The effect of stress state on ductility in the moderate stress triaxiality regime of medium and high strength steels

Experiments on double notched tube specimens subjected to tension and torsion were conducted by Barsoum and Faleskog (2007) [8,9]. In this study a complementary experimental investigation was conducted on tensile round circumferentially notched bar specimens. The results from the current study were compared with the results from the double circumferentially notched tube specimens with stress triaxiality larger than 0.7 in order to asses the influence of the Lode parameter on ductility in the moderate stress triaxiality regime. The effective plastic strain, the stress triaxiality T and the Lode parameter L were determined at the center of the notch up to the point of onset of failure by means of finite element. The influence of the Lode parameter on the failure strain was significant for the high strength and low hardening material, whereas for the medium strength and high hardening material the influence of the Lode parameter was less distinguished. The experimental results were then analyzed with the micromechanical model proposed by Barsoum and Faleskog (2011) [15], which is based on the assumption that ductile failure is a consequence of that plastic deformation localizes into a band. The band consists of a square array of equally sized cells, with a spherical void located in the center of each cell, which allows for studying a single 3D unit cell with fully periodic boundary conditions. The unit cell is subjected to a proportional loading such that it resembles the stress state, in terms of T and L, from the experiments. The micromechanical model captures the experimental trend and the influence of L on ductility very well. It is found that the Lode parameter sensitivity increases by the combination of increase in the yield strength and decrease in strain hardening. The fractographical analysis reveals that this Lode parameter sensitivity is associated with the failure characteristics of the material.