Effects Of Residual Stress Field On Fatigue Crack Propagation Behavior At Welded Joints Of Ultrahigh Strength Steel

To meet requirements of the ship and Marine structures becoming larger andlightweight, alloy steel of higher strength was widely used. Usually ships andmarine are large welded structures. As the strength raised, the weld ability of thesteels will meet big challenges. Researchs show that the fatigue fracture of moststructures occurred in welded joints. However, researchs on fatigue crack growthbehavior of ultrahigh strength steel were poor. The welding imperfection like coldcrack will reduce the fatigue life of the welded structures. What’s more, residualstress has important influence on the fatigue crack propagation behavior of weldedjoints.In this thesis, the fatigue crack growth rate of the welded joints withoutresidual stress was tested first, residual stress about the plate butt welding and theFatigue crack propagation process after the welding were analysed by the finiteelement numerical simulation. Effects of the residual stress on the maximum of theapplied stress and stress ratio were studied in this paper too. In addition, a modifiedformula about fatigue crack propagation rate was concluded. The main contents ofthis dissertation are as follows.Changes in the metallographic structure and performance of the joints that wasin welding state and annealing state were studied, which proved that reliefannealing process had no influence on the metallographic structure and performancebut reduced the residual stress. The fatigue crack growth rate of the parent materialand annealed welded joint was tested. The result showed that SIF range and FCGRdata present high linear relationship under log-log coordinate system. In the weldbead, the da/dN-K curve moved to the left side as the stress ratio increased. In thebase material, due to the crack closure effect the curves presented differentperformance. A modified formula about fatigue crack propagation rate wasconcluded as follow: da/dN=C[f(R) K]m, f(R) was the function of stress ratio R.The redistribution of the residual stress along with the fatigue crackpropagation was obtained by the finite element numerical simulation. The resultsindicated that residual stress redistributed along with the propagation of the fatiguecrack. The stress peak reduced gradually. The result of the Kresunder residual stress field showed that as the crack extended, the value of Kresgradually reduced. At thesame time, the influence of residual stress field on crack propagation faded away.The FCGR curves of the welded joints under the effects of the residual stressfield were acquired by the center crack tensile test. The result indicated that underdifferent stress ratio the curves discretely presented. The improved formula ofFCGR da/dN=C[f(Reff) K]mthat modified by Kreswas proposed and it exhibitedgood condensed effect, in which the Reffwas the effective stress ratio.