Master S-N curve approach to fatigue prediction of breathing web panels

Abstract

The master S-N curve approach has been previously introduced for assessing the fatigue life of welded components; particularly for weldments in pipelines and marine structures (ASME Sec. VIII Div. 2 and API 579/ASME FFS-1). The approach relies on several past successful industrial applications and has offered two major advancements in fatigue analysis of welded joints using finite element. These advancements include an equivalent structural stress method which provides a mesh-insensitive quantity adjacent to the joint's weld line, and the S-N curve analysis based on the equivalent structural stress. In the present study, the master S-N curve approach is assessed as a possible new methodology to deal with the breathing induced fatigue analysis of steel ‘plate girders’. In fact, when plate girders of steel or composite bridges are subjected to repeated loading of vehicles, the web panels designed according to tension field action undergo relatively repeated large out-of-plane deformations. This phenomenon is known as breathing effect and can result in fatigue cracking at welded web plate boundaries. In this work, first, mesh-insensitivity of the structural stress method is demonstrated for the breathing web of a plate girder. Secondly, through available experimental data, the accuracy of the master S-N curve for fatigue life evaluation of breathing cases is validated. Finally, the application of the master S-N curve approach for fatigue analysis of breathing webs through FE simulation of multiple plate girders is illustrated and the effect of initial out-of-plane displacement as an important geometrical parameter in the girders' fatigue behavior is investigated.