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Process parameter estimation in sheet metal forming using a finite element inverse method (유한요소 역 해석을 이용한 박판금속 성형의 공정변수 예측)
Sheet metal forming processes experience very complicated deformations effected by process parameters such as the blank shape, blank holding force, bead force, die geometry, sheet thickness, friction, lubrication, and so on. Although these process parameters influence the deformation mechanism and the quality of deformed parts, the optimum values of process parameters are determined by intuition and experience through trial and error. A more systematic method such as finite element analysis can simulate complicated sheet metal forming processes and provide useful information. However, general finite element analysis is generally carried out with given process parameters and thus requires numerical trial and error with enormous time and cost to determine the optimum values of process parameters. For this reason, some approaches to find directly the process parameters have been developed. However, most design purpose approaches have shortcomings such as geometric restriction, neglecting variation of material properties due to deformation path, and path-independent boundary conditions which causes deformation error. In order to overcome such problems, a new finite element inverse method is introduced for direct prediction of the blank shapes and strain distributions from desired final shapes. The finite element inverse method enables the determination of process parameters within a small error range in good computing time before the process design. The finite element inverse method in this papers adopts Hencky's deformation theory. Hill's anisotropic yield criterion, and simplified boundary conditions. The proposed initial guess scheme using linear inverse mapping enables applications of general shapes to the finite element inverse method effectively. The finite element inverse method with the optimization scheme can calculate non-shape parameters directly as well as shape parameters. The (one step) inverse method is, then, extended to the multi-step inverse method in order to reduce the amount of error. The present algorithm has been implemented in a finite element code and applied to several sheet metal forming examples for demonstration of its validity. As a bench mark test, the algorithm was first applied to drawing of cylindrical and square cup. Blank shapes and thickness strain distributions were obtained. And then, experiments were carried out using blank specimens prepared for the calculated blank shape; Secondly, simulations of more complicated geometries such as oil pan and front fender were conducted as a demonstration of the programs capability and versatility. Third, non-shape parameters such as blank holding force and bead force were also estimated directly with a proper scheme optimizing objective functions. Finally, the multi-step inverse method was applied to a square cup. The error induced by the one step inverse analysis was reduced significantly by the three step analysis. The process parameters of the above examples were calculated rapidly within a small range of error. Consequently, these examples fully demonstrate that the developed algorithm is a good finite element code for the purpose of process design.