Free Expansion Bulge Testing of Tubes For Automotive Hydroform Applications

Free expansion of straight tubes is the simplest test to evaluate tube properties for hydroforming applications and to provide basic understanding of the mechanics of tube hydroforming. A circular cylindrical tube is sealed at both ends and fluid, usually water, is pumped into the tube to increase its internal pressure to bulge and burst the tube. Previous numerical simulations of the free expansion tube test were limited to modeling the midsection of the tube under various assumptions of deformation path. The simulation results obtained deviated from the experimental results under all simulation conditions considered. A new model is developed in this paper in which the whole tube is simulated instead of considering only its mid-section. Judged by the pressure-expansion relations, the model accurately predicted free expansion hydroforming tests results. Burst predictions were made using the plastic strain criterion, the traditional forming limit diagram and the stress-based forming limit diagram. Detailed discussions concerning the specification of the tube properties in hydroforming and the application of the burst prediction criteria in hydroforming is presented.