Impact and Crashworthiness Laboratory. Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT), Cambridge MA, USADepartment of Mechanical and Process Engineering, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
M B Gorji;D Mohr
The proper numerical treatment of strain and stress distributions are an indispensable prerequisite of predicting the fracture initiation in deep drawing operations. The formability of sheet metal is generally limited by the so-called Forming Limit Curve (FLC) that is broadly accepted in forming community. However, fracture actually occurs at higher strains than the membrane instability predicted by the FLC. In the present paper, the results of the well-known Nakazima experiments (which are traditionally used to determine a FLC) are explored beyond the necking limit to determine the multi-axial fracture response. Indeed, the strains at the fracture initiation have been determined using a hybrid experimental-numerical approach, i.e. through the numerical simulation of each experiment. To increase the robustness of the hybrid experimental-numerical technique, we also measured the post-mortem thickness of all failed specimens. Moreover, in view of characterizing the local plasticity, the non-quadratic full stress constitutive model along a recently Hosford-Coulomb fracture criterion are employed. The candidate tests to determine the fracture parameters are the Nakazima test and a special configuration of the cup drawing test, which fails under the out-of-plane shear loading range. The obtained complex constitute model are then applied to predict the fracture in a new designed triangle shape part for deep drawing. This complex new triangular deep drawing shape is fabricated to validate the constitutive model and predict a crack initiation.