Experimental Study of Biaxial Load-Unload Behavior of DP590 Steel Sheets

Biaxial load-unload tests under radial paths in the true stress space were carried out for DP590 steel sheets using specially-designed cruciform specimens. Depending on the specific path, over 15% equivalent logarithmic plastic strain was achieved so that the load-unload behavior was successfully probed at relatively high strain levels. It was found that the stress-strain response at the initial load/unload follows the predicted linearly elastic response very well and that subsequently the slope decays. Following this initial phase, a second linear response is observed, which ultimately leads to the non-linear plastic response. 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. At higher strains, this ratio is approximately 25%, indicating the inaccuracy of springback simulations when a linearly elastic unloading response is assumed. For each load-unload cycle, the dissipated energy density tends to increase with the progression of prestrain. The plastic work contours covering the first quadrant of the stress space were successfully constructed and the directions of the plastic strain rates were then calculated. A good agreement with the experimental facts was found by adopting the anisotropic yield function Yld2000-2D.