UMAT Secondary Development Of Constitutive Equation And Finite Element Analysis For Metallic Time-dependent Springback After Deformation

Springback is the most difficult defect to control in the sheet stamping forming andtube forming process. With the continuous development of modern industrial technology,high-strength sheets are applied in the automobile body manufacturing largely, higherrequirements are also put forward in the tube bending processing technology, which madethe problem of springback increasingly prominent. But for a long time, researches onspringback problems have been limited to the instantaneous springback caused bymacroscopic elastic stress releasing after the workpiece unloading, and the phenomenon ofthe workpiece’s shape changing over time has been ignored, which is called thetime-dependent springback.As a phenomenon which has not yet been cognitive, the time-dependent springbackhas aroused the concern of the international plastic field. Judging from the present study,the time-dependent springback phenomenon mainly exists in the bending formingspringback of high-strength sheet used in cars and stainless steel tube1Cr18Ni9Ti used inthe aerospace. With the background of sheet metal and tube bending forming springbackproblem, aimed at the behavior of the time-dependent springback, this paper carries ondeeply studying using experiment research and the method of theoretical modelingcombined with numerical analysis. The main contents of the paper are as follows:(1)According to the Wiechert mechanical model assumption of isotropic space andrelevant theory of viscoelasticity, the constitutive model of the time-dependent springbackof metal deformation is established. Based on finite element method, this constitutivemodel is discretized, thus the stress and time increment form and energy update form of thethree dimensional constitutive model are achieved. Using Fortran programming language,UMAT secondary development of the constitutive model is implemented in the finiteelement software ABAQUS;(2)Mechanical properties and viscoelasticity parameters of AC170PX aluminum alloysheets used in cars and stainless steel tube1Cr18Ni9Ti used in the aerospace are obtainedby the tensile experiment and relaxation experiment under different strain levels, providingnecessary material parameters for the finite element simulation.(3) The finite element simulations about the three-point bending test of AC170PX plate and swing traction bending experiment of1Cr18Ni9Ti tube are carried out, and duringthe time-dependent springback simulation, the written UMAT subroutine was used.Through comparing the results of simulation and experiment, the constitutive model isproved to be correct and reliable, and several laws with respect to the time-dependentspringback of plates and pipes are thereby obtained.(4) The explanation for the appearance of the time-dependent springback is given fromthe point of residual stress by combination of the sheet bending theory and finite elementsimulation results.In this study, an effective constitutive model and the numerical method of thetime-dependent springback of metal bending are established. Besides, the laws and causesof the metal time-dependent springback are discussed, providing an effective means forresearching the phenomenon of metallic time-dependent springback after deformation.