Fatigue Evaluation Procedures for Bi-axial Loaded Plate Joints Using the Battelle Structural Stress Method

Previously, Battelle has investigated the behaviour of a series of related multi-axial weld fatigue situations and developed effective analysis procedures. The first was a flange-to-tube joint under torsional-dominant loading. The second was a multi-axial stress state under uniaxial loading consisting of a tensile-loaded plate with either a fillet welded pipe or angled attachment. For multi-axial fatigue evaluation, an effective equivalent structural stress range (ΔEESS), defined as a combination of the equivalent normal structural stress range and the equivalent in-plane shear structural stress range in a von Mises form, derived from the Battelle structural stress method (BSSM), was formulated and used to clearly illustrate the differences in the behaviour of these two situations.As an extension of these multi-axial fatigue situations, the BSSM for fatigue life evaluation is examined herein for bi-axial loading of welded plate joints that otherwise require special treatment. However, when the BSSM is applied to bi-axially loaded plate joints, the failure location can be predicted correctly, and fatigue life predictions using the master S-N curve approach are very accurate compared with fatigue test results from the literature. This is because the BSSM captures the bi-axial loading effects, including the ratio of the load magnitudes and their phase difference that define the local variations of ΔEESS within the joint and thus define the failure location of the joint.In this investigation, it was found that the failure locations are influenced by the maximum normal structural stress (normal to weld line) not by the in-plane shear structural stress (parallel to weld line) which becomes negligible. In addition, it was also found that the cycles-to-failure data from the subject joint types were 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 predictions for these plate joints under bi-axial loading.