The determination of flow stress of tubular material for hydrofoming applications

Tube hydroforming (THF) process is nowadays a developed and successful way for forming
of complex shapes with less operations required compared to conventional tube forming
processes. FE simulations are powerful tools which allow a remarkable saving in time and
money when developing a feasibility study or a prototype phase for a new THF operation.
However the successful use of FE simulations depends on several factors such as interface
friction or material properties. While in the past flow stress used as input in FE simulations
was generally obtained from tensile test conducted on the sheet prior to rolling and welding
operations in case of rolled and welded tubes, or tensile test conducted on the whole tube
for extruded or seamless tubes, in the last years tube bulge test has been widely used. In
this research test experiments, coupled with suitable analytical model of the process, are
used to identify the flowstress of tubes (under biaxial stress state) by measuring geometrical
features of the tube under study as the fluid pressure increases. In the present paper a new
approach to tube bulge test is described. The innovative aspect is related to the fact that the
tube ends are blocked and so the equilibrium expression in axial direction normally used to
calculate stresses is no more valid. The stress state is therefore derived from the flow rule
and the volume constancy.
The new proposed analytical model was validated by means of FE simulations. Results of
test conducted on seamless tubes show that bulge test allows to obtain material properties
for high strain thus avoiding possible errors in extrapolating flow stress for FE simulations.