Simplified measurement of the strain to fracture for plane strain tension: On the use of 2D DIC for dual hole plane strain tension mini Nakazima specimens with dihedral punch

A plane-strain-tension stress state leads to a minimum in ductility in most micro-mechanical and phenomenological fracture models as well as Forming Limit Curves. Hence, this stress state plays a crucial role in many applications and a reliable measurement of the strain to fracture under plane strain tension is of particular importance when calibrating modern fracture initiation models. Recently, a new experimental technique has been proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The basic configuration of the novel setup includes a dihedral punch applying out-of-plane loading onto a Nakazima-type disc-shaped specimen with two symmetric circular cut-outs. 3D Digital Image Correlation (DIC) is used to measure the surface strains of the specimen up to fracture. In contrast to the widely used V-Bending test, the maximum obtainable strain is not limited when using this set up, and fracture will always initiate after proportional loading in a plane strain tension stress state. In the present study, a major simplification of the testing methodology is proposed, reducing from 3D DIC to a simple 2D DIC method for measuring the fracture strain. Comparisons are presented on three metals, a 1.5 mm thick DP600 steel, a 0.8 mm thick DP450 steel and a 1.2 mm thick AA2024 aluminum alloy.