Modified Wöhler curve method, theory of critical distances and Eurocode 3: A novel engineering procedure to predict the lifetime of steel welded joints subjected to both uniaxial and multiaxial fatigue loading

In order to propose a procedure suitable for assessing steel welded joints by post-processing simple linear-elastic finite element (FE) models, this paper summarises an attempt to formalise a novel approach based on the use of the modified Wöhler curve method (MWCM) applied along with the theory of critical distances (TCD). This engineering procedure was initially calibrated by using the standard curves supplied by Eurocode 3. Subsequently, an unifying value for the multiaxial critical distance was calculated by taking full advantage from the notch-stress intensity factor (N-SIF) approach. The accuracy and reliability of the devised method was systematically checked by using a large amount of data taken from the literature and generated by testing steel welded details both under uniaxial and multiaxial nominal loading. In particular, both standard cruciform joints and structural details having complex geometries were considered and the devised procedure was applied to specimens tested both in as-welded and in stress-relieved condition. This approach was seen to be highly accurate, giving estimates falling within the widest standard scatter band between the two used to calibrate the approach itself. In particular, our method proved to be capable of correctly taking into account the scale effect as well as the degree of non-proportionality of the stress field damaging the fatigue process zone. Such results are very interesting and promising, especially in light of the fact that the numerical effort which has to be made to apply the proposed approach is the same as the one needed to apply other existing linear-elastic approaches.