| Fatigue of steel bridge design and fatigue life prediction research in our country islagging behind, for steel bridge fatigue design specifications primarily with reference toforeign norms and norms of railway steel bridge, steel bridge fatigue loading standards are notyet on the form and details of the fatigue life of many structures SN curve requirements, thewelded structures and welded joint fatigue test data very lack of plaque. Therefore, to steelbridge welded joints and base metal fatigue damage behavior of experimental research isnecessary, it will steel bridge fatigue design and fatigue life prediction to provide basic designdata.This paper mainly study on hardness and microstructure of welded joints of the bridgesteel Q345qD, based on which, mainly made a research on the fatigue performance and thecorresponding fracture mechanism and fatigue crack growth rate of parent material and thewelded joints. The results are as follows:(1)Through the tests of the mechanical properties for welded joints, getting the hardnessdistribution map of the butt joints, according to which we can learned that the hardness valuein the center of the welding is low, the highest hardness point exists in the recrystallized zonesof the heat affected zones, and there exists the lowest rigidity in part of recrystallized zones.The lowest point of hardness is in the weak area of the butt joints.(2)Through the fatigue strength tests of the base metal and the welded joints withdifferent forms, the S-N curves of different samples were obtained. And making theconclusion that in the2000000cycles, with the reliability of50%and97.7%, the fatiguelimits of the parent material corresponding were249.74MPa and204.75MPa; the fatiguestrength of butt joints were respectively176.11MPa and146.88MPa; fatigue limits of thewelded joints with the excess removed were respectively183.4MPa and150.75MPa.(3)Through the observation and analysis to the fatigue fractures, the fatigue crack of thebase specimen originated from the defects of the specimen surface or near surface material;the fatigue crack of the butt joint specimen originated from the stress concentration of thewelded toe; the fatigue crack of the welded joints with the excess removed originated from the heat-affected zone.(4)Through the tests of fatigue crack growth rate, obtaining the fatigue crack growth ratecurves in different regions of the Q345qD steel for bridge. According to the comparativeanalysis to the obtained curves of each regions, we can learn that the fatigue crack growth ratein different butt joints of Q345qD steel for bridge are different with the same stress ratio, theexpansion rate of heat affected zone is rapid, transverse weld and parent material are slower,weld metal has the slowest speed.(5)Through the analysis to fracture morphology of fatigue crack growth rate test sample,we can get following conclusions: the fatigue source area of fatigue crack originated from thelocation of the prefabricated crack, extension mechanisms of crack spreading area givepriority to fatigue strips;there are micro equiaxed dimples in the area of base metal’s transientfracture zone, showing ductile fracture.There are seriously obvious cleavage characteristics inthe areas of heat affected zone and the transient fault zone in weld and the transverse one,showing some brittle fracture. |
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