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  • A Multi-Scale Theoretical Scheme for Metal Deformation
  • Abstract A conceptual theoretical scheme for single crystal metal deformation is presented consisting of multi-scale models from dislocation dynamics to the continuum constitutive relations. The scheme rests on the fundamental observations that deformation is characterized by partially ordered internal dislocation wall structures, discontinuous strain bursts in time, and strain localization in a surface slip band structure. A percolation strain model corresponds to elementary slip line burst events, with percolation parameters to be supplied from experiments and dislocation dynamics studies of wall structures. A model for localization of the slip lines into bands is proposed (for suitable loadings) which envisions channels for…
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  • The Influence of Crystal Surfaces on Dislocation Interactions in Mesoscopic Plasticity: A Combined Dislocation Dynamics- Finite Element Approach
  • Abstract We focus here on the direct coupling of Dislocation Dynamics (DD) computer simulations with the Finite Element Method (FEM) to simulate plastic deformation of micro-scale structures, and investigate the influence of crystal surfaces on dislocation motion. A series of three-dimensional (3-d) DD simulations of BCC single crystals with a single shear loop in the (101)-[111] slip system are first presented. The purpose of these simulations is to explore the relationship between loop force distributions and the proximity of the loop to the crystal boundary. Traction boundary conditions on a single crystal model are satisfied through the superposition of the "image''…
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  • Atomistic Measures of Materials Strength
  • Abstract We examine the role of atomistic simulations in multiscale modeling of mechanical behavior of stressed solids. Theoretical strength is defined through modes of structural instability which, in the long wavelength limit, are specified by criteria involving elastic stiffness coefficients and the applied stress; more generally, strength can be characterized by the onset of soft vibrational modes in the deformed lattice. Alternatively, MD simulation of stress-strain response provides a direct measure of the effects of small-scale microstructure on strength, as illustrated by results on SiC in single crystal, amorphous, and nanocrystalline phases. A Hall-Petch type scaling is introduced to estimate strength…
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  • Dislocation Nucleation and Propagation During Thin Film Deposition Under Tension
  • Abstract Using molecular dynamics method, we study the nucleation of dislocations and their subsequent propagation during the deposition of tungsten thin films under tension. Aiming to reveal the generic mechanisms of dislocation nucleation during the deposition of polycrystalline thin films, the case of tungsten on a substrate of the same material is considered. The substrate is under uniaxial tension along the [111] direction, with the thermodynamically favored (01ˉˉ1) surface being horizontal. The simulation results indicate that the nucleation starts with a surface step, where a surface atom is pressed into the film along the [111ˉˉ] direction. This process leads…
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  • The Core Structure and Energy of the 90° Partial Dislocation in Si
  • Abstract The 90° partial dislocation in Si is studied using a combination of Tersoff potentials and isotropic elasticity theory. Both periodic supercells and cylindrical cells are employed and the results compared. The dislocation core radius is extracted by fitting the results of atomic scale calculations to an expression for the elastic energy of the dislocation. The energy differences between two proposed reconstructions of the dislocation core are computed and found to depend systematically on the stress field imposed on the dislocation. It is suggested that hydrostatic stresses may introduce a core transformation.
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  • Computer Simulation of Fundamental Behaviors of Point Defects, Clusters and Interaction with Dislocations in Fe and Ni
  • Abstract In order to investigate the interaction of point defects with a dislocation, an interstitial cluster or a SFT (stacking fault tetrahedron), computer simulation has been carried out in model Fe and Ni crystals. The capture zone (the region where the interaction energy is larger than kT) was determined for various interactions. Calculated capture zone for T =500°C for SIAs (crowdion and dumbbell) around a straight edge dislocation is larger than that for a vacancy in both Fe and Ni. Capture zones for Ni are larger than those for Fe, suggesting that Ni (fcc) has a larger dislocation bias factor than…
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  • A Variational Multiscale Method to Embed Micromechanical Surface Laws in the Macromechanical Continuum Formulation
  • Abstract The embedding of micromechanical models in the macromechanical formulation of continuum solid mechanics can be treated by a variational multiscale method. A scale separation is introduced on the displacement field into coarse and fine scale components. The fine scale displacement is governed by the desired micromechanical model. Working within the variational framework, the fine scale displacement field is eliminated by expressing it in terms of the coarse scale displacement and the remaining fields in the problem. The resulting macromechanical formulation is posed solely in terms of the coarse scale displacements, but is influenced by the fine scale; thereby it has…
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  • The Emerging Role of Multiscale Modeling in Nano- and Micro-mechanics of Materials
  • Abstract As a result of surging interest in finding fundamental descriptions for the strength and failure properties of materials, and the exciting prospects of designing materials from their atomic level, an international symposium on Multiscale Modeling was convened at ICES'2K in Los Angeles during August 23 - 25, 2000. In this symposium, 23 speakers with research interests spanning fields as diverse as traditional mechanics, physics, chemistry and materials science have given talks at this symposium. The topics of discussion were focused on sub-continuum modeling of the mechanics of materials, taking into account the atomic structure of solid materials. The main motivation…
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  • Generation-Phase Simulation of Dynamic Crack Bifurcation Phenomenon Using Moving Finite Element Method Based on Delaunay Automatic Triangulation
  • Abstract The governing condition of dynamic crack bifurcation phenomena had not been fully elucidated until our recent experimental studies. We found from the experimental results that the energy flux per unit time into a propagating crack tip or into a fracture process zone governs the crack bifurcation. Regarding the numerical simulation of dynamic crack bifurcation, to the authors' knowledge, no accurate simulations have been carried out, due to several unresolved difficulties. In order to overcome the difficulties, for the analysis of dynamic crack bifurcation, we developed a moving finite element method based on Delaunay automatic triangulation. Using the moving finite element…
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  • Meshless BEM for Three-dimensional Stokes Flows
  • Abstract This paper describes a combination of the dual reciprocity method (DRM) and the method of fundamental solution (MFS) as a meshless BEM (DRM-MFS) to solve three-dimensional Stokes flow problems by the velocity-vorticity formulation, where the DRM is based on the compactly supported, positive definite radial basis functions (CS-PD-RBF). In the velocity-vorticity formulation, both of the diffusion type vorticity equations and the Poisson type velocity equations are solved by DRM-MFS. Here a typical internal cubic cavity flow and an external flow past a sphere are presented. The results are acceptable. Furthermore, this paper provides a preliminary work for applications to the…
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