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Influence of Shot Peening and Post Heat Treatment on Fatigue Life of Welded Tubular Joint and Determination of Residual Stress Relaxation During Fatigue Loading

Asemani, Hamid Reza | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43523 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. In the present work, the residual stress distribution in welded tubular X-joints of offshore platformshas been investigated. For this purpose a 3D thermo-mechanical finite element model for welding process has been proposed using Goldak thermal model. Residual stress of the joint is obtained using this model. The maximum temperature of the welding process and the maximum residual stress are 2050 °C and 370 MPa, respectively.
    Then residual stress distribution of post weld heat treatment, fatigue loading and shot peening has been studied too. In order to simulate the post weld heat treatment, a decoupled thermo-mechanical analysis has been performed. The temperature of the furnace and the period of heating and cooling have been considered 600 ° C and one hour, respectively. Tensile residual stresses reduced by 22 percent after post weld heat treatment.
    To investigate residual stress relaxation during fatigue loading, a commercial software and programming language are utilized. Out of palne bending loading has been assumed where the stress ratio was 0.1 and the frequency of the loading was considered 5. After the first cycle, the value of the maximum tensile stress reduced by 30 percent and relaxation of the residual stresses during loading in next cycles decreased intensively.
    Shot peening process, has been simulated. In this process, normal stress in x direction was compressive and up to 220 MPa in depth.
    Fatigue crack growth in each set of residual stresses has been studied and compared to each other. Using FRANC3D, 3D modeling of fatigue crack growth was done and the life of the join before and after each process was obtained. Referring to the obtained results, life of this joint increased up to 2.3 times after shot peening and may be concluded that the effect of post weld heat treatment is ignorable when cracks are of small length. Taking relaxation of the residual stresses into account in simulation, fatigue life increased up to 1.4 times.
  9. Keywords:
  10. Residual Stress ; Heat Treatment ; Shot Peening ; Goldak Thermal Model ; Welded Tubular joint

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