Establishment of a new hybrid fracture criterion for cross wedge rolling. (1st February 2020) - Pater, Zbigniew
Highlights: The stress states in the axial zone of parts produced by cross wedge rolling are analyzed; these stress states are, for the first time ever, analyzed in the stress triaxiality–Lode angle parameter plane. A new hybrid fracture criterion combining two fracture mechanisms, i.e. void formation and shear fracture, is proposed. A new rotary compression-based method for damage function calibration is developed; the critical damage function of hot-formed C45 steel is determined by the proposed hybrid fracture criterion. CWR tests are performed, using fracture-causing parameters. The new fracture criterion is verified; it is found that the new criterion helps predict axial fracture with high accuracy. Abstract: This paper relates to the problem of fracture in cross wedge rolling (CWR). A FEM analysis is performed of the stresses in the axial zone of the workpiece formed by this method. Thirteen CWR cases are analyzed for different values of the forming angle, α=<15°; 35°>, spreading angle, β=<4°; 12°>, and reduction ratio, δ=<1.1; 1.9>. Numerical results demonstrate that the stresses in the CWR process are characterized by the mean stress triaxiality ηav =<0.075; 0.334> and the mean Lode angle parameter Θav <−0.905; −0.415>. Under this stress state, fracture may occur as a result of both void formation and shear fracture. A new hybrid criterion of fracture is proposed, which combines 2 well-known fracture criteria, i.e. the maximum shear stress criterion and the Oh criterion (normalized Cockcroft-Latham criterion). It should, however, be mentioned that the effect of the above criteria on the final value of the proposed hybrid criterion depends on the stress triaxiality. The critical value of the proposed fracture criterion is determined by a new calibration test based on rotary compression, in which the stress state is similar to that in parts produced by the CWR method. The critical value of the new fracture criterion is determined for the C45 grade steel in the temperature range of 950–1150 °C. Results demonstrate that increasing the forming temperature leads to an increase in the critical damage function. To verify the proposed fracture criterion, CWR tests are performed under conditions leading to fracture of the material (C45 steel). Experimental findings and numerical results obtained by the new hybrid fracture criterion show a very high agreement in terms of both quality (crack occurrence prediction) and quantity (crack length prediction).