Lange, Christian (2006) Experimental study and numerical simulation of the hemming process of automotive parts. PhD thesis Sciences et Génie des Matériaux, ENSMP - CEMEF Centre de Mise en Forme des Matériaux, ENSMP.
Abstract
Hemming is an assembly process using to join sheet metal parts for closures of automotive bodies such as side doors, hoods and tailgates. An outer sheet metal is bent onto an inner sheet metal in three steps: flanging, pre-hemming and hemming. But hemming generates a reduction in size of the outer panel (roll-in), aspect defects and some times cracks of the sheet metals. In order to minimize the development cost and time, PSA Peugeot Citroën for simulates numerically the process with the finite element code OPTRIS v6.1 based on a shell element formulation. Because of bad predictions of the simulations, the aim of this work is to improve numerical simulations by a better understanding and modeling of material model used for sheet metals and friction model between the outer part and the hemming dies. Because the radius of curvature of the outer sheet is comparable to its thickness, we estimate the performance of shell elements to model such large strains.
Uni-axial tensile tests coupled with a strain measurement system using correlation of digital images are performed with micro-hardness tests on hemmed parts. The aim is to determine the monotonic hardening curves at strain levels similar to those obtained in the extension part of the hemmed sheet (around 0.5 for 1 mm thick aluminum alloy sheets and 0.7 for 0.7 mm thick steel sheets). Otherwise, 4 point bending and bending/unbending tests are performed for the material characterization in non monotonic straining conditions similar to those in hemming. The tests show the relevance of the monotonic hardening laws deduced from tensile and micro-hardness tests. Moreover, they allow the Bauschinger effect characterization by the identification of the coefficients of mixed isotropic/kinematic hardening models.
An experimental device has been developed for the hemming tests of flat parts with straight edge. It permits the measurement of friction between the outer sheet and the hemming dies is set up. The tests show that the classical Coulomb friction model is sufficient to describe tribology in hemming. This characterization method of friction is adapted to the semi-industrial hemming tests of flat parts with curved edge.
The numerical simulations of hemming of flat parts with straight edge are performed with a mechanical model based on bending theory and the finite element codes OPTRIS v6.1 and FORGE2® (solid elements in plane strain conditions). A sensitivity analysis show the strong sensitivity of the roll-in to the friction coefficient and the necessity of modelling the several steps of the hemming process and of taking into account kinematic hardening in material model. Finally, the similarity of the numerical roll-in obtained with OPTRIS v6.1 and FORGE2® demonstrates that the shell elements are able to model hemming process.
| Item Type: | PhD Thesis (PhD) |
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| Thesis Supervisor: | Felder, Eric |
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| Date: | April 2006 |
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| Board of examiners: | Manach, Pierre-Yves and Rauch, Edgar and Auger, Patrice and Brunet, Michel and Massoni, Elisabeth |
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| Ecole Doctorale: | ED 364 SCIENCES FONDAMENTALES ET APPLIQUEES |
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| Discipline: | Sciences et Génie des Matériaux |
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| Collection (Fonds): | ENSMP |
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| Institution: | ENSMP |
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| Department: | ENSMP - CEMEF Centre de Mise en Forme des Matériaux |
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| Subjects: | 4. Materials Science, Mechanics and Mechanical Engineering |
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| Uncontrolled Keywords: | Hemming, Numerical simulation, Finite elements, Rheological tests, Bending, Friction, Sertissage, Simulation numérique, éléments finis, Essais rhéologiques, Pliage, Frottement |
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