Identification Of Post-necking Flow Curve For Sheet Metals By Inverse Method

The Finite Element Method and Computer Aided Simulation software have been widely used to shorten research cycle-time and reduce investment in automobile development projects. Simulation of sheet metal forming and mechanical behaviors in car collision involving large deformation requires the flow curve (i.e. the true stress-strain relation) over large strains. A simple tensile test using contact extensometer is not enough to achieve a more integrated true stress-strain curve for large strains.This paper introduced a method for obtaining the flow curve of sheet metals over a large range of strain through the combination of Finite Element Analyses and simple tensile test.(1) Steel plates are commonly used in manufacturing car body. The automobile industry consumes more and more high-strength steel and aluminum as the generalization of the lightweight technology. This paper selected three kinds of typical material as objects of study. Firstly, the standard samples were designed and machined according to the national standard. Original tension force-elongation curves of three materials were obtained with the help of extensometer. Engineering stress-strain curves, true stress-strain curves and true stress-plastic strain curves can be transformed through corresponding formula. The sheet metals were cut into small rectangular pieces using a mechanical shearing machine along the rolling direction (RD), transverse direction (TD) and diagonal direction (DD). Coefficients of nominal anisotropy in three directions were measured considering the influence of material anisotropy.(2) The general purpose finite element simulation software ABAQUS/Standard was chosen to simulate the tensile test. ABAQUS processes excellent nonlinear simulation performance. The appropriate Finite Element Model for accurate simulation of the anisotropic plastic deformation during diffuse necking was determined. Different element sizes and element types were repeatedly attempted when creating an accurate and effective Finite Element model. The final determined model contains reasonable structural model, element factors balancing simulation accuracy and efficiency, material model with anisotropic yielding property and output item corresponding to experimental tension force-elongation curves.(3) An inversed method has been proposed, which combines experiments and simulations. Based on experimental curves, data points in flow curves were constantly revised and extrapolated until simulated tension force-elongation curves match with corresponding curves of experiment.(4) Different hardening functions were evaluated for their capabilities in approximating the entire flow stress curves up to localized necking. After comparing the fitting results of different equations, a modified Hockett-Sherby function was proposed and its performance was demonstrated. At last, stress triaxiality is briefly discussed in accordance with the simulation result of Q195 tensile test.