Study On The Multiaxial High Cycle Fatigue Assessment Method Of SWATH

Fatigue failure is one of the main failure modes of naval architecture and oceanstructures, and it often occurs on the welded structures. The connecting bridge structure ofSWATH has complex geometry and is always subjected to multiaxial fatigue loading, whichlead to the state of multiaxial stresses situation. Therefore, research on multiaxial fatigue forthe connecting bridge structure of SWATH is very necessary.A new approach to estimate multiaxial high cycle fatigue for welded structures based onthe critical plane method called MWCM (Modified W ohlerCurve Method) is introduced inpresent thesis. Experimental data of flange-tube welded joint under proportional andnon-proportional combined bending-torsion loading is used to validate the feasibility andaccuracy of MWCM. And compare MWCM with Von Mises equivalent stress hypothesismode and methods from Eurocode3and IIW recommendations. The result demonstrate thatMWCM performed well to evaluate multiaxial fatigue life for welded structures subjected toboth proportional and non-proportional multiaxial loads.The multiaxial accumulated fatigue damage calculation formula of MWCM is deducedfor ship and offshore structures based on the S-N curve, Miner rule and Weibull distributionfor the long term stress range.In the last chapter, research on multiaxial high cycle fatigue for SWATH is done. FEMstress analysis shows that the connecting bridge structure is subjected to proportionalmultiaxial stress, the effect of shear stress shouldn’t be neglected, and the multiaxial fatigueanalysis methods must be carried out to assess its fatigue strength. Then MWCM andmaximum principal stress range method are used to evaluate multiaxial fatigue life, and theresults are compared. The result shows that the multiaxial high cycle fatigue damage life ofthe connecting bridge structure of SWATH evaluated with MWCM and the maximumprincipal stress range method are almost equal.