Analysis of the influence of stress triaxiality on formability of hole-flanging by single-stage SPIF

摘要

Traditionally the fracture in sheet metal forming is characterized by the fracture forming limit (FFL) curve typically obtained by using conventional Nakajima tests. This curve is implicitly assumed a material property. Single point incremental forming (SPIF) is a novel and flexible forming process characterized by the ability to suppress local necking and develop stable plastic deformation up to sheet fracture. In many cases, these fracture strains are clearly above the conventional FFL. The current work presents a numerical study of the evolution of stress triaxiality in SPIF in the (epsilon) over bar - (eta) over bar space. The simulations are validated with hole-flanging tests by single-stage SPIF over AA7075-O sheet of 1.6 mm thickness. The difference in the average stress triaxiality at fracture exhibited in SPIF and Nakajima tests would allow explaining the enhancement on formability observed in incremental sheet forming.