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Residual Stress Determination of Multi-pass Welding Process, Stress Relieving Procedures and Its Effect on Crack Propagation

Chamani, Mohammad | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42336 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farahi, Gholamhossein
  7. Abstract:
  8. One of the most important issues in welding is residual stresses. It is desirable to produce less residual stresses when possible and to reduce them which remain in the structure. At the present, various methods have been developed to reduce the residual stresses. The most important methods are heat treatment, shot peening, and grinding. The main problem in using these methods is cost, and for large structures these methods are often impractical. In recent decades, a technique called vibratory stress relief has been developed. It is used for large structures without limitation of size and weight; besides, the cost of this process is much less in comparison with other conventional methods such as heat treatment (for the large structures is less than 1%). However, due to lack of knowledge of this process about reasons for reducing the residual stress and lack of enough model or information that how much residual stresses are reduced after stress relaxation, this method is not widely used in comparison with expensive thermal stress relief method. The main objective of this study is to evaluate the effect of vibratory stress relief on reduction of residual stress in the weld, comparison with the thermal stress relief method and also the effect of these methods to increase the fatigue life welded structure. For this purpose, a 3D elastic - plastic finite element model for welding process is conducted by using Goldak thermal model in ANSYS. Then the effect of vibratory stress relief for resonant and non-resonant frequencies on the residual stress is investigated; also for comparison with previous method, post weld heat treatment is simulated. Finally, the effect of these two stress relaxation methods on the fatigue life is assessed by using damage mechanic methods. For this purpose, fatigue crack growth under dynamic loading is simulated by using Franc3D software, and welded structure life is obtained before and after the relaxation process.
  9. Keywords:
  10. Welding Residual Stress ; Post Weld Heat Treatment Cracking (PWHT) ; Fatigue Crack Growth Rate ; Goldak Thermal Model ; Vibratory Stress Relief

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