Fatigue life prediction of as-welded steel structures using phase-field modelling: The role of material properties and weld geometry

Offshore welded steel structures are typically constructed using S355 low-carbon steel due to its low cost and durability. Within these structures, fatigue is often the limiting design factor, with particular attention given to weldments where fatigue cracks tend to nucleate. Design standards such as DNV-RP-C203 aid engineers by providing fatigue S-N design curves, however a key limitation is the use of a single as-welded S-N curve regardless of manufacturing quality, leading to overly conservative designs. This paper investigates how variations in global material properties and weld geometry contributes to scatter in fatigue S-N curves. A phase-field model is used to predict this scatter in welded S355 plates based on expected variation in material properties, Stress Concentration Factor (SCF) at the weld toe, and other influencing factors. Material parameters are varied in a Monte-Carlo simulation serving as a virtual test campaign, predicting fatigue lives across realistic ranges of material properties and weld geometries using high-precision phase-field simulations. The tests replicate the Structural Life-cycle Industrial Collaboration (SLIC) joint industry project on large-scale double V-notch butt welds in the as-welded condition. Results show that phase-field modelling with realistic input data can enhance fatigue design by incorporating asset-specific material properties and weld geometries without additional experiments. Moreover, the model captures the gradual transition from as-welded to flush ground conditions by reducing the SCF toward unity. The findings support tailored design of future offshore structures and enable life-extension of existing assets by reducing excess conservatism in current design curves.