Paper Title
EFFECT OF THE TORSION PRE-STRAIN ON STRESS TRIAXIALITY OF THE MILD STEEL

Abstract
Abstract - The aim of the research was to describe the effect of torsion pre-strain on the fracture characteristics of the steel ISO 11 375 during tensile testing of round barwith and without groove. Both specimens were subjected to three levels of torsional plastic pre-strain and then tensile experiments were performed. This procedure was solved by numerical modellingusing the ANSYS code based on the constitutive elastoplastic equations determined from the tensile test of the smooth specimen without pre-strain. The experimental and calculated engineering stress-strain diagrams practically coincided with those of the FEM model. The dependences of the stress triaxialityη = σh /σi (σh is the hydrostatic stress, σi is the stress intensity) on the equivalent strain ε for smooth specimens and specimens with groove with three levels of the pre-strain were determined. For the region beyond the strength limit the dependence of triaxiality on the true strain was calculated using the Bridgman correction. The coordinates of the fracture locus points were determined from the stress-triaxiality dependencies and approximated by the power function. All torsional pre-strains caused a decrease of the tensile fracture strain in smooth specimens, which can be explained by the increase in the triaxiality observed in the FEM models. In the notched specimens, no systematic increase in fracture strain was observed although the triaxiality decreased for the two highest pre-strains level. Keywords - Torsional Pre-Strain, Tensile Test, Bridgman Correction, Triaxiality Space, Stress Triaxiality, Stress-Strain Curve.