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Ductile fracture is one of the most common failure modes in hot metal forming. It can be predicted by means of so-called damage
functions that describe the relation between stress, deformation and fracture initiation. A practical use of these functions requires
the knowledge of the critical damage value of the material that is determined by calibration tests based on compression, tension
and torsion. For the prediction to be correct, one must ensure that the modelled and real stresses are in agreement. Previous studies
did not offer any effective test for determining critical values of damage under changing load conditions that occur in cross and
skew rolling processes, among others. To compensate for this knowledge gap, researchers at the Lublin University of Technology
have developed a new test consisting in rotary compression of a test-piece in a cavity between the tools, which is described in this
paper. In the proposed test, a cylindrical test-piece is rolled over a cavity (impression) created by grooves on two mating tools.
The cavity height is smaller than the test-piece diameter. At the critical value of the forming length, the state of stress induced
thereby in the test-piece axis causes fracture. Knowing the critical forming length, it is possible to determine the critical value of
damage by numerical modelling. The practical application of the proposed test is illustrated through the case of C45 grade steel
subjected to forming in the temperature range 950–1150 °C. The analysis makes use of the normalized Cockcroft-Latham (NCL)
criterion of ductile fracture.
