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High and Low Cycle Fatigue Behavior of Severe Shot Peened Welded Joints

Ahmadikia, Behnam | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49275 (05)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein
  7. Abstract:
  8. Fatigue failure is deemed as one of the most dominant failure mechanisms of metal components applied in the industry, as in more than 90% of cases which have led to failure, fatigue mechanism has played the major role. Fusion welding is one of the most widespread joining techniques and nowadays is extensively used in various industries including transportation, machine construction, petroleum, and offshore structures. Material properties undergo inevitable changes during welding due to non-uniform heating and cooling as well as addition of a new material and, as a result, an inhomogeneity in the base metal. That being so, investigation of welded joints subjected to cyclic loading that may cause fatigue failure is clearly of a great importance. Materials resistance to fatigue crack growth is profoundly related to their microstructures and surface properties. In addition, experimental studies demonstrate that most failures initiate at the point in which surface cracks appear. Therefore, control and improvement of surface properties may have a significant effect on fatigue life extension of metal components in industry. Surface nanocrystallization is one of the most successful surface modification techniques that improves elastic and surface properties of metals. Several methods have already been presented for transforming coarse-grained surface structure of a given material into a nanocrystallized structure among which Severe Shot Peening (SSP) stands out. Creating significant plastic deformation at the surface, SSP process is used widely due to its high rate of production, high efficiency, and lower costs compared to other similar processes.
    This study intends to investigate the effects of surface nanocrystallization on the low cycle and high cycle fatigue behavior of welded joints. For this purpose, welded specimens of 316L stainless steel underwent severe plastic deformation by means of SSP process. X-ray diffraction measurements confirmed the generation of a nanocrystallized surface layer and the average size of grains at the surface of the specimens was measured to be 21nm. After their surface structures were refined to Nano-scale dimensions, specimens were subjected to cyclic tension-tension loading in both high and low cycle fatigue regimes. Furthermore, welding and severe shot peening processes were simulated and the resulting residual stress field was determined using finite element analysis. Results of this study indicate that severe shot peening enhances fatigue life of welded joints in the very high cycle regime significantly. With increasing stress levels in lower lives, however, beneficial effects of SSP diminish steadily until in lives less than 50,000 cycles fatigue properties of shot peened welded joints are examined to be inferior to that of not-peened specimens. In addition, results of FE analysis demonstrate that reduction of fatigue life of welded joints in moderate and low cycle regimes is due to relaxation of compressive residual stress as well as surface roughness created during severe plastic deformations
  9. Keywords:
  10. Microstructure ; Welded Joint ; Severe Shot-peening ; Finite Element Analysis ; Nanocrystallization ; Fatigue Behaviour

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