Punch in a Punch: Validating FLC and fracture models for severe strain path changes

While generating experimental linear loading strain paths is still required for the identification of Forming and Fracture Limit Curves, non-linear loading paths are necessary to validate models for industrial applications. Commonly non-linear loading paths are achieved by interrupting oversized uniaxial or biaxial tensile experiments and extracting pre-strained specimens for further forming or fracture testing. Due to the inherent multiple manufacturing steps, this method is challenging to automate, which denies the generation of large datasets for deep analysis. The present study demonstrates that severely non-linear loading paths can be obtained in a high-throughput manner from a single specimen by means of a telescopic forming approach—specifically, a punch-in-a-punch system—within an automated Nakazima setup. Two steels and two aluminium alloys are tested, each using sets of seven Nakazima specimens, subjected to a two-step forming process. The first step is an interrupted Marciniak forming test. The displacement is then stopped and held while a secondary piston is moved out of the Marciniak punch's inner part, effectively generating a second loading path.