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Mechanical Behavior Analysis of Micro-Tubes Used in Fabrication of Magnesium Stents using the Crystal Plasticity Method

Mirzakhani, Amin | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56542 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Assempour, Ahmad
  7. Abstract:
  8. Properties of stents are significantly dependent on the mechanical properties of the microtubes used in their construction. Therefore, investigating the influence of various factors on the mechanical behavior of magnesium microtubes employed in the fabrication of biodegradable stents can play an indispensable role in the development of the emerging industry of biodegradable stent manufacturing. In this dissertation, the effects of different parameters on the mechanical behavior of magnesium microtubes used in the construction of biodegradable stents have been examined. To achieve this, the objectives of this dissertation have been divided into three main sections. In the first section, a suitable modeling approach based on the Crystal Plasticity Finite Element (CPFE) method has been introduced to investigate the mechanical behavior of extruded magnesium microtubes. In the second section, the effects of microstructural parameters such as grain size, crystallographic texture, and the amount of yttrium (Y) alloying element on the mechanical behavior of magnesium microtubes were examined using the CPFE modeling approach from the first section. In the third section, a new combined approach based on experimental results and CPFE modeling, considering the size effects, has been used to investigate the effect of initial magnesium microtube fabrication methods on the mechanical behavior of stent struts. In general, the results indicate that the mechanical behavior of magnesium alloys significantly changes with variations in microstructural parameters. Addition of the Y element significantly enhances the yield strength (YS), ultimate tensile strength (UTS), and ductility (UE) by refining the grain size and weakening the crystallographic texture. Additionally, due to the use of Severe Plastic Deformation (SPD) processes in producing magnesium microtubes, considering the weak mechanical properties of magnesium compared to corrosion-resistant metals, investigating the influence of different manufacturing methods on the mechanical behavior of magnesium microtubes is important. For this purpose, three manufacturing methods with different process temperatures and numbers of SPD passes were used to produce initial magnesium microtubes for stent fabrication. Subsequently, the mechanical behavior of stent struts, which are the remaining part of the microtubes after laser cutting, was evaluated using a new combined approach based on experimental data and CPFE modeling. In this thesis, CPFE modeling was performed for two groups of struts. The first group of struts was considered in the extrusion direction (ED), and the second group was considered at 45 degrees to the ED. Comparison of the results showed that processing magnesium microtubes with two passes of SPD at 300 ̊C in the third manufacturing method leads to the production of both groups of struts with an excellent combination of YS, UTS, and UE
  9. Keywords:
  10. Crystal Plasticity ; Microtube ; Stent ; Strut and Tie Model ; Texture Analysis ; Grain Size ; Rare Earth Element ; Finite Element Method ; Severe Plastic Deformation

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