A New Testing Method for the Determination of Ductile Fracture in Cold Forming Processes

Abstract

The primary objective of this study is to show that ductile fracture mechanism in cold forming is highly dependent on the state of stress resulting from the process. So, it is convenient to use identification testing procedure and well defined method for determining equivalent strain at fracture. We propose upsetting test of cylinders and tensile test of cylindrical notched specimens for determining ductility limits in forming processes. The most important, capability offered by these tests is to obtain a triaxiality increasing from negative values to large positive values, such as in extrusion or bulk forming. Investigations realized on steels used in cold forming lead us the conclusion that in all the cases, there is a void nucleation and void growth in the material. However, in the same material, we can observe generalized void nucleation or a localization in small shear bands. So, the application of ductile fracture criteria or failure models with these assumptions needs knowledge of the state of stress in the material. In order to prove such an approach, a simulation of upsetting test and tensile test of cylindrical notched specimens has been realized in using IBM analysis. Eased on experimental results, the authors propose using a failure model based on instability of plastic flow at low triaxiality and using a failure model based on growth of voids at high triaxiality. Furthermore, it has been pointed out that the initial structure of the material is not a significant parameter in the occurrence of ductile fracture.