Calibration of Modified Mohr-Coulomb Ductile Fracture Criterion Using Tube Hydroforming Process

Abstract

An accurate material model significantly improves the simulation accuracy of tube hydroforming processes. To precisely calibrate a ductile fracture criterion, it is necessary to conduct fracture experiments that encompass a broad spectrum of stress conditions. Furthermore, a more accurate calibration is achieved when applying tube geometry-based tests, which present a more exact reflection of process conditions. Comprehensive phenomenological criteria with a wide coverage of stress states have not been used in the tube hydroforming process. Moreover, the need for a comprehensive calibration process based on tube geometry has not been underlined due to the simplicity of the considered criteria. This research provides a novel environment where innovative fracture experiments with nested tubes are designed and performed in a single setup. Certain cut-outs with special geometries are designed on the outer AA6063-T6 tube, considered for fracture. The inner tube, made of ductile metal, acts as a medium to transfer hydraulic pressure to the outer tube. Iterative finite element simulation was applied to design the geometry of cut-outs on the outer tube. The modified Mohr-Coulomb criterion (MMC3) is calibrated using these fracture experiments. A comparison between the experimental and simulation results showed the efficiency of the fracture criterion calibrated by the proposed tests, where the pressure, bulge height, and tube thickness at fracture onset were predicted with average errors of less than 13%, 9%, and 3%, respectively.