High-cycle fatigue behaviour of as-welded high-strength steel joints
Liyanage, Anuranga (2026)
Diplomityö
2026
School of Energy Systems, Konetekniikka
Kaikki oikeudet pidätetään.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2026061672350
https://urn.fi/URN:NBN:fi-fe2026061672350
Tiivistelmä
In this study, the high-cycle fatigue behaviour of as-welded high-strength steel joints was investigated. Particular focus was placed on the knee-point behaviour of the S–N curve, which remains insufficiently understood. The primary objective was to investigate the HCF behaviour and knee-point position of as-welded non-load-carrying cruciform joints fabricated from S700 high-strength steel.
Experimental fatigue testing was conducted under constant-amplitude axial loading at stress ratios R = 0.1 and R = 0.5 with a total number of 21 specimens. The specimens were characterised using 3D laser scanning for numerical work using real weld geometries, X-ray diffraction for residual stress measurement and fractographic examination using scanning electron microscopy. The fatigue data were evaluated using both global and local assessment methods. It includes the nominal stress method, effective notch stress method, theory of critical distance and 4R method.
The results showed that the knee-point of the as-welded occurred at a higher stress amplitude than indicated by the IIW recommendations. The knee-point was identified at approximately two million cycles for R = 0.1 and eight million cycles for R = 0.5. The 4R method accounted for the mean stress effect, which identified the data from both stress ratios into a single S-N curve with a common knee-point at two million cycles. Fractography analysis showed that the scatter in fatigue life was primarily caused by subsurface weld imperfections. Overall, geometry stress concentration and subsurface defects were the dominant factors controlling the effective notch stress life of high-strength steel weldments.
Experimental fatigue testing was conducted under constant-amplitude axial loading at stress ratios R = 0.1 and R = 0.5 with a total number of 21 specimens. The specimens were characterised using 3D laser scanning for numerical work using real weld geometries, X-ray diffraction for residual stress measurement and fractographic examination using scanning electron microscopy. The fatigue data were evaluated using both global and local assessment methods. It includes the nominal stress method, effective notch stress method, theory of critical distance and 4R method.
The results showed that the knee-point of the as-welded occurred at a higher stress amplitude than indicated by the IIW recommendations. The knee-point was identified at approximately two million cycles for R = 0.1 and eight million cycles for R = 0.5. The 4R method accounted for the mean stress effect, which identified the data from both stress ratios into a single S-N curve with a common knee-point at two million cycles. Fractography analysis showed that the scatter in fatigue life was primarily caused by subsurface weld imperfections. Overall, geometry stress concentration and subsurface defects were the dominant factors controlling the effective notch stress life of high-strength steel weldments.