Establishment And Application Of A Kind Of Yield Criterion For Sheet Forming

Current yield criteria are obtained from the transformation of the original yield criteria, or adopt higher order functions without physical significance to make the results more close to the test points. While the current yield criteria are almost global variables functions of stress, different loading paths will be affected by each other when predicting the yield locus. So it is hard to get precise prediction in all test points. In this paper, the whole locus is sectioned based on the relations between the octahedral shear stress and the loading path, then in the closed yield locus of2D principal stress space, the Hill48yield criterion is modeled by sections based on the thought of kinematic hardening. It means that the new yield locus can be attained by using modified functions, and the parameters of the yield criterion are calculated by the data obtained from the uniaxial tensile tests and biaxial tensile tests. This is a new idea to create the anisotropic yield criterion.The uniaxial loading specimens (uniaxial tensile specimens and plane strain specimens),biaxial loading specimens and their corresponding loading direction with certain angles from rolling direction are designed to get the coefficient of modified function in the new yield criterion. All the experiments can be completed easily in the existing experimental conditions. For the experimental data which are difficult to obtain are calculated by other yield models or reasonable assumptions.In order to verify the reliability and universal applicability of the new yield criterion, the aluminum alloy.2024-o and5754M and two kinds of steel sheets, SPEN and BH220which are commonly used under symmetrical tension and compression are chosen. Mg alloy AZ31B (close-hexagonal crystal HCP), mid-carbon low alloy steel (body-centered cubic crystal BCC) and2090T3(face-centered cubic crystal FCC) which are commonly used under asymmetrical tension and compression are selected in the paper. The calculated results of the new yield criterion are compared with the experimental results. In order to study the differences between the new yield criterion and the existing anisotropic yield criteria, Barlat2000and Hosford are selected to make a comparison in calculated results for symmetrical tension and compression materials, such as AA6022T4、2090T3、Al-2.5wt%Mg. CPB06ex02model, which is more accurate, is selected to be compared in calculated results for asymmetrical tension and compression materials, such as AZ31B and mid-carbon low alloy steel. The results show that the yield locus prediction of the new yield criterion is higher in accuracy, which not only meet the reliability and universal applicability but also have high consistency with other existing calculation results. In the practical forming process of sheet metal, the sheet metal is often in a complex stress state containing shear stress, which means that the directions of the principal stress are not coincidence with rolling directions. So this situation cannot be predicted with the yield criterion created in2D principal stress space. These experimental results, which are needed to solve the coefficients of modified function in every segmentation, are represented to Fourier function of angles between principal stress directions and rolling directions. And the capability of this method to describe the anisotropic coefficients is compared with the calculated results of Hill48and Barlat89.This new yield criterion is implemented into the finite element simulation software ABAQUS by writing a user material subroutine. By compared the results between the simulation and the experiments of the biaxial cruciform tensile specimens which have a certain angle with the rolling direction, the validity of the embedded yield criterion is verified. At last, this program is applied in the practical engineering.