Towards sensitive prediction of wrinkling instability in sheet metal forming by introducing evolution of triple nonlinearity: Tube forming

Wrinkling instability is one of key defects restricting the formability of sheet metallic materials How to sensitively predict the wrinkling occurring is one of the most fundamental and challenging issues in sheet metal forming operations to ensure defect-free deformation. In the study, taking typical axial compression and draw bending of tubular materials as the cases, the buckling modes of sheet metal forming under less and more degrees of constraints are experimentally compared, then a hybrid integrated prediction framework of wrinkling in sheet metal forming is constructed by introducing evolution of triple nonlinearity in geometry, material and boundary conditions, viz., multiple geometric micro-imperfections (GMI), anisotropic/asymmetric properties (AAP) and explicit FE algorithm, respectively. The results show that transferred wrinkling modes may occur for less constrained process, while the multiple constraints make the wrinkling occur in a higher order buckling mode consuming more energy. Multiple GMI modes constructed by linear buckling analysis are superimposed into explicit FE models, viz., the first three buckling modes superposed for axial compression, while the higher order buckling mode analogous to actual wrinkling selected for draw bending. Three constitutive models, viz, Mises, normal anisotropic Hill’48-r and stress invariant based anisotropic/asymmetric models (SIM), are comparatively implemented in the explicit FE models. Compared with the perfect model without introducing GMI, the GMI-implemented FE model provides more sensitive prediction of wrinkling for both forming cases. The SIM-implemented FE model predicts the instability more sensitively than other constitutive models in tension-compression asymmetry dominated bending case. By coupling the GMI and SIM into explicit FE models, the desired wrinkling prediction capability for both forming cases is further confirmed since evolution of triple nonlinearity is simultaneously considered.