Describing tube formability during pulsating hydroforming using forming limit diagrams

Pulsating hydroforming is a novel forming technique that applies pulsating hydraulic pressure to deform tubular materials. Larger expansions and more uniform wall thicknesses in tubes have reportedly been achieved using this technique. However, periodic oscillations of hydraulic pressure acting on the tubes during pulsating hydroforming make the tube deformation behaviour and formability unpredictable. Forming limit diagrams, which consist of two forming limit curves in a major–minor strain coordinate system, are widely used to indicate the formability of sheet materials in plastic deformation. The comparable use of forming limit diagrams to indicate the formability of tubular materials under the pulsating action of hydroforming has not been previously established. In this study, pulsating and non-pulsating hydro-bulging experiments were performed on SS304 stainless steel tubes. Under distinct tension–compression and tension–tension strain states with and without active axial feeding, the forming limit curves for the deformed tubes were constructed based on the experimental data. The effects of various hydraulic pressure pulsating parameters, including pulsating amplitude and frequency, on the forming limit curves were analysed and compared. The experimental results showed that each of the forming limit curves under pulsating hydro-bulging was higher than the forming limit curves under non-pulsating hydro-bulging, thereby confirming the influence of the pulsating parameters. In general, the height of the forming limit curves increased as the pulsating amplitude and frequency increased, largely independent of the tension–compression and tension–tension states. Overall, the results showed that the proposed method for determining the forming limit curves (and the subsequent forming limit diagram) for tubes during pulsating hydro-bulging is feasible.