Characterization and modeling of a vehicle wheel under crash-relevant conditions

The challenge of crash simulation

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In case of frontal crash, vehicle wheels play an important role for the vehicle crash behavior, especially in case of small overlap crash, because this is where the wheel may be pushed into the passenger cabin.

Crash simulations help to reduce the number of elaborate and costly crash tests. For this purpose, valid simulation models have to be created for the single vehicle components.

Within the scope of a bachelor thesis, a vehicle wheel was experimentally characterized and numerically simulated at Fraunhofer EMI in cooperation with the Karlsruhe Institute of Technology (KIT). The objective was to make a reliable description of the deformation and failure behavior of the wheel under crash-relevant loads possible and to apply this to a crash simulation.

Material model

A 16-inch aluminum wheel was chosen for the described project. A thorough analysis of the mechanical properties of the rim material showed that the measured material parameters depend on the position. There are mainly two reasons why the component is very heterogeneous:

• During the production of a cast component, differing material structures and strengths emerge from the cooling process.
• In the course of this, cavities and bubbles form within the material. 

It was possible to confirm these insights microscopically by micrographs of the structures and macroscopically within the samples. For the characterization experiments, samples were taken from the rim spokes, band and hub. In all three areas, differing material properties were measured.

Furthermore, the experiments showed that especially the failure behavior under tension differs from that under pressure, as no failure was observed under compressive loads.

Experiment at the component crash-test facility

The numerical model could be validated thanks to  a component experiment at the component crash-test facility of Fraunhofer EMI. The test setup was designed such that the wheel had a maximum area of freedom for deformation. During the test, the impactor force acting on the wheel was measured as well as the impactor path. At the same time, a high-speed camera recorded the component test.

Simulation and validation of the model

Experiment and simulation are in good agreement. The challenge to generate reproducible test results was successfully met.

This research emphasizes essential aspects of the modeling of massive cast components:

• The deformation differences shown by the different samples from rim, hub and spoke that were detected during the material analysis have meanwhile been integrated into the calculations.
• It is a further insight that the load case has to be considered with regard to triaxiality, especially, concerning failure under tension and pressure. To this end, the GISSMO damage model in LS-DYNA was used.
• As differing mesh sizes were used for the material modeling (0.5 millimeters) and for the component simulation (5 millimeters), the mesh-size dependency was treated by a so-called regularization curve.