Fatigue Performance Of Welded Joints Under Combined Cycle Fatigue With Ultrasonic Component

The loading configuration experienced by many important welded structuresconsists of low cycle fatigue (LCF) and high cycle fatigue (HCF), namely, combinedcycle fatigue(CCF). The objective of this paper was to study the fatigue properties ofwelded joint in Q345and Q235B steel.To simulate the CCF loading, a CCF testing apparatus that is capable of providinginteractive LCF loading and HCF loading was developed. The HCF loading wasprovided by a ultrasonic fatigue testing apparatus based on DSP.Base metal specimens and welded joint specimens in Q345and Q235B steelwere tested by the ultrasonic fatigue testing apparatus at stress ratio R=-1. The testingresults show that the S-N curves of both base metal and welded joints descendcontinuously in super long life region. Fatigue fracture can still happen on thespecimen and there is no fatigue limit between10~7and10~9cycles. The fatiguestrength of welded joist is far less than that of base metal in super long life region.All Cracks initiate at the surface or subsurface of base metal specimens andwelded joint specimens in Q345and Q235B steel. No crack initiation at internalinclusions was found.Thin butt specimens in Q345and Q235B steel were tested under pure LCF, pureHCF and CCF loading by the CCF testing apparatus. The results indicate theapplication of the CCF is detrimental to fatigue life under conditions of stress ratio of0.5and maximum stress of420MPa.Using ultrasonic peening method to treat the toe zone of the welded specimens.The CCF testing results show that the CCF strength of the specimens treated byultrasonic peening is higher than that of specimens as welded greatly.The CCF fracture were analyzed by Scanning Electron Microscope. There arelots of secondary cracks and fatigue steps on the fracture surface. Obvious riverstriations can be found on crack propagation zone.A cumulative linear damage analysis was performed to estimate fatigue life of thewelded joint fatigued by CCF loading. The results indicate that the strong couplingeffect between the two loading components will be underestimated using Miner ruleestimating the fatigue life of the sepcimens. There is an obvious relation between the coupling effect and the amplitude ratio of HCF loading to LCF loading.