Biaxial unloading and springback behavior of dual-phase DP590 steel using cruciform specimens

The unloading behavior of a dual-phase steel (DP590) from a biaxial state of stress was probed using a newly-designed cruciform specimen. The specimen was designed to develop uniform and relatively large plastic strains (over 15% equivalent logarithmic plastic strain) in the gage section, before failure. Nine radial loading paths in the 1st quadrant of the plane stress space were probed. The experiments involved repeated loading and unloading up to failure. At every unloading, the initial response was found to agree with the linear, orthotropically elastic response of the undeformed material. This first linear response was followed by a second one, at a reduced slope. Beyond that, the recorded response was fully non-linear. The same sequence of events was observed during each reloading. The biaxial non-linear strain recovery components εxnl and εynl were measured to be on average approximately 11% of the elastic strains εxe and εye, respectively. This ratio was found to increase with plastic deformation. Subsequently, these biaxial experiments were used to calibrate the Yld2000–2D yield function.