Efficient approach to plastic wrinkling prediction for multi-intersecting stiffened panels in spherical die forming

Incremental die forming is a novel flexible forming process for fabricating large integral panels in which a pair of smaller tools is used to press the large structure multiple times at different positions. However, during each pressing, the material of the flange region shrinks in the circumferential direction because of the flow into the die, and thus it tends to wrinkle. In this study, the wrinkling behavior in the flange region of the polygonal panel deformed in a relatively smaller spherical die was predicted by a theoretical approach that determined the moment when the circumferential stress reached the critical wrinkle limit. The stress distribution of the flange region was derived by considering the plastic behavior reflected by the plastic modulus and shear stress component caused by the nonaxisymmetric shape. The critical circumferential wrinkle stress of the stiffened flange skin was calculated using the energy method. Both stresses are related to the radial dimension of the panel edge, which varies with the punch stroke. Therefore, the critical stroke during the pressing process was obtained and verified by finite element model simulations and experiments on regular hexagonal and square flanged panels, proving the accuracy and efficiency of the approach. The factors affecting the wrinkling behavior, including the forming radius, die size, and panel size, were then investigated through a theoretical analysis and simulations. This approach provides a valuable reference for the design of appropriate punch strokes that can prevent severe flange wrinkles during incremental die forming.