2010 年 51 巻 594 号 p. 700-705
The effect of strain gradient on the stretch flange deformation limit of steel sheets was investigated by hole expansion test under some forming conditions and by FEM. Hole expansion ratio was changed by changing initial hole diameter and punch shapes. It was clarified that limit deformation strain increases as it does not depend on initial hole diameter or punch shape, and thus strain gradient increases. The strain gradient along the direction toward the maximum principal strain did not affect limit deformation strain. The limit deformation thickness strain of the stretch flange portion could be changed by changing strain gradient. The sensitivity of the effect of strain gradient on limit deformation strain tended to increase as material yield ratio increased. These results suggest that the formability of stretch flange deformation area should be determined from not only the maximum principle strain but also the strain gradient and blanking clearance.