42nd International Deep Drawing Research Group Conference - IDDRG 2023

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 limit deformation thickness strain of the stretch flange portion could be changed by changing strain gradient. 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. Then, the accuracy of stretch flange formability prediction by considering the strain gradient for actual car body parts in mass production were investigated. The maximum principle strain and the strain gradient of the parts were calculated by the FEM and measured by the press parts. These results suggest that the formability of stretch flange deformation area for actual car body parts could not be predicted by the maximum principle strain or the thickness reduction. The accuracy of stretch flange formability prediction by considering strain gradient for actual car body parts could be verified by the investigations.