As pointed out previously in Ch. 2, sheet-metal forming differs a lot from bulk-metal forming in that the stress condition in a sheet-metal forming operation usually is characterized by tensile stress, whereas most commonly in bulk-metal forming there is compressive stress. Because of this, necking and fracturing of the workpiece material is a much larger problem in sheet-metal forming than in bulk-metal forming. A useful tool for optimization of sheet-metal forming processes, so that they can be conducted without problems due to necking or fracture, is circle grid analysis. The principles applied in circle grid analysis will therefore be described in this chapter. After this, some technological test methods applied in order to test the formability of sheet metals will be described: plane strain stretching, biaxial stretching, and the Erichsen test.

The principles used when forming-limit diagrams (FLDs) are made will be explained. It will also be shown how such diagrams and circle grid analysis can be used in order to optimize the deformation conditions in a typical industrial sheet-metal forming operation.

Elastic springback is a phenomenon of great importance in sheet-metal forming. Commonly the cross section of the sheet does not become fully plasticized in such processes. In bending, the midlayer of the sheet will deform elastically only, and will therefore cause elastic springback in the material when the forming load is removed.

Sheet metals used for forming are commonly anisotropic, i.e., they have different properties in different directions.