Novel Punch Design for Nonlinear Strain Path Generation and Evaluation Methods

The forming limit curve (FLC) is a common method to assess material formability in sheet metal forming processes. It is determined with the Nakajima or Marciniak test according to ISO 12004-2 [1]. The disadvantage of these test procedures is that the results are only valid for linear strain paths. In most real sheet metal forming processes, like deep-drawing of complex car body parts or multi-step processes, nonlinear strain paths exist. It is well-known that the classic FLC cannot describe material failure for nonlinear strain paths.At the Institute for Metal Forming Technology (IFU), new punch geometries have been developed to realise specific nonlinear strain paths in a standard Nakajima testing environment. The formability of sheet materials under nonlinear loading can be determined more accurately when using these new punch geometries than with the classic Nakajima test setup. Different strain paths can be realised depending on the specimen and the punch design, in order to evaluate the formability of the material according to strain conditions as they occur in real forming processes.Within this paper, the results of different punch geometries have been tested using the mild deep-drawing steel DC04. The strain conditions before crack initiation are compared to the standard FLC and to the newly developed IFU-FLC criterion, which can predict material failure under nonlinear strain paths.