Effect of pre-strain on stretch flange deformation limit of steel sheets

Abstract

The effect of pre-strain on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various pre-strain conditions. Pre-strain was given by stretch forming with a cylindrical punch. The strain ratio and amount of pre-strain were controlled by changing the blank shape and forming height. The hole expansion tests were carried out with a conical punch and cylindrical punch in order to clarify the effect of the strain gradient. The effect of the strain ratio of pre-strain on the stretch flange deformation limit can be expressed by equivalent strain. In the case of higher strain gradients, the stretch flange deformation limit increased with increasing pre-strain. However, at lower strain gradients, the stretch flange deformation limit was almost constant. These results suggest that the formability of the stretch flange deformation area given by pre-strain should be determined from not only the total equivalent strain but also the strain gradient and the amount of pre-strain expressed by equivalent strain.

Accuracy of stretch flange formability prediction by considering strain gradient for actual car body parts

“…Thus, there will also be no subsequent studies which verify stretch flange formability considering the influence of the strain gradient of the stretch flange deformation portion in FEM forming simulations. In this situation, Iizuka et al [5][6][7] investigated the effect of the strain gradient on the stretch flange deformation limit strain of steel sheets. The results clarified the fact that the stretch flange deformation limit strain basically depends on the strain gradient in the radial direction.…”