Research Papers

Formability Assessment of Prestrained Automotive Grade Steel Sheets Using Stress Based and Polar Effective Plastic Strain-Forming Limit Diagram

[+] Author and Article Information
S. Basak

Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
West Bengal 721302, India
e-mail: shamik.iit@gmail.com

S. K. Panda

Assistant Professor
Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
West Bengal 721302, India
e-mail: sushanta.panda@mech.iitkgp.ernet.in

Y. N. Zhou

Department of Mechanical and
Mechatronics Engineering,
University of Waterloo,
200 University Avenue West,
Waterloo, ON N2L 3G1, Canada
e-mail: nzhou@uwaterloo.ca

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received March 29, 2015; final manuscript received May 21, 2015; published online June 26, 2015. Assoc. Editor: Tetsuya Ohashi.

J. Eng. Mater. Technol 137(4), 041006 (Oct 01, 2015) (12 pages) Paper No: MATS-15-1073; doi: 10.1115/1.4030786 History: Received March 29, 2015; Revised May 21, 2015; Online June 26, 2015

Accurate prediction of the formability in multistage forming process is very challenging due to the dynamic shift of limiting strain during the different stages depending on the tooling geometry and selection of the process parameters. Hence, in the present work, a mathematical framework is proposed for the estimation of stress based and polar effective plastic strain-forming limit diagram (σ- and PEPS-FLD) using the Barlat-89 anisotropic plasticity theory in conjunction with three different hardening laws such as Hollomon, Swift, and modified Voce equation. Two-stage stretch forming setup had been designed and fabricated to first prestrain in an in-plane stretch forming setup, and, subsequently, limiting dome height (LDH) testing was carried out on the prestrained blanks in the second stage to evaluate the formability. The finite element (FE) analysis of these two-stage forming process was carried out in ls-dyna for automotive grade dual-phase (DP) and interstitial-free (IF) steels, and the σ-FLD and PEPS-FLD were used as damage model to predict failure. The predicted forming behaviors, such as LDH, thinning development, and the load progression, were validated with the experimental results. It was found that the LDH in the second stage decreased with increase in the prestrain amount, and both the σ-FLD and PEPS-FLD could be able to predict the formability considering the deformation histories in the present multistage forming process with complex strain path.

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Fig. 1

Proposed mathematical framework for construction of σ-FLD and PEPS-FLD

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Fig. 2

Comparison of forming limit of DP600 and IF steels: (a) experimental ε-FLD [22,23] and (b) estimated σ-FLD using Barlat-89 yield criterion in conjunction with hardening laws

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Fig. 3

Predicted (a) decoupled theoretical ε-FLD for different prestrain conditions obtained from σ-FLD and (b) the convergence of PEPS-FLDs of all decoupled ε-FLDs

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Fig. 4

Microstructure observed in scanning electron microscopy: (a) DP600 and (b) IF

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Fig. 5

The schematic of the tooling for the multistage forming operation: (a) in-plane prestraining and (b) out-of-plane stretch forming for LDH test (all dimensions are in mm)

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Fig. 6

Numerical process sequences for two-stage forming operation: (a) quarter symmetry model of in-plane stretch forming setup, (b) deformed specimen after 11.3% prestraining, (c) quarter symmetry model of LDH testing of prestrained blank, and (d) deformed dome indicating the location of maximum thinning and failure

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Fig. 7

Deformation path during LDH testing of DP600 as-received material plotted inside (a) as-received ε-FLD, (b) estimated σ-FLD, and (c) PEPS-FLD

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Fig. 8

The deformed cups obtained from LDH testing of different prestrained materials

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Fig. 9

Comparison of predicted LDH by different FLDs at different prestrained conditions: (a) DP600 and (b) IF

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Fig. 10

(a) Strain path, (b) stress path, and (c) PEPS path during forming of 6.3% prestrained DP600 material

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Fig. 11

Comparison of thinning development in DP600 and IF materials in LDH testing: (a) as-received material condition and (b) 11.3% prestrained condition

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Fig. 12

Variation of maximum thinning location for different prestrained sheet materials

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Fig. 13

Validation of FE predicted load-progression curve for as-received and prestrained materials



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