Fatigue Evaluation Procedures for Multi-Axial Stress State in Welded Joints

Abstract

The Battelle structural stress method (BSSM) for fatigue life evaluation is examined for multi-axial stress states that develop due to uniaxial loading in welded structures. The resultant multi-axial stress state due to simple uniaxial loading is easily observed in common joint types such as a plate with a welded tube or a plate with an angled attachment. In these joint types, under simple loading, the stress distribution at the location of failure along the weld line shows significant in-plane shear stress (parallel to the weld line) as well as normal stress (normal to weld line). Although the fatigue data, as exemplified by the inverse slope of the S-N curve for the subject joints under uniaxial loading, are observed to be similar to that for normal-loading-mode dominant (Mode I) failures in welded joints, when only the normal structural stress is considered for these joints the predictions of both the fatigue failure location and the fatigue life using the master S-N curve approach are inaccurate because the in-plane shear stress plays a significant role in the development of the crack. The slope of fatigue data exhibited in S-N curves taken from weld fatigue data for resultant multi-axial stress state generated by uniaxial loading is different from multi-axial fatigue loading conditions for tubular joints as discussed in the recent work [OMAE2014-23459]. In this article, the fatigue behavior of welded joints with multi-axial stress states is evaluated using an effective equivalent structural stress range parameter that is formulated as a von Mises form of the combined normal and in-plane shear equivalent structural stress ranges. When the effective equivalent structural stress range parameter is employed, the fatigue failure location can be predicted correctly. It is also found that the cycles-to-failure data from the subject joint types are comparable with the master S-N curve for Mode I loading dominant behavior (inverse slope of 3.125). Therefore, the master S-N curve that was developed for Mode I failures can be equally applicable for fatigue life prediction for these joints by replacing the equivalent structural stress range with effective equivalent structural stress range on the ordinate axis.