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Hierarchical Multi-Scale Modeling of Large Plastic Deformation with Application in Powder Compaction

Rezaei Sameti, Amir | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51249 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. The hierarchical multi-scale approach is one of the most powerful techniques that takes the advantage of different scales and succeeds the limitations of each method in a way that the large systems in coarse-scale can be simulated with atomic precision. In this thesis, the hierarchical atomistic-continuum multi-scale method is developed for modeling the phenomena with non-homogenous deformation, large deformation and plastic behavior. In this regard at first, an atomistic-based higher-order continuum model is formulated in the framework of nonlinear finite element method to present the geometrically nonlinear behavior of nano-structures. The efficiency of higher-order Cauchy-Born hypothesis was studied by performing a comparison between the results of classical and higher-order Cauchy-Born hypothesis with those of the fully atomistic model for the homogeneous and non-homogeneous atomistic structures. It was shown that in the non-homogeneous atomistic structure, the higher-order multi-scale model represent the proper consistency with the results of fully atomistic model since the contribution of higher-order terms is significant for evaluation of the atomic positions in the Cauchy-Born hypothesis. In the next section, a hierarchical RVE-based continuum-atomistic multi-scale procedure is developed to model the nonlinear behavior of nano-materials. In this case, the presented several numerical examples confirmed the capability and versatility of the proposed computational algorithm in modeling the geometrical and material nonlinearities of problem. Also, a comprehensive study is performed on the mechanical behavior of atomistic-RVE which reveals that various parameters, e.g. RVE size, vacancy in atomistic structure and etc., can affect the mechanical response of atomistic-RVE. In the next sections of thesis, the forming process of nano-powders as one of the newfound methods in powder metallurgy is investigated numerically. Initially, this investigation is performed via the molecular dynamics method and the impression of different parameters on the compaction process is evaluated. The obtained results reveals that the compaction process can be divided into three distinct stages that can be characterized as transitional restacking stage, plastic deformation stage and cold working and particle attrition stage. Because of the high computational cost of fully atomistic method, the presented RVE-based atomistic-continuum hierarchical multi-scale approach is employed for modeling the compaction process of nano-powders in large specimens. In this regard, along with the numerical simulations of nano-powder compaction process with highly nonlinear behavior, the impression of various parameters on the mechanical behavior of employed atomistic-RVE is investigated. According to the numerical simulations, it is observed clearly that the employed multi-scale technique enable to capture the highly nonlinear behavior of nano-powders during the compaction process. Also, on the basis of the investigations on the atomistic-RVE, it is shown that the number and size of nano-particles, point defects in the nano-particles structure and etc. can have the significant influence on the mechanical response of atomistic-RVE which should be determined in consistency with the considered materials
  9. Keywords:
  10. Computational Nanomechanics ; Multiscale Modeling ; Large Deformation ; Plastic Deformation ; Representative Volume Element ; Powder Compaction ; Computational Mechanics

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