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Title: Role of residual stress relaxation in fracture of stainless steel
Author: Son Cao Do
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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This thesis presents a study about weld residual stresses created in Esshete 1250 stainless steel cylinders and the impact of the residual stresses on fracture of the Esshete cylinders. A series of residual stress measurements in welded Esshete cylinders using both Deep Hole Drilling and incremental Deep Hole Drilling techniques are presented. These measurements are compared against independent measurements using other techniques such as Neutron Diffraction and the Contour Method on almost identical cylinders. This is to obtain a confidence on residual stress measurement results using different techniques currently available for steel. There is another stainless steel material which is studied in this thesis. That is 316L stainless steel. Residual stress measurements were also carried out on welded cylinders made of 316L material and compared with independent results. The author of this thesis carried out only the Deep Hole Drilling and incremental Deep Hole Drilling measurements. Measurement results using other techniques were obtained from other researchers such as those from the Open University. The study on the impact of the residual stresses on fracture of the welded cylinders was however done only on Esshete 1250 material. The Esshete welded cylinders were in two conditions: as-welded and thermal aged. The thermal aged condition was obtained by putting the Esshete cylinder to a furnace for 10,000 or 20,000 hours at 650°C and this is to simulate the thermal ageing effect from service at a power station. The residual stress measurements were carried out on both conditions to find out residual stress before and after thermal ageing. The conclusions were that at as-welded condition, the peak tensile value weld residual stress can exceed the yield stress of Esshete material at room temperature; repair welds increased membrane stress in weld residual stress in both axial and hoop directions; and thermal ageing relaxed significantly the weld residual stress in both girth and repair welds. The 316L cylinders are in as-welded condition only and one conclusion was that the repair weld in 316L welded cylinders also increased the membrane stress of the weld residual stress field in both axial and hoop directions. Highly tensile residual stresses were measured in a repair weld of an Esshete 1250 cylinder and motivated an investigation into how these residual stresses affect the fracture of the Esshete cylinder and how they relax with plasticity. This is because the weld residual stress has a high membrane stress of 350MPa in the axial direction and 410MPa in the hoop direction. The repair weld residual stress is found to be almost constantly tensile at 350MPa in the axial direction and 410 MPa in the hoop direction with a peak tensile stress in Esshete steel welds was found to be approximately 550MPa. The highly tensile residual stresses present in the repair welded Esshete cylinder are detrimental to the integrity of the cylinder as a crack subjected to this residual stress field is more likely to grow under external loading than if the residual stress is less tensile or the membrane component is less tensile. As a result, when an engineer accesses the structural integrity of a welded structure, tensile residual stress with a significantly tensile membrane component will require much attention and may lead to a redesign. The influence of such a highly tensile residual stress field is investigated in this thesis using Finite Element analyses. A conclusion from this study is that the repair weld residual stress has a significant effect on the fracture of the Esshete welded cylinder. The presence of tensile residual stresses were shown to promote the onset of yielding in the cracked cylinder earlier than would be the case if residual stresses were not present. However, for higher applied loads the simulations with and without residual stresses converged which indicated that the accumulated plasticity has relaxed the residual stresses completely. This is indeed a shortcoming of current structural integrity assessment codes which is the inability to predict relaxation of residual stresses with plasticity. As an example, a widely used industrial code for structural integrity assessments is R6 [1]. It is a well developed tool to predict fracture or failure of a predefined structure such as a pipe with a circumferential crack or an axial crack. It is also able to include the presence of residual stress in assessments and predict conservatively fracture or failure of a structure with the presence of residual stress. R6 is unable to predict how a residual stress field would relax with plasticity. The axial residual stress in an Esshete repair welded cylinder was also found to have similar features to a long-range residual stress field based on mapping results presented in Chapters 5 and 6. As one focus of the investigation on residual stress impact on fracture of the Esshete cylinder is how the residual stresses relax with plasticity during fracture, an idealised long-range residual stress model along with an improved experimental rig were employed to illustrate how a long-range residual stress relaxes with plasticity and the relationship of that relaxation with elastic follow-up. The long-range residual stress model and experimental rig were concluded to provide an excellent representation of how the imposed long-range residual stress relaxes with plasticity. Measured residual stress relaxation was also compared against predictions provided by the idealised model. The main part of this thesis study on residual stress impact on fracture of a welded Esshete cylinder comes from a large-scale four point bending experiment. A 600mm long Esshete steel cylinder containing a repair weld with a part-circumferential through thickness crack introduced underwent the large scale four point bending. Simulations for this four point bending experiment were carried out, with and without residual stresses. Prior to these simulation results, techniques to map measured residual stresses into 3D finite element models were developed and are explained in this thesis. The FE simulations with and without residual stresses illustrated how residual stresses influence the fracture of the long cylinder. Axial residual stress relaxation was shown using a crack affected zone concept. This concept also allows estimates of elastic follow-up created in a cracked cylinder, ,and was found to be about 1.3. This value corresponds to rapid residual stress relaxation. The relationship between the elastic follow-up of a crack affected zone with the relaxation of the axial residual stress is also discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available