Non-Linear strain paths in Sheet Metal Forming

Abstract: Today's automotive requirements have resulted in complex Sheet Metal Forming (SMF) processes of Sheet Metal (SM) with reduced formability, and thus it is crucial to be able to predict formability accurately to prevent material failure during SMF. Formability predictions today utilize Forming Limit Curves (FLC)s in Finite Element Analysis (FEA), but  FLCs are not valid for the Non-Linear Strain Paths (NLSP)s generated during SMF. One purpose of this thesis is thus to increase the knowledge on FP handling NLSP,  which was obtained through providing suggestions of failure models for handling NLSP effects, based upon literature on the subject. Generating NLSP experimentally is both time and material costly with the conventional method, thus the second purpose of this thesis was to increase the knowledge on test procedures for generating NLSP in SM. Based upon the findings of Chandramohan \cite{chandramohan_study_2021} five test procedures for generating NLSP were put forward, and the Nakajima test with modified punch geometry was chosen for further study.   In this thesis, the NLSP characteristics of two modified punch geometries were evaluated by FEA performed using LS-DYNA. For the FEA three specimens with blank width of 50, 100 and 200 mm was used, and the anisotropic Barlat yld2000  was used as the material model. This material model was calibrated to material data of Mild steel CR4, Aluminium alloy AA6016, and Dual-phase steel DP800. The results for all materials showcased similar reacquiring general NLSP characteristics at the corners of the punch features, which are unfavorable positions when failure by necking is evaluated, and thus it was concluded that the tested punch geometries are not favorable and more development of the punch geometry is needed.