NEW STRAIN HARDENING MODEL FOR SHEET METALS AND ITS APPLICATION ON PREDICTING SPRINGBACK OF TITANIUM SHEET

摘要

In the field of numerical studies for sheet metal forming, strain hardening equation strongly influences on the computational results, especially in term of spring-back as well as forming limit diagram of sheet metal. This study presents a new strain hardening model named as Kim-Tuan model in order to characterize hardening behavior of sheet metals in all ranges of strain. To verify the advantage of proposed model in comparison with other well-known strain hardening models for the task of capturing the hardening behavior of sheet metals, a series of uniaxial tensile tests of industrial sheet materials are performed to achieve the stress-strain relation. The studied materials for this task are AL6016-T4, DP980 and commercially pure titanium (CP Ti) sheets that are examples for the face centered cubic, body centered cubic, and hexagonal closed packed structure metals, respectively. Furthermore, the strain hardening equation of CP Ti sheet in form of Kim-Tuan model is applied into a ABAQUS finite element code to predict spring-back amount in bending test for this material in order to highlight the benefit of proposed equation in numerical field. For this goal, Hill quadratic yield function with non-associated flow rule is adopted to describe yield locus of CP Ti sheet. The evolution of mixed isotropic-kinematic hardening is modeled based on nonlinear kinematic hardening theory of Armstrong-Frederick formulation. Additionally, semi-implicit stress integration scheme of return mapping algorithm is used to compute the stress over each time increment. To evaluate the accuracy of the prediction, bending testes are carried out and compared with computational results. It is seen that spring-back prediction is highly precise with experimental data and it is concluded that the proposed hardening model can be applied in the field of numerical study of spring-back for CP Ti sheet material.