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A Coupling Atomistic-continuum Approach for Modeling Dislocation in Plastic Behavior of Nano-structures

Omrani Pournava, Amir Mohsen | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: English
  3. Document No: 49939 (53)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Khoei, Amir Reza; Jahanshahi, Mohsan
  7. Abstract:
  8. In this study, a novel multi-scale hierarchical method has been employed to explore the role of edge dislocation on Nano-plates with hexagonal atomic structure in large deformation. multiscale hierarchical atomistic/molecular dynamics (MD) finite element (FE) coupling methods are proposed to demonstrate the impact of dislocation on mechanical properties of Magnesium in large deformation. The atomic nonlinear elastic parameters are attained via computing first-order derivation of stress with respect to strain of Representative Volume Element (RVE). To associate between atomistic and continuum level, the mechanical characteristics are captured in the atomistic scale and transferred to the continuum level directly by using Abaqus subroutine UMAT. The different number of instances with primary edge dislocations was made and different strain rate is considered under the 0 kelvin temperature to evaluate Magnesium’s nonlinear elastic constants. Primary edge dislocations are created by proper adjustment of atomic positions to resemble dislocations and then the application of equations of motion to the relaxed configuration of this instances. The interatomic potential used for atomistic simulation is Finnis-Sinclair Embedded-Atom-Method (FS-EAM) as many-body interatomic potential and the Nose-Hoover thermostat has been executed to adjust the inflection of temperature. Stress-strain curve of different representative volume elements under simultaneously engineering strain in axes X, Y, and XY loading have been provided and also by fitting a second-order polynomial to elastic constants, it has obtained space in atomistic scale and then data has been transferred to the macro level. nonlinear elastic constants are achieved by computing the first derivation of stress per unit volume with respect to strain. the variation of nonlinear elastic constants for Nano-crystalline RVE’s at different number of primary edge dislocations have been obtained. With the purpose of providing a relation between various quantities in Nano-scale to their counterparts in macro-scale, computed material properties from molecular dynamics simulation have been transferred to each gauss points of finite element mesh by using UMAT subroutine. The numerical results clearly show the behavior of material in the presence of primary dislocations. The analysis of structure of crystal defects for instance dislocation and grain boundaries requires consideration of inharmonic effects on the scale of micro. Molecular dynamic based on interatomic interaction makes a powerful and accurate tool of analysis on this scale available. Dislocations are one of the most important classes of defects in crystals. They have significant effects on the physical properties of crystals. They could be primarily generated during the formation of a crystal or during the loading instance. Since dislocation is a change in perfect crystal structure, it is possible to recognize in the molecular level. However, the high computation cost of the MD level has led researchers to use the multi-scale methods in dislocation studies. Example of application of these techniques might be found in the work of. The future study will be focused on the development of multi-scale method to mechanism of dislocation emission to investigate the plasticity behavior of Nano-Crystalline structure
  9. Keywords:
  10. Molecular Dynamics ; Strain Localization ; Large Deformation ; Mechanical Behavior ; Dislocation Emission ; Edge Dislocation ; Computational Nanomechanics ; Multiscale Finite Volume Method

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