Characterisation of residual stress in welded structures with application in low cycle fatigue loading regime

Abstract

Welded steel structures are important for modern industrial systems, yet they are often put under the cyclic operational loads that make fatigue a main cause of structural failure. A key factor of fatigue behaviour is the presence of welding created residual stresses, which are often tensile at important areas such as the weld toe. This thesis studies the early residual stress conditions in welded steel specimens under a low cycle fatigue (LCF) loading regime. The study focuses the limitations of conventional fatigue check guidelines for using the accountable residual stress measurement methodology.

The measured methodology focused on testing specimens produced by three common welding processes: FCAW, SMAW, SAW. High resolution characterization was done by using X ray Diffraction (XRD) merged with Convolutional Multiple Whole Profile (CMWP) analysis to measure early residual stresses and dislocation densities (ρ). Microstructural and fracture related checks were done using Scanning Electron Microscopy (SEM) to find fatigue damage mechanisms and crack initiation paths. These measured results were used as key data for various fatigue prediction methods, such as Effective Notch Stress (ENS) method, enabling the analysis of local stress conditions throughout the fatigue process.

The results showed that SMAW produced the highest early tensile residual stress (+362 MPa), followed by FCAW (+321 MPa) and SAW (+252 MPa). CMWP analysis showed that dislocation density changes in two clear stages during cyclic loading, reaching a steady condition after approximately 10³ cycles. For the tested samples, steady dislocation densities ranged from 8.9×10¹⁴ m⁻² to 1.42×10¹⁵ m⁻² depending on the welding process and microstructural area.

In conclusion, by combining detailed microstructural characterization with realistic residual stress measurement method considering the acknowledged errors of some methods, this thesis shows a stronger method for studying the structural strength of lightweight steel parts. The results show the importance of considering process specific stress conditions and microstructural growth to improve the reliability and durability of welded structures in high capacity engineering applications.