Abstract: In tube hydroforming, how to properly evaluate bursting or wrinkling is an important problem in engineering applications. Forming limit diagram has been widely used as an effective method to estimate the hydro-formability. Based on tube ellipse bulging test, measurement method of the limit strain at tension-tension strain zone of forming limit diagram is developed. The forming limit diagram of tube hydroforming can be achieved by means of the free bulging test and the ellipse bulging test. A flexible test setup with the design of insert structure for tube hydroforming limit test is developed, and the hydroforming limit tests of laser welded tube with different specification are completed using the test setup. The results show that the thickness and diameter of laser welded tubes made from the same brand sheet material not only affect the limit strain at the plane strain state, but also affect the shape of forming limit curve, and the value of strain ratio at tension-compression strain zone is decreased with decreasing the diameter-to-thickness ratio.

Key words: Formability, Forming limit diagram, Hydroforming, Laser welded tube, Test method, driving style recognition, energy management strategy, equivalent consumption minimization, HEV