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Research On Fast Finite Element Simulation Of Sheet, Tube And Profile Forming

Posted on:2011-03-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:L J FuFull Text:PDF
GTID:1101360305456767Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Sheet, tube and profile formings take up a large proportion in the material forming, being widely used in the fields such as automobile, aerospace, aviation and shipping etc. Particularly, profile bending and tube hydroforming play an important role in the metal manufacturing to meet the needs of lightweight, reduction of energy consumption and cost. In the study of numerical simulations concerning such formings, the incremental analysis and the one- and multi- step simulation have complemented each other. Incremental analysis is a relatively accurate finite element computation method for sheet, tube and profile forming since the deformation history, contacts and frictions between blank and forming tools, and other factors influencing forming and product performance can be considered in the round, while the one- and multi-step approach is a kind of fast finite element computation method and can be used to inspect formability and possible forming defects, optimize blank shape without the information of tools and dies, or only with simplified tool surfaces.However, most one- and multi-step approaches are only applied in the sheet metal forming with a flat blank. This paper is to achieve a more widely-used fast simulation system named OMSTEP, with which sheet, tube and profile forming proecesses can be generally simulated forward or inversely with bending effects being considered. The main research content is concluded as follows.When the bending effects are considered in the traditional one-step approaches which are based on the sheet metal forming, the resulted formulation is limited in the one-step inverse simulation with flat initial configuration, not suited to the general forming processes of sheet, tube and profile. Therefore, the BST shell element is introduced here to describe deformation without sacrificing the efficientcy of computation and the simplicity of formulation. In the BST element, the membrane compuation results equal to those gained by traditional one-step membrane elements and the curvature is calculated without using rotational DOF, not limited either by the condition whether the initial blank is flat or by the inverse simulation. Then based on the extreme principle, total deformation theory adopted for one-step and the constitutive equations from scalar prediction-return mapping method adopted for multi-step, the basic fast FE inverse and forward formulation is deduced.Traditional methods to get initial guess solutions are all proposed for the sheet metal forming, among which the geometrical methods are not generally suited while the parameterization method in computer graphics can only be used after mapping the edge of sheet onto plane. So the edge tweaking method, which is shape-preserved, is firstly adopted here to map the outer edge of sheet, and then combined with the parameterization method to get initial satisfactory mesh of sheet by solving linear equations. It is a new issue to get initial guess solutions for profile bending and tube hydroforming. Two mehods are presented in this dissertation. One is a simple geometrical method in which the mesh is mapped by developing the sweepling line (center line) of profile and the other is an effective and general-suited parameterization method in which the column coordinate system is used to map mesh. Besides, a simple effective method is put forward to get good initial guess solutions for mid-configures. Traditional sliding constrait algorithm is mainly proposed for the sheet metal forming too, not suited to profile and tube forming. Therefore, a new contact sliding constraint method is presented, in which the nearest constraint surface element of a node is found by contact search, and then the node normal stiffness is calculated by the penalty method and the friction is computed by the spring element. This method avoids the problem of geometrical adjustment of coordiate in traditional sliding constraints and is suited to profile and tube forming. Some attempts are also made to employ the penalty method to treat real contacts between blank and tools for multi-step forward simulation. Effective inside-outside algorithm is utilized to search contact pairs, the regularized friction model is investigated and some parameters are set up to deal with the problem of big penetration.Sheet metal forimg processes are simulated inversely and forward by the developed in-house one- and multi- step program OMSTEP. It is shown that the one-step inverse simulation is highly efficient and easy to use and the simulation results can be improved by the multi-step approach. Besides, the numerical examples show that the convergence can be more easily achived by the sliding constraint than by the penalty method. However, the wrinkling can be oberserved directly only when the penalty method is used to treat real contacts.The applications of one- and multi- step approach are extended to general forming processes of profile and tube, which is hard for the traditional one- and multi- step approach based on the sheet metal forming. The experiment of three point bending of rectangular tube, in which the cross-sectional distortions are inspected, and the experiment of stretch bending of profile with a complicated cross-section, in which the variation of thickness and the springback are inspected, is conducted to inspect the suitability of OMSTEP system to the profile bending. The variation of wall thickness and cross-section distortions gained from the one-step simulation show good agreements with the incremental analyses and experimental results, but the detailed deformation tendency can only be achieved correctly from multi-step simulations since the mid-configurations are increased and the deformation history is considered. The springback values based on the results of multi-step simulation are also closer to the experimental results than those based on the one-step simulation. In the simulation of profile bending, the sliding constraint is adopted to avoid the contacts between the fillers and profile. In addition, the inverse simulation affords optimized tube blank for the forward simulation in the one step simulation of hydroforming while the multi-step approach matches with the multi-stage hydroforming.
Keywords/Search Tags:one-step approach, multi-step approach, inverse and forward finite element simulation, sheet metal forming, profile bending, tube hydroforming
PDF Full Text Request
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