Research On Finite Element Simulation Of Bulk Metal Forming Based On Deformation Theory

Bulk metal forming plays an important role in metal processing and manufacturing, which is a complicated physical process with ternary nonlinearity of geometry, material and boundary condition. At present, finite element (FE) simulation technique based on incremental theory is widely applied in the study of bulk metal forming. The history of deformation process could be traced accurately in the incremental FE approach, which is very important for analysis of bulk forming problems with intensive nonlinearity. However, incremental FE approach, in which analyzed model is complex and computing effort is considerable, couldn't provide analyzed results rapidly, which couldn't be suitable for initial concept design of product and predictive analysis of process technic.Deformation theory is the integral of incremental theory in the condition of simple loading, which couldn't be used in general situations theoretically. However, the deformation plasticity solution is preferred for many applications of complex loading problems due to the mathematical simplicity. It is indicated that deformation theory could provide very close results compared with incremental theory for many problems deviating proportional loading path by plentiful analysis of practical problems.Deformation theory has the potential of applications for problems deviating proportional loading and has its own characters of computation accuracy. Deformation theory has been applied for sheet metal forming FE simulation successfully, by which one-step and multi-step simulation methods could provide results in close agreement with incremental FE predictions with very simple model and very short computing time. Compared with sheet forming processes, the deformation is larger and the deformation path and the contact history of the deformed part are more complicated in bulk metal forming processes. So it is more difficult to apply the deformation theory to bulk forming simulation. Consequently, despite the remarkable progresses that have been achieved in FE approach based on deformation theory for sheet metal forming, the applications of that in bulk metal forming are seldom. The research on application of deformation theory to bulk metal forming FE simulation is carried out in the dissertation, the main works and innovations are:A bulk metal forming FE approach based on deformation theory is put forward. The strain and stress under large deformation condition and deformation constitutive relationship are defined. This approach is implemented to minimize approximated plastic potential derived from the total plastic work and the equivalent external work in static equilibrium for incompressible rigid-plastic materials based on constraint variational principle under the principle of extremum work. The rigid-plastic deformation FE equations are deduced.Several key techniques of bulk metal forming deformation FE approach are studied. The fictitious sliding constraint is proposed for dealing with boundary condition of large-step computation. The estimation of equivalent external work is put forward. The initial solution is present by presetting configuration and mapping meshes. The local constraint of freedom is proposed for bulk forming problems with approximate rigid region in order to improve the convergence of deformation large-step computation.The one-step simulation scheme based on deformation theory is proposed and one-step FE simulation program is developed. One-step simulation, which neglecting deformation history, is performed by FE calculation in one large step between the initial and final configurations of process. The multi-step simulation scheme based on deformation theory is proposed and multi-step FE simulation program is developed. Multi-step simulation, in which the computing step is divided by considering variation of deformation path and contact situation in whole process, is performed by FE calculation in multiple large steps. In multi-step simulation, it is realized to track the variation of entire deformation path by using deformation theory and consider the variation of contact history to improve the accuracy of external work estimation, which result in application of deformation FE approach to bulk forming problems more generally.The one-step and multi-step numerical simulations are performed by taking plain strain and axisymmetric thick-walled cylinder forming under inner pressure and plain strain compression as examples to analyze the effect of deformation path, material's hardening and boundary condition on deformation FE calculation. The applicability of deformation theory in bulk metal forming FE simulation is discussed.The one-step forward simulations of cylindrical upsetting and flat plate rolling are performed. As for one-step simulation of flat plate rolling, the treatment of friction boundary problem between workpiece and roller during process is put forward. The results of one-step simulation are compared with those of incremental simulation and experiment to verify the correctness of the proposed approach.The multi-step forward simulations of cylindrical bar forward extrusion and rectangular billet elongation are performed. The experiment of cylindrical bar forward extrusion is carried out. The results of multi-step simulation are compared with those obtained by incremental simulation and experiment to verify the feasibility and validity of the proposed approach.