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Material Modeling of DIN1.6959 Steel and Determination of Stresses in Autofrettaged Thick-Walled Tubes

Hosseinian, Enayatollah | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39050 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein; Asempour, Ahmad
  7. Abstract:
  8. In hydraulic autoferttage, the tube is subjected to high internal pressure and the resulting residual stresses increase the pressure capacity and fatigue lifetime of the tube. High strength steels are used for construction of such tubes. Material behavior of these steels is very complicated due to pronounced Bauschinger effect. DIN1.6959 steel is frequently used for construction of autofrettaged tubes in some countries such as Germany, Switzerland and Iran. In this work, the material behavior of this steel was modeled accurately using uniaxial tension-compression tests and mechanical properties and material behavior of this steel also were compared with A723 and HB7 steels. As a general rule, material behavior of DIN1.6959 can be considered as an average of A723 and HB7 steels’ behavior. Two numerical methods and one analytical solution were presented for the solution of autofrettaged thick-walled tubes. Proposed numerical methods are capable of solving autofrettaged tubes under plane stress, plane strain, open end and closed end conditions employing Tresca and von-Mises yield criteria and complicated material behaviors. New analytical method solves autofrettaged tubes with constant axial condition for incompressible materials under von-Mises yield criterion. Results of these new methods were compared with other numerical and analytical solutions. The results show that new methods are accurate and very efficient. Moreover, autofrettage of tubes analyzed using accurate material models of DIN1.6959, A723 and HB7 steels. Results indicate that ideal models overestimate compressive residual stresses nearly 50% relative to accurate models. Also fatigue lifetime of autofrettage tubes utilizing ideal models is about 30 times more than accurate models. As well, investigation of autofrettaged tubes shows that for radius ratio smaller than two, the plane stress and open end conditions have similar results and also for radius ratio bigger than two, the plane strain and closed end conditions are approximately alike. In addition, autofrettaged tubes were examined using compressible and incompressible assumptions. Results indicate that autofrettage pressure, tangential and radial stresses are approximately the same for the both cases but axial stress and all components of strains are completely different under these assumptions
  9. Keywords:
  10. Residual Stress ; Autofrettage Process ; Materials Modeling ; DIN 1.6959 Steel ; Thick-Walled Tube

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