Application of ductile fracture criteria in spin-forming and tube-bending processes

As one of the principal failures that occur in metal spin-forming and tube-bending processes, ductile fracturing restricts the process of metal forming. It is difficult to predict the rupture and forming limit for the complex processes of spin forming and tube bending. Thus, in this paper we aim to choose reasonable ductile fracture criteria to accurately predict the time, position and distribution of the damage that occurs in these processes. By embedding the Lemaitre and Cockcroft–Latham (C&L) criteria into VUMAT (user material subroutine of the ABAQUS/explicit finite element method (FEM) code), finite element models are built and validated for predicting rupture failure and forming limit in the shear spin-forming, splitting spin-forming and tube-bending processes. The results show that the Lemaitre criterion is better than the C&L criterion at accurately predicting the position at which damage will occur and its distribution for both spin forming and tube bending. The maximum feed ratio varies within a small range with the splitting angle of the roller. There is an obvious difference between the distribution, variation and level of the damage that can occur for a normal bending radius without rupturing, and that for a very small bending radius, where there is a greater risk of rupturing. For tube bending using a very small bending radius and without a mandrel in the tube, the pressure die velocity increases, so the flattening of the bending area increases to some extent, which makes it opposite of normal tube bending.