The main aim of the present work is to investigate, through the strain energy density method, the fatigue behaviour of high strength welded joints realised employed in hydraulic runner blades. The geometries, found in literature, present the welding bead machined in order to have no geometrical discontinuities in the specimen realising a wide fitting radius between the two welded plates; the only critical geometrical discontinuity in the specimen is given by the lack of penetration that leads to an internal crack‐like defect. The specimens presented failure both from the weld toe and from the weld root depending on the amount of welding penetration. The results, summarised in this work with the strain energy density method, show clearly the possibility to consider a unique master curve for this kind of joints regardless of the failure initiation point. Acquiring, through a finite element model, the strain energy density value both at the weld toe and at the weld root and comparing them, the method is proved to be adequate to detect the most critical area of the joint. A first fatigue master curve in terms of cyclic averaged strain energy density value is provided for the fatigue design of high strength welded joints.

In the present paper, the influence of the mean stress on the fatigue behavior of smooth and notched medium carbon steel components (C45 grade) is investigated through conventional and more advanced design methodologies. Original experimental data are provided for two notched specimen geometries and three different high load ratios (R = {0.7, 0.8, 0.9}). To assess the effect of the mean stress in a larger range, the bulk of fatigue data has been widened through more than 150 tests on C45 steel drawn from literature. The considered experimental data ranged between a load ratio of −2 and 0.9. The mean stress effect has been assessed in a first attempt through well‐established literature models resulting in a data correlation characterized by a large scatter. Finally, an attempt to uniquely consider both geometrical and load ratio effects has been performed through a numerical application of the strain energy density (SED) method. Elevated load ratios (R ≥ 0.7) prove to be highly detrimental for C45 steel with a fatigue life reduction of up to 3 orders of magnitude; however, this effect can be considered with a sufficient degree of accuracy through the SED method.