Investigation of non-proportional load paths by using a cruciform specimen in a conventional Nakajima test

Abstract

The prediction of necking in sheet-metal forming is one of the most important tasks in the simulation of forming processes. The most common way to predict the necking for linear load paths is the Forming Limit Diagram (FLD) according to ISO 12004-2. This criterion is valid only for linear load paths. For non-proportional loading paths, Volk introduced a Generalized Forming Limit Concept (GFLC). This model enables the prediction of the formability for any non-proportional loading paths. The experimental investigation of those loading paths is still very challenging and complex. It involves a wide range of testing equipment for the pre- and post-strain of specimens. Jocham introduced a new method that allows the investigation of various non-proportional loading paths by using a cruciform specimen and a draw bead tool in a conventional Nakajima test setup. The height of the draw beads can be varied to create different load paths in the specimen centre. Nevertheless it is necessary for the thinnest part of the specimen to be in the middle of the specimen. This is achieved by milling an indentation into the centre. This mechanical procedure affects the formability of the material. In this paper, the cruciform specimens are laser weld from three single metal sheets in order to minimize the influence of the manufacturing process on the specimen. With this procedure, it is possible to test unmanufactured sheets. However, the crack still occurs in the centre of the specimen. With the new cruciform specimen, we are able to create arbitrary non-proportional strain paths.