Car Compatibility in Frontal Crashes: New Methods to Determine the Influence of Mass, Structure, Stiffness, and Geometry, and their Interactions on Injuries

Doctoral thesis, 1998

Vehicle-to-vehicle crash compatibility affects vehicle compartment acceleration and intrusion, which are the two main causes of car occupant injuries. A better understanding of compatibility effects is needed to establish safety priorities and improve occupant and partner protection. For this purpose, three new methods were developed to study individual compatibility effects on occupant and partner protection of car structure, mass, and local stiffness, respectively. Additionally, an integrated vehicle-occupant mathematical model was developed and used to estimate the relative significance of and interactions between vehicle mass, front structure and geometry. An epidemiology study was carried out with Volvo crash material to determine the individual effect of overlap amount on injury risk in frontal offset crashes. Age, gender and impact severity biased the injury effects of overlap amount. These biases were minimized using new techniques to sort the crash data, which resulted in a data-set of 654 cases. In crashes with an equivalent barrier speed > 32 km/h, most injured (40-43 %) were caused by frontal crashes with 1/3 to 2/3 overlap, whereas the highest injury risk was found for < 1/3 overlap at 62% for MAIS2+ and 44% for MAIS3+ injuries. The results indicate that a 40% overlap test is a satisfactory test condition to address occupant protection in frontal offset crashes. A new analytical model was developed to study individual mass effects on the average of occupant and partner fatality rates, as well as on the total number of fatalities. A vehicle fleet mass of 600 to 2000 kg was hypothetically changed over a period of 15 years, using different strategies. The results showed that a homogenous car fleet would have a 59% lower fatality rate. The fatality rate increased by 11% when the baseline cars were downsized by 300 kg, but the downsizing strategy could limit the number of fatalities during the transition. Mass effects were relatively low compared to those of inherent vehicle protection and impact speed. The higher fatality rate of a lighter fleet can be compensated for by narrowing the fleet mass distribution, by simultaneous improvements of inherent vehicle protection, or by impact speed reductions. Repeated frontal crash-tests against a load-cell barrier were conducted to measure local longitudinal and shear stiffness of the vehicle front structure and passenger compartment. The repeated tests well approximated the total crush of a high-speed test for similar energy equivalent speeds. However, the power-train significantly increased barrier force and compartment intrusion in higher speed tests. The new tests provide local stiffness data of front structure and compartment, which can improve crash reconstruction and for compatibility research. The test method assesses mass and (local) stiffness aggressivity. A new integrated vehicle-occupant multi-body model was developed and used in a multi-faceted compatibility parameter study. The model used the local longitudinal and shear stiffness from the repeated tests as input data. The model was validated with frontal crash-tests in terms of vehicle acceleration and compartment intrusion. It was used to examine the relative significance of and interactions between compatibility parameters on the average injury risk of occupant and partner, in frontal offset car-to-car and rigid barrier crashes. The parameters included vehicle mass, front longitudinal stiffness and bumper-level, and were increased by 20% in one of the cars. Stiffness was indicated as the main compatibility parameter with a 16.4% risk increase, followed by the bumper-level (6.2% increase). Mass effects were relatively low with an average risk reduction of 1.8%, while any combination of incompatibility reduced safety. A more compatible fleet could consist of relatively stiff light vehicles and soft heavy cars with similar bumper levels. The new research methods helped to identify important compatibility parameters and to study their individual injury effects on crash safety. In this way, the complexity of the compatibility issue was reduced. The new vehicle-occupant model was able to predict both vehicle acceleration and local passenger compartment intrusion. The model showed to be valid, flexible and simple, which supported its applicability and efficiency in compatibility research. The model can determine the relative significance of and interactions between various compatibility factors, and can be useful in optimization studies.