Jun 14 2007
Light-metal rims are popular with car fans. Fiber-composite materials offer a new option, but these wheels have not yet been approved due to a lack of suitable testing methods. Researchers can reliably simulate how they will cope with hitting the curb and negotiating bends.
Automobile wheel rims have to be able to take a lot of punishment, including curb impact, potholes and road salt. Before they can be fitted to cars, therefore, they have to undergo reliable testing to ascertain whether they are strong enough and meet the quality requirements. Standardized testing methods exist for conventional rims made of steel or aluminum. Fiber-reinforced plastics have reached the stage where they could be used for making wheels, but there is doubt as to whether the testing methods for metal wheels can be applied to them. As a result, plastic wheels have not yet been approved by Germany’s technical inspectorate, the TÜV. Manufacturer BTE Hybrid-Tech GmbH has plenty of reason to be confident, however: the wheel rims made of fiber-plastic composite which it produced in 2001 have successfully come through tests over distances of 250,000 kilometers. The advantage of plastic wheels is that the material combines high specific strength with low weight, and it looks good as well. It consists of two main components – a matrix and reinforcing fibers. Through their interaction the two components achieve better properties in the composite material than they do separately.
The legal situation for plastic wheels could change soon, giving the designers of lightweight rims more latitude in their choice of material. Working in conjunction with colleagues from four other Fraunhofer institutes, research engineers at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt are developing a simulation method which reliably predicts the quality of the rims. “First we produce a computed tomography image of the wheel,” explains Dr. Andreas Büter, Head of Department at the LBF. “The image enables us to ascertain the length, alignment, curvature and density of the fibers. These parameters are crucial for the strength and load capacity of the material.” On the basis of these results the research scientists simulate the microstructure of the material, a virtual unitary cell in which they can for the first time depict arbitrary material configuration. Applying the results from the unitary cell, they use a numerical component model to simulate how the wheel will handle bends in the road or hitting the curb and how it would behave in a crash. “We calculate the stress and elongation occurring in the material under various loadings,” says Büter. “We know from experiments what stresses the material can withstand without being damaged and what elongations will damage it. This enables us to make a reliable assessment of plastic wheels.”