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A new springback cause analysis method was developed for sheet press forming with measured 3-dimentional shapes of dies and a formed part. In conventional springback cause analysis, a stress distribution is calculated by elasto-plastic FEM analysis of press forming. The newly developed method can be directly applied to a press formed part and its forming dies: an elastic FEM analysis of press forming is conducted using measured shapes of both a pressed part and forming dies, and a stress distribution which causes springback is obtained; and then a springback cause analysis is carried out based on the stress distribution. This calculation method is easier than that of conventional press forming simulation because plastic material properties and friction parameters are not required in its procedure. The philosophy of the new method and an example of application to an automotive part are described in this paper.

Springback is one of the most serious problems in high-strength steel-sheet forming to produce automotive body parts. A springback-root-cause analysis method was developed to identify the areas of stresses at the bottom dead point, which are the most influential in springback. The analysis method of using springback driving stresses, that is, the difference between stresses before and after springback, is more accurate to eliminate the effect of the residual stresses on springback. This analysis method was applied to both a simple model and the forming of an actual part to verify this analysis method. The areas of stresses that have a major impact on springback were identified. A countermeasure against the actual part springback based on this result was devised and it was clarified that the countermeasure is effective on the springback.

The effect of strain gradient on the stretch flange deformation limit of steel sheets was investigated by hole expansion test under some forming conditions and by FEM. Hole expansion ratio was changed by changing initial hole diameter and punch shapes. It was clarified that a limit stretch flange deformation strain basically depends on not the initial hole diameter and the punch shape by themselves but the strain gradient in the radial direction. The strain gradient along the direction toward the maximum principal strain did not affect limit deformation strain. The limit deformation thickness strain of the stretch flange portion could be changed by changing strain gradient. These results suggest that the formability of stretch flange deformation area should be determined from not only the maximum principle strain but also the strain gradient and blanking clearance.

The present paper deals with a computer-aided process design for tension leveling based on finite element analysis. A FE code developed specifically for that purpose, which involves a sophisticated constitutive model of cyclic plasticity, enables the accurate performing of a numerical simulation. The very short computation time in numerical simulation by this FE code is due to a novel calculation algorithm to solve the steady elasto-plasticity problem based on the incremental deformation theory of plasticity. From the results of numerical simulation for some tension leveling processes, it has been concluded that a series of work rolls should be set in the final stage of the leveling process such that the roll-intermesh gradually decreases along the stream line and approaches zero at the final roll.