# Your search: "author:Papadopoulos, A"

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## Scholarly Works (200 results)

Shape memory alloys have found diverse applications in several engineering

systems including biomedical devices and thermal actuators. This

is due to their superelastic and shape-memory behavior, which occur as a result of

solid-solid transformations from a parent phase to several variants of the product

phases. The most commonly used shape-memory alloy is a nearly equiatomic

NiTi alloy known as Nitinol. Much research has been devoted to modeling

polycrystalline Nitinol under various thermomechanical loading conditions. As a result,

several phenomenological and micromechanics-based models have been proposed

to characterize the complex behavior of Nitinol in both monocrystalline and

textured polycrystalline form.

In this work, a multiscale thermomechanical model for Nitinol is developed

that takes into account the temperature-dependent multivariant phase transformations

at the single-crystal level and the interaction between various crystals in a textured

polycrystalline aggregate. The single-crystal thermomechanical model

is relevant to both thermal loading and mechanical loading at high strain-rates.

The coupled thermomechanical problem is solved using a monolithic approach

in a finite-element framework. Specializing this model to isothermal conditions leads to a

temperature-independent mechanical response, which is suitable for quasistatic

mechanical loading. Most models in literature account only for isothermal stress-induced

phase transformations between the *austenite* and multivariant

*martensite* phases in Nitinol. In this work, such a constitutive model

is extended to include the formation of intermediate multivariant

*rhombohedral* phase as well. In order to model the macroscopic

response of polycrystalline Nitinol, first a statistics-based method is developed

to determine the optimum size of Representative Volume Element (RVE) meant

for solving the microscale problem. The macroscale constitutive response is

then derived through computational homogenization of this RVE response.

A finite element-on-finite element architecture is employed to solve

this multiscale problem accurately. Representative numerical

simulations are performed in order to validate the modeling approach with

several experiments on thin-walled tubes.

- 1 supplemental PDF

Cell motility, and in particular crawling, is a complex process, with many as yet unknown details (\textit{e.g.}, sources of viscosity in the cell, precise structure of focal adhesions). However, various models of the motility of cells are emerging. These models concentrate on different aspects of cellular behavior, from the motion of a single cell itself, to -taxis behaviors, to population models. Models of single cells seem to vary significantly in their intended scope and the level of detail included.

Most single cells are far too large and complex to be globally amenable to fine-scale modeling. At the same time, cells are subject to external and internal influences that are connected to their fine-scale structure. This work presents a continuum model, including the use of a continuum theory of surface growth, that will predict the crawling motion of the cell, with consideration made for the appropriate fine-scale dependencies.

This research addresses several modeling aspects. At the continuum level, the relationships between force and displacement in the bulk of the cell are modeled using the balance laws developed in continuum mechanics. Allowances are made for the treatment of and interaction between multiple protein species, as well as for the addition of various terms into the balance laws (\textit{e.g.}, stresses generated by protein interactions). Various assumptions regarding the nature of cell crawling itself and its modeling are discussed. For instance, the extension of the lamellipod/detachment of the cell is viewed as a growth/resorption process. The model is derived without reference to dimensionality.

The second component of the presentation concerns the numerical implementation of the cell motility model. This is accomplished using finite elements, with special features (\textit{i.e.}, ALE, discontinuous elements) being used to handle certain stages of the motility. In particular, the growth assumption used to model the crawling motility is represented using ALE, while the strong discontinuities that arise out of the growth model are represented using the discontinuous elements. Results from representative finite element simulations are shown to illustrate the modeling capabilities.

The mechanical behavior of cortical actin cytoskeleton, a network of cross-linked semiflexible actin filaments (F-actin), is of interest to biologists and engineers since it plays a key role in many fundamental cellular properties and processes, such ascell shape and motility. The dynamics of flexible filaments in low-Reynolds number viscous shear flow has also gained much interest in the last decades in a wide variety of applications involving biological systems like DNA, polymers and proteins. In this dissertation, two numerical algorithms based on the Immersed Boundary Method (IBM) are proposed and implemented for the study of the actin cortex and the dynamics of semiflexible filaments immersed in low-Reynolds flows.

A new, modified, and more computationally efficient version of the IBM that combines the Coarse-Graining Method (CGM) with IBM is developed for modeling of inextensible filaments in shear flow at low Reynolds numbers. The various two-dimensional orbit regimes of flexible filaments are studied and the results of the proposed method are validated with theoretical results and previous works, numerical and experimental, showing excellent agreement. They are subsequently used to develop a prediction model using Artificial Neural Networks (ANN) to effectively forecast the orbit regime of a filament in shear flow with different parameters.

An extension of the traditional IBM to include a stochastic stress tensor is also proposed in order to model the thermal fluctuations in the cytoplasmic fluid surrounding the actin cortex. The theoretical values for time-averaged contraction for a single inextensible filament under hydrodynamic thermal fluctuations are verified through numerical simulation. The mechanical behavior of the actin cortex and its elasticity when subjected to shear flow is investigated, illustrating a stiffening of the cross-linked network with increasing strain under shear flow, as other experimental and numerical studies have shown. By implementing the proposed extension of the IBM, the behavior and interaction of passive F-actin under thermal fluctuations is also studied in the current work, where a trend of filaments to spread out is observed.

The diffusion of liquid and gas through porous solids is of considerable technological interest and has been investigated for decades in a wide spectrum of disciplines encompassing chemical, civil, mechanical, and petroleum engineering. Porous solids of interest are made of either natural materials (e.g., soil, sand) or man-made materials (e.g., industrial filters, membranes). In both cases, liquids (e.g., water, crude oil) and gases (e.g., air, oxygen, natural gas) are driven through the voids in the porous solid by naturally or artificially induced pressure. Nafion⃝R is an important example of a well-characterized man-made porous medium due to its extensive use in proton- exchange membrane fuel cells. Here, while the fuel cell is in operation, a mixture of air and water diffuses through the pores of a Nafion⃝R membrane. The efficiency of the fuel cell is affected by the variation in water concentration. In addition, high water concentration has been experimen- tally shown to cause substantial volumetric deformation (swelling) of the membrane, which may compromise the integrity of the device.

In this dissertation, a continuum approach for modeling diffusion of fluid through a porous elastic solid is proposed. All balance laws are formulated relative to the frame of a macroscopic solid resulting from the homogenization of the dry solid and the voids. When modeling only liquid diffusion through the macroscopic solid, the displacement of the macroscopic solid and the liquid volume fraction are chosen to characterize the state of the porous medium, and Fick's law is used as the governing equation for liquid flow. When modeling multiphase diffusion through the macroscopic solid, the displacement of the solid, the gas pressure and the liquid saturation are chosen as state variables, and both fluid diffusions are assumed to follow Darcy's law. Both single phase and multiphase diffusion models are implemented in the finite element method, and tested with various loading conditions on different types of materials. Numerical simulation results are presented to show the predictive capability of the two models.

This thesis presents a class of homogeneous non-equilibrium molecular dynamics (HNEMD) methods for obtaining the heat transport coefficient that relates the heat flux and temperature gradient in the linear irreversible regime. These methods are based on the linear response theory of statistical mechanics. The proposed HNEMD methods are parallelizable, and yield better statistical averages at lower overall computational cost than the existing direct and Green-Kubo methods.

The HNEMD method, as it was initially proposed, is applicable only to single-species systems with two-body interactions. In this thesis, the HNEMD method is extended to single species systems with many-body interactions, and is applied to silicon systems where three-body interactions are taken into account.

The HNEMD method developed for single-species systems is inadequate for obtaining the heat transport coefficient of multi-species systems. A further development of the HNEMD method, the Mixture-HNEMD (M-HNEMD) method, is presented for multi-species systems with many-body interactions. This M-HNEMD method satisfies all the requirements of linear response theory and is compatible with periodic boundary conditions. Applications of the M-HNEMD method to liquid argon-krypton systems with two-body interactions and to perfectly crystalline gallium-nitride systems with three-body interactions are presented, and the results are consistent with the results from the Green-Kubo method. This is the first HNEMD method which can be used for calculating the heat-transport coefficient of multi-species systems.

The expressions for stress tensor and heat-flux vector needed for the development of HNEMD method for single-species systems and of the M-HNEMD method for multi-species systems with many-body interactions require an extension of the statistical mechanical theory of transport processes proposed by Irving and Kirkwood. This extension forms an integral part of the thesis.

In this work, molecular dynamics modeling is used to study the mechanical

properties of PPTA crystallites, which are the fundamental microstructural

building blocks of polymer aramid fibers such as Kevlar. Particular focus is

given to constant strain rate axial loading simulations of PPTA crystallites,

which is motivated by the rate-dependent mechanical properties observed in

some experiments with aramid fibers. In order to accommodate the covalent

bond rupture that occurs in loading a crystallite to failure, the reactive

bond order force field ReaxFF is employed to conduct the simulations.

Two major topics are addressed: The first is the general behavior of PPTA

crystallites under strain rate loading. Constant strain rate loading

simulations of crystalline PPTA reveal that the crystal failure strain

increases with increasing strain rate, while the modulus is not affected by

the strain rate. Increasing temperature lowers both the modulus and the

failure strain. The simulations also identify the C--N bond connecting the

aromatic rings as weakest primary bond along

the backbone of the PPTA chain. The effect of chain-end defects on PPTA

micromechanics is explored, and it is found that the presence of a chain-end

defect transfers load to the adjacent chains in the hydrogen-bonded sheet in

which the defect resides, but does not influence the behavior of any other

chains in the crystal. Chain-end defects are found to lower the strength of

the crystal when clustered together, inducing bond failure via stress

concentrations arising from the load transfer to bonds in adjacent chains near

the defect site. The second topic addressed is the nature of primary and

secondary bond failure in crystalline PPTA. Failure of both types of bonds is

found to be stochastic in nature and driven by thermal fluctuations of the

bonds within the crystal. A model is proposed which uses reliability theory

to model bonds under constant strain rate loading as components with

time-dependent failure rate functions. The model is shown to work well for

predicting the onset of primary backbone bond failure, as well as the onset of

secondary bond failure via chain slippage for the case of isolated

non-interacting chain-end defects.