Study On Fatigue Degradation Effect Of Surface Defects And Shot Peening Of Deck-rib Welded Joints Of Stell Girder

Welding is one of the most commonly used connection methods in orthotropic steel bridge deck(OSD)structures,which has the dual functions of connection and force transmission.But in the welding process,welding defects caused by its complexity and randomness are inevitable.Undercut is a surface volume defect,it will reduce the effective cross-sectional area of the base material and cause stress concentration.The stress concentration caused by the undercut defect,coupled with the cyclic load,makes the fatigue cracks easy to initiate and propagate at the defect location.The undercut defect has a much greater impact on the fatigue strength of the component than the static strength,and significantly reduces the fatigue resistance and service quality of the deck-rib welded joints.How to qualitatively analyze the deteriorating effect of the undercut defect on the deck-rib welded joints and clarify the strengthening mechanism of the shot peening impact on the deckrib welded joints is the basic premise to ensure the safe service of the structure and improve the effectiveness of shot peening.This thesis takes 36 finite element models with undercut defect as the research object,and carries out a series of researches on the above problems.The main contents include:(1)The finite element model of single shot and multiple shot peening is established in ABAQUS,and the influence of shot diameter D,shot impact velocity v,and impact times n on the residual stress field distribution and surface roughness of shot peening is analyzed.(2)36 specimen models with different undercut shapes are established in ABAQUS.The effect of undercut defect on the stress concentration degree of the deck-rib welded joints is explored through the stress concentration factor index,and the crack propagation characteristics and fatigue life are discussed by the stress intensity factor amplitude index.The results show: the undercut length l and undercut width w have a great influence on the stress concentration factor,the undercut angle ? has almost no effect on the stress concentration factor;due to the stress concentration caused by the undercut defect,compared with the no defect(ND)model,the amplitude of the equivalent stress intensity factor at each point of the initial crack front and the middle crack is larger.The existence of undercut defect reduces the fatigue performance of the deck-rib welded joints by about 25%.(3)In this thesis,the relaxation effect of the residual stress of shot peening impact under cyclic loading is explored,and the redistribution analysis of the residual stress of shot peening impact during fatigue crack growth is carried out.The results show: after 100 cycles of loading,the relaxation ratio is basically stable at about 80%,and it is considered that the residual stress of shot peening has reached a stable state.In the process of fatigue crack growth,the stress near the crack tip increases sharply to reach the peak stress,and the greater the shot peening strength,the greater the peak stress.The stress in the area away from the crack tip has a certain degree of relaxation,and as the fatigue crack expands,the degree of relaxation becomes larger and larger.(4)After considering the relaxation effect of the residual stress of shot peening under cyclic loading and the redistribution of the residual stress during fatigue crack propagation,the strengthening mechanism of shot peening is studied,and the fatigue life before and after shot peening is compared and analyzed.The results show: after the shot peening treatment,the amplitude of the equivalent stress intensity factor within a certain depth is significantly reduced compared with that before the shot peening treatment,indicating that the fatigue crack growth has been significantly inhibited in the early stage.After shot peening,the fatigue life of 36 specimen models with undercut defect increases significantly,with an average increase of 17.86%,indicating that shot peening has a significant effect on improving the fatigue performance of specimen models.The research results can provide a scientific basis for the service safety of steel bridge structures and the improvement of shot peening efficiency.