Failure by void coalescence in metallic materials containing primary and secondary voids subject to intense shearing

Abstract

Failure under intense shearing at close to zero stress triaxiality is widely observed for ductile metallic materials, and is identified in experiments as smeared-out dimples on the fracture surface. Numerical cell-model studies of equal sized voids have revealed that the mechanism governing this shear failure mode boils down to the interaction between primary voids which rotate and elongate until coalescence occurs under severe plastic deformation of the internal ligaments. The objective of this paper is to analyze this failure mechanism of primary voids and to study the effect of smaller secondary damage that co-exists with or nucleation in the ligaments between larger voids that coalesce during intense shearing. A numerical cell-model study is carried out to gain a parametric understanding of the overall material response for different initial conditions of the two void populations, subject to shear dominated loading. To account for both length scales involved in this study, a continuum model that includes the softening effect of damage evolution in shear is used to represent the matrix material surrounding the primary voids. Here, a recently extended Gurson-type model is used, which represents the effect of the small secondary voids under the low triaxiality loading conditions considered. This work suggests a failure mechanism for materials that contain voids on two different length scales, subject to intense shearing, in terms of; (i) the interaction of the primary voids, and (ii) the material softening of the ligaments due to the evolution of secondary damage. It is found that coalescence of primary voids under shear loading is severely affected by the presence of smaller secondary voids or defects in the ligaments. The change in overall ductility is presented for a wide range of initial material conditions, and an empirical correlation with the peak load is reported.