| The effect of changing strain paths has been investigated by determining forming limit diagrams (FLD) of sheets prestrained in uniaxial, plane-strain and biaxial tension. It was found that 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.; The experimental FLDs have been compared with calculations following the Marciniak and Kuczynski approach, extended to incorporate path changes. The experimental trends were best reproduced when the calculations were made using a high exponent yield criterion.; A simple experimental set-up for obtaining plane-strain tension in rectangular sheet specimens is described. It was found that there is good agreement between experimental and calculated forces in in-plane plane-strain experiments assuming isotropic power law hardening if a high exponent yield criterion is used.; Finally, an example of the importance of strain-path changes in a stamping of an aluminum automobile part is presented. |
