Mechanics of fracture in single point incremental forming

Single point incremental forming (SPIF) is a sheet metal forming technique which has gained considerable interest in the research community due to its enhanced formability, greater process flexibility and reduced forming forces. However, a significant impediment in the industrial adoption of this process is the accurate prediction of fracture during the forming process. This work uses a recently developed fracture model combined with finite element analyses to predict the occurrence of fracture in SPIF of two shapes, a cone and a funnel. Experiments are performed to validate predictions from FEA in terms of forming forces, thinning and fracture depths. In addition to showing excellent predictions, the primary deformation mechanism in SPIF is compared to that in conventional forming process with a larger geometry-specific punch, using the deformation history obtained from FEA. It is found that both through-the-thickness shear and local bending of the sheet around the tool play a role in fracture in the SPIF process. Additionally, it is shown that in-spite of higher shear in SPIF, which should have a retarding effect on damage accumulation, high local bending of the sheet around the SPIF tool causes greater damage accumulation in SPIF than in conventional forming. Analysis of material instability shows that the higher rate of damage causes earlier growth of material instability in SPIF. A new theory, named the ‘noodle’ theory, is proposed to show that the local nature of deformation is primarily responsible for increased formability observed in SPIF, in-spite of greater damage accumulation as compared to conventional forming.