Micromechanism of Ductile Crack Initiation in Structural Steels Based on Void Nucleation and Growth.

Abstract

The effect of the specimen geometry and the inclusion content on the critical condition for ductile crack initiation was determined using notched round bar tensile specimens, and the critical void volume fraction and secondary void nucleation effect were investigated using the Gurson-Tvergaard constitutive model. It is shown that the secondary void nucleation from pearlite nodules plays a dominant role in ductile crack initiation, and void nucleation and void growth behavior are strongly affected by the stress triaxiality and the plastic strain. In the large plastic strain and low stress triaxiality region, a large number of secondary voids were nucleated in the early stage of deformation and void volume fraction grew to a critical value in spite of the relatively low void growth rate at low stress triaxiality. On the other hand, in the low plastic strain and high triaxial stress region, void volume can grow rapidly even though the volume of secondary nucleated void was smaller than that of high plastic strain region. Critical void volume fraction decreases largely with increasing in MnS content in the high triaxial stress region; however, the effect of MnS content is small in the low stress triaxiality and large plastic strain region.

Micromechanical Modeling of Ductile Crack Initiation Behavior of Two Phase Steels.

“…Zhang and Niemi 32) investigated the critical void volume fraction under various triaxial stress states based on the Thomason's plastic limit-load criterion 33) combined with the G-T model for determining void growth behavior, and showed that the critical void volume fraction for materials with relatively large initial void volume fraction, f 0 , is affected by stress triaxiality, while f c is almost constant for materials with small values of f 0 . Ishikawa et al 17) also suggested a constant critical void volume fraction for steels with a small MnS content by comparing experimental investigations and calculation using the G-T model.…”

“…However, ductile cracking in relatively clean ferrite-pearlite steels is dominated by secondary void nucleation from pearlite nodules. 17) Therefore, a characteristic length for ductile cracking is related to the mean spacing of pearlite nodules, as illustrated in Fig. 11.…”

“…8,9,10,17,21) The volume fraction of nucleated void, f N , is closely related to microstructural characteristics of the ferritepearlite steels. 17) If all of the pearlite nodules in the critical region crack and act as secondary nucleated voids, the volume fraction of the nucleated voids, f N , is given by…”

“…4) However, as refining techniques in the steel making process have been improved, recent commercially-produced structural steels contain much fewer inclusions. 16) Ishikawa et al 17) showed that the ductile crack initiation in ferritepearlite steels with very small amounts of MnS inclusions, and ductile crack initiation in regions of strain concentration, but with a low stress triaxiality, are both controlled by secondary void nucleation from pearlite nodules. It was also pointed out that the parameters in the G-T model should be closely related with the details of the microscopic behavior of materials.…”

“…The void nucleation parameters in the G-T model are closely related to microstructural characteristics of the ferrite and pearlite phases. 17) The void nucleation strain, one of the major parameters in the G-T model, was determined in relation to the microscopic behavior of the pearlite phase. In order to simulate the microscopic behavior of two-phase steels with different microstructures, finite element unit cell models based on a Voronoi tessellation of the BCC lattice 18,19) were used.…”