Numerical Analysis of Necking in Stretch-Bending Based on Modified Maximum Force Criteria

The necking criteria based on the maximum force principle, such as the Swift and Hill necking models, have been extensively used in the past to study sheet formability. Many extensions or modifications of these criteria have been proposed to improve necking predictions under only stretching conditions. Recently, the authors proposed two approaches to predict necking under stretch-bending conditions: a generalisation of classical maximum force criteria to stretch-bending; and a necking criterion based on critical distance concepts, suggesting that necking is controlled by the damage of a critical material volume located at the inner side of the sheet. Both approaches were evaluated with analytical models and the failure was successfully predicted in different materials, such as steel, brass and aluminium. This work presents a numerical study of the proposed necking criteria in stretch-bending processes with different punch radii. The simulations are validated with experimental tests over H240LA steel sheets.