Tubular hydroforming process for a majority of automotive parts is a complex process where the initially straight tube undergoes a series of pre-bending, pre-forming, die closure and then the final pressurization. The pre-bending process is usually carried out by a rotary bending machine and utilizes an inside mandrel to prevent the tube from collapsing during bending. Because of the high pressure exerted by the mandrel in the normal direction of the tube sheet, the failure mechanism for the tube in the subsequent hydroforming process will differ significantly from that experienced in sheet metal stamping process, which can be well characterized by the Forming Limit Diagram (FLD). The presentation begins with experimental evidence of material failure during hydroforming following pre-bending, and continues with two predictive models for analysis. One is a Gurson-type void growth model, and the other a damage based model which takes into account the effect of hydrostatic pressure in damage growth. Numerical examples are given to illustrate the effectiveness of both models and are compared to experimental data. Their applicability in an industrial production environment is discussed.

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