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Reasons for Scatter in Crash Simulation Results

In crash simulation, small changes of the model or boundary conditions may result in substantial changes of the simulation results. For a BMW car model, the node positions of the crashed model show differences of up to 14 cm between several executions on a parallel machine for the same input deck. For the Dodge Neon testcase, small variations of the barrier position result in substantial scatter of the intrusion. Detailed investigations of several models have shown, that in some cases numerical effects might be responsible for the scatter in the results. In most cases, however, the instable behaviour of the simulation results is caused by bifurcations. These bifurcations result from numerical algorithms or are a feature of the car design. In the Neon model the scatter is a result of the interaction between the axle and the engine block. In the case of the BMW car model, the scattering of the simulation results is a direct consequence of buckling of the longitudinal rail. A slight redesign of this part causes stable results for parallel machines. Stable crash behaviour of a car model is a design target for the following reasons: • Simulation results might be misleading, when the impact of changes of the model or model parameters is investigated. • The numerical model is always only an idealized representation of the real car design. A stable crash behaviour simplifies the prediction of the crash behaviour of the real car from simulation results for the idealized model. • Smaller bounds for the scattering of the characteristic crash values will improve the possibilities of the engineer to find the best compromise for the car design with respect to the targets of the different load cases. Due to the nature of crash simulation many parts might show instable behaviour. Usually, only a small subset has a real impact on those values, which measure the crash behaviour (like intrusion). Measuring the scatter of simulation results for these characteristic values is a first step. In order to improve the design, it is necessary to trace this scatter back to its origin in space and time. DIFFCRASH is a tool, which allows one to measure scatter and to trace this scatter back to its origin. It allows the engineer, to understand the mechanisms of propagation and amplification of scatter during the crash itself as a basis for the improvement of the stability of the car design.