Micromechanical modeling of ductile crack initiation behavior of two phase steels

The effects of the volume fraction and the morphology of a second phase on ductile crack initiation behavior were determined by notched round bar tensile specimens using ferrite-pearlite steels which contain quite small amounts of MnS inclusions. Nominal strain to crack initiation was increased by decreasing pearlite volume fraction, and by the controlled rolling, which produces an elongated microstructure. The Gurson-Tvergaard (G-T) constitutive model was used to investigate the micromechanism of ductile crack initiation behavior. For evaluating the void nucleation strain, an axisymmetric unit cell model based on a Voronoi tessellation of the BCC lattice (V-BCC model) was applied to determine the microscopic strain inside the pearlite phase which controls secondary void nucleation. The parameter representing the volume fraction of nucleated void, f(N), was evaluated by fitting the numerical solution to experimental data for nominal stress/nominal strain curves of the notched round bar specimen. It was found that steels with lower pearlite volume fractions or elongated pearlite nodules have lower f(N), and the void growth rate is lower for the steels with lower f(N), which requires a large amount of plastic strain for void growth. Ductile cracking was initiated in the region having the highest void volume fraction for all steels. It was shown that the critical void volume fraction for ductile crack initiation is independent of stress triaxiality, and the steels with lower f(N) show smaller critical void volume fractions.