- Main
Multiscale Crystal Defect Dynamics: A Crystal Plasticity Theory Based on Dislocation Pattern Dynamics
- LYU, DANDAN
- Advisor(s): Li, Shaofan
Abstract
Understanding the mechanism of plasticity is of great significance in material science,
structure design as well as manufacture. For example, the mechanism of fatigue, one critical
origin for mechanical failures, is governed by plastic deformation. In terms of simulating
plasticity, multiscale methods have drawn a lot of attention by bridging simulations at different
scales and yielding high-quality atomistic properties at affordable computational resources.
The main limitation of some multiscale methods is that the accuracy in much of the continuum
region is inherently limited to the accuracy of the coarse-scale model, even though much
effort has been made to improve the existing multiscale models by developing adaptive
refinement models. It is known that crystal defects play an important role in
determining material properties at macroscale. Crystal defects have microstructure,
and this microstructure should be related to the microstructure of the original
crystal. Hence each type of crystals may have similar defects due to the same failure
mechanism originated from the same microstructure, if they are under the same loading
conditions. In this dissertation, a multiscale crystal defect dynamics (MCDD) model
is proposed that modelling defects by considering their intrinsic microstructure derived
from the microstructure or dislocation patterns of the original perfect crystal. The main
novelties of present work are: (1) the discrete exterior calculus and algebraic topology
theory are used to construct a scale-up (coarse-grained) dual lattice model for crystal
defects, which may represent all possible defect modes inside a crystal; (2) a higher
order Cauchy-Born rule (up to the fourth order) is adopted to construct atomistic-informed
constitutive relations for various defect process zones, and (3) an hierarchical strain
gradient theory based finite element formulation is developed to support an hierarchical
multiscale process zone model for various defects in a unified formulation. The efficiency
of MCDD computational algorithm allows us to simulate dynamic defect evolution at large
scale while taking into account atomistic interaction. The MCDD model has been validated
by comparing the results of MCDD simulations with that of molecular dynamics (MD)in the
cases of nanoindentation, uniaxial tension and simple shear. Numerical simulations have
shown that MCDD can capture not only material failure but also inelastic deformation in
a multiscale continuum simulation by considering the atomistic interaction.
In addition, simulation of anisotropy demonstrates that MCDD is capable of capturing
the influence of loading axis orientation and the loading directionality on dislocation nucleation.
Main Content
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