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Simulation of Crack Propagation in Ductile Metals Under Dynamic Cyclic Loading by Adaptive Finite Element Method and Continuum Damage Mechanics Model

Eghbalian, Mahdad | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 44475 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. Crack nucleation and growth is unfavorable in many industrial and every day-life cases. designers’ effort is to prevent or delay it by taking into account safety and maintenance considerations; but in some industrial operations, the main target is to form a crack in a part to achieve a particular shape; and designers’ duty is to control the way it happens. so numerical modeling of this phenomena has many useful applications in preventing the structures’ failure and designing the production processes for industrial goods; and because of this, a great attention has been paid to it in the last two decades. a situation usually encountered in every day-life is the earthquake excitation which threatens the safety of structures and industrial equipment. in this case the crack growth and failure behavior of ductile materials (such as structural steel) under dynamic cyclic loading becomes important from design point of view.
    The goal of this dissertation is to simulate the crack nucleation and growth process in ductile materials under dynamic cyclic loading. several investigations in this field reveal that the offspring of ductile fracture is the nucleation and growth of microcracks, which in macroscopic scale show themselves in the form of a softening material behavior. in order to simulate this softening behavior, ductile fracture assumptions and continuum damage mechanics is utilized. two approaches in simulating the material failure by ductile fracture can be considered. The first approach is to introduce the crack as a softened region and the second one is to introduce it as a discontinuity in the displacement field. none of the aforementioned approaches can cover all the aspects of the ductile fracture correctly. so in this investigation a combination of these two approaches is utilized. in this manner both the softening behavior of the material prior to crack initiation and the discontinuity in the displacement field after that, is modeled. this softening behavior is simulated using the damage concept and its’ effect on the constitutive equations of the material. there are numerous methods for modeling displacement discontinuous field. in this investigation Adaptive Finite Element method is used for this purpose; because the geometry of the crack can be modeled by updating the domain discretization. moreover optimizing the domain mesh can be achieved by reducing the error, besides this, it prevents element distortion and so a proper mesh is available during the loading period.
    In order to model the material behavior under cyclic loading, some hardening rules are introduced and their robustness is investigated through comparing them with experimental results. also to obtain a more realistic behavior under dynamic loading, viscoplastic framework is used. besides this a discussion on different phenomena effective on the damage and crack behavior under dynamic cyclic loading such as the crack closure effect or the crack faces penetration, will be made. simplifying assumptions are used to take into account the crack closure effect that besides their simplicity, give reliable results. moreover for a crack under compressive loading there is the possibility of the penetration of the crack faces which is prevented by implementing a robust contact constraint algorithm. the reliability of the overall finite element framework used is finally analyzed in some problems
  9. Keywords:
  10. Crack Propagation ; Viscoplasticity ; Cyclic Loading ; Damage Mechanics Model ; Adaptive Finite Element Method ; Dynamic Loading

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