Takeshi Uemori

Elasto‐Plasticity Behavior of Type 5000 and 6000 Aluminum Alloy Sheets and Its Constitutive Modeling

To examine the deformation characteristic of type 5000 and 6000 aluminum alloy sheets, uniaxial tension, biaxial stretching and in‐plane cyclic tension‐compression experiments were performed, and from these, r‐values (r0, r45 and r90), yield loci and cyclic stress‐strain responses were obtained. For the accurate description of anisotropies of the materials, high‐ordered anisotropic yield functions, such as Gotoh’s biquadratic yield function and Barlat’s Yld2000‐2d, are necessary. Furthermore, for the simulation of cyclic behavior, an advanced kinematic hardening model, such as Yoshida‐Uemori model (Y‐U model), should be employed. The effect of the selection of material models on the accuracy of the springback prediction was discussed by performing hat bending FE simulation using several yield functions and two types of hardening laws (the isotropic hardening model and Y‐U model).

Plastic deformation behavior of high strength steel sheet under non-proportional loading and its modeling

This paper deals with plastic deformations of a high tensile strength steel sheet (HTSS sheet) under biaxial stress condition including strain path. Using a cruciform specimen of a HTSS sheet of 780MPa-TS, experiments under proportional and non-proportional loadings were investigated. Numerical simulations of stress-strain responses for several strain paths after biaxial stretching were conducted using a large-strain cyclic plasticity model (Yoshida-Uemori model). The results of numerical simulation agrees well the corresponding experimental results, which is attributed to the accurate modeling of the backstress evolution of the anisotropic yield function.

FLD OF AZ31 SHEET UNDER WARM STRETCHING AND ITS PREDICTION

Forming Limit Diagrams (FLDs) of a magnesium alloy (AZ31) sheet at various forming speeds (3 to 300 mm·mm-1) at several temperatures of 100-250°C were investigated by performing a punch stretch-forming test. The forming limit strains increased with temperature rise and with decreasing forming speed, where the effect of forming speed was stronger at higher temperatures. To describe such a characteristic of FLD of AZ31, the Marciniak-Kucznski type forming limit analysis was conducted using the Backofen-type constitutive equation (c = Cenem). In this analysis, the damage evolution in the necking zone was taken into account based on Oyanes ductile fracture criterion. The numerical results of the FLD show a good agreement with the corresponding experimental observations.

Die-Bending of Adhesively Bonded Sheet Metals

In this paper, the deformation behavior of adhesive layer in die-bending for adhesively bonded sheet metals was investigated by experiments and finite element method (FEM). We paid special attention to the bending/unbending shear deformation of the adhesive layer during the die-bending of adhesively bonded sheet metals by using highly ductile acrylic adhesive. Major results obtained are summarized as follows: (1) The bending/unbending shear deformation of the adhesive layer was observed during the die-bending. (2) The punch radius has a large influence on the die-bending in the adhesively bonded sheet metals. (3) It is desirable to perform die-bending at high speed as well as air-bending.

Butt Joining of High Strength Steel Sheet and Dissimilar Metal Sheet by Shot Peening

In the shot peening process, the substrate undergoes large plastic deformation near the surface due to the hit with many shots. A large plastic deformation characterized by a shear droop occurs at the edge of the substrate. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and then the contact area are shot-peened, the sheets can be joined due to the plastic flow generated by a large plastic deformation during shot peening. This method is similar to joining by caulking. The aim of this paper is to investigate the butt joining of high strength steel and dissimilar metal sheets using a shot peening process. The shot velocity and the coverage were controlled in the experiment. The shots used were made of high carbon cast steel and cemented carbide with an average diameter of 0.1 mm. The sheets were high strength steel and aluminum alloys. The influences of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. Tensile test was also examined to evaluate bond strength. It was found that the present method can be used to enhance the butt joining of high strength steel and dissimilar metal sheets.

Implicit Stress Integration and Consistent Tangent Matrix for Yoshida’s 6th Order Polynomial Yield Function Combined with Yoshida-Uemori Kinematic Hardening Rule

This paper describes fully implicit stress integration scheme for Yoshida’s 6thorder yield function combined with Yoshida-Uemori kinematic hardening model and its consistent tangent matrix. Cutting plane method was employed for accurate integrations of stress and state variables appeared in Yoshida-Uemori model. In the present scheme, equivalent plastic strain, stress tensor and all the state variables are treated as independent variables in order to handle the 6th order yield function which is not the J2 yield function, and the equilibriums for each variables are solved for the stress integration. Subsequently, exact consistent tangent matrix which is necessary for implicit static finite element simulation was obtained. The proposed scheme was implemented into finite element code LS-DYNA and deep drawing process for aluminum alloy sheet was calculated. The earing appearance after drawing was compared with the experiment.

Finite Element Analysis of High Tensile Strength Steel Sheet by Using Complex Step Derivative Approximations

The framework for the complex step derivative approximations (hereafter CDSA) to calculate the consistent tangent moduli is studied. The present methods is one of the most effective methods to implement any material constitutive equations to the commercial finite element codes and does not suffer from calculation conditions and errors. In order to confirm the efficiency of CDSA, we developed the user subroutine code based on the CDSA using associative J2 flow rules with general nonlinear isotropic hardening rules that is commonly and widely utilized in commercial finite element codes. In this study, the user material subroutine ‘Hypela2’ of MSC.Marc (ver.2013.0.0) was utilized. The finite element calculation result by the proposal method shows a good agreement with the corresponding result by the MSC.Marc default setting. Also we apply the Yoshida-Uemori back stress model to the CDSA and evaluate this new technique to predict the deformation behavior of high tensile strength steel sheet.

Deformation Behavior of Adhesive Layer in Stretch-Bending/Unbending for Adhesively Bonded Sheet Metals

In this paper, the deformation behavior of adhesive layer in stretch-bending/unbending for adhesively bonded sheet metals was investigated by experiments and finite element method (FEM). We paid special attention to the cyclic shear deformation of the adhesive layers during the plastic working. Major results obtained are summarized as follows: (1) When the adhesively bonded sheet metals is bent and pulled out at a 90° angle, shear deformation due to bending of the adhesive layer starts shortly before reaching the die corner and unbending starts at the middle of the corner. (2) The die radius has a large influence on the bending behavior. (3) It is possible to suppress shear deformation of the adhesive layer by using a material with small tensile strength as one of the two adherends.

Springback of Aluminum Alloy Sheet Metal and its Modeling

Aluminum alloy sheet metals have been widely utilized for a light weight construction of automobile. However, Aluminum sheet metals still remain one of the difficult materials to predict the accurate final shapes after press forming processes, because of several mechanical weak features such as lower Youngs modulus, strong plastic anisotropy of yield stress, Lankford values, and so on. In order to solve the problems, the present author has developed a new constitutive model called Modified Yoshida-Uemori model. The present model can describe accurate non-proportional hardening behaviors of Aluminum alloy sheet metals. In the present research, several experimental procedures were carried out to reveal the mechanical properties of Aluminum alloy sheet metals. From the comparison between experimental data and the corresponding calculated results by our constitutive model, the performance of our model was evaluated. In addition to the above mentioned research, the evaluation of some springback analyses were also carried out. The calculated results show good agreements with the corresponding experimental data.

Description of anisotropy and the Bauschinger effect on various types of steel sheets

To describe the anisotropy of sheets a sixth-order polynomial type 3D yield function is proposed. The yield function is constructed as a sum of several components of the Cazacu-Barlat function (2001) which was derived as an extension of the J2-J3 Drucker yield criterion (1949) to orthotropy using the linear transformation of the stress deviator. In this framework of modeling, the convexity of the yield locus is perfectly guaranteed. The model was validated by comparing the numerical predictions of planar anisotropy of r-values and flow stress directionality, as well as the shape of yield loci, with the corresponding experimental data on several types of steel sheets (high r-valued IF steel and SPCE, and high strength steel sheets of 440-980MPa TS grades). For most of steel sheets, the model of the sum of two J2 components, which involve eight anisotropic coefficients, is sufficient for the description of their anisotropies. For the description of the Bauchinger effect and cyclic workhardening characteristic, Yoshida-Uemori kinematic hardening model (2002a, 2002b, 2003) was employed, which includes a new proposal to describe non-saturation type workhardening.