An accurate determination method for constitutive model of anisotropic tubular materials with DIC-based controlled biaxial tensile test

• A novel controlled biaxial tensile testing system was developed using digital image correlation (DIC) as a measurement tool. • The constitutive model of tubular materials considering the evolution of anisotropic characteristics was calibrated accurately. • The accuracy of the calibrated constitutive model was verified in engineering practices by compiling the model into VUMAT for simulating typical free tube hydro-bulging. • The developed DIC-based controlled biaxial tensile test is an effective method to determine an appropriate anisotropic constitutive model for tubular materials. The reliability of numerical simulation results vitally depends on the accurate determination of the constitutive model, which requires accurate and sufficient experimental data. To address the current limitations of the range and accuracy of the experimental data of tubular materials, a novel controlled biaxial tensile testing device was developed using digital image correlation (DIC) system as the deformation measuring tool. Using such a device, arbitrary biaxial tensile stress path can be accurately applied to tubular specimens, and the strain can be accurately and continuously measured over the whole deformation process. Firstly, the accurate experimental data of AA6061-O tubes were obtained through the controlled biaxial tensile tests. Then, using the experimental data, the constitutive model considering the evolution of anisotropic characteristics was calibrated based on the Yld2000-2d yield criterion. Finally, the accuracy of the calibrated constitutive model was verified in engineering practices by compiling the constitutive model into finite element software as a user material subroutine (VUMAT) for simulating tube hydro-bulging. Excellent agreements were achieved between experimentally-determined and predicted bulge height, bulge profile, and pole thickness of bulged tubes over the whole hydro-bulging process. It is demonstrated that the DIC-based controlled biaxial tensile test is effective to accurately calibrate the constitutive model of tubular materials, which can be used for both the research of plastic deformation behavior and practical application of hydroforming industry.