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Evaluation of Deformation and Formability Behaviors of the Ferritic-Pearlitic Steel Sheets by Employing the Experimental Approaches and Micromechanical Modeling

Isavand, Samaneh | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53747 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Assempour, Ahmad
  7. Abstract:
  8. A broad range of plain carbon steels have ferrite-pearlite microstructures, which are used in many industrial applications due to the good strength and formability provided by the typical combination of ferrite grains and pearlite colonies. The mechanical behavior of ferrite-pearlite steels inherently originates from the configurations of ferrite grains and pearlite colonies, which comprised alternating lamellae of cementite with randomly distributed orientations in a ferrite matrix. In this thesis, three different experimental-numerical approaches were employed to investigate the deformation behavior, damage mechanisms, and formability of the ferrite-pearlite steels. At the first step, two different ferritic-pearlitic microstructurs (non-banded and banded) were produced using heat treatments. The microstructure-based finite element (FE) simulations were conducted using the representative volume elements (RVEs). Both ferrite and pearlite were assumed to be isotropic and homogeneous in this approach. In order to predict the forming limit diagrams (FLDs), the microstructural inhomogeneity and Johnson-Cook damage criteria were applied to the RVEs under different loading paths. Then, the predicted stress-strain curves and FLDs were compared with the results of the tensile and Nakazima tests. At the second step, the previous numerical approach was improved by using the crystal plasticity (CP) model for ferrite. At the last step, microstructural stress/strain evolution, strain localization and damage mechanisms in the ferrite-pearlite steels were studied through a high-resolution integrated experimental-numerical approach. In this approach, ferrite and cementite lamellae were also considered. In order to model the microstructure, ferrite crystal orientations and ferrite and cementite lamellar morphologies were measured by electron backscatter diffraction (EBSD) and SEM micrographs of ferrite and cementite lamellar morphologies. Then, the experimental strain field was calculated employing the in-situ SEM tensile testing using the digital image correlation (DIC). The microstructure was simulated based on the experimental data using the CP model. In this research, the influences of microstructural morphology, pearlite volume fraction, pearlite interlamellar spacing, ferrite crystallographic orientations, and cementite lamellar orientations were also studied by creating synthetic microstructures. The findings indicate that the initial microstructural inhomogeneity criterion adequately enables to predict the plastic instability in the ferritic-pearlitic steel sheets without using any damage or failure criterion (with an error of less than 15%). Furthermore, three important damage mechanisms including ferrite-pearlite interface decohesion, ferrite-cementite interface plasticity and damage, and pearlite/cementite cracking were observed in the ferritic-pearlitic microstructure
  9. Keywords:
  10. Pearlite ; Strain Localization ; Forming Limit Diagram (FLD) ; Digital Image Correlation ; Crystal Plasticity ; In-Situ Tensile Test ; Scanning Electron Microscopy (SEM) ; Ferrite

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