Prediction of Formability of Bi-axial Pre-strained Dual Phase Steel Sheets Using Stress-Based Forming Limit Diagram

Abstract

Dual phase (DP) steel is of great interest for automotive part manufacturers due to its excellent combinations of strength and formability. Complex components involving three-dimensional stampings are usually fabricated through multistage sheet forming operations. The ability of a sheet metal to be deformed into a specific desired shape by distributing strain over arbitrary tool surface depends on complex interaction of material, process and design variables. The strain-based forming limit diagram (ε-FLD) is often used as a measure of formability in the press shop due to convenience of measuring the limiting strain. However, it was reported by previous researchers that the ε-FLD of sheet metal shifts after pre-strain due to the initial forming operations. Hence, this work proposes a mathematical framework for constructing σ-FLD of different pre-strained sheets incorporating Barlat-89 yield criterion with different hardening laws. The formability of biaxially pre-strained DP600 was evaluated experimentally in two stages. The forming behaviour of pre-strained material was predicted by finite element model using the σ-FLD, and the predicted results matched very closely with the experimental data. It was also observed that the σ-FLD was robust and underwent insignificant changes due to the change in the pre-strain path.

Micromechanical Effect of Martensite Attributes on Forming Limits of Dual-Phase Steels Investigated by Crystal Plasticity-Based Numerical Simulations

“…The Keeler-Brazier model depends on the major true strain value FLD0, true on the thickness of the virtual samples, and the strain hardening coefficient when the minor true strain equals zero [13]. It presents Equations (A1)-(A3) to predict the FLDs following the activities, as shown in Figure A2, where major true strain E 1 is dependent on values of thickness t and strain hardening coefficient n.…”

“…Analytical models commonly applied to predict and plot FLDs by adopting major and minor limit strains are Swift, Chow-Hosford, Keeler-Brazier, and Marciniak-Kuczynski [11]. These approaches have been analyzed and compared in depth in research work by Basak and G Béres [12,13]. These approaches depend on some variables to develop the mathematical model and corresponding limit strains, i.e., Swift models require strain ratios and strain hardening coefficient values.…”

Formability analysis of pre-strained AA5754-O sheet metal using Yld96 plasticity theory: Role of amount and direction of uni-axial pre-strain

“…Recently, Basak et al [30,31] estimated the strain path independent σ-FLD of Al 2008 T4 sheets using the Barlat-89 anisotropy plasticity theory and Swift hardening law, and further the formability of bi-axial pre-strained DP600 and IF steel sheets were predicted successfully implementing in finite element model. It is noteworthy that auto-body panels were stamped using rigid dies and punches mounted in a double action hydraulic press, and the final critical shapes were obtained in multiple stages.…”