Vehicle stiffness is one of the three major factors in vehicle to vehicle compatibility in a frontal crash; the other two factors are vehicle mass and frontal geometry. Vehicle to vehicle compatibility in turn is an increasingly important topic due to the rapid change in the size and characteristics of the automotive fleet, particularly the increase of the percentage of trucks and SUVs.
Due to the non-linear nature of the mechanics of vehicle structure, frontal stiffness is not a properly defined metric. This research is aimed at developing a well defined method to quantify frontal stiffness for vehicle-to-vehicle crash compatibility. The method to be developed should predict crash outcome and controlling the defined metric should improve the crash outcome. The criterion that is used to judge the aggressivity of a vehicle in this method is the amount of deformation caused to the vulnerable vehicles when crashed with the subject vehicle.
The methodology was developed to evaluate the force deformation data from National Highway Traffic Safety Administration (NHTSA)'s New Car Assessment Program (NCAP) for vehicle to vehicle frontal crash compatibility. This methodology relied on a simplified single-mass single-structure model. Crash simulations of models of vehicles of different mass and structure characteristics were conducted.
It was found that the deformation in the vulnerable vehicles depended on the force levels of the aggressive vehicles more than the approximated stiffness values. Vehicles frontal structures were represented in energy dissipated vs. maximum force reached graphs instead of a force vs. deformation graphs. A less aggressive -more compatible- vehicle would dissipate more energy in deforming its frontal structure before reaching a certain force level. Conversely, a more aggressive vehicle would reach high force levels before dissipating a lot of energy.
It is recommended that aggressive vehicle should dissipate half of the energy of a particular severity crash (25-30 mph) before reaching a certain amount of crush forces. These forces should be lower than the force levels required to collapse the occupant compartment of the vulnerable vehicles (500-600kN).