Investigation of different techniques to evaluate the FLC from experiments and FE simulations

摘要

The formability of sheet metal is highly related to the material's resistance to strain localisation and fracture. The Forming Limit Curve (FLC) is one way to evaluate the tendency to instabilities during the forming operation of a material for different strain states. The Nakazima test is a common technique used to experimentally determine the limiting strains. In this paper a slightly modified version of the proposed ISO-standard used at Outokumpu/Avesta Research Centre (ARC) is presented. The method considers the limiting principal strains before and after failure has occurred. The obtained results from the present approach are compared with previous internal methods used at the company. The previous internal methods consisted of evaluating the limiting principal strains at the maximum punch force before fracture or at a set punch distance before fracture. An austenitic stainless steel grade (254 SMO~R) is used in the study. The method will in this work be called the Interpolation Method (Evl). A Finite Element (FE) model of the Nakazima test is modelled in LS-DYNA? with the goal to be able to simulate the experimental test. In order to compare the FLC between the experimental and the numerical results, one instability indicator is proposed based on the onset of fracture in the FE model.