THE INFLUENCE OF STRAIN-PATH CHANGES ON FORMING LIMIT DIAGRAMS OF A1 6111 T-4

The effects of changing strain-paths on forming limits of aluminum alloy 6111 T4 have been investigated by determining forming limit diagrams (FLDs) of specimens prestrained to several levels in uniaxial, plane strain and biaxial tension, parallel and perpendicular to the prior rolling direction. Prestraining in biaxial tension generally lowers the entire FLD, whereas prestraining in uniaxial tension raises the limits on the right hand side of the FLD without much effect on the left hand side, when the direction of the largest principal strain does not change. If the directions of the principal strains are rotated, prestraining in uniaxial or plane strain tension lowers the forming limits for most of the FLD range. A general finding was that, after prestraining, the amount of the additional plane strain deformation possible before failure depends on the effective strain during prestrain, regardless of the original strain-path. Finally, an example of the importance of strain-path changes in a stamping of an aluminum automobile part is presented.