Experimental Method for Multistage Loading Tests with Various Prestrain Paths

Two-stage loading tests with various prestrain paths between simple shear and equibiaxial tension are essential for understanding multistage sheet metal forming processes. Yet, as to each existing experimental method, two-stage loading tests can only be realized with one type of prestrain path. Worse still, two-stage loading tests with various pre-deformation paths between simple shear and uniaxial tension are unachievable through current experimental methods. In this paper, an experimental method with controllable prestrain paths is proposed to resolve these problems. The principle of the experimental method for achieving two-stage loading tests with various prestrain paths is first given. Then a tension and shear biaxial loading method is established and specimens suitable for the two-stage loading tests are determined through the finite element analyses of the strain field uniformity, accuracy of the calculated plastic work per unit volume and strain paths of specimens under different loading conditions. Subsequently, experiments on 6K21-T4 aluminum sheet are performed to validate the feasibility and capability of the proposed method. It is found that the mechanical properties and behavior of the material subjected to non-linear strain paths with different prestrain paths between simple shear and plane strain tension can be obtained by changing the loading conditions acting on the boundaries of the new flat specimen. Therefore, the proposed experimental method can be used for multistage loading tests of sheet metals with various prestrain paths, especially with different prestrain paths between simple shear and uniaxial tension, which solves the problems mentioned above.