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Amir R. Khoei
(To download a complete list of Publications
click here)
Research
interests |
Computational Nano-mechanics
Computational plasticity (FEM, X_FEM and Meshless
methods)
Computational Fracture Mechanics
Computational Geomechanics
Large deformation analysis
Error estimation and adaptivity for plasticity
problems
Contact and frictional modelling
Strain localization analysis |
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Modeling the nano-indentation
test on the pure aluminum substrate with different crystal
orientations; (a) the multiscale model, (b) a comparison of the
multiscale result and experimental data, (c) the von-Mises
stress contours on the surface of the aluminum substrate (See:
International Journal of Mechanical Sciences, 108897,
2024). |
Computational Nano-Mechanics
The purpose of my research activities in computational nano-mechanics
is to investigate the characteristics of the material at nano-scale. The
current research is based on the Molecular Dynamics simulations
and the non-linear finite element simulations. Molecular
Dynamics simulation is used as a standard tool in computational
nano-mechanics. The research is conducted by developing a
Molecular Dynamics software which is able to simulate
multi-particle systems in canonical and micro-canonical
ensembles. Furthermore, a machine learning-based multiscale
approach is developed for modeling the crystalline nano-structures.
To this end,
a computational atomistic-continuum multiscale
framework is developed based on the machine learning (ML)
architecture to capture the nonlinear behavior of nano-crystalline
structures. The dataset of the ML process is derived from
analyzing the atomistic representative volume element (RVE)
through the molecular dynamics method under various deformation
paths. The multilayer perceptron neural network approach (MLP–ANN)
is employed in the optimization algorithm of the ML process to
train the atomistic dataset. It is shown that the proposed
ML-based method enhances the multiscale analysis to capture the
nonlinear behavior of nano-crystalline structures without
computational complexities associated with conventional
multiscale methods. The results highlight that the proposed
multiscale framework can be utilized as a constitutive model for
the large-scale analysis with a great accuracy and unique
efficiency.
Multiscale modeling
of
a
perforated cantilever micro-beam in bending
(See:
International Journal of Mechanical Sciences, 239,
107858, 2023). |
The crack
trajectory for a plate with two holes;
a)
Geometry of the plate,
b) Adaptive FEM method,
C) Extended–FEM
technique,
d) The von-Mises stress contour (see
A.R.
Khoei,
Extended Finite Element Method, Theory and Applications, John
Wiley, 2015) |
Computational Fracture Mechanics
A comprehensive research work has been performed on fracture
mechanics with special regards to crack growth and crack
propagation in the brittle, cohesive and ductile materials. The
numerical simulations are based on the adaptive FEM analysis and
the X-FEM technique, which have been verified by the results of
experimental tests. The method of adaptive finite element
procedure is used to refine the mesh at the crack tip region
based on the estimated error. The simulation of crack
propagation was performed in 2D and 3D problems for both the
elastic crack growth and the damage-plastic crack propagation.
The other technique is based on the extended-FEM method, which
is a state-of-the-art computational approach in FEM, and
eliminates the necessity of using mesh generators in the
analysis of continuum mechanics problems involving any kind of
interfaces, such as bi-materials, contact problems, and
fractured media.
Moreover, a dynamic large deformation X-FEM method is proposed
for modeling the full process of dynamic ductile fracture based
on a nonlocal damage visco-plasticity model. The effect of
inertia is modeled using an explicit central difference scheme
which is enhanced through the use of mass lumping, reduced
integration with hourglass control, and numerical damping. The
material nonlinearity and the flow stress dependency on strain
rate, hardening and temperature are modeled with the
Johnson–Cook visco-plastic model. The micro void nucleation,
growth and coalescence are modeled macroscopically with an
isotropic damage model. The localization phenomenon due to the
damage and thermal softening is suppressed by using the visco-plastic
regularization in combination with a nonlocal visco-plastic
model. The large deformation and large strain formulations are
implemented within the X-FEM framework to model the macro crack
discontinuities using an updated Lagrangian approach. Crack
propagation and crack direction criteria are applied and the
issues relating to the combination of X-FEM with the damage
visco-plasticity model are addressed.
An enriched–FEM modeling of
dynamic ductile fracture problems with a nonlocal damage–viscoplasticity
model for a double notch problem (see
Finite
Elements in Analysis and Design,
99,
49-67,
2015) |
The
contact–impact of two elastic rings; The deformed
configurations at various time steps (see:
Computer Methods in Applied Mechanics and Engineering,
269, 198-221,
2014) |
Computational Contact Mechanics
The main area of research in computational solid mechanics
encompasses large deformations, plasticity behaviour and contact
problems for various solid mechanics including: metal forming
applications and shear band localization problems. Both
classical and Cosserat continuum mechanics have been employed in
the framework of FEM analysis, adaptive FEM simulation, meshless
technique, X-FEM modeling, and ALE formulation. Recent
achievements in computational modeling of large plastic
deformations are based on a three-invariant cap plasticity model
and an enriched arbitrary Lagrangian-Eulerian finite element
technique. A generalized cap plasticity with the isotropic-kinematic
hardening rule is developed within the framework of an enriched
arbitrary Lagrangian-Eulerian FE formulation. An enriched finite
element method is implemented based on the extended FEM
technique to capture the arbitrary interfaces independent of
element boundaries. The process is accomplished by performing a
splitting operator to separate the material (Lagrangian) phase
from convective (Eulerian) phase, and partitioning the
Lagrangian and relocated meshes with some sub-quadrilaterals
whose Gauss points are used for integration of the domain of
elements. The enriched ALE-FEM technique is applied to simulate
the friction between two bodies by imposing the contact
constraints and modifying the contact properties of frictional
slip through the node-to-surface contact algorithm in the
concept of penalty and augmented-Lagrange approaches.
Large deformation contact
modeling of a block with a curved interface problem based
on the phantom-node X-FEM technique (see: Computational
Mechanics,
61,
449–469, 2018) |
An enriched–FEM
modeling of the hydro-fracture approaching a pre-existing
natural fault together with the contours of vertical stress,
vertical deformation and horizontal deformation at the
intersection of hydro-fracture with the fully-cemented
natural fault (Engineering Fracture Mechanics, 212,
269-290, 2019)
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Computational Geomechanics
Research activities in computational geotechniques have been
developed based on the FEM and X-FEM modeling of deforming
porous medium interacting with the flow of two immiscible
wetting and non-wetting pore fluids. The governing equations are
derived for unsaturated soils, such as the earth and rockfill
dams, within the framework of the generalized Biot’s theory. An
integrated software environment is designed for
multi-disciplinary computational modeling of geotechnical
problems, called SUT-DAM (Advances Engineering Software,
37, 728-753, 2006). The SUT-DAM is designed in
both popularity and functionality with the development of
user-friendly pre- and post-processing software. In SUT-DAM, a
numerical model is developed based on a Lagrangian finite
element formulation for large deformation dynamic analysis of
saturated and partially saturated soils. An adaptive FEM
strategy is used by employing an error estimator, adaptive mesh
refinement, and data transfer operator. The SUT-DAM supports
different yield criteria, including the classical and advanced
constitutive models, such as the Pastor-Zienkiewicz and cap
plasticity models.
Furthermore, an enriched finite element method is developed to
model the interaction between the hydraulically-driven fracture
and the naturally-cemented fault within the porous medium. The
inflow and continuity equations of the fluid phase are solved
throughout the discontinuities in conjunction with the momentum
balance equation of the bulk using a sequential manner based on
the staggered Newton algorithm. The frictional contact behavior
along the overlapped zone of the naturally-cemented fault is
modeled through the extended–FEM penalty scheme. The effect of
cementation bond along the natural fault is incorporated
employing a modified Coulomb law to derive the cementation
tractions. Various interaction scenarios are recognized based on
an approach accounting for the effect of fracturing fluid
diversion into the intersected fracture network.
A nine-spot well
pattern with a hydraulically fractured production and different
natural fracture pattern; Contours of water saturation in
fractured reservoir after 7 years of water injection (Advances
in Water Resources,
94,
510-528,
2016) |
Software Design |
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SUT_DAM
(version 4.0)
An integrated software environment for geotechnical engineering
For
more information about the software see the following paper:
A.R. Khoei,
S.A. Gharehbaghi, A.R. Azami and A.R. Tabarraie,
‘SUT-DAM:
An integrated software environment for multi-disciplinary
geotechnical engineering’,
Advances Engineering Software,
37, 728-753, 2006. |
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