Study On The Multiaxial Fatigue Criteria Of Welded Joints Based On The Notch Stress Approach

In the naval architecture and ocean engineering field,most of the welded structures are subjected to multiaxial fatigue loads,and fatigue damage is one of the main forms of destruction.So far,there has an abundance of criteria for the fatigue assessment of welded joints under multiaxial loads.However,there do not appear to be much consensus as to which multiaxial fatigue criterion applies to a wider range and has higher evaluation accuracy.For that reason,it is of great importance to perform a comparison and evaluation of commonly applied multiaxial fatigue criteria,and to revise the criteria for the purpose of proposing a more applicable multiaxial fatigue criterion.In this paper,the notch stress approach according to the IIW is applied to multiaxial fatigue assessment of welded joints in order to obtain a criterion that can be widely used for the fatigue assessment of welded joints.Firstly,the notch stress approach is used to reanalyze the uniaxial fatigue strength of welded joints,the difference of the results between the notch stress approach and the nominal stress approach is compared,and the applicability of the uniaxial S-N curve recommended by the IIW is analyzed through different welded joints.The results show that: The S-N curve recommended by the IIW is suitable for the fatigue assessment of the fillet welded joints,but not for the fatigue assessment of the butt welded joints.A new notch stress S-N curve(FAT215/4)applicable for the butt welded joints is proposed according to the evaluation results of the joints.Then,the notch stress approach is applied to the multiaxial fatigue assessment of welded joints.Simplified formulas for calculating each notch stress component under the notch local coordinate system is proposed aiming at the characteristics of the notch stress approach,on this basis,six commonly used stress-based multiaxial fatigue criteria(von Mises criterion,IIW criterion,EESH criterion,C-S criterion,Findley criterion,and MWCM criterion)are used.Two types of multiaxial loading are considered: proportional(in-phase)loading and non-proportional(out-of-phase)loading.Among the six criteria,Findley criterion is the most accurate by comparative analysis of the safety and accuracy of the evaluation results.Finally,after the analysis of the damage parameters in Findley criterion,a modified Findley criterion is proposed considering the effect of the ratio of the normal stress range to the shear stress range at the critical plane and the phase difference(non-proportional)between multiaxial loads on the multiaxial fatigue strength of welded joints,this new criterion uses the IIW-and EESH criterion for reference.Then,the modified Findley criterion is used to reanalyze the multiaxial fatigue strength of welded joints.The result shows that: The evaluation result is more accurate using the modified Findley criterion to evaluate multiaxial fatigue strength of welded joints.A new notch stress S-N curve(FAT373/4.8)is proposed when using the modified Findley criterion.