Spring back evaluation for high and ultra high strength sheet steels with the bending under tension machine

The Bending Under Tension (BUT) machine is used for evaluation of spring back in sheet metal forming. A strip is drawn over a rotating cylindrical die with different restraining forces. The strip is allowed to spring back after drawing and the curvature of the strip is measured. The loading sequence is typical to wall sections of pressed components, side wall curl, where the material has been bent, unbent and finally unloaded. The test was performed on four high and ultra high strength steels with tensile strengths in the range from 800 MPa to 1300 MPa. A clear separation of the data for the four steels was demonstrated. It was analysed to what extent the differences in spring back between the different steels was related to the differences in tensile strengths. A finite element simulation model was used to simulate the strip curvature after BUT testing. The material parameters of the model were fitted to monotonic uniaxial and equibiaxial tests and uniaxial cyclic tests. The model could describe the experimental data in a satisfactory way.