Evaluation of the cold formability of high-strength low-alloy steel plates with the modified Bai-Wierzbicki damage model

While strength and toughness properties of construction steels are major mechanical properties with respect to the safety assessment of components, increasingly often requirements are defined on the processing properties of materials, in particular cold formability properties. So far, the cold formability of structural steels is characterized in terms of three-point bending tests, which result in very expensive experimental effort and limited understanding of the material behaviour under various stress states. In this paper, the cold formability of a structural steel S355J2+N is characterized under various stress states in a laboratory scale by a recently proposed hybrid damage mechanics approach. A damage initiation criterion that considers the influence of stress triaxiality and Lode angle is used to describe the onset of material degradation in a microstructural scale in the approach. Subsequent damage evolution is followed to further quantify the accumulation of damage till the final fracture. A series of laboratory tests is designed to calibrate the model parameters as well as to verify the calibration. Good agreement of experimental and numerical force-displacement responses proves the predicative capability of the model. With the laboratory findings, the industrial scale three-point bending test is simulated to predict the forming limit. In addition, the numerical study reveals that the stress state of the critical element in three-point bending test coincides with plane-strain tension, which results in a simpler method to characterize the bendability to reduce the experimental effort.