Bending

Technology suitable for both serial and small quantity production

In industrial application, micro bending processes are frequently used in case of spring, clamp or lead frame production. Considering the characteristics of this process, namely part dimension being close to sheet thickness, conventional FE-simulation programs, which assume plain strain condition in the deformed area, are not applicable. In order to overcome this, a first model has been developed that enables the calculation under plane stress conditions or in the improved version considering the anisotropy of the material. The analyses of the experimental results have shown that the process forces relative to the size decrease with miniaturization in case of small grains. In case of large grains (only few grains over the sheet thickness), the force is increasing again. This effect has been confirmed by investigations performing scaled bending experiments on metallic foils. It has been shown, that depending from the thickness of the sheet (scaled from 200 microns down to 25 microns) and from the material structure, bending moment and thus the spring back angle increases when scaling down. This confirms the previously described theory of strain gradient plasticity. When scaling down the foil thickness, two contrary effects appear: the effect of a reduction of the flow stress due to an increasing fraction of surface grains on the overall volume and the effect of an increasing flow stress caused by the increasing density of geometrically necessary dislocations. When the foil thickness is getting smaller, the latter effect gains superior influence resulting in both, higher normalized bending moment and higher spring back angle.

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Metals Processing

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