Inverse identification method for determining stress–strain curves of sheet metals at high temperatures

Abstract

Because of the strong dynamic softening effect of sheet metals at high temperatures, necking occurs early and continues for longer periods. Therefore, obtaining a flow curve after necking has occurred is necessary. The high-temperature flow behavior of materials typically exhibits the coupled effects of work hardening, softening, strain-rate sensitivity, and damage, and using the traditional inverse identification method is quite challenging. This study presents a finite element (FE) inverse identification method that considers work hardening, softening, damage, and strain-rate sensitivity. The method uses a coupled viscoplastic-damage constitutive model as the modified object, and the parameters of the damage equation are iteratively modified by the FE method until the simulated force–displacement curve is consistent with that of the experiment. The identification procedure is demonstrated using 7075 aluminum alloy, and a complete high-temperature flow curve prior to fracture is obtained. The obtained flow curve is applied to analyze the strain field distribution in a hot tensile process and to predict the fracture occurrence and thickness distribution under an isothermal bulging test. Experimental and simulation results are shown to be in good agreement, thus proving the validity of the proposed method.