Fuel Tank Design at Inner Pressure V.A. Kolupaev1,∗, M. Bulla2 1 Mechanics & Simulation, Department of Plastics, Fraunhofer Institute for Structural Durability and System Reliability (LBF), Schloßgartenstr. 6, D-64289 Darmstadt, Germany 2 Altair Engineering Inc., Nasdaq, Josef-Lammerting-Allee 10, D-50933 Cologne, Germany ∗ vladimir.kolupaev@lbf.fraunhofer.de Keywords: Burzyński-plane, triaxiality, Poisson’s ratio Multiaxial stress states in components are evaluated using stress analysis methods: stress angle, triaxiality factor, equivalent stress. However, when designing with VON MISES criterion adjusted based on the tensile test, under-dimensioning may occur for multiaxial tensile loading. Such loading cases are crucial in the design of the tanks under internal pressure. Generalized strength criteria require data from additional tests: shear, compression, equibiaxial tension, etc. However, these tests are often costly and may be subject to excessive scatter, sometimes beyond the accuracy of the design. The basic idea of this work is to define the safety at the multiaxial tensile loading in the absence of measured data. For this purpose, the deviation 5. . . 10% from the predicted value with VON MISES criterion is set at the balanced biaxial tensile loading. The required parameter is calculated using this deviation and can be expressed as a plastic POISSON’s ratio. Three adapted strength criteria are implemented for design: BERG, original HUBER and modified HUBER [1, 2] These criteria describe pressure-insensitivity under compression but limit the hydrostatic tensile stress. The limit surfaces represent the scaled VON MISES cylinder in the principal stress space C1 -capped with SCHLEICHER ellipsoid at the different cross sections I 1 =σ T 0 (uniaxial tension, BERG criterion: no uniaxial compression / uniaxial tension difference), I1 =σ S 30 (shear, HUBER criterion) or I1 =σ C 60 (uniaxial compression, modified HUBER criterion). The subscripts describe the stress angle θ of the corresponding loads. The stress analysis is performed in a post processor of commercial FEM software Altair Radioss® and Ansys®. In order to determine the weak points of the designed component, the deviation of the proposed stress criteria from VON MISES criterion is varied in the dimensioning phase. The impact of this method is demonstrated with an automobile fuel tank made of HDPE. The selection of the criterion depends on the modelling concept: conservative (BERG), universal (HUBER) or progressive (modified HUBER). Based on the introduced criteria, optimization of thermoplastic components can lead to new design concepts. Our evaluations with the discussed criteria have shown, that approx. 10% deviation from the predicted value with VON MISES criterion at the equibiaxial tension can be recommended as a first step in the pre-design. These results are transferrable to similar components. 24
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