Steel and copper flow stress determination for THF applications

Tube hydroforming (THF) process is nowadays a developed and successful way for forming complex shapes with less operations required compared to conventional tube forming processes. Since THF is a recently developed technology, there is a lack of knowledge on the process which can be overcome by using FE simulations. In fact, simulation allows a remarkable saving in time and money when developing a feasibility study or a prototype for new THF operations. The reliability of FE simulations depends on several factors such as interface friction and material properties. The classical method used for evaluating the material flow stress is the tensile test. This is a simple test which stresses the material in only one direction, differently from the actual tube stress state in THF. A biaxial stress state can be achieved by using the tube bulge test, which, consequently gives more reliable data for determining the material flow stress. The present paper describes the experiments and the analytical model of tube bulge test for the identification of the flow stress under a biaxial stress state. The innovative aspect of the proposed approach is related to the fact that the tube ends are blocked and, in the analytical model, the stress state is derived from the flow rule and the volume constancy. The experimental data of bulge curvature and tube thickness are used by the analytical model to calculate the material stress-strain relation. Results of tests conducted on different tube materials show that bulge test allows to obtain material properties for high strain thus avoiding possible errors in extrapolating flow stress for FE simulations.