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Reliability Study of Steel Welded Joints after Post-Weld Heat-Treatment and Shot-peening

Hosseinimehr, Mohammad Ali | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49717 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. Welding as one of the widespread joining techniques, is used in shipbuilding industries, steel bridges, water and gas pipelines, and offshore structures. Also, fatigue plays the major role in more than 90% of welded joints failure. Furthermore, their unserviceability due to failure can incur severe monetary consequences. Unlike deterministic approach, utilizing reliability methods in addition to failure and retrofit cost modeling, enable decision making under uncertainty which leads to an optimal decision. The objective of this research is to create a reliability model of weldedjoints after post-weld heat-treatment and shot-peening using related and reliable experimental data. For this purpose, first, 316L steel, which is a frequently used austenitic stainless steel in industry, was selected and butt welded. Then, high cycle fatigue specimens were cut from the plate. The specimens were divided into three categories, from which the first set was kept apart without any post-treatment. The surface of the second category was nano-structured using severe shot-peening with 19.5A intensity and 1400% coverage, and the grain size was reduced to 12 nm in order to study the surface grain size reduction on fatigue life. In third category, the specimens were stress-relieved using post-weld heat-treatment. Then, high cycle fatigue tests were carried out for all of the three categories. The results show that severe shot-peening improves the fatigue behavior after 50,000 cycles and has a reverse effect before it. Post weld heat treatment also results a better fatigue behavior than severe shotpeening after 100,000 cycles. Thereafter, a Bayesian regression model by means of fatigue life data was created in order to be used as a probabilistic model. The reliability problem was solved by first order reliability method (FORM) with first curvature modification and its accuracy was verified using Monte Carlo sampling method. Also, stress-life curves with respect to various reliability indices were plotted and compared. These curves alongside with determination of target reliability index, enable one to make a rational and realistic decisions in the case of fatigue failure
  9. Keywords:
  10. Welding ; Reliability ; Fatigue ; Past-Weld Heat Treatment ; Heat Treatment ; Severe Shot-peening

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