Analysis of wrinkling during sheet hydroforming of curved surface shell considering reverse bulging effect

Wrinkling in unsupported region is a worthy problem to be solved in sheet metal forming process. Sheet hydroforming is advantageous in the prevention of unsupported wrinkles. However, the simply increasing of liquid pressure is not enough to suppress the wrinkling even though with the occurrence of "reverse bulging effect".1 In order to predict and control the wrinkling quantitatively in unsupported region for thin-walled shells with curved surface, a theoretical model on critical wrinkling stress was proposed by considering proper "reverse bulging effect" based on energy method. The influence of liquid pressure and other parameters on the critical wrinkling stress was analyzed. The critical loading path of the liquid pressure to control wrinkling was obtained by combining critical wrinkling stresses and circumferential stresses. An experimental setup for an extremely thin-walled shell with semi-ellipsoidal geometry was designed and manufactured to verify the theoretical model. It is found that at a certain punch stroke, the magnitude of the critical wrinkling stress increases and that of circumferential compressive stress decreases with the improvement of the liquid pressure. The critical loading path can be utilized to get well formed shells with a ratio of thickness to diameter equals 0.27% in the experiments. The proposed method can be applied to predict and control wrinkling in unsupported region for hydroforming of thin-walled shell with high accuracy and considerably reduced simulation time.