Abstract
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.
Highlights
One of the predominant failure modes in metal forming is ductile fracture which is associated with nucleation, growth and coalescence of voids
These authors used Forge 2005 to determine the effect of basic parameters of cross wedge rolling (CWR) on the damage value calculated by the normalized Cockcroft-Latham (NCL) criterion
A comparison of the numerical and experimental results demonstrates that the use of the critical damage values determined by tensile tests in cross rolling processes is incorrect and leads to erroneous conclusions
Summary
One of the predominant failure modes in metal forming is ductile fracture which is associated with nucleation, growth and coalescence of voids. Given the difficulty with determining the critical damage value in hot metalworking conditions, no wonder the number of publications devoted to this problem is scarce This can be observed with respect to cross and skew rolling processes in which variable stresses occurring in the workpiece axis lead to fracture, a phenomenon which is known as the Mannesmann effect. Regarding the use of phenomenological models of ductile fracture in the analysis of cross and skew rolling processes, it must be noted that the first work on this problem was published by Piedrahita et al in 2005 [27] These authors used Forge 2005 to determine the effect of basic parameters of cross wedge rolling (CWR) on the damage value calculated by the NCL criterion. A comparison of the numerical and experimental results demonstrates that the use of the critical damage values determined by tensile tests in cross rolling processes is incorrect and leads to erroneous conclusions. This study proposes a test which consists in performing rotary compression in a cavity between the tools
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