Year

2009

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering - Faculty of Engineering

Abstract

The field of fracture mechanics can generally be divided into two groups: (i) the study of material behaviour prior to crack and (ii) the developing of crack opening criteria. Even though various studies have been done in both subjects there are still gaps that need to be bridged. This thesis aims at combining both groups of the modelling of dynamic fracture in crystalline materials at a reasonable cost of computational time. A model of crystal plasticity finite element method has been formulated to account for the effects of lattice structure in the crystalline materials. The model has been applied to simulate tensile deformation around a notch tip in both single crystals and polycrystalline aggregates. By comparing with observations from various experiments, the model has been proved to be able to accurately capture the material’s behaviours around a notch tip undergoing tensile load. Particularly, this model is among the very few, if not the first, that accurately predicts various experimental observations of two notch tip orientations (010)[101] and (010)[100] that are widely found in the literature. This study has also developed a crack opening criterion that is dependent upon the evolution of the lattice structure. The core of this new criterion is an atomic interaction model that estimates energies of the interface of an fcc bicrystal. Results of grain boundary energy of and symmetrical tilt boundaries of an aluminum bicrystal obtained from the atomic interaction model agree very well with those from molecular dynamics simulations. The newly developed criterion has been applied to the modelling of crack opening and crack growth in a region around the notch tip in single crystals. Elements in the finite element mesh satisfying the criterion are removed from the mesh by using the element removal technique in Abaqus/Standard. Missing elements effectively act as voids in the material. Thus crack opening (in terms of void nucleation) and the subsequent crack growth (in terms of coalescence of new and existing voids) are captured naturally. The newly developed methodology to model crack opening has been applied to predict mode I crack growth around a notch tip in Cube and Brass oriented fcc single crystals. The obtained results show similar behaviours of crack growth with those from molecular dynamics simulations of single crystals having the same lattice orientations. The thodology to model crack opening that has been proposed in this thesis is original. It enables the explicit modelling of crack growth without presuming a crack path. Also, a predefined crack opening criterion, which could be erroneous, that has been used in many finite element simulations of fracture is avoided. To the best of the author’s knowledge, the criterion of crack opening that depends on the structure of the interface of two misoriented lattices is presented in this study for the first time. The current thesis focuses into modelling tensile deformation and the subsequent fracture in fcc crystals. The methodology that has been proposed however can be readily applied to crystalline materials of various lattice structures with minor modifications.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.