Finite element simulation of the tube hydroforming process - bending, preforming and hydroforming

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Finite element simulation of the tube hydroforming process - bending, preforming and hydroforming

Trana, K.

This paper covers finite element simulations of the complete hydroforming process for the lower part of an A-pillar for a car. Simulations were carried out for the bending, preforming and hydroforming process using the MPP version of the explicite code LS-DYNA. The study focuses on the development of a practical simulation procedure for the entire hydroforming process and how the bending and preforming operation influence the hydroforming result. The simulation results were also compared with experimental results with respect to the thickness distribution and the agreement proved to be good. The study shows the importance of the bending operation and that it is necessary to take it into account when performing hydroforming simulations. In this case, the mandrel geometry (cylindrical, flexible) in the bending tool plays an important role. Changing from a cylindrical mandrel to a mandrel with balls connected with ball links determines if the tube will manage the hydroforming without bursting. The evaluation of the forming limit using a strain based forming limit diagram (FLD) should be done with great care because of the broken strainpaths. A crude fracture criterion is the necking tendencies, which does not admit any use of safety margins, however. The necking effect is also highly mesh dependent. Thus, a stress based FLD might be a better approach than the conventional forming limit curve (FLC). The hardening model used surely influenced the simulation results due to broken strainpaths. If mixed hardening instead of pure isotropic hardening was used, the Bauschinger effect could be taken into account. Another way to improve the simulation results would be to use material data from tensile tests of the tubes instead of using sheet properties. Especially the tube yield stress differs from the sheet yield stress. This study indicates that processes with multiple forming operations can be simulated with good accuracy.