Masaki Urabe

Identification of material parameters in constitutive model for sheet metals from cyclic bending tests

Abstract This paper deals with the identification of material parameters in a constitutive model for sheet metals using the bending moment versus curvature diagrams obtained by cyclic bending tests. The model can describe the cyclic strain hardening by the isotropic hardening and the Bauschinger effect by the kinematic hardening. An optimization technique based on the iterative multipoint approximation concept was used for the identification of the material parameters. This paper describes the experimentation, the fundamentals and the technique of the identification problem, and the verification of this approach.

Inverse approach to identification of material parameters of cyclic elasto-plasticity for component layers of a bimetallic sheet

Abstract The present paper proposes a novel approach to the identification of the mechanical properties of individual component layers of a bimetallic sheet. In this approach, a set of material parameters in a constitutive model of cyclic elasto-plasticity are identified for the two layers of the sheet simultaneously by minimizing the difference between the experimental results and the corresponding results of numerical simulation. This method has an advantage of using the experimental data (tensile load vs strain curve in the uniaxial tension test and the bending moment vs curvature diagram in the cyclic bending test) for a whole bimetallic sheet but not for individual component layers. An optimization technique based on the iterative multipoint approximation concept is used for the identification of the material parameters. This paper describes the experimentation, the fundamentals and the technique of the identification, and the verification of this approach using two types of constitutive models (the Chaboche-Rousselier and the Prager models) for an aluminum clad stainless steel sheet.

Computer-aided process design for the tension levelling of metallic strips

Abstract 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.

Improvement of Shape Accuracy in Press-Formed Parts of High-Strength Steel by Springback-Root-Cause Analysis

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.

Simple Springback Cause Analysis Using Measured Shapes of Dies and Pressed Part

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.

Progress in press forming computer aided analysis for high strength steel sheet applications

The development of press-forming analysis technologies is important to expand the application of high strength steel sheets to automotive body structures. In general, there are various problems in the forming process of high strength steel sheets. In this study the improvements in the prediction accuracy of stretch-flange-fracture and springback were especially focused. In terms of the prediction accuracy of stretch-flange-fracture, a new stretch-flange-fracture prediction technology was developed based on a maximum principal strain gradient. It enables the accurate prediction of stretch- flange-fracture in press-forming of practical parts. On the other hand, springback prediction technologies were developed to solve springback problems. It is very important to clarify the root cause of springback in order to control. Therefore, a new method of springback factor analysis was developed, which can extract the areas and residual stresses which have major impacts on springback at press-forming.

Identification of Material Parameters in Constitutive Models of Cyclic Plasticity from Bending Tests of Sheet Metals

This paper deals with the identification of material parameters in constitutive models of cyclic plasticity for sheet metals using the bending moment vs. curvature diagrams obtained by cyclic bending tests. After a brief discussion on the identification for some simple plasticity models, a new identification method based on the iterative multipoint approximation concept is presented for complicated elastic-plastic consititutive models incorporating many numbers of material parameters. As an example, eight material parameters in Chaboche-Rosseliers model were identified by this method. This approach has been verified by comparing the simulated stress-strain curve using the constitutive model incorporating the identified material parameters with the experimental curve determined by uniaxial tension.

Identification of mechanical properties of component layers in a bimetallic sheet by mixed experimental-numerical approach

Publisher Summary This chapter proposes a novel approach for the identification of the mechanical properties of the individual component layers of a bimetallic sheet. This method has an advantage of using the experimental data that is tensile load versus strain curve in the uniaxial tension test, and the bending moment versus curvature diagram in the cyclic bending test for a whole bimetallic sheet, but not for individual component layers. A set of material parameters in each of two types constitutive model of cyclic plasticity for individual component layer was identified by minimizing the difference between the experimental results and the corresponding results of numerical simulation. An optimization technique based on the iterative multipoint approximation concept was used for the identification of the material parameters.