The development of a novel forming limit diagram under nonlinear loading paths in tube hydroforming

Indexed by:Journal Papers

Journal:INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES

Included Journals:EI、SCIE

Volume:172

ISSN No.:0020-7403

Key Words:Forming limit; Nonlinear loading; Tube hydroforming; Hydro-bulging test

Abstract:The loading paths of tubular parts containing the characteristic of variable diameter during hydroforming are nonlinear and similar to the loading paths during tube hydro-bulging tests. Therefore, the forming limit determined by hydro-bulging tests is very suitable for guiding the forming of this kind of parts. However, traditional forming limit diagram (FLD) cannot present the nonlinear loading paths. In this paper, a theoretical analysis model for tube hydro-bulging tests is developed first. And then the stress paths and strain paths during bulging with fixed-ends and free-ends are analyzed through the theoretical model and finite element simulations. The results show that as the length-diameter ratio of the bulging zone ranges from 1.0 to 3.0, stress paths and strain paths are obviously nonlinear. For hydro-bulging with fixed-ends, the axial-hoop stress ratio changes gradually from 0.5 towards 1.0, and the strain state changes from plane strain state towards equal-biaxial tension state. For hydro-bulging with free-ends, the stress ratio changes gradually from 0 towards 0.5, and the strain state changes from simple tension state towards plane strain state. In order to give a better characterization of the forming limit of tubes under nonlinear loading paths, a novel FLD combining the geometric parameter of the tubular part is proposed. Two FLDs of an AA6061 aluminum alloy seamless tube under different end conditions are determined by experiments. The results show that both in hydro-bulging with fixed-ends and with free-ends, a higher forming limit is expected when using a smaller length-diameter ratio. As the new FLD can give direct presentation of limit strains together with the end condition and the length-diameter ratio in tube hydroforming, it can be used directly for the analysis and optimization of actual tube hydroforming processes, instead of the simple evaluation of the formability of raw materials.

Date of Publication:2020-04-15