Non-linear Unloading Behavior Of Sheet And Its Influence On Stamping Springback

During the stamping process of body parts,springback has been one of the main factors affecting the accuracy of forming prediction.In the general numerical simulation of forming prediction,the elastic modulus is treated as constant value in the unloading process.However,the actual sheet loading-unloading tests show that elastic modulus were varied with the plastic strain during unloading,there are non-linear strain,resulting in low springback prediction accuracy.In this paper,the sheets with different thickness for body parts stamping is taken as the raw material of study.Through the experiment and numerical simulation,non-linear unloading behavior of sheets with different thickness sheets were studied,and it’s effects on springback in the stamping,to provide springback prediction and services for TRB(Tailor Rolled Blank)stamping parts of vehicles.The main work includes,Firstly,the discretization method is used to replace TRB with a number of different equal thicknesses sheets.Uniaxial tensile tests were carried out on sheets with different thickness and different orientations.and the variation rules of yield strength,elongation and r value with thickness were obtained,provided basic performance data for accurate stamping simulation of different thickness sheets.Then,the loading-unloading tests were carried out in several ways.The true stress-strain curve were analyzed,verified the existence of non-linear recovery strain of CR340LA sheet during unloading;investigated the relationship between nonlinear recovery strain with material deformation speed,different orientation and loading mode;analyzed the relationship between the nonlinear recovery strain,elastic recovery strain,and the total strain recovery,mathematical model were used to quantitatively describe the unloading modulus variation.Finally,sheet’s U-bend springback tests with different thicknesses and two numerical simulations(conventional elastic modulus and varying unloading modulus)were implemented.In order to consider the effect of material anisotropy on springback,three different orientations of springback tests and numerical simulations were carried out for each thickness sheets.The experimental and simulation results show that the anisotropy of the material has no effect on the springback,and the springback of the sheets decreases with the increase of the thickness under this paper condition.The conventional elastic modulus can predict the springback of the sheets with different thickness,but compared with actual springback results,deviations are large relatively,which increasingly unable to meet the springback prediction accuracy requirements in current production conditions.The springback simulation results with unloading modulus are closer to the actual test results.Hence,the importance of considering the material nonlinear recovery strain to improve the accuracy in springback prediction is verified.