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Mechanical Approach (mechanical + approach)

Distribution by Scientific Domains

Medical Sciences	33%

Selected Abstracts

Continuum Mechanical Approach to Sintering of Nanocrystalline Zirconia,

ADVANCED ENGINEERING MATERIALS, Issue 10 2005
R. Zuo
Nanocrystalline 3,mol,% yttria-stabilized zirconia was sinter-forged isothermally under varying external uniaxial stresses. The applied uniaxial stresses were relatively low, compared to the intrinsic sintering stress of the material studied. Uniaxial sintering stresses and uniaxial viscosities were experimentally determined as function of density by means of a continuum mechanical approach which involves measuring the sintering rate of a free-sintered specimen, and a specimen sintered under the application of an external uniaxial stress. The uniaxial viscosity increased strongly with density only in the final stage sintering regime. The magnitude of the uniaxial sintering stress exhibited a decrease with density. [source]

Past, Present and Future Insights into the Understanding and Treatment of Osteoarthritis: Molecular and Mechanical Approaches

INTERNATIONAL JOURNAL OF EXPERIMENTAL PATHOLOGY, Issue 1 2009
Article first published online: 19 JAN 200
First page of article [source]

Constituent Particle Break-Up During Hot Rolling of AA 5182,

ADVANCED ENGINEERING MATERIALS, Issue 1-2 2010
Nicolas Moulin
Aluminum sheet is currently used for body panels on a number of mass-produced vehicles, in particular for closure panels. AA5xxx alloys always contain coarse inter-metallic particles (Alx(Fe,Mn)ySi, Mg2Si) after casting. In the present work inter-metallic particle break-up during hot reversible rolling of AA5182 alloy sheets has been analyzed. The sizes and shapes of inter-metallic particles in as-cast and industrially hot rolled AA5182 alloys sheets were characterized by 3D X-ray tomography observations. The relation between particle break-up and particle morphology was then analyzed statistically and by a micromechanical finite element (FE)-based model. The essential outcomes of the statistical approach may be summarized as follows. The inter-metallic particle population may be described by five morphological parameters. Secondly the comparison of the particle morphology in as cast and industrially rolled sheets leads to the definition of five classes. The evolution of each particle class as function of the rolling strain is provided. The statistical analysis shows which particles break-up. The stresses and strains in inter-metallic particles, embedded in an elasto-viscoplastic aluminum matrix submitted to plane strain compression, were analyzed by an FE model. A new failure criterion was proposed. The essential outcomes of the mechanical approach are as follows: a precise description of stress concentration mechanisms in nonconvex particles, a close description of the parameters controlling particle break-up, and finally a simplified classification of the failure behavior. [source]

Continuum Mechanical Approach to Sintering of Nanocrystalline Zirconia,

Fully quantum mechanical energy optimization for protein,ligand structure

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2004
Yun Xiang
Abstract We present a quantum mechanical approach to study protein,ligand binding structure with application to a Adipocyte lipid-binding protein complexed with Propanoic Acid. The present approach employs a recently develop molecular fractionation with a conjugate caps (MFCC) method to compute protein,ligand interaction energy and performs energy optimization using the quasi-Newton method. The MFCC method enables us to compute fully quantum mechanical ab initio protein,ligand interaction energy and its gradients that are used in energy minimization. This quantum optimization approach is applied to study the Adipocyte lipid-binding protein complexed with Propanoic Acid system, a complex system consisting of a 2057-atom protein and a 10-atom ligand. The MFCC calculation is carried out at the Hartree,Fock level with a 3-21G basis set. The quantum optimized structure of this complex is in good agreement with the experimental crystal structure. The quantum energy calculation is implemented in a parallel program that dramatically speeds up the MFCC calculation for the protein,ligand system. Similarly good agreement between MFCC optimized structure and the experimental structure is also obtained for the streptavidin,biotin complex. Due to heavy computational cost, the quantum energy minimization is carried out in a six-dimensional space that corresponds to the rigid-body protein,ligand interaction. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1431,1437, 2004 [source]

OH hydrogen abstraction reactions from alanine and glycine: A quantum mechanical approach

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2001
Annia Galano
Abstract Density functional theory (B3LYP and BHandHLYP) and unrestricted second-order Møller,Plesset (MP2) calculations have been performed using 3-21G, 6-31G(d,p), and 6-311 G(2d,2p) basis sets, to study the OH hydrogen abstraction reaction from alanine and glycine. The structures of the different stationary points are discussed. Ring-like structures are found for all the transition states. Reaction profiles are modeled including the formation of prereactive complexes, and very low or negative net energy barriers are obtained depending on the method and on the reacting site. ZPE and thermal corrections to the energy for all the species, and BSSE corrections for B3LYP activation energies are included. A complex mechanism involving the formation of a prereactive complex is proposed, and the rate coefficients for the overall reactions are calculated using classical transition state theory. The predicted values of the rate coefficients are 3.54×108 L,mol,1,s,1 for glycine and 1.38×109 L,mol,1,s,1 for alanine. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1138,1153, 2001 [source]

A comparison of quantum chemical models for calculating NMR shielding parameters in peptides: Mixed basis set and ONIOM methods combined with a complete basis set extrapolation

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2006
Seongho Moon
Abstract This article compares several quantum mechanical approaches to the computation of chemical shielding tensors in peptide fragments. First, we describe the effects of basis set quality up to the complete basis set (CBS) limit and level of theory (HF, MP2, and DFT) for four different atoms in trans N -methylacetamide. For both isotropic shielding and shielding anisotropy, the MP2 results in the CBS limit show the best agreement with experiment. The HF values show quite a different tendency to MP2, and even in the CBS limit they are far from experiment for not only the isotropic shielding of carbonyl carbon but also most shielding anisotropies. In most cases, the DFT values differ systematically from MP2, and small basis-set (double- or triple-zeta) results are often fortuitously in better agreement with the experiment than the CBS ones. Second, we compare the mixed basis set and ONIOM methods, combined with CBS extrapolation, for chemical shielding calculations at a DFT level using various model peptides. From the results, it is shown that the mixed basis set method provides better results than ONIOM, compared to CBS calculations using the nonpartitioned full systems. The information studied here will be useful in guiding the selection of proper quantum chemical models, which are in a tradeoff between accuracy and cost, for shielding studies of peptides and proteins. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 825,836, 2006 [source]

Transmission electron microscopy and theoretical analysis of AuCu nanoparticles: Atomic distribution and dynamic behavior

MICROSCOPY RESEARCH AND TECHNIQUE, Issue 7 2006
J.A. Ascencio
Abstract Though the application of bimetallic nanoparticles is becoming increasingly important, the local atomistic structure of such alloyed particles, which is critical for tailoring their properties, is not yet very clearly understood. In this work, we present detailed study on the atomistic structure of Au,Cu nanoparticles so as to determine their most stable configurations and the conditions for obtaining clusters of different structural variants. The dynamic behavior of these nanoparticles upon local heating is investigated. AuCu nanoparticles are characterized by high resolution transmission electron microscopy (HRTEM) and energy filtering elemental composition mapping (EFECM), which allowed us to study the internal structure and the elemental distribution in the particles. Quantum mechanical approaches and classic molecular dynamics methods are applied to model the structure and to determine the lowest energy configurations, the corresponding electronic structures, and understand structural transition of clusters upon heating, supported by experimental evidences. Our theoretical results demonstrate only the core/shell bimetallic structure have negative heat of formation, both for decahedra and octahedral, and energetically favoring core/shell structure is with Au covering the core of Cu, whose reverse core/shell structure is not stable and may transform back at a certain temperature. Experimental evidences corroborate these structures and their structural changes upon heating, demonstrating the possibility to manipulate the structure of such bimetallic nanoparticles using extra stimulating energy, which is in accordance with the calculated coherence energy proportions between the different configurations. Microsc. Res. Tech., 2006. © 2006 Wiley-Liss, Inc. [source]

Update on Atrial Fibrillation: Part I

CLINICAL CARDIOLOGY, Issue 2 2008
Irina Savelieva M.D.
Abstract Atrial fibrillation (AF) is an epidemic, affecting 1% to 1.5% of the population in the developed world. Projected data from the population-based studies suggest that the prevalence of AF will grow at least 3-fold by 2050. The health and economic burden imposed by AF and AF-related morbidity is enormous. Atrial fibrillation has a multiplicity of causes ranging from genetic to degenerative, but hypertension and heart failure are the commonest and epidemiologically most prevalent conditions associated with AF as both have been shown to create an arrhythmogenic substrate. Several theories emerged regarding the mechanism of AF, which can be combined into two groups: the single focus hypothesis and the multiple sources hypothesis. Several lines of evidence point to the relevance of both hypotheses to the mechanism of AF, probably with a different degree of involvement depending on the variety of AF (paroxysmal or persistent). Sustained AF alters electrophysiological and structural properties of the atrial myocardium such that the atria become more susceptible to the initiation and maintenance of the arrhythmia, a process known as atrial remodeling. Angiotensin II has been recognized as a key element in atrial remodeling in association with AF opening the possibility of exploitation of "upstream" therapies to prevent or delay atrial remodeling. The clinical significance of AF lies predominantly in a 5-fold increased risk of stroke. The limitations of warfarin prompted the development of new antithrombotic drugs, which include anticoagulants, such as direct oral thrombin inhibitors (dabigatran) and factor Xa inhibitors (rivaroxaban, apixaban). Novel mechanical approaches for the prevention of cardioembolic stroke have recently been evaluated: percutaneous left atrial appendage occluders, minimally invasive surgical isolation of the left atrial appendage, and implantation of carotid filtering devices. Copyright © 2008 Wiley Periodicals, Inc. [source]