Loading of mini-Nakazima specimens with a dihedral punch: Determining the strain to fracture for plane strain tension through stretch-bending

Abstract

A new experimental technique is proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The proposed technique makes use of a mini-Nakazima specimen that is clamped onto a 30 mm diameter die and subjected to out-of-plane loading through a dihedral punch. While other techniques for determining the strain to fracture for plane strain tension loading (e.g. notched tension or V-bending) suffer from limitations with regards to the thickness and ductility of the material to be characterized, the mini-Nakazima experiments are more robust and universally applicable. Experiments are performed on mini-Nakazima, notched tension and V-bending specimens extracted from 1.2 mm thick aluminum 2024-T351, 0.8 mm thick DP450 and 1.6 mm thick DP980 steel. In addition, detailed numerical simulations are performed for each experiment. The hybrid experimental-numerical results show the limitations of existing experimental techniques and demonstrate the reliability of the proposed stretch-bending technique. As a by-product, it is shown that the Yld2000-3D yield function with associated flow rule provides a reasonably accurate description of the large deformation response of aluminum 2024-T351. Equally good predictions are obtained for the DP450 and DP980 steels when using a von Mises yield function in conjunction with a non-associated Hill’48 flow rule. Furthermore, to characterize the effects of the stress triaxiality and the Lode angle parameter on the fracture response of the above materials, the strains to fracture are determined for simple shear and equi-biaxial tension.