The FLC, enhanced fromavbility, and incremental sheet forming

The FLC is a well known concept in the sheet metal forming world. It is used to map the material’s formability and the make-ability of a product. The FLC is valid only within certain restrictions. These restrictions are: A: a straight strain path; B: absence of bending; C: absence of through-thickness shear; D: a condition of plane stress. The formability of a material can be increased significantly if one is allowed to violate any of these restrictions, meaning either: use a complex strain path, incorporate bending, incorporate through-thickness shear, or apply a contact stress. Both shear and contact stress change the stress state, and both lower the yield stress in tension and raise the necking limit up to a certain level. Bending creates a non-uniform stress distribution over the thickness of the sheet, resulting in a reduction of the yield force in tension, and it creates a range of stable elongation depending on the sheet thickness at each passage of the punch. The effect of a complex strain path depends on the particular situation; in incremental sheet forming it is based on non-isotropic hardening. In general it will not be possible to create such conditions in the entire product at once. However it is possible to do this intentionally in a small, restricted zone by creating special situations there. By moving this zone over the entire product the whole part can be made with increased formability. This technique of incremental forming is explained briefly. The special conditions around the punch indeed violate the FLC restrictions mentioned above. The enhanced formability obtained in incremental sheet forming is illustrated with many examples.