Modified Wöhler Curve Method and multiaxial fatigue assessment of thin welded joints

The present paper is concerned with the use of the Modified Wöhler Curve Method to estimate fatigue lifetime of thin welded joints of both steel and aluminium subjected to in-phase and out-of-phase multiaxial fatigue loading. The Modified Wöhler Curve Method postulates that, in welded connections subjected to in-service complex time-variable loading, fatigue damage reaches its maximum value on that material plane experiencing the maximum range of the shear stress amplitude, such a stress quantity being calculated according to the Maximum Variance concept. The most important peculiarity of the above multiaxial fatigue criterion is that it can be applied by performing the stress analysis in terms of both nominal and local quantities, where in the latter case the relevant stress state at the assumed critical locations can be estimated according to either the reference radius concept or the Theory of Critical Distances. The accuracy and reliability of our multiaxial fatigue criterion was systematically checked through several experimental results taken from the literature and generated by testing, under in-phase and out-of-phase biaxial loading, welded joints of both steel and aluminium having thickness of the main tube lower than 5 mm. Such a systematic validation exercise allowed us to prove that the Modified Wöhler Curve Method is a powerful tool suitable for performing the fatigue assessment of thin welded joints, this holding true independently of the strategy adopted to perform the stress analysis. Finally, a microstructural motivation of the length scales included in the Theory of Critical Distances can be established by linking this technique to gradient mechanics, as we will argue.