Equivalent Hot Spot Stress Method In Multiaxial Fatigue Assessment Of Complex Welded Joints

Most welded structures in ship and offshore structures are in the state of multiaxual fatigue situations because of having complex geometry and subjecting to mutiaxial fatigue loading. Nevertheless, the studies on the uniaxial fatigue are too centralized which is a primary cause of uncontrollable deviation and high dispersion for multiaxial fatigue strength assessment of complex welded joints.The hot spot stress approach had been successfully used in design of ship and offshore structures. The current hot spot stress approach still has some limitations to the multiaxial fatigue strength assessment of complex welded joints. The reference stress for fatigue assessment is that of surface stress without any information about the stress gradient into the thickness. As for the mutiaxial fatigue strength of complex welded joints, it is necessary to analyze the complicated stress state near the weld toe which cannot be described by the traditional extrapolation approach.In this paper, a new stress estimation approach for the multiaxial fatigue strength assessment of complex welded joints is proposed, in which an equivalent hot spot stress is defined at a point (called Zero Point) in the plate thickness direction where the nonlinear stress due to the notch effect of the weld is equal to zero. Firstly, influence factors and change laws of the Zero Point are discussed, and a fitting equation for determining the Zero Point is established. Then, published fatigue tests data and numerical stress results of some welded joints are used for comparison between the proposed equivalent hot spot stress approach and the current hot spot stress approach. The equivalent hot spot stress approach is verified to be consistent with the current hot spot stress approach and also have higher accuracy for being able to consider the thickness effect on the fatigue strength to a certain extent. Although the zero point is affected by the dimension of the weld toe, from the prospective of engineering, the dimension effect can be neglected with the error analysis result of equivalent hot spot stress no more than 10%.With the combination of the new method which can provide an effective measure of a stress stage and the Modified Wohler Curve Method (MWCM), the multiaxial fatigue assessment life of welded joints is convenient to be predicted under given loading path.