Leg injuries are the most prevalent type of injury associated with vehicle-to-pedestrian collisions. Further, leg injuries can result in long term debilitation of the injured party. The potential for pedestrian leg injury with respect to the front-end design of an automotive vehicle is currently evaluated using a component impactor consisting only of a thigh, knee, and lower leg. The simplicity of this test allows for relatively inexpensive testing and repeatable results. However, the lack of upper body mass (UBM) is known to reduce the biofidelity of the legform's response.
This study investigated the effects of the UBM on the measured impact response of the lower extremity. Three-dimensional Madymo simulations were performed using full body pedestrian and leg models, which consisted only of a single leg with the pelvis, torso, head, and other extremities removed. Initially, five output measures were recorded for these models during simulated impacts. These measures, which include femur shear and moment, tibia shear and moment, and proximal tibia acceleration, have been shown to correlate to injury and are therefore important quantities in assessment of injury risk. The full body impact simulations were used to generate a target response. The properties of the UBM, or UBM design, include the mass, moment of inertia, and center of gravity (CG) height. By determining the UBM design which produced a leg model response most similar to the full body response, the optimum UBM design would be obtained. An optimization was performed using Madymizer, one of the programs within the Madymo suite, which determined the optimum design to be a 10.93 kg mass, with a moment of inertia of 0.0698 kg-m2, attached 0.462 m above the knee.
The optimum design would present many difficulties if fabricated and used in practice. As a result, a new design was sought which would still produce marked improvement in the legform's ability to assess vehicle aggressiveness while also being experimentally viable. Interaction plots were used to determine the correlations between the UBM design parameters and the output measures of the leg. A practical design space was obtained from these correlations. A series of simulations were then performed using combinations of these parameters. Evaluating the performance of each combination, as well as the geometric size and shape that it would require, a practical UBM design was selected. The practical UBM design consisted of a 4 kg mass, a moment of inertia of 0.03 kg-m2, with an attachment locating the CG at 0.40 m above the knee.
Finally, the robustness of the practical design was studied using vehicle-to-pedestrian simulations. The vehicle geometry was varied parametrically in order to assess whether the leg with the attached UBM more accurately assessed vehicle aggressiveness than the leg model alone could. The results showed that by including the practical UBM design, the leg model was more capable of vehicle assessment for all vehicle designs. Further, the leg with UBM model was capable of assessing bumper heights above the knee and accurately measuring femur shear and moment, whereas the leg model could not.