A simple method to determine pressure curve for sheet hydro-forming and experimental verification

Abstract

A simple method to determine optimal pressure curve for sheet hydro-forming has been proposed. The determination of the pressure curve is carried out by characterizing the curve three parameters, e.g. initial pressure, final pressure and pressure path. Through the study of previous works, general shape of the pressure curve for hydro-forming is monotonically increasing with respect to a punch stroke. Since the penetration volume to a pressure chamber by a punch is of the similar characteristics as the punch stroke, and the shape of the pressure–stroke curve is non-linear in general, the pressure path is assumed to be proportional to the penetration volume by a punch, rather than punch stroke. During the hydro-forming experiment, it is observed that the major cause of failures in the early forming stage is related to lifting of a blank from a die surface due to the penetration of punch. Now the initial pressure is determined from the lifting-prevention condition. From the initial pressure, the assumption of pressure path and some chosen trial final pressure values, the pressure curves are interpolated in a straightforward manners. Then, FE analyses are carried out to predict whether failure occurs or not. From the result of FE analysis, optimal final pressure curve has been determined among the trial final pressure curves. The corresponding experiment has been carried out with an experimental hydro-forming press where the pressure is controlled by a proportional relief valve. The press is relatively inexpensive, under an open-loop control system. Through the comparison of experiment and analysis, the predicted pressure curve has been verified an optimal for the successful forming since no defect has been observed.