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Prediction of Strain, Stress, and Microstructure in Sever Plastic Deformation of Equal Channel Angular Extrusion

Narooei, Keivan | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42727 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. Sever plastic deformation can be used for decreasing the grain size and increasing the mechanical properties. In this method a square or circular cross section specimen is passed from two intersecting channel with equal cross section. Thereby, a large plastic strain without change of cross section is produced in the specimen. This large strain is the main reason for the interest of researchers in many fields. In the experimental research fields, researchers can study the response of material in the large plastic strain range, investigation of stage III of hardening and after it, change of microstructure to submicron and Nano, and increase of strength simultaneously with preserving of ductility. In the theoretical backgrounds, by invention of sever plastic deformation evolutions in dislocation, crystal plasticity, constitutive relations in large strain can be observed. In the current research for the prediction of microstructure several problem was proposed. Using the streamline field and combining with upper bound theorem a two and three dimensional admissible velocity-strain field has been proposed to calculate the strain filed and required extrusion force, firstly. In the numerical investigation, because of the large mesh distortion and singularity of velocity field due to sharp corner of intersection of two channels, the Arbitrary Lagrangian-Eulerian method has been used. Regarding the stationary of mesh around the sharp corner, a modified method for the solving of the convection phase has been used. For solving the contact problem between the die and specimen due to large sliding between them, the node to segment method has been used. For improving the convergence the nonlinear term in the first iteration has not been considered in the step of finding the contact-non contact nodes. To satisfy of Patch-test and improving the convergence rate the two phase algorithm, in which the position of master-slave body has been changed, is used. In addition to Misses plasticity, the crystal-plasticity method has been used for the simulation of large strain. For solving the stress singularity in the stress space of crystal plasticity, the Singular Value Decomposition (SVD) was used. Finally, using the Toth et al. formulation, the grain size was predicted. The simulation results were compared with the experimental and numerical results of other researchers and in addition to good correlations, the nonphysical insight of some interpretation was corrected. Also, for the first time the kinematically strain field, that was proposed by other researchers, was proved by the using of ALE method
  9. Keywords:
  10. Arbitrary Lagrangian-Eulerian Method ; Contact Mechanics ; Nonlinear Finite Element Method ; Singularity ; Penalty Function ; Crystal Plasticity

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