On the Effects of Stress State and Microstructure Induced Anisotropies on Ductile Fracture

Abstract

A major challenge that is currently facing the mechanics of materials community is the accurate prediction of fracture in advanced ductile materials. The intertwined effects of intrinsic and extrinsic factors make ductile fracture one of the most complex phenomena in materials mechanics. Intrinsic factors include large plastic deformations, induced anisotropies, microstructural evolution, and stress state effects. Extrinsic factors relate to the effect of boundary conditions and to the onset of plastic instabilities, either material (e.g., shear bands) or structural (e.g., necking). This dissertation sheds light on three fundamental topics - effect of non proportional loadings, anisotropic ductile fracture and failure by shear localization. First, by means of a simple fracture model, a generic shape of the fracture strain versus average triaxiality locus and previously published experimental results are rationalized. Then, a more elaborate ductile fracture model is utilized to carry out three-dimensional finite element simulations of damage accumulation to failure in initially crack-free specimens under certain symmetry considerations. The results reveal an emerging competition between intrinsic and structural effects imparted by plastic anisotropy. Finally, full 3D simulations are carried out when the triads of loading and plastic anisotropy are misoriented. The simulations reveal, for the first time, failure by shear band formation in initially round notched bars, reminiscent of experimental observations in Al or Mg alloys. These insights have practical and theoretical consequences and will aid the implementation of improved models of ductile fracture with accurate predictive capabilities and the design of safer structural components in the aerospace, automotive and energy industries.