The bending dependency of forming limit diagrams

The forming limit diagram (FLD) is commonly used in the forming industry to predict localized necking in sheet metals. However, experimental and mathematical determination of forming limit curves is based on in-plane deformation without taking a bending component into account. This paper addresses the question whether a forming limit diagram predicts localized necking too conservatively or too liberally for different amounts of superimposed bending. Furthermore a methodology is proposed on how to determine the influence of bending quantitatively and how to incorporate the results into a FLD. Finite element analysis (FEA) is used to model the angular stretch bend test (ASBT), where a strip of sheet metal is locked at both ends and a tool with a radius stretches and bends the center of the strip until failure. By using different radii, the sample is exposed to different amounts of superimposed bending during the stretching. The FEA model is verified by experimental work using a dual phase steel (DP600). Applying a strain rate dependent identification method for determining the onset of localized necking, the FEA model is utilized to access the forming limit strains for different stretch bending conditions. Thereby the Keeler-Brazier FLD is extended by a third dimension, the superimposed bending dimension, to assess the effect of bending on the forming limit of sheet metal parts. The more severe the superimposed bending, the higher the formability of the steel.