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Fatigue behaviour of high-strength dissimilar welded joints with thickness steps
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School of Engineering |
Master's thesis
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en
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128
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Abstract
Achieving sustainable and lightweight marine structures needs the efficient use of high-strength steel. This approach poses optimized structures using a combination of normal- and high-strength steels and varying plate thicknesses. In this context, the fatigue behaviour of high-strength dissimilar welded joints with thickness step is crucial, as it has not been extensively addressed in earlier investigations.
Therefore, the aim of this thesis is to clarify key factors that characterize the fatigue failure of strength-dissimilar butt-welded joints made of 20 mm S690 and 15 mm AH36. For this aim, the thesis work has focused on three detailed analyses: first, structural stress analysis accounting for global misalignment; second, notch stress analysis accounting for local weld geometry; and third, material microscopic analysis. The critical failure location in the welds is discussed using the results of these analyses.
As the first analysis, structural stress at the failure location was obtained using finite element simulations considering the accurate specimen geometry measured by an optical 3D scanning system. Structural stress reduces S-N data scatter significantly, and the 1 mm stress method assesses the failure location better than other structural methods. The second analysis focused on effective notch stress, considering various weld geometry parameters such as weld notch radius and undercut depth, captured through high-resolution line confocal imaging. The effective notch stress method provided a more accurate assessment of failure locations than structural stress methods. In the third analysis, it was identified that fatigue failure occurred at the weld toe of bevel side, where the heat-affected zone and weld metal boundary are situated. This area exhibited relatively lower microhardness values, with grain size analysis and sub-grain structure examination showing a clear correlation with hardness values on the failure bevel side, underscoring the significance of sub-grain analysis in understanding material behaviour.
Consequently, the current thesis confirms that the combined effects of global misalignment and local weld geometry are key dominant parameters for fatigue failure behaviour. To also link the influence of local material properties with fatigue failure assessment, future research should focus on advanced fatigue life modelling that explicitly considers the surface integrity effects.
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Supervisor
Remes, HeikkiThesis advisor
Ono, YukiLehto, Pauli