Formability limits by local buckling in thin-walled tubes

This paper presents a new methodology to determine the formability limits by local buckling in thin-walled metal tubes that is an extension of a method that was earlier developed by the authors to characterize wrinkling in metal sheets. The method consists of subjecting a set of rings with different lengths extracted from the tubes to axial compression and combining experimental measurements with finite element modeling to determine the critical in-plane surface strains at the onset of buckling. Digital image correlation is used to track the experimental evolution of the in-plane strains with the normalized height of the plastic hinges and to validate the finite element model for subsequent utilization in the determination of the critical strains at the onset of buckling. Results show the effectiveness of the proposed methodology to obtain, for the first time ever, the formability limits by local buckling of thin-walled tubes in both principal strain and effective strain versus stress triaxiality spaces.