Based on the plastic membrane theory and static equilibrium equations,a novel
hydro-
bulging approach is proposed with curve-fitting of the experimental data to establish the
flow stress equation of a
thin-walled tube in hydro-bulging(THB) process,where the site measurements of stress and strain distribution,as well as the profile curvature radii in meridian direction of deformed tube are eliminated.A simple hydro-bulging
test tooling under a conventional single action press without any external hydraulic pressure,but internal one,is developed to obtain required material parameters for the flow stress equations.A series of free bulging experiments on stainless steel tube and low carbon steel tubes with two unrestrained ends are carried out,and the flow stress equations for the two tubular materials are then established in biaxial stress state.The results indicate that an extended flow stress curve in a much wider range of strain than that in
tensile test can be obtained,the flow stress curves by either this approach or tensile test are close under small strain,but bifurcate greatly as strain increases,and the total flow stress level,hardening exponent n-value and strength coefficient K-value in the approach are obviously larger than those by tensile test of corresponding sheet blank.