The automotive industry is required to meet improved fuel efficiency standards
and stricter emission controls. Aluminum tube hydroforming is particularly well suited in
meeting the goals of lighter, more fuel-efficient and less polluting cars. Its wider use in
industry is hindered however by the reduced ductility and more complex constitutive
behavior of aluminum in comparison to the steels that it is meant to replace. This study
aims to address these issues by improving the understanding of the limitations of the
process as applied to aluminum alloys.
A series of hydroforming experiments were conducted in a custom testing facility,
designed and constructed for the purposes of this project. At the same time, several levels
of modeling of the process, of increasing complexity, were developed. A comparison of
these models to the experiments revealed a serious deficiency in predicting burst, which
was found experimentally to be one of the main limiting factors of the process. This
discrepancy between theory and experiment was linked to the adoption of the von Mises
yield function for the material at hand. This prompted a separate study, combining experiments and analysis, to calibrate alternative, non-quadratic anisotropic yield
functions and assess their performance in predicting burst. The experiments involved
testing tubes under combined internal pressure and axial load to failure using various
proportional and non-proportional loading paths (free inflation). A number of state of the
art yield functions were then implemented in numerical models of these experiments and
calibrated to reproduce the induced strain paths and failure strains.
The constitutive models were subsequently employed in the finite element models
of the hydroforming experiments. The results demonstrate that localized wall thinning in
the presence of contact, as it occurs in hydroforming as well as other sheet metal forming
problems, is a fully 3D process requiring appropriate modeling with solid elements. This
success also required the use of non-quadratic yield functions in the constitutive
modeling, although the anisotropy present did not play as profound a role as it did in the
simulation of the free inflation experiments. In addition, corresponding shell element
calculations were deficient in capturing this type of localization that precipitates failure,
irrespective of the sophistication of the constitutive model adopted. This finding
contradicts current practice in modeling of sheet metal forming, where the thin-walled
assumption is customarily adopted. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6557 |
| Date | 19 October 2009 |
| Creators | Korkolis, Ioannis |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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