On localization and void coalescence as a precursor to ductile fracture

Abstract

Two modes of plastic flow localization commonly occur in the ductile fracture of structural metals undergoing damage and failure by the mechanism involving void nucleation, growth and coalescence. The first mode consists of a macroscopic localization, usually linked to the softening effect of void nucleation and growth, in either a normal band or a shear band where the thickness of the band is comparable to void spacing. The second mode is coalescence with plastic strain localizing to the ligaments between voids by an internal necking process. The ductility of a material is tied to the strain at macroscopic localization, as this marks the limit of uniform straining at the macroscopic scale. The question addressed is whether macroscopic localization occurs prior to void coalescence or whether the two occur simultaneously. The relation between these two modes of localization is studied quantitatively in this paper using a three-dimensional elastic-plastic computational model representing a doubly periodic array of voids within a band confined between two semi-infinite outer blocks of the same material but without voids. At sufficiently high stress triaxiality, a clear separation exists between the two modes of localization. At lower stress triaxialities, the model predicts that the onset of macroscopic localization and coalescence occur simultaneously.

Slant strained band development during flat to slant crack transition in AA 2198 T8 sheet: in situ 3D measurements

“…In comparison to the previously reported results (Morgeneyer et al 2014b), it can be stated that even in the area closer to the notch root the same mechanisms prevail, namely, the onset and development of slant bands precedes damage, which points out that plasticity is the driving mech- These high levels of plasticity cause damage nucleation and growth leading to the final failure. This type of ductile failure corresponds to Mechanism 2 identified by Tekoglu et al (2015). However, compared with ROI 1, the strained bands in ROI 2 are more numerous and decrease in number when moving away from the notch root.…”

Micromechanically-motivated phenomenological Hosford–Coulomb model for predicting ductile fracture initiation at low stress triaxialities

“…1). As discussed by Tekoglu et al (2015), polycrystalline materials may also fail due to (i) localized plastic flow only (e.g. necking up to zero cross-sectional area), (ii) localization of plastic flow after damage-free deformation, followed by void growth and nucleation inside the primary band of localization, and (iii) direct coalescence (secondary localization) without any prior occurrence of primary localization.…”

Critical hardening rate model for predicting path-dependent ductile fracture

“…Such simulation models can be supplemented with coalescence criteria (see review by Benzerga and Leblond 2010) to account for localization events at the mesoscale. Note that the latter may even occur before macroscopic localization (Tekoglu et al 2015).…”

“…Gurson 1977;Gologanu et al 1993;Benzerga and Besson 2001;Monchiet et al 2008) the evolution law for the void volume fraction (and other possible microstructural state variables) is loading path sensitive and failure predictions with microstructurally-informed coalescence criteria (e.g. Thomason 1985;Pardoen and Hutchinson 2000;Benzerga 2002;Tekoglu et al 2015) are then loading path dependent. However, given the limited benefits of Gurson type of models as far as predicting the elasto-plastic response of sheet metal is concerned, the combination of non-porous plasticity models with damage indicator models provides an attractive framework for predicting loading path dependent fracture initiation in industrial practice.…”