Coupled Finite Element And Meshless Method Model And Simulation For Sheet Visco-elasto-plastc Flexible-die Forming

Viscous pressure forming (VPF) is different from sheet rigid-die forming andgaseous or hydro forming process. Not only because of the coupled deformationcharacteristics between sheet metal and flexible-die in the forming process, but also theinfluence of flexible die’s properties on the formability of sheet metal. This coupleddeformation is difficult to solved analytically, it is necessary to utilize numerical analysismethod to make an in-depth study. Due to the large deformation of flexible-die in theforming process, it often leads to a mesh distortion when finite element method is used,remeshing for bulk forming of flexible-die is difficult and will affect the calculationaccuracy. In this paper, a coupled finite element and meshless method was proposed toanalyse viscous pressure forming and sheet metal polyurethane rubber forming. The keytechniques such as the visco-elasto-plastic constitutive model of viscous medium,frictional contact treatment between sheet metal and flexible-die and adhesive stressmodel between viscous medium and sheet metal were also studied. Based on the abovetheories, a coupled analysis program was developed to forecast sheet metal flexible-dieforming numerically. The validity of the model and forecast program was verifiedthrough experimental research.First of all, a new numerical analysis method which coupled finite elementmethod and meshless method was proposed to analyse sheet visco-elasto-plasticflexible-die forming process. In this method, the elastoplastic deformation of sheetmetal was analyzed with finite element method and the bulk deformation of flexible-die was analyzed with element-free Galerkin method, it can make full use of theirrespective advantages of these two algorithms. The elasto-plastic constitutiveequation of sheet metal, visco-elasto-plastic constitutive equation of semisolidviscous medium and hyperelastic constitutive equation of solid polyurethane rubberwere given firstly. Then a coupled FEM-EFGM formulation of sheet flexible-dieforming process was established based on updated Lagrangian formulation, and itsvalidity was verified through numerical examples.Secondly, it contains not only contact between sheet metal and die, but also contactbetween two coupled deformation bodies (flexible-die and sheet metal) in sheet flexible-die forming process, which is an important difference with rigid-die forming process.According to the sliding friction between the sheet metal and die, adhesive stressbetween semisolid viscous medium and sheet metal and frictional contact between solidflexibile-die and sheet, corresponding numerical models were established respectively.The interface contact treatment between sheet metal and flexible was a key problem because of different discrete ways of these two coupled deformation bodies. A staticexplicit algorithm was adopted to solve multi-body coupled deformation problems, thesize of the time step was controlled to keep the material state and contact state stable ineach loading step which can aviod the disadvantage of misconverged iteration in a static-implicit algorithm. The calculation of stress and strain increment of sheet metal andflexible-die in incremental process were also illustrated.Thirdly, visco-elasto-plastic constitutive model of semi-solid viscous medium werestudied. Viscous mediums with six different molecular weights (550000g/mol,600000g/mol,650000g/mol,700000g/mol,800000g/mol, and900000g/mol) wereseparately carried on oscillation, creep-recovery and shear viscosity experiments byusing dynamic shear rheometer (DSR). Through analyzing the results of the experiment,visco-elasto-plastic constitutive model parameters of viscous mediums with differentmolecular weights were obtained. Furthermore, a relationship between molecular weightof different viscous mediums and visco-elasto-plastic constitutive model parameters wasestablished, it can provide a guide for analysing the deformation behavior of viscousmediums in viscous pressure forming process.Finally, by using the coupled FEM-EFGM analysis program, sheet elastic flexible-die bulging, sheet polyurethane rubber bulging and viscous pressure bulging process withdifferent molecular weights were forecasted numerically. Viscous pressure bulgingprocesses of Al1060sheet with different viscosity viscous mediums were forecastednumerically and experimentally. The advantages of coupled FEM-EFGM program indealing with large bulk deformation were investigated through forecasting sheet elasticflexible-die bulging numerically. By forecasting1Cr18Ni9Ti stainless steel sheetpolyurethane rubber bulging process, the strain and wall-thickness distribution of bulgingparts were analyzed at different dome heights. Compared with experimental results, thevalidity of the coupled FEM-EFGM forecast program was obtained. The influence ofviscosity on the configuration, stress, strain distribution and thickness reduction ratio ofbulging parts were investigated in detail. Both the numerical forcast analysis and theexperimental result showed that high viscosity viscous medium can change the maximumstrain location and make a more uniform wall-thickness distribution. Viscous pressureforming processes of conical part were also forecasted by the coupled FEM-EFGMprogram, the experimental results and numerical ones were in good agreement.