A deep knowledge of the production process is needed, in order to achieve quality and safety requirements in a structural component. As a matter of fact, manufacturing processes can introduce defects such as residual stresses, internal and superficial imperfections. Together with the inherent geometric variations, such as notches, cracks or defects in general, it is often difficult to precisely characterise the structural strength of such parts. In this context, residual stresses play an important role, especially in welded structures. The evaluation of residual stresses is typically performed using both experimental and numerical methods. Both present strengths and drawbacks which demand their combined usage to achieve a consistent and meaningful evaluation of the residual stresses. Within this scope, this PhD thesis presents an evaluation of residual stresses in a pipe-to-plate welded joint and studied their influence on the fatigue life of torsionally and bending loaded components. In the first part, the finite element method was used to assess the capability of different thermal methods used to simulate a single pass of the gas metal arc welding process in reproducing the temperature distribution around the weld bead. Results of the simulations were compared to experimental measurements of the surface temperature close to the weld region. The considered thermal techniques adopted different levels of complexity, from the basic implementation of a constant initial temperature assigned to a given material volume, to the more comprehensive and widespread Goldak's double-ellipsoid model. The study shows that, close to the weld seam, very similar thermal behaviours can be achieved by employing each one of the analysed methods. Secondly, considering the constant initial temperature method, the comparison between experimental measurements and numerical simulations showed a fairly good agreement, suggesting that a relatively simple method (i.e., requiring the setting of only one parameter) can be used to efficiently reproduce the thermal history of a welding process. In the second part, the study of residual stresses for a S355JR carbon steel pipe-to-plate welded joint is presented. Numerical simulations and experimental tests were both employed in order to gain wide-ranging knowledge. Numerical simulations were performed with the software Ansys through uncoupled thermal-structural simulations in order to evaluate the stresses, strains and temperature at each node of the finite element model for each phase of the simulation. Temperature-dependent elastic-plastic material properties were adopted in combination with the \textit{element birth \& death} method to simulate the welding process. Two different numerical approaches were implemented for reproducing the weld seam solidification process. The obtained results were discussed and compared with experimental data, in terms of relaxed radial strains measured nearby the seam weld, due to a material removal procedure. The third part investigates the influence of residual stresses on the fatigue life of the welded joints. Influencing factors such as geometric discontinuities and the material heterogeneous microstructure were considered. Experimental tests on as-welded and stress relieved specimens with fully reversed torsion and bending loading conditions were carried out. Experimental results showed how residual stresses exhibited an influence mainly on torsionally loaded components. Numerically, the uncoupled thermal-structural finite element simulation presented in the chapters before was used to assess the complete residual stress field within the specimens. Secondly, residual stresses were mapped and included as initial condition in numerical models intended for fatigue damage factors calculation. Finally, experimental results were then used to corroborate numerical models and verify their efficacy in assessing fatigue endurance. In the last part of this work, a preliminary numerical study of a notched specimen geometry is presented. The work attempts to reproduce residual stress conditions comparable to those found on the welded joint critical notch section on notched specimens thus in order to explain the results observed on welded specimens. Indeed, by varying notch radius and opening angle of a cylindrical specimen, it is possible to obtain a stress gradient similar to that obtained after the welding process at the weld notches. The use of simplified geometry allows easier analysis and a possible improved understanding of the processes taking place within the material.

Influence of residual stresses on the fatigue life of welded joints

Andrea Chiocca
Primo
2021-01-01

Abstract

A deep knowledge of the production process is needed, in order to achieve quality and safety requirements in a structural component. As a matter of fact, manufacturing processes can introduce defects such as residual stresses, internal and superficial imperfections. Together with the inherent geometric variations, such as notches, cracks or defects in general, it is often difficult to precisely characterise the structural strength of such parts. In this context, residual stresses play an important role, especially in welded structures. The evaluation of residual stresses is typically performed using both experimental and numerical methods. Both present strengths and drawbacks which demand their combined usage to achieve a consistent and meaningful evaluation of the residual stresses. Within this scope, this PhD thesis presents an evaluation of residual stresses in a pipe-to-plate welded joint and studied their influence on the fatigue life of torsionally and bending loaded components. In the first part, the finite element method was used to assess the capability of different thermal methods used to simulate a single pass of the gas metal arc welding process in reproducing the temperature distribution around the weld bead. Results of the simulations were compared to experimental measurements of the surface temperature close to the weld region. The considered thermal techniques adopted different levels of complexity, from the basic implementation of a constant initial temperature assigned to a given material volume, to the more comprehensive and widespread Goldak's double-ellipsoid model. The study shows that, close to the weld seam, very similar thermal behaviours can be achieved by employing each one of the analysed methods. Secondly, considering the constant initial temperature method, the comparison between experimental measurements and numerical simulations showed a fairly good agreement, suggesting that a relatively simple method (i.e., requiring the setting of only one parameter) can be used to efficiently reproduce the thermal history of a welding process. In the second part, the study of residual stresses for a S355JR carbon steel pipe-to-plate welded joint is presented. Numerical simulations and experimental tests were both employed in order to gain wide-ranging knowledge. Numerical simulations were performed with the software Ansys through uncoupled thermal-structural simulations in order to evaluate the stresses, strains and temperature at each node of the finite element model for each phase of the simulation. Temperature-dependent elastic-plastic material properties were adopted in combination with the \textit{element birth \& death} method to simulate the welding process. Two different numerical approaches were implemented for reproducing the weld seam solidification process. The obtained results were discussed and compared with experimental data, in terms of relaxed radial strains measured nearby the seam weld, due to a material removal procedure. The third part investigates the influence of residual stresses on the fatigue life of the welded joints. Influencing factors such as geometric discontinuities and the material heterogeneous microstructure were considered. Experimental tests on as-welded and stress relieved specimens with fully reversed torsion and bending loading conditions were carried out. Experimental results showed how residual stresses exhibited an influence mainly on torsionally loaded components. Numerically, the uncoupled thermal-structural finite element simulation presented in the chapters before was used to assess the complete residual stress field within the specimens. Secondly, residual stresses were mapped and included as initial condition in numerical models intended for fatigue damage factors calculation. Finally, experimental results were then used to corroborate numerical models and verify their efficacy in assessing fatigue endurance. In the last part of this work, a preliminary numerical study of a notched specimen geometry is presented. The work attempts to reproduce residual stress conditions comparable to those found on the welded joint critical notch section on notched specimens thus in order to explain the results observed on welded specimens. Indeed, by varying notch radius and opening angle of a cylindrical specimen, it is possible to obtain a stress gradient similar to that obtained after the welding process at the weld notches. The use of simplified geometry allows easier analysis and a possible improved understanding of the processes taking place within the material.
2021
File in questo prodotto:
File Dimensione Formato  
Thesis_compressed.pdf

embargo fino al 03/11/2024

Descrizione: Tesi finale
Tipologia: Versione finale editoriale
Licenza: Tutti i diritti riservati (All rights reserved)
Dimensione 5.93 MB
Formato Adobe PDF
5.93 MB Adobe PDF   Visualizza/Apri   Richiedi una copia

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/1150119
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact