Abstract: Transformation relation between limit strain and limit stress is derived on the basis of the plastic theory and Hill79 yield criterion. Then, the conventional strain-based forming limit diagram (FLD) of 2008T4 Aluminum alloy sheet is converted into stress-based forming limit stress diagram (FLSD). This FLSD is used in conjunction with LS-DYNA finite element simulation to predict the onset of necking and the limit pressure in T-shaped tube hydroforming. The prediction results using FLSD and FLD are also compared. The research indicates that two predicted forming limits are obviously different. Although the necking occurs at the same location in both prediction results, the limit pressures make a great difference. And the limit pressure predicted by FLSD compares well with the experimental result. In view of the strong path-dependence of the FLD, it will be more convenient and reliable to use FLSD criterion in tube hydroforming analysis.

Key words: Forming limit diagram, Forming limit stress diagram, Hydroforming, Numerical simulation