Micromechanical modeling of ductile crack initiation behavior of two phasesteels

The effects of the volume fraction and the morphology of a second phase on ductile crack initiation behavior were determined by notched round bar tens ile specimens using ferrite-pearlite steels which contain quite small amoun ts of MnS inclusions. Nominal strain to crack initiation was increased by d ecreasing pearlite volume fraction, and by the controlled rolling, which pr oduces an elongated microstructure. The Gurson-Tvergaard (G-T) constitutive model was used to investigate the micromechanism of ductile crack initiati on behavior. For evaluating the void nucleation strain, an axisymmetric uni t cell model based on a Voronoi tessellation of the BCC lattice (V-BCC mode l) was applied to determine the microscopic strain inside the pearlite phas e which controls secondary void nucleation. The parameter representing the volume fraction of nucleated void, f(N), was evaluated by fitting the numer ical solution to experimental data for nominal stress/nominal strain curves of the notched round bar specimen. It was found that steels with lower pea rlite volume fractions or elongated pearlite nodules have lower f(N), and t he 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 ini tiated in the region having the highest void volume fraction for all steels . It was shown that the critical void volume fraction for ductile crack ini tiation is independent of stress triaxiality, and the steels with lower f(N ) show smaller critical void volume fractions.