비선형 변형 경로 실험을 통한 균질 이방성 경화 모델 검증

Abstract

During sheet metal forming processes, the material undergoes quasi-linear and non-linear strain paths. A non-linear strain path causes large fluctuations in the hardening behavior of a material that produce phenomena such as the Bauschinger effect, the transient stage with high hardening rate or permanent softening. It is very important to capture these phenomena in the constitutive equations in order to conduct accurate sheet metal forming process simulations. Recently, the Homogeneous Anisotropic Hardening (HAH) model was proposed in MML to describe the hardening behavior of materials subjected to non-linear strain path. Unlike conventional kinematic hardening, in which the yield surface translates in stress space, the HAH model predicts the hardening behavior solely based on the distortion of the yield surface. To verify the validity of the HAH model, non-linear strain path experiments were conducted in this work. The first strain path, called pre-strain, was always uniaxial tension in the sheet transverse direction (90º from rolling direction). For the pre-strain, large tensile specimens were clamped in a suitable gripping system and deformed in order to produce a sufficiently large size area with a uniform strain distribution from which new specimens were machined. After these pre-strain tests, new specimens were subjected to different modes of deformation, namely, uniaxial tension in directions different from the transverse direction, simple shear and biaxial tests with different load ratios. The experimental results, the flow curves and the yield surfaces, were compared with those simulated by the HAH model. These comparisons allow the assessment of the accuracy and shortcomings model, which are very useful to improve the HAH formulation.