Investigation of forming stress limit in tube hydroforming of automobile framework

The forming limit stress diagram(FLSD) which established with limit stress is independent on the strain paths and it is practical to use as the criterion of forming limit under cmplex strain paths.The forming limit of 3A21 Aluminum alloy sheet was tested and the forming limit diagram(FLD) was determined.Then the FLSD of 3A21 was constituted by transformation formulas between limit strain and limit stress.This FLSD was used in conjunction with LS -DYNA finite element simulation to predict the onset of fracture and limit forming pressure in tube hydroforming.The simulation result compares well with the experimental result.The FLSD is able to predict the forming limit of tube hydroforming with remarkable accuracy.