Evaluation of Experimental Forming Limit Curves and Investigation of Strain Rate Sensitivity for the Start of Local Necking

Abstract

The failure prediction in sheet metal forming is typically realized by evaluating the so called forming limit curves (FLC). Up to now, the FLC determination was performed with failed specimens of Nakajima or Marciniak test setups. Standard methods determine the failure by considering the occurrence of cracking and do not consider the possibility of time continuous recording of the Nakajima test. Consequently forming limit curves which have been evaluated in such way are often “laboratory dependent” and deviate for identical materials significantly. This contribution presents an algorithm for a fully automatic and time‐dependent determination of the beginning plastic instability based on physical effects. The algorithm is based on the evaluation of the strain distribution based on the displacement field which is evaluated by optical measurement and treated as a mesh of a finite element calculation. The critical deformation states are then defined by 2D‐consideration of the strain distribution and their time derivatives using a numerical evaluation procedure for detecting the beginning of the localization. The effectiveness of the proposed algorithm will be presented for different materials used for the Numisheet’08 Benchmark‐1 with Nakajima test. Additionally the effect of strain rate sensitivity on the beginning instability of local necking is discussed. It can be shown that the strain rate sensitivity is from major importance and should not be neglected for the forming simulation of sheet metal materials.