Fundamental differences exist between sheet metal forming and hydroforming processes. Sheet metal forming is basically a one step metal fabrication process. Almost all plastic deformation of an originally flat blank is introduced when the punch is moved normal to a clamped sheet metal. Hydroforming, however, consists of multiple steps of tube making, pre-bending, crushing, pressurization, etc. Each of the above mentioned steps can introduce permanent plastic deformations. The forming limit diagram obtained for sheet metal forming may or may not be used in hydroforming evaluations.
A failure criterion is proposed for predicting bursting failures in tube hydroforming. The tube material's stress-strain curve, obtainable from uniaxial tensile test and subjected to some postulations under large stress/strain states, is used in judging the failure. The failure prediction is based on comparing the true stress/strain states developed in the tube during hydroforming to the above mentioned stress-strain curve. A lower estimate and an upper estimate for the bursting failure of the tube are obtained. The former is obtained employing the maximum load carrying capacity of the material, while the later is based on the total elongation of the material. The criterion is path independent and most suitable for applications in conjunction with numerical simulations.
Applications of the proposed criterion were made to unsupported long tubes with fixed ends and free ends cases. The tubes are made of typical AKDQ steel with known stress-strain relations and forming limit diagram. Results obtained by applying the developed bursting criterion for these simple loading cases are comparable to those obtained from using forming limit diagram.