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The onset of macroscopic strain localization limits the ductility of many ductile materials. For porous ductile materials, two distinct mechanisms of macroscopic localization have been identified: void growth induced softening and void coalescence. In this work we focus on analyzing the influence of material's strain rate sensitivity (SRS) on the two mechanisms of macroscopic localization or ductile failure as a function of the imposed stress triaxiality. To this end, three dimensional finite element calculations of unit cells have been carried out to model void growth and coalescence in an infinite block containing a periodic distribution of initially spherical voids in a band. The matrix material of the unit cell is considered to follow a strain rate dependent elastic perfectly plastic flow response. The unit cell calculations are carried out for a range of SRS parameter, imposed stress triaxiality and initial orientations of the voided band. Our results show that both the critical porosity and strain at the onset of localization and coalescence are strongly influenced by the SRS parameter and the imposed stress triaxiality values. Furthermore, the relative effect of the SRS parameter is found to increases with the increasing value of the imposed stress triaxiality.
ductility; fracture mechanisms; elastic-viscoplastic material; porous material; finite elements