Injury Biomechanics in Car-Pedestrian Collisions: Development, Validation and Application of Human-Body Mathematical Models

Doctoral thesis, 1997

The aim of this study was to develop and validate human-body mathematical models which can be used to simulate the dynamic responses of pedestrians in a car impact. The main focus has been on simulations of bumper impact to the lower extremities, since lower extremity injuries most frequently occur in car-pedestrian accidents and result in long-term consequences and high social costs. Two approaches were used: (1) modeling with the multibody system (MBS); (2) modeling with the finite element method (FEM). Different models were developed, including a human-like knee joint MBS model, a breakable leg MBS model, a complete human-body MBS model, and a lower extremity skeleton FEM model. The MBS models were implemented by the MADYMO 3D program, and the FEM model by DYNA3D program. The models were validated by tests with biological subjects. The models enabled simulations of the responses of the lower extremity to a lateral bumper impact to be carried out. The injury mechanisms and the prediction of injury risk of the knee-leg complex was the focus of this study. Furthermore, simulations of full scale car-pedestrian impacts were done to predict the risk of pedestrian injuries in car accidents and to investigate influences of car-front parameters on the risk of pedestrian injuries. The human-like knee model gives insights into the injury mechanisms of the knee in a lateral impact. The knee responses were analyzed in terms of ligament strain, condyle contact force, and transverse dislocation between articular surfaces. The correlation between the outcome from simulations and injuries observed in tests with biological subjects was established. Calculated ligament strains greater than 20% were related to ligament failure in tests, and condyle contact forces greater than 6 kN were related to condyle fracture. Predicted transverse dislocations of 8-9 mm between articular surfaces in simulations confirmed the findings in the high-speed film analysis of the knee responses to lateral impacts at 15-20 km/h. The knee-injury mechanisms can be summarized as ligament tension and condyle compression due to a combination of shearing, bending, and torsional loading applied to the knee joint. The knee injuries are dominated by bending load. The breakable leg model filled a gap in modeling leg fracture and improved the sensitivity of the pedestrian model. The simulation of the knee responses associated with leg fracture in car-pedestrian impacts indicated that the impact response and injury mechanism of the knee in a lateral bumper impact with the upper part of the leg are both dependent on whether or not the leg is fractured. For a more realistic model of the lower extremity skeleton, the FEM model was used. It contributed to a better understanding of the impact responses of the knee-leg complex by means of an analysis of stress distribution within the simulated structures. The stress analysis with the FEM model indicated that the calculated tensile stress of 160 MPa in a lateral impact at 31 km/h correlates well with the ultimate tensile strength of the leg bone determined in biological tests. The complete human-body model was used to simulate car-pedestrian impacts. The effects of the car-front on pedestrian responses were evaluated in terms of the bumper height and stiffness, the bumper-lead distance, hood-edge height and stiffness. The effects of bumper and hood edge on knee-leg responses were identified. It was found that the head responses are significantly influenced by the height of the hood-edge and less influenced by the bumper. The developed models demonstrated capabilities for predicting the risk of pedestrian injuries in an impact with a vehicle. For instance, the breakable leg model can be used to predict the risk of long bone fractures. From the knee model, the forces and moments transferred through the knee, and the strain of the knee ligaments can be calculated to evaluate the knee failures. For the head, the linear acceleration, the HIC value and the angular acceleration can be calculated to predict risk of head injuries. The models are thus valuable tools to acquire better knowledge of impact biomechanics in car-pedestrian accidents and help assess the performance of vehicle front structures and develop safety countermeasures.