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CONTENTS
Volume 4, Number 3, September 2011
 

Abstract
A multiscale modeling scheme that addresses the influence of the nanoparticle size in nanocomposites consisting of nano-sized spherical particles embedded in a polymer matrix is presented. A micromechanics-based constitutive model for nanoparticle-reinforced polymer composites is derived by incorporating the Eshelby tensor considering the interface effects (Duan et al. 2005a) into the ensemblevolume average method (Ju and Chen 1994). A numerical investigation is carried out to validate the proposed micromechanics-based constitutive model, and a parametric study on the interface moduli is conducted to investigate the effect of interface moduli on the overall behavior of the composites. In addition, molecular dynamics (MD) simulations are performed to determine the mechanical properties of the nanoparticles and polymer. Finally, the overall elastic moduli of the nanoparticle-reinforced polymer composites are estimated using the proposed multiscale approach combining the ensemble-volume average method and the MD simulation. The predictive capability of the proposed multiscale approach has been demonstrated through the multiscale numerical simulations.

Key Words
multiscale modeling; molecular dynamics; micromechanics-based constitutive model; nanoparticles; polymer composites.

Address
B.R. Kim: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea
S.H. Pyo: Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI, 48109-2125, USA
G. Lemaire and H.K. Lee: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

Abstract
Accurate estimations of pre-failure deformations and post-failure responses of geostructures require that the simulation tool possesses at least three main ingredients: 1) a constitutive model that is able to describe the macroscopic stress-strain-strength behavior of soils subjected to complex stress/strain paths over a wide range of confining pressures and densities, 2) an embedded length scale that accounts for the intricate physical phenomena that occur at the grain size scale in the soil, and 3) a computational platform that allows the analysis to be carried out beyond the development of an initially

Key Words
post-failure response; granular soil; micropolar plasticity; localization; shear banding.

Address
Majid T. Manzari and Karma Yonten: Civil and Environmental Engineering Department, The George Washington University,
Washington, DC 20052, USA

Abstract
International efforts have focused recently on the development of tungsten surfaces that can intercept energetic ionized and neutral atoms, and heat fluxes in the divertor region of magnetic fusion confinement devices. The combination of transient heating and local swelling due to implanted helium and hydrogen atoms has been experimentally shown to lead to severe surface and sub-surface damage. We present here a computational model to determine the relationship between the thermo-mechanical loading conditions, and the onset of damage and failure of tungsten surfaces. The model is based on thermoelasticity, coupled with a grain boundary damage mode that includes contact cohesive elements for grain boundary sliding and fracture. This mechanics model is also coupled with a transient heat conduction model for temperature distributions following rapid thermal pulses. Results of the computational model are compared to experiments on tungsten bombarded with energetic helium and deuterium particle fluxes.

Key Words
tungsten; Plasma-Facing Components (PFC); helium ions; cracks; grain boundary damage; swelling.

Address
Tamer Crosby and Nasr M. Ghoniem: Department of Mechanical & Aerospace Engineering, University of California at Los Angeles (UCLA), 420 Westwood Plaza, Los Angeles, CA 90095-1597, USA

Abstract
Recent advances in scientific computing enable the full atomistic simulation of DNA molecules. However, there exists length and time scale limitations in molecular dynamics (MD) simulation for large DNA molecules. In this work, a two-level homogenization of DNA molecules is proposed. A wavelet projection method is first introduced to form a coarse-grained DNA molecule represented with superatoms. The coarsened MD model offers a simplified molecular structure for the continuum description of DNA molecules. The coarsened DNA molecular structure is then homogenized into a threedimensional beam with embedded molecular properties. The methods to determine the elasticity constants in the continuum model are also presented. The proposed continuum model is adopted for the study of mechanical behavior of DNA loop.

Key Words
multiscale homogenization; wavelet projection method; coarse-graining; DNA molecules; molecular dynamics.

Address
Hailong Teng: Civil & Environmental Engineering Department, University of California, Los Angeles (UCLA), Los Angeles, USA
Livermore Software Technology Company (LSTC), Livermore, USA
Chung-Hao Lee and Jiun-Shyan Chen: Civil & Environmental Engineering Department, University of California, Los Angeles (UCLA), Los Angeles, USA

Abstract
The dynamic mechanical behavior of silicone rubber reinforced with multi-walled carbon nanotubes (MWCNTs) has been investigated in this study. The MWCNT-reinforced nanocomposites are tested in compression mode through dynamic mechanical analysis (DMA). Multiple effects including MWCNT loading, testing frequency, dynamic strain amplitude, and pre-strain level are taken into consideration. Results show that, by adding 5 wt% of MWCNTs, the dynamic stiffness and damping coefficient of the silicone rubber are significantly enhanced. It is further observed that the dynamic mechanical properties of the nanocomposites are sensitive to dynamic strain amplitude but only slightly affected by pre-strains.

Key Words
nanocomposites; carbon nanotubes; mechanical behavior; damping.

Address
Rui Li and L.Z. Sun: Department of Civil and Environmental Engineering, University of California, Irvine, CA 92697-2175, USA


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