Numerical evaluation of measurement accuracy of additional shear strain on tubular-type biaxial bulge test with an angle between axis of anisotropy and axis of principal stress

Biaxial bulge tests using tubular specimens can measure a more significant range of plastic deformations than biaxial tensile tests using cruciform specimens. Biaxial bulge tests are typically performed by coinciding the axes of anisotropy of the material and principal stress. The effect of the shear stress on the anisotropic yield function is measured using uniaxial tensile tests inclined from the axis of anisotropy. To comprehensively measure the yield surface under shear stress, biaxial stress tests with an angle between the axes of anisotropy and principal stress must be conducted. However, the measurement accuracy of the stress and strain in biaxial bulge tests inclined from the axis of anisotropy has not been verified. In this study, biaxial bulge tests of anisotropic sheet metals were analysed using the finite element method as a virtual experiment to verify that the biaxial stress and plastic strain increments can be adequately measured. Biaxial bulge tests without torque were performed using a tubular specimen prepared with an angle between the axes of the anisotropy and orientation of the tubular specimen. The additional shear strain caused by the inclination angle was measured as twist of the tubular specimens. The stress and strain increments obtained from the measurable quantities were compared with the yield surface model used in the finite element analysis. The results confirm that the stress and strain increments in anisotropic materials can be appropriately measured, even in biaxial bulge tests with the inclination angle.