Yield surface characterization for lightweight alloy sheets via an improved combined shear–tension experimental method

Combined shear and tension (CST) tests are important experimental methods for characterizing yield surfaces for metal sheets, which is vital to ensure the effectiveness of the constitutive models employed in finite element simulation. However, the existing CST experimental method with a reduced thickness specimen, designed for advanced high strength steel sheets, is not suitable for accurately characterizing yield surfaces for lightweight alloy sheets, such as aluminum alloy sheets. In this paper, an improved experimental method employing CST loading along with an appropriate full-thickness specimen is proposed to address the problem. To establish the proposed experimental method, an appropriate full-thickness specimen is presented through finite element method and combined with a newly developed biaxial testing machine. To verify the effectiveness and feasibility of the improved experimental method, virtual simulations and real experiments on the proposed full-thickness specimen obtained from 6K21-T4 aluminum sheets under different CST loading cases are conducted. Research results show that the yield surfaces of the aluminum alloy sheets between simple shear and plane strain (SSPS) can be described accurately by employing the improved experimental method. In addition, according to the experimental results, the prediction capability of the Yld2000 and Hill48 yield criteria is studied. It is found that the commonly used Yld2000 yield criterion cannot accurately predict the yield behavior of the aluminum alloy sheets under shear-dominant loading.