To determine a workability diagram for bulk metal forming processes, it is proposed to use experiments in which fracture occurs at a free surface. A good example of such experiments is an upsetting test with profiled dies. An advantage of this method is that fracture conditions are formulated in terms of strain components whereas stresses are excluded using an exact theoretical solution. In the present study three axisymmetric upsetting tests, with flat dies and with conical dies, are carried out to determine the strain to fracture. The method proposed is then combined with these experimental data to find an approximation to the workability diagram of a tool steel.

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Narrow layers of severe plastic deformation are often generated near frictional interfaces in deformation processes as a result of shear deformation caused by friction. This results in material behavior that is very different from that encountered in conventional tests. To develop models capable of predicting the behavior of material near frictional surfaces, it is necessary to design and carry out tests that account for typical features of deformation processes in a narrow sub-surface layer. In the present paper, upsetting of steel specimens between conical and flat dies is used as such a test. The objective of the paper is to correlate the thickness of the layer of severe plastic deformation generated near the friction surface and the die angle using a new criterion for determining the boundary between the layer of severe plastic deformation and the bulk.

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The results of three different upsetting tests on steel C1530 are presented. The die design is shown to have a marked effect on free surface fracture. A popular empirical ductile fracture criterion (Oyane's criterion) is assessed by means of the experimental data obtained and by a theoretical approach for determining the workability diagram from free surface fracture data. It is shown that the criterion is not appropriate for the material investigated. The range of free surface conditions where experimental fracture data are required for formulating an accurate fracture criterion is specified and a number of tests to create such conditions is proposed.