THUMS model to verify the validity of the whole pedestrian model.4. Investigation of injury biomechanics tests based on load characteristics of pedestrians. Dynamic three-point bending tests were performed utilizing pig legs at higher velocities like those in actual vehicle-to-pedestrian crashes. The results indicated that increasing degree of impact force between pig legs in tests and pedestrian femur in simulations was consistent with impact velocity increasing, and strain rate of pedestrian long bones was accurate. Furthermore, valuable experience was gained for conducting experiments on injury biomechanics of human body from these tests.5. Investigation of injury mechanisms of pedestrian lower extremity and risk factors affecting pedestrian lower extremity based on finite element model. Due to the detailed kinematics and dynamics response of tissues of pedestrian during crash simulation can be obtained from the finite element analysis, the validated FE models of pedestrian and vehicle were used to develop the crash model of pedestrian with a SUV as well as a sedan car. And then the biomechanical responses of femur, tibia, fibula, and knee joint were analyzed in detail based on those models. The injuries and its locations of bones and/or ligaments could be predicted with the results of bending moments of long bones and knee joint and stress/strain diagrams. The corresponding structure resulting in injuries of the pedestrian will be found out easily. Furthermore, the simulation results could be used to improve pedestrian safety of vehicular front structures. Simulation results show that, impact velocity played an important role on the injury of pedestrian lower extremity, while the influence of vehicle style was relatively not significant. Therefore, the structure parameters of frontal end of vehicles need to be taken as the main factors to study the injury risk of pedestrian lower extremity in future study. Furthermore, it is important to control the relative position between femur and tibia on biomechanical experiment of the knee joint, because the shearing displacement of the knee joint affects the maximum bending moment of the knee joint. |
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