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CONTENTS
Volume 1, Number 4, December 2008
 

Abstract
The interfacial thermal resistance of pristine and defective carbon nanotubes (CNTs) embedded in low-density polyethylene matrix is studied in this paper. Interface thermal resistance in nanosystems is one of the most important factors that lead to the large variation in thermal conductivities in literature and the novelty of this paper lies in the estimation of the interfacial thermal resistance for defective nanotubes-systems. Thermal properties of CNT nanostructures are estimated using molecular dynamics (MD) simulations and the simulations were carried out for various temperatures by rescaling the velocities of carbon atoms in the nanotube. This paper also deals with the mesoscale thermal conductivities of composite systems, using effective medium theories by considering the size effect in the form of interfacial thermal resistance and also using the conventional micromechanical methods like Hashin-Shtrikman bounds and Wakashima-Tsukamoto estimates.

Key Words
carbon nanotube; molecular dynamics; universal force field potential; interfacial thermal resistance; stone-wales defect; effective medium theories; thermal conductivity.

Address
V. U. Unnikrishnan; Advanced Computational Mechanics Laboratory, Department of Mechanical Engineering,
Texas A&M University, College Station, TX 77843-3123, USA
J. N. Reddy and D. Banerjee; Multi Phase Flows and Heat Transfer Laboratory, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA
F. Rostam-Abadi; U.S. Army Tank-Automotive and Armaments Command, AMSTA-TR-R, Warren, MI 48397, USA

Abstract
Atomistic simulation of nanoindentation with spherical indenters was carried out to study dislocation structures, mean contact pressure, and nanohardness of Au and Al thin films. Slip vectors and atomic stresses were used to characterize the dislocation processes. Two different characteristics were found in the induced dislocation structures: wide-spread slip activities in Al, and confined and intact structures in Au. For both samples, the mean contact pressure varied significantly during the early stages of indentation but reached a steady value soon after the first apparent load drop. This indicates that the nanohardness of Al and Au is not affected by the indentation depth for spherical indenters, even at the atomistic scale.

Key Words
nanohardness; contact pressure; contact area; nanoindentation; molecular dynamics; size effects; dislocation.

Address
Chuin-Shan Chen, Chien-Kai Wang and Shu-Wei Chang; Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan

Abstract
This paper employs the reproducing kernel (RK) approximation for evaluation of field theory-based incompatibility tensor in a polycrystalline plasticity simulation. The modulation patterns, which is interpreted as mimicking geometrical-type dislocation substructures, are obtained based on the proposed method. Comparisons are made using FEM and RK based approximation methods among different support sizes and other evaluation conditions of the strain gradients. It is demonstrated that the evolution of the modulation patterns needs to be accurately calculated at each time step to yield a correct physical interpretation. The effect of the higher order strain derivative processing zone on the predicted modulation patterns is also discussed.

Key Words
incompatibility tensor; field theory; reproducing kernel approximation; dislocation substructure; crystal plasticity; finite element method.

Address
Y. Aoyagi; Department of Mechanical Engineering, Keio University, 3-14-1, Hiyoshi Kohoku-Ku, Yokohama, 223-8522, Japan
T. Hasebe; Department of Mechanical Engineering, Kobe University, 1-1, Rokkodai Nada, Kobe, 657-8501, Japan
P. C. Guan and J. S. Chen; Civil & Environmental Engineering Department, UCLA, 5731 Bolter Hall, Los Angeles, CA 90095, USA

Abstract
This paper examined the stability of high-dense dislocation substructures (HDDSs) associated with martensite laths in High Cr steels supposed to be used for FBR, based on a series of dislocation dynamics (DD) simulations. The DD simulations considered interactions of dislocations with impurity atoms and precipitates which substantially stabilize the structure. For simulating the dissociation processes, a point defect model is developed and implemented into a discrete DD code. Wall structure composed of high dense dislocations with and without small precipitates were artificially constructed in a simulation cell, and the stability/instability conditions of the walls were systematically investigated in the light of experimentally observed coarsening behavior of the precipitates, i.e., stress dependency of the coarsening rate and the effect of external stress. The effect of stress-dependent coarsening of the precipitates together with application of external stress on the subsequent behavior of initially stabilized dislocation structures was examined.

Key Words
dislocation dynamics; dislocation substructure; creep strength; high Cr steel; multiscale modeling; field theory.

Address
M. Yamada, T. Hasebe and Y. Tomita; Graduate School of Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe, 657-8501, Japan
T. Onizawa; Japan Atomic Energy Agency, Core and Structural Group, Advanced Nuclear System Research and Development Directorate, 4002 Narita, O-arai, Ibaraki 311-1393, Japan

Abstract
In this paper a general approach for studying the motion of a cantilever beam interacting with a 2D fluid flow is presented. The fluid is solved by a general purpose commercial computational fluid dynamics (CFD) package (FLUENT 6.2), while the structure is managed by means of a dedicated finite element method solver, coded in FLUENT as a user-defined function (UDF). A weak fluid structure interaction coupling scheme is adopted exchanging information at the end of each time step. An arbitrary cantilever beam can be introduced in the CFD mesh with its wetted boundaries specified; the cantilever can also interact with specified rigid and flexible walls through use of a non-linear contact algorithm. After a brief review of relevant scientific contributions, some test cases and application examples are presented.

Key Words
cantilever beam; computational fluid dynamics; finite element methods; fluid-structure interaction; weak coupling.

Address
Riccardo Baudille and Marco Evangelos Biancolini; Department of Mechanical Engineering University of Rome \"Tor Vergata\" Via del Politecnico 1, 00133 Roma, Italy

Abstract
In this work, a nano-size rocksalt crystal (magnesium oxide) is considered as a continuous collection of unit cells, while each unit cell consists of discrete atoms; and modeled by a multiscale concurrent atomic/continuum field theory. The response of the crystal to an electromagnetic (EM) wave is studied. Finite element analysis is performed by solving the governing equations of the multiscale theory. Due to the applied EM field, the inhomogeneous motions of discrete atoms in the polarizable crystal give rise to the change of microstructure and the polarization wave. The relation between the natural frequency of this system and the driving frequency of the applied EM field is found and discussed.

Key Words
atomic/continuum modeling; finite element; polarization; microstructure.

Address
Yajie Lei, James D. Lee and Xiaowei Zeng; Department of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC 20052, USA


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