Verification of accuracy of yield functions of sheet steels under shear strains in uniaxial tensile tests in multiple directions

Abstract

Sheet steels have more or less plastic anisotropy. It is therefore important to consider the anisotropy of a sheet steel in FE analysis of press forming. There are many anisotropic yield functions proposed such as Hill'48, Blart's Yld functions and Gotoh's orthotropic yield function. These yield functions have material parameters to express the anisotropy of a sheet steel, and the parameters need to be calibrated based on r-values and stresses in tension tests in various directions, and/or stresses and strains in bulge tests or in biaxial tension tests. The accuracy of a calibrated yield function should be confirmed by using the results of another material test which is not used in the calibration. However, some functions have many parameters and need many calibration tests. A new method to check parameters of yield functions was developed making use of shear strains in a uniaxial tension test. In this study, shear strains were measured by DIC in uniaxial tension tests in various directions. The results, the ratios of a share strain to the tensile strain, were compared with theoretical values from the calibrated yield functions.