Improvement in Accuracy of Failure Prediction in Sheet Hydroforming of Square Cups Using Stress-Based Forming Limit Diagram

Prediction of failure in sheet metal forming processes accurately is very important for successful production and optimization of parameters. A major problem of conventional strain-based forming limit diagrams (FLDs) is their inability to predict failure accurately in processes such as sheet hydroforming where there is a change in strain path and mode of deformation. In the present work, a stress-based forming limit diagram has been developed for AA 5182 alloy sheets modifying the analytical procedure, proposed by Stoughton, to determine forming limits in stress space from failure strains incorporating anisotropy using Balart’s yield criterion. The developed stress-based FLD has been used to predict failure in sheet hydroforming of square cups. Results are compared with Hill’s quadratic yield criterion. Significant difference has been found in failure prediction between strain-based and stress-based criteria when they are applied to sheet hydroforming. A change in strain path has been observed at the critical corner regions in hydroforming of square cups due to initial drawing and then biaxial stretching during calibration. The experimental validation clearly showed that accuracy in failure prediction can be improved in sheet hydroforming by using a stress-based forming limit diagram.