Flow curves up to high strains considering load reversal and damage

Oliver Hering,A Erman Tekkaya,Felix Kolpak

doi:10.1007/s12289-018-01466-z

Oliver Hering, A Erman Tekkaya + Show 1 more

Open Access

https://doi.org/10.1007/s12289-018-01466-z

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Abstract

The new idea is to produce specimens by forward rod extrusion, where in the core of the extrudate a deviatoric tension-loading is present, which is superposed by an adjustable hydrostatic pressure. Various damage levels are hence possible in the extrudate. Conducting tensile and upsetting tests with the pre-strained specimens both the influence of a load reversal as well as the material weakening through ductile damage on the resulting flow curve is explored. Not only can the results be utilized to identify flow curves of materials up to high strains (ε > 1.7), but also to get new insights into the plastic material behaviour, which can be used for generating or adapting new damage models as well as kinematic hardening models under cold forging conditions. The proposed method was first assessed by means of analytical and numerical methods and then validated experimentally, by the example of the typical cold forging steel 16MnCrS5.

Highlights

The flow curve is the relation between the flow stress and the true plastic strain
The procedure of flow curve evaluation for large strains by means of tensile tests on material pre-strained by forward rod extrusion will be discussed in the following
A new experimental method has been proposed to evaluate flow curves of materials by tensile tests on specimens prestrained by forward rod extrusion

Summary

Introduction

The flow curve is the relation between the flow stress and the true plastic strain. In the field of metal forming, flow curves are necessary for the prediction of forming forces, tool deflection, material flow as well as the resulting product properties of the produced parts. Since in cold forging, the hydrostatic pressure is usually large, a high variety of complex part geometries can be produced. In order to predict the plastic material behaviour under cold forging conditions by means of numerical analyses with sufficient accuracy, the flow curve needs to be defined up to the large strains that occur during the forming process. In the case of tensile tests the maximum plastic strain is given by the onset of necking, when the true strain ε is equal to the corresponding hardening exponent n of the workpiece material.

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