Prediction of ductile damage and fracture in the single- and multi-stage incremental hole-flanging processes using a new damage accumulation law

Incremental hole-flanging (IHF) is a process in which a sheet with a pre-cut hole is flanged by the single point incremental forming (SPIF) process. Fracture prediction in IHF, such as SPIF, is associated with many challenges due to the deformation mechanisms. The purpose of this paper is to overcome the existing limitations and challenges and, thus, to predict accurately failure in single- and multi-stage IHF processes. To this end, the modified Mohr–Coulomb (MMC) criterion was implemented using an appropriate user subroutine in a finite element method (FEM) model. The AA6061-T6 aluminum alloy sheet, which has low formability and is fractured from its free edges in the IHF process, was examined as an example. Initially, a linear damage accumulation law, in which the prediction error is high due to the nonlinear stress and strain states in the IHF, was used to predict the fracture. Therefore, in the next step, a nonlinear damage accumulation function was utilized. While the nonlinear accumulation accurately predicts the single-stage IHF fracture, it is not able to predict the fracture well in the multi-stage IHF. It was observed that in multi-stage IHF, the rate of damage accumulation decreases with increasing the number of forming stages. Accordingly, a new nonlinear damage accumulation rule was developed. Experimental and numerical results indicated acceptable accuracy of the proposed nonlinear accumulation in the fracture prediction in the single- and multi-stage IHF processes.