Formability limits of high-strength H240LA steel sheets under stress/strain gradients

Abstract

Conventional sheet metal forming processes usually involve the simultaneous action of stretching and bending, inducing strain/stress gradients across the sheet thickness and complex strain paths, which decisively influence the onset and development of the failure mechanism by necking. Therefore, an accurate assessment of formability requires the use of strain path independent tools that take in to account the bending effects on the sheet failure. The use of the theory of critical distance (TCD), widely used in fatigue and fracture mechanics to account the effect of the stress/strain gradients in metal failure, has been satisfactorily analysed by the authors to predict failure by necking under stretch–bending conditions. The current work develops, using and experimental and numerical analysis, two failure models for strain/stress gradient situations which combine both the use of path-independent forming limit diagrams, particularly the forming limit stress diagram (FLSC) and the equivalent plastic strain based forming limit curve (epFLC), along with the concepts of critical distance to account for the effect of bending in the onset of necking. Both developed models are used to predict numerically the failure in a series of stretch–bending tests over H240LA steel sheets. Their predictive capabilities are discussed and compared in the light of the experimental results.