Production of bistable fully closed metallic shells by introducing residual stresses during bending processes

Bistable fully closed metallic shells are of great interest as lightweight construction components, which could be transported without enclosure and unfolded at the construction place. The effect of bistability in metallic shell structures is achieved by a specific distribution of residual stresses over the shell thickness, which can be introduced by bending processes. So far no fully closed structures have been achieved in metallic shells. Hence, the aim of this paper is the identification of suitable bending radii combinations in order to produce a bistable shell with a fully closed tube shape in the deployed state. Therefore, a Finite Element (FE) model was established to determine whether bistability occurs in dependence on the bending radii. The numerical model indicates, that bistability can be achieved for 0.2 mm thick spring steel 1.1274 (AISI 1095) by applying two die-bending operations with small bending radii of 6–8 mm. Accompanying experiments with a closed-die incremental bending tool were performed. A good transferability of the numerical results regarding the occurrence of bistability and the calculated residual stresses is shown. For the second aspect, X-ray diffraction measurements of residual stresses were used for comparison. Finally, the present work presents a process route to generate a bistable shell with a fully closed tube shape structure in the deployed state, having a radius of 40.95 mm.