Stress-based wrinkling criteria and experimental verifications in sheet metal forming

The prediction of wrinkling, one of the major failure modes in sheet metal forming, has been a challenging issue. This work aims at developing a reliable and efficient stress-based wrinkling predictor, which can be easily incorporated into any FEM codes or simplified stress analysis for rapid assessment of manufacturability.; In an effort to assess the onset of flange wrinkling with normal constraint, an analytical model combining energy method and plastic bending theory is established. The critical buckling stress and wavelength are obtained as functions of normal pressure, in-plane tension, sheet thickness and material properties. The present results agree very well with those obtained from the numerical approach, and have demonstrated the excellent predictive capability of the model by comparing with the experiments.; Another compressive instability problem is sheet wrinkling without normal constraints. A general wrinkling criterion utilizing energy equality and the effective dimensions of the region undergoing circumferential compression is proposed Based on a flat plate or curved sheet model with appropriate boundary conditions, the critical buckling stress is obtained as a function of material properties, effective dimensions and in-plane stress ratio. Investigations of three forming cases demonstrate excellent agreements between the present predictions and experiments, and also prove that the introduction of the effective dimensions is critical to the determination of critical buckling condition. The present approach is employed to predict the onset of wrinkling in shrink flanging based on a curved sheet model and also shows good agreements with the experiments. Another application is wrinkling phenomenon in the thin-walled tube bending. A doubly curved sheet model is established to obtain the minimum bending radius without wrinkling in the bending process. The wrinkling limit and sensitivity analysis on material properties and geometry effects for these forming operations provide a guideline in designing/optimizing forming parameters. A novel wedge strip test is designed to experimentally investigate the wrinkling behavior of sheet metal under various boundary conditions and specimen geometry. It is observed that various boundary restrictions yield different but consistent buckling modes and also affect the onset and growth of buckling. The present criterion is also validated by the experimental results.