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.