Ultra low cycle fatigue of welded steel joints under multiaxial loading

Ultra low cycle fatigue (ULCF) is a phenomenon that can be summarily defined as the deterioration of material properties under high amplitude cyclic loading, under which failure is reached typically within ten to twenty cycles. This phenomenon is observed in extreme loading scenarios such as earthquakes or exceptional winds where large cyclic strains are expected. Particularly sensitive are structural components that are designed to absorb the loading’s energy through plastic dissipation, like moment or shear links in eccentrically braced frames. The demands on such components can vary significantly throughout its duration due to their variable multi-directional nature and, as such, it is important to assess the resistance of structural joints under multiaxial states. This paper will present an experimental campaign done on a welded high strength steel (S770QL) tube to plate joint. This joint was submitted to high cyclic bending, pure torsional and combined proportional bending and torsional loadings. Weld profile and strain measurements at the weld toe were performed using digital image correlation (DIC). An analysis of more than sixty test results will be presented so as to provide an adequate resistance model for ULCF life estimations.