Analytical model for tube hydro-bulging tests, part II: Linear model for pole thickness and its application

Hydro-bulging test is a specialized method to obtain the mechanical properties of tubular materials, which is very important for the cases when tubular blank is formed under bi-axial stress states. However, this test is currently applied with different testing conditions and analytical models. In addition, the deformation is often interrupted in order to measure the pole thickness change during testing, which may has considerable effect on final results obtained. Based on the models for stress components and bulging zone profile presented in an accompanying paper, i.e., Part I [19], analytical model for pole thickness during hydro-bulging test is presented and analyzed. A linear model for pole thickness change is proposed for the first time, and is successfully validated by experiments of AA6061 aluminum tube and SUS409 steel tube. Based on the linear model, the difficulty for pole thickness measuring during hydro-bulging is conquered. Then, a new testing method termed as linear-model based hydro-bulging test, is developed and applied for extruded aluminum alloy tube and roll-welded steel tube. It is demonstrated that the new testing method is more feasible and effective in characterizing the deformation behavior of tubular materials under bi-axial stress states.