Formability Evaluation Based on Stress Triaxiality in Incremental Forming and Stretch Press Forming

The formability of die-based press forming (stretch forming) and that of incremental forming (IF) were compared using the ductile fracture equation and stress triaxiality to investigate whether IF’s superior formability could be accurately evaluated. Experiments were conducted to determine the fracture conditions of the test material in each forming method, and finite element method analysis was performed on the basis using experimental results to obtain the stress triaxiality. In press forming, the stress triaxiality remained nearly constant during forming, indicating that the evaluation using the ductile fracture equation was feasible. In IF, the stress triaxiality around the tool was found to be the same as that in press forming, making it difficult to evaluate its high formability using the ductile fracture equation. Focusing on the formed areas of the test material, the stress triaxiality was not constant and changed over time during the forming process. Damage analysis using GISSMO, which accounts for temporal variations in stress triaxiality, was performed. This analysis revealed that both IF and press forming could be effectively evaluated, and that IF’s superior formability could also be appropriately represented.