Stress-based necking and failure for incremental sheet forming

Incremental sheet forming (ISF) is a flexible process in which sheet metal is formed by a progression of localized deformation. However, the overall mechanics is complicated and special conditions, such as bending under tension, cyclic bending & unbending, and shear deformation occur during the process which contribute to the overall enhancement of formability. The research investigates the deformation mechanisms in ISF with relation to necking and failure. A strain-based forming limit criterion is widely used in sheet-metal forming industry to predict necking. However, this criterion is strictly valid only when the strain path is linear throughout the deformation process. Strain path in ISF is often found to be severely nonlinear throughout the deformation history. Therefore, the practice of using a strain-based forming limit criterion often leads to erroneous assessments of formability and failure prediction. On the other hands, stressbased forming limit is insensitive against any changes in the strain path and hence it is used to model the necking limit which is combined with the fracture limit based on maximum shear stress (MSS) criterion (Stoughton and Yoon, 2011). Simulation model is evaluated for a single point incremental forming using AA 6022-T43, and checked the accuracy against experiments carried out with an ABB robot. The proposed model has given a good scientific basis for the development of ISF and its usability over conventional sheet forming process.