For fatigue design it is necessary to provide guidelines on how to calculate fatigue damage at weld toes based on S-N data when the principal stress direction is different from that of the normal direction to the weld toe. Such stress conditions are found at details in different types of plated structures. Some different fatigue criteria for these stress conditions are presented in design standards on fatigue design. Criteria used by the International Institute of Welding (IIW), Eurocode, British Standard and in the DNV standards have been assessed against some relevant fatigue test data presented in the literature. Only proportional loading conditions have been considered here. (By proportional loading is understood that the principal stress direction is kept constant during a load cycle). An alternative equation for calculation of an equivalent or effective stress range based on stress normal to the weld toe and shear stress at the weld toe has been proposed. The proposed methodology can be used for nominal S-N curves and it can be used together with a hot spot stress S-N curve with stresses read out from finite element analysis. The different design criteria are presented in this paper together with recommendations on analysis procedure.
