On the effect of stress state on the failure limits of hole-flanged parts formed by SPIF

Abstract

Single Point Incremental Forming (SPIF) is a novel and flexible forming technology that has been used in the last few years to obtain a variety of customized sheet parts. One of the most valuable advantages of this process is its ability to suppress the localized necking of the sheet, allowing a stable plastic deformation up to the ductile fracture of the sheet. Traditionally the fracture in sheet metal forming is characterized by the conventional Fracture Forming Limit (FFL) curve, obtained with Nakazima or Marziniak tests. However, in many cases the SPIF processes exhibit fracture strains clearly above the conventional FFL, showing an unexpected gain of formability. It is well known that ductile fracture in metals is highly affected by the stress state in the material. Therefore, among others, this fact could contribute to explain the experimental differences observed in the FFL obtained by conventional and incremental operations. The present work develops a numerical model in ANSYS to study the mechanics of the fracture process during the incremental deformation operation. The simulations focus on the hole-flanging operation by SPIF in AA7075-O metal sheets of 1.6mm thickness, recently experimentally analysed by the authors. Different configurations of pre-cut hole diameters are simulated. The evolution of the strain paths and the hydrostatic stress in the SPIF process are analysed and discussed. These variables help to explain the apparent enhancement of formability observed in SPIF processes beyond the conventional FFL curve.