ABSTRACT
Strain based Forming Limit Diagrams (FLD) are used to predict a failure in sheet metal forming. The main problem for strain based forming limit diagrams is its inability to predict failures accurately in various sheet forming operations where there is a change in strain path and mode of deformation. Whereas the stress based forming limit diagram can reveal formability limit of material more precisely. Strain based criterion is valid only when the strain path is linear throughout the whole deformation process. Stress based forming limit criterion is not dependant on strain path. So stress based criterion can be used for predicting forming limits in forming operations where strain ratio does not remain constant. There are several situations in forming operations where stress based FLD can be applied to predict the failure correctly like incremental forming, hydroforming, multistage forming, deep drawing of pockets etc. Different yield criterion is used to transform strain based FLD into stress based FLD. For incremental sheet forming process conventional Forming Limit Curve(FLC) is not used as it is prepared considering necking limit whereas ISF process is limited by fracture. Hence, in literature it is termed as Fracture Forming Line(FFL). In this paper, first strain based Fracture Forming Line(FFL) is established by generating different shapes by incremental sheet forming process. From strain based FFL, stress based FFL is prepared using Hills quadratic criterion.
