Modelling of Factors Influencing Foot Ankle Responses and Injuries in Car Crashes

Doktorsavhandling, 2003

Foot and ankle injuries in frontal car collisions are believed to be caused mainly by occupant contact with the floor and toepan, occupant interaction with the pedals, and the lower leg being deformed between the floor and knee bolster. A better understanding of the causes of foot and ankle injuries is required to design cars for improved safety of the legs. In this study, mathematical models were developed, evaluated by crash tests, and used in parametric studies to examine the factors that influence the injury outcome. The models will serve as a basis for the development of principles for countermeasures effective in a variety of crash situations. Dynamic tests with biological materials were conducted to obtain more information about injury tolerances of the human ankle in dorsiflexion loading. Mathematical vehicle-occupant models were developed in the multi body and finite element software MADYMO. The models were assessed by both crash tests and sled tests. Parametric studies were made using the mathematical models according to statistical methods. Two approaches were used in the mathematical modelling: investigation of the toepan design, and examination of the safety performance of the same car with and without safety systems. All of the toepan design parameters tested were found to have an effect on lower leg responses: the dummy responses also differed significantly depending on the foot position, occupant size and crash severity. To design cars for improved lower leg safety, efforts should be made to reduce the toepan acceleration and intrusion distance in particular. However, the simulations in this study showed that there is an important interaction between the timing of the peak acceleration and the intrusion distance: when the right foot of the driver was resting on the toepan at the time of the crash, the axial force in the right tibia increased with toepan acceleration; the intrusion magnitude was of minor importance. However, when the right foot rested initially on an accelerator that was displaced during the crash, the axial force increased with the intrusion magnitude, and toepan acceleration was of minor importance. Moreover, this study showed that the local deformation of the toepan is a likely cause of lower leg injuries and must be reduced. The interaction effects of the parameters investigated were also obvious in the examination of safety systems. Although an airbag in the toepan area might considerably reduce the tibial responses, the designers have to be aware that using a foot airbag optimized for a severe crash situation can actually increase the load on the lower leg in less severe crash situations. In this study, the foot-ankle load increased in 50% of all crash situations. Therefore, it is desirable to be able to measure the occupant size and crash severity and to adjust the airbag inflation in a crash situation. In the biological tests, lower leg specimens were struck at the plantar side of the foot and the ankle responses were measured. The average dorsiflexion angle at the time of injury was 42° and the average ankle moment in tests with fractures was 61 Nm. The mathematical modelling techniques used in this study proved to be useful tools for investigating lower leg responses in car crashes. The models developed can simulate the complex shape of toepan intrusion and the interaction between the occupant and the vehicle. The investigation of the causes of injury provided valuable information for designing vehicles with better lower leg safety.