A robust experimental technique to determine the strain to fracture for plane strain tension

Abstract

The stress state of plane strain tension plays a crucial role in many forming and crash applications. Under plane stress conditions (in thickness direction), most micromechanical and phenomenological models predict a minimum in ductility for a plane strain tension loading (in the plane of the sheet). When looking at Forming Limit Diagrams (FLD) or at modern stress state dependent fracture initiation models, a "plane strain ductility valley" exists between uniaxial and equi-biaxial tension. Given that the ductility reaches a minimum for plane strain tension, the reliable measurement of the strain to fracture for plane strain tension is particularly crucial when calibrating modern fracture initiation models. Many experimental techniques have been proposed in the past, but a standardized universal experimental technique for plane strain tension testing is still missing to date. It is the goal of the present work to develop a robust experimental technique for determining the strain to fracture for plane strain tension. Emphasis is placed on finding a technique that is universally applicable i.e. that provides reliable results irrespective of the material thickness or ductility. In addition, it should be easily automatized in order to be included in up-to-date test protocol dedicated to the prediction of the behavior from large database analysis. The experiments are designed such that the material is subject to proportional loading, i.e. the stress state remains constant throughout the entire loading history until fracture initiates. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes three different types of plane strain tension experiments (flat notched tension, V-bending and the newly-proposed stretch-bending of mini-Nakazima specimens) to elucidate their differences and limitations, and to demonstrate that the newly-proposed technique is the only one that yields meaningful results for all three materials.