Gihyun Bae

Effect of the Yield Stress and r‐value Distribution on the Earing Profile of Cup Drawing with Yld2000‐2d Yield Function

This paper deals with the effect of the yield stress and r‐value distribution on the earing in the cup drawing. The anisotropic yield function, Yld2000‐2d yield function, is selected to describe the anisotropy of two metal sheets, 719B and AA5182‐O. The tool dimension is referred from the Benchmark problem of NUMISHEET’2002. The Downhill Simplex method is applied to identify the anisotropic coefficients in Yld2000‐2d yield function. Simulations of the drawing process are performed to investigate the earing profile of two materials. The earing profiles obtained from simulations are compared with the analytical model developed by Hosford and Caddell. Simulations are conducted with respect to the change of the yield stress and r‐value distribution, respectively. The correlation between the anisotropy and the earing tendency is investigated based on simulation data. Finally, the earing mechanism is analyzed through the deformation process of the blank during the cup deep drawing. It can be concluded that ears locate at angular positions with lower yield stress and higher r‐value while the valleys appear at the angular position with higher yield stress and lower r‐value. The effect of the yield stress distribution is more important for the cup height distribution than that of the r‐value distribution.

Accuracy Analysis of Anisotropic Yield Functions based on the Root-Mean Square Error

This paper evaluates the accuracy of popular anisotropic yield functions based on the root-mean square error (RMSE) of the yield stresses and the R-values. The yield functions include Hill48, Yld89, Yld91, Yld96, Yld2000-2d, BBC2000 and Yld2000-18p yield criteria. Two kind steels and five kind aluminum alloys are selected for the accuracy evaluation. The anisotropic coefficients in yield functions are computed from the experimental data. The downhill simplex method is utilized for the parameter evaluation for the yield function except Hill48 and Yld89 yield functions after the error functions are constructed. The yield stresses and the R-values at every 15°from the rolling direction (RD) and the yield stress and R-value at equibiaxial tension conditions are predicted from each yield function. The predicted yield stresses and R-values are then compared with the experimental data. The root-mean square errors (RMSE) are computed to quantitatively evaluate the yield function. The RMSEs are calculated for the yield stresses and the R-values separately because the yield stress difference is much smaller that the difference in the R-values. The RMSEs of different yield functions are compared for each material. The Hill48 and Yld89 yield functions are the worst choices for the anisotropic description of the yield stress anisotropy while Yld91 yield function is the last choice for the modeling of the R-value directionality. Yld2000-2d and BBC2000 yield function have the same accuracy on the modeling of both the yield stress anisotropy and the R-value anisotropy. The best choice is Yld2000-18 yield function to accurately describe the yield tress and R-value directionalities of sheet metals.

A Simulation-Based Prediction Model of the Restraining and Normal Force of Draw-Beads with a Normalization Method

This paper proposes a simulation-based prediction model to predict the restraining and normal force of drawbeads for the sheet metal forming process. A reliable prediction model is constructed for the equivalent drawbead by a modified DOE (Design of Experiment) method, which consists of the Box-Behnken design and a simplified full factorial design. To construct prediction models of draw-bead forces, draw-bead forces are first calculated by finite element analysis and confirmed by experiments followed by an approximation with second order regression equations in various design cases. To increase the accuracy of prediction models, normalization of draw-bead forces is conducted based on the effectiveness ratio of design variables in a regression analysis. The normalized draw-bead forces are then approximated by second order regression equations again. The accuracy of the prediction models constructed is verified by comparing the prediction results with the simulation results in the entire design space.

Intermesh effect on roller straightening process of single layer steel cord

Abstract This paper deals with the effect of the intermesh on the roller straightening process of a single layer steel cord. A single layer steel cord, which is constructed with three drawn wires, is selected as a target structure for stranding experiments. Intermeshes at the inlet and outlet of the roller straightening device are selected as design parameters. A design table is constructed for experiments with the variation of two intermeshes of the roller straightening device. Experiments were carried out to measure the three assessment items for a single layer steel cord such as the residual torsion of a steel cord, the arc height of a steel cord and the preform of a single wire in each design case for the quantitative evaluation of a steel cord quality. The sensitivity analysis is performed for design parameters based on the data measured. Prediction equations of the three assessment items are constructed from the regression analysis. The prediction equations constructed can be utilised as a design guideline for the roller straightening process of a single layer steel cord.