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Research Papers

Evaluation of Formability in Bending/Hemming of Aluminum Alloys Using Plane-Strain Tensile Tests

[+] Author and Article Information
Guosong Lin, S. Jack Hu

Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109

Wayne Cai

Manufacturing Systems Research Laboratory, General Motors R&D Center, Warren, MI 48090

J. Manuf. Sci. Eng 131(5), 051009 (Sep 23, 2009) (9 pages) doi:10.1115/1.3123316 History: Received May 24, 2006; Revised November 05, 2008; Published September 23, 2009

This paper presents a fundamental study for the characterization of formability in bending/hemming, or “bendability/hemmability,” of automotive aluminum alloys. Based on the strain/stress nature of the hemline (bent corner) surfaces, maximum surface strain was proposed as the hemming fracture criterion, and it is believed that bendability/hemmability of aluminum alloys can be directly approximated by the fracture strains from plane-strain tensile tests. The criterion was verified by comparing hemming and plane-strain tensile experiments on AA6111-T4 and AAx611-T4 with fracture strains measured, respectively, via a modified angled line method and thickness reductions. After a verification of material hardening laws, the identified failure criterion was implemented into 2D and 3D finite element hemming simulations for fracture predictions. The findings provide the basis for the design of aluminum hems under formality constraints, and it is expected that the fundamental theory and established methodology are applicable to bendability/hemmability evaluations for general sheet materials.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Three-step hemming process

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Figure 2

Hemline surface cracking

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Figure 3

Prebending using wrap bend tester

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Figure 4

Final hemming using MTS testing machine

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Figure 5

Principal of the ALM for strain measurement

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Figure 6

Verification of the M-ALM principle through FE simulation: (a) deformed line pattern and calculated strain at h/t0=3.5, and (b) maximum strain history calculated by the ALM and M-ALM at various hemming heights

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Figure 7

Evolution of strain and surface quality for a typical hemmability test

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Figure 8

Image process procedures for surface quality and strain analysis

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Figure 9

Fracture strain measurement for plane-strain tensile test

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Figure 10

Fracture strain measurement for uniaxial tensile test

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Figure 11

Fracture strains from hemming, plane-strain, and uniaxial tensile tests

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Figure 12

Extrapolations of flow stress curves for AA6111-T4 and AAx611-T4

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Figure 13

Simulated fracture strains and experimental measurements

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Figure 14

Failure criterion implementation in hemming simulations

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