Research On High-Fidelity Constitutive Model For Tailor Rolled Blank

Applying car parts with continuous transition thickness designed with actual load distribution is an effective way to reduce car weight.When TRB(Tailor rolled blank)are punched for car parts,different thickness zones perform different mechanical properties,leading to different unloading processes and different Bauschinger effects.To gain more accurate predictive results in the related FEM analysis,more detailed research on mechanical properties,unloading process and Bauschinger effect should be carried out.In this paper,HC260LA TRB is taken as the research object.The goal is to build a more accurate material model and to improve the springback simulation prediction accuracy of high strength steel plate and TRB during the parts cold stamping,so as to better guide the mold design.To achieve this goal,the discretization method is used to study the area of varying thickness of TRB,and the mechanical properties of the area of varying thickness are replaced by the properties of six thickness plate.This paper mainly focuses on the of nonlinear unloading effect and Bauschinger effect and the influence of these two factors on the precision of numerical simulation springback prediction.Details are as follows:(1)Using 2.95 mm HC260LA plate as raw material,they are rolled into different thickness plates by controlled reduction,then heat treated under the same annealing procedures to ensure the properties(yield strength,tensile strength,elongation,plastic strain ratio)of various thickness plates to meet the requirements of user.(2)The tensile-unloading experiment is carried out,the relationship between nonlinear unloading phenomenon and strain rate,loading-unloading history,different orientation in plane,pre-strain amount is explored by analyzing the stress-strain curve.The unloading modulus decreased by about 20%at most with the increase of plastic strain(1.4mm plant,90° direction),which indicates that it is necessary to take the phenomenon of nonlinear unloading phenomenon into consideration in the simulation of springback prediction.(3)Tensile-compression test was carried out.The stress-strain curve was analyzed to study the relationship between the Bauschinger effect and the pre-strain,the in-plane orientation,the thickness distribution.Bauschinger effect increases with the increase of pre-strain,but the growth rate is slowing down.The Bauschinger effect decreased in the order of 90°,45° and 0° direction,and Bauschinger strain ratio in 90° direction was 30%higher than 45° direction and 41%higher than 0° direction.From the perspective of the permanent softening,90° direction was 12%higher than 45° direction and 17%higher than 0° direction.(4)The stretching-three point bending-springback experiment of flat blank was carried out.In the meantime,Four kind of constitutive models(Neither the nonlinear unloading phenomenon or the Bauschinger effect is considered,The nonlinear unloading phenomenon is considered,The Bauschinger effect is considered,Both the phenomenon of nonlinear unloading and the Bauschinger effect are considered)were used to simulate the experimental process,and the simulation results were compared with the experimental results.The comparison of the simulation results and experimental results shows that with the nonlinear unloading phenomenon considered,the springback given by the simulation is increased,with the Bauschinger effect considered,the springback given by the simulation is reduced.The experimental springback angle was 8°43',the difference between the simple constitutive model(Neither the nonlinear unloading phenomenon or the Bauschinger effect is considered)and the experimental result was 3°51',and the difference between the high-fidelity constitutive model(Both the phenomenon of nonlinear unloading and the Bauschinger effect are considered)and the experimental result was 2°11'.With the phenomenon of nonlinear unloading and the Bauschinger effect considered,the springback prediction accuracy is greatly improved compared with the results given by simple constitutive model simulation.(5)The tensile-bending-springback experiment of TRB was performed and simulation with two kind of constitutive model(Neither the nonlinear unloading phenomenon or the Bauschinger effect is considered,Both the phenomenon of nonlinear unloading and the Bauschinger effect are considered)are completed.The change of the opening before and after the springback in experiment is 2.82 mm.The difference between the predicted result using the simple constitutive model and the experimental value was 1.30 mm,and the difference between the simulation result using the high-fidelity constitutive model and the experimental value was 0.42 mm.The application of high-fidelity constitutive model to HC260LA TRB deformation simulation can improve the prediction accuracy of springback.