Inclined rotary forming (IRF) combines bending and axial compression with rotation and is employed for forming flange shaft-structured parts from round rod billets. Although the forming load is smaller than that in conventional processes, cracks may develop during this process. However, it is difficult to predict the onset of cracks using conventional ductile fracture prediction equations. In this paper, the ductile fractures were predicted using the anisotropic ductile damage equations that the present authors previously developed. The proposed equation was expressed as a second-rank tensor resulting from the product of the stress tensor and the strain increment tensor. Referring to previously published results on the compression of cylindrical specimens, the difference between diagonal and longitudinal sidewall cracks was assumed by considering the anisotropy of ductile damage. The proposed equation was then applied to the IRF process, where diagonal cracks developed at the outer rim of the flange. Consequently, it was found that the proposed anisotropic damage model can predict the onset of diagonal cracks on the outer rim and the effect of the lubrication condition on the onset of these cracks.

In the present study, the ductile fracture behavior occurring during the cold forging of hollow parts was tested under various back-pressure force conditions. From the experimental result, it was found that the position and direction of ductile fracture can be changed in accordance with the applied back-pressure force. Numerical simulations were carried out to determine the appropriate ductile fracture criteria. The inverse calculation technique was applied to improve the simulation accuracy, adjusting the flow stress data. Two types of ductile fracture model, the traditional Cockcroft-Latham model and its modification, were evaluated. In the modified model, the damage value was selected from among several damage values calculated using stress and strain components transferred to different directions. From the result, the modified ductile fracture model predicted the ductile fracture behavior more accurately compared with the traditional Cockcroft-Latham model.