Theoretical experimental analysis of fracture initiation at the free surface in upsetting between conical and flat dies

Abstract

The ductile fracture criterion based on the workability diagram and an average value of the triaxiality ratio over the strain path is quite flexible for fitting to experimental data. Moreover, its advantage is that the fracture initiation at free surfaces can be predicted without measuring stress. It is sufficient to find the distribution of insurface strains from experiment and, then, theoretical arguments can be used to find the strain to fracture for rigid plastic, hardening material obeying Mises' yield criterion and its associated flow rule. Such an experimental/theoretical method is adopted in the present paper to investigate the fracture initiation in upsetting of hollow cylinders between conical and flat dies. A distinguished feature of this process, as compared to many conventional processes used in ductile fracture studies, is that there is no plane of symmetry and the fracture initiation occurs at the line of the intersection of two traction free surfaces. The primary objective of the present paper is to understand an effect of this geometric singularity on the applicability of ductile fracture criteria. © 2008 Verlag Stahleisen GmbH, Düsseldorf.