Influence of the stress history and of the Lode angle on the determination of the ductile fracture locus for two steel alloys

The fracture locus of ductile materials is primarily related to the stress triaxiality. Recently, the Lode angle was also found to be relevant in the fracture locus determination. In order to characterize a material in terms of fracture locus, usually several tests, spanning a wide range of stress triaxiality and Lode angle values, have to be performed using different specimen geometries, obtained both from round bars and sheets. In this paper the fracture loci of two steel alloys (S500MC and 22MnB4) are obtained using a simplified procedure, based on the Bai–Wierzbicki model. The procedure considers, for experimental tests, only flat specimens, avoiding the use of round specimen of the same material which were not available. Experimental tests were performed on differently shaped specimens and the results were used to train finite element (FE) models. The triaxiality and Lode angle histories, computed for each test by the FE model, were used to obtain the fracture loci following two different approaches: the proportional loading approach and the non-proportional loading approach. The former does not consider how the stress triaxiality and Lode angle vary during the test, while the latter considers their history in the computation of the fracture locus. The results show how the Lode angle influences the fracture locus, especially for 22MnB4, and how non-proportional loading approach is more accurate to compute the damage for the different specimen geometries.