JEC COMPOSITES MAGAZINE - Issue #45 - November/December 2008 - 26

FEATURE “Automotive” Non-linear simulation of a lightweight composite rim Formula Student is a world-wide racing series where student teams build single-seater racing cars and compete in a series of races. For their car, the TU-DART team developed a carbon-fibre composite rim, using finite element modelling to optimize rim geometry and laminate structure. DR MARTIN FLEISCHHAUER R&D MANAGER COMPOSCIENCE JAN KOENEN RACE ENGINEER TU-DART-RACING I n the Formula Student programme, teams of student sponsored by industrial organizations and university institutes develop high-tech racing cars. The vehicles are able to hold their own in terms of engineering and performance against those competing in the "big racing series". Besides vehicle performance, aspects being rated are vehicle styling and the concept’s innovative character, economic efficiency and marketability. Carbon-fibre monocoque chassis have become a standard feature in vehicle design, and the use of CFRP is now extending to components such as drive shafts, transverse control arms, and peripheral engine parts. For its latest racing car, called the Gamma 08, the Darmstadt Technical University’s TU-DART team has developed a 38-cm carbon-fibre composite rim and introduced it on the road (Fig. 1). The objective was to replace the current aluminium rims and to further reduce the rim weight by 3 kg. The team’s design work was sponsored by German engineering service provider compoScience, a specialist in the design and calculation of fibre-reinforced components. The first stage consisted in defining the service loads on the rim. To this end, the loads and load frequencies were determined during measurement runs. These test runs established that most of the load on the rim − more than 1.7 g − is caused by transverse acceleration stress. Maximum load combinations were determined and the frequency of the loads over the service life of the rim was calculated based on the resulting data. Transferring the wheel loads into the rim via the tyre was a particular challenge for the designers. To avoid having to model the tyre, the loads were distributed to the two rim flanges using a distribution function that was determined experimentally in advance with the help of a strain-gauged rim. Apart from the service loads, particular attention was paid to the loads on the rim during tyre mounting and demounting. Neglecting these loads during the design phase could lead to rim damage during assembly. Finite element modelling To optimize the rim-geometry details for the three-spoke design, an isotropic three-dimensional FE model with constant wall thickness was created at the outset, to which a combination of maximum side load and maximum vertical wheel force was introduced. To optimize the initial geometry, the main stresses were analysed, and local stress maxima were reduced by modifying the rim geometry. In particular, isotropic calculation of the geometry model served to plan the laminate structure. Optimum fibre orientation was determined based on the main stress trajectories (Fig. 2). Following geometry optimization, another FE model was created − in this case, a model with layered shell elements. For this, the rim was divided into areas with constant component thickness and constant layer structure, and a first laminate design was created based on the isotropically determined main stress directions. The rim was built up from unidirectional HT carbonfibre prepreg layers 0.3 mm thick. Woven fabric prepreg of the same thickness was used for the visible layer. Fig. 1: The Formula Student racing car of Darmstadt Technical University. jec composites magazine / No45 November-December 2008

JEC COMPOSITES MAGAZINE - Issue #45 - November/December 2008

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