Among several parameters affecting the fatigue life of the welded joints, the size effect and weld quality are the main focus of thecurrent study. Fatigue design technical documents consider the thickness effect by reducing the S-N curve for joints thicker thanthe reference thickness value. Yet, limited attention has been paid to the thinness effect, where the specimens gain life due todecreasing thickness. Understanding the connection between weld profile geometry and fatigue strength is also imperative forassessing welded joints against repeated loads, as geometrical irregularities lead to local stress raising and decrease fatigue life.In this study, several fatigue test series were performed on non-load carrying welded joints with 2- and 16-mm thicknesses underuniaxial constant amplitude loading, resulting in S-N curves to analyse the influence of thickness on fatigue strength. For each testspecimen, the variation of weld shape along the weld seam is measured using a laser scanning tool. Assessment of fatigue testresults and weld profile measured data helps better understand the uncertainty and variation in fatigue life and the relationshipbetween fatigue strength with specimen thickness and the weld quality. The results indicate that there is a beneficial effect fromreducing thickness and increased weld quality.

Plate thickness influences the fatigue performance of welded components. In fatigue design standards and recommendations, the thickness effect and fatigue strength reduction have been considered by modifying the S–N curve for plates thicker than a reference thickness. However, increasing fatigue strength due to the thinness effect is often disregarded. The current study focuses on the size effect in fatigue of butt welded and non-load carrying cruciform welded joints under constant amplitude tension load. Literature data is evaluated using the effective notch stress method with a reference radius of 1 mm, which is used for all finite element models to ensure that FAT-value corresponding to a 1 mm notch radius remains constant across all models. A probabilistic assessment of the results using the weakest-link theory is applied to improve the prediction accuracy of thinner members outside the recommended thickness range of the used radius. The method reduces the S–N data scatter in comparison to the variation of test data and shows applicability also for thinner members. A comparison of the size effect for the current method with extrapolated values from standards and recommendations shows a difference in the size effect for thinner members.

Safe design against unstable fractures in load-bearing structures is crucial at sub-zero temperatures where the brittle fracture toughness can be unfavourable, especially for high-stress designs incorporating ultra-high-strength steels. The brittle fracture toughness of surface cracks in structural steel with a minimum yield strength of 1300 MPa is, for this reason, tested in the present study at sub-zero temperatures. The realistic flaws are compared with single-edge notched specimens (SEN(B)) from thicker plates with the same chemical composition, using a representative fracture toughness for a three-dimensional crack front according to the Master Curve method. A novel approach determines the latter without considering the local temperature and constraint variation through empirical relations. The experimental result shows a difference in the reference temperature between the two specimen types, which likely is the natural variation of the manufactured materials and/or a loss of constraint due to the difference in the scaled specimen deformation level.