| Increased infrastructure coverage and multiple speeds of rail transport have raised new challenges in fatigue design and life assessment of welded steel bridges.Welded steel bridges have the characteristics of light weight and fast installation which could effectively solve the problem of winter construction in northern China.However,the problem of low-temperature brittle fracture of steel structure requires enough attention.The strength of structural steel usually grows in low temperature environment,while the plasticity and toughness is significantly reduced.The existing design codes or specifications control the low temperature impact toughness of steel for low-temperature fracture design and the fatigue performance of welded steel bridges subjected to reciprocating traffic loads in low-temperature environments is not taken into account which increased uncertainty in fatigue design and life assessment.Morever,the existing study of the low temperature fatigue fracture of steel structures is only a small number of verification tests for some specific projects,whose number of specimens could not meet the basic requirements of fatigue testing and test results are highly discrete.It is difficult to provide reference value for other engineering applications.In this paper,using the most widely used bridge steel Q345 qD base metal and its butt welds as the research object,the static tensile tests,impact tests,axial high cycle fatigue tests and other related tests at room temperature and low temperature were carried out by 55 tensile specimens,63 impact specimens and 228 fatigue specimens.The changes of the basic mechanical properties and fatigue properties with the decrease of the ambient temperature were studied,and mainly completed work and the results are as follows:(1)The room and low temperature static tensile tests were carried out on the base metal,butt welds and welded joints.The strength,plasticity and toughness of the three types of specimens at 20,0,-20,-40 and-60 ? were obtained.The changes of the strength index and the plasticity index with the ambient temperature were studied,and the expression of the yield stress,ultimate stress and yield ratio of the material with the ambient temperature as the independent variable was obatained.(2)The room and low temperature static tensile tests were carried out on the base metal,butt welds and heated affect zone.The impact toughness of the three types of specimens at 20,0,-20,-40 and-60 ? were obtained.The variation law of impact toughness with ambient temperature was studied,and the expression of ambient temperature as independent variable was obtained.The ductile brittle variable temperature of the three parts was also calibrated.(3)The room and low temperature axial high cycle fatigue tests were carried out on the base metal and its butt welds.The fatigue life data and S-N curves of the Q345 qD base metal and its butt welds of specimens at 20,0,-20,-40 and-60 ? were obtained.Furthermore,the confidence coefficient ? and the unilateral tolerance factor(7)p,,v(8)k? was introduced to receive the P-S-N safety fatigue design curve cluster with confidence ? = 90%.(4)The fatigue limit of the base metal and its butt welds was measured under the condition of specified fatigue life Nf = 5 million times using the up-and-down fatigue strength test method with eighteen Q345 qD base metal specimens and thirteen butt welds specimens.Based on the analysis of the fatigue data of the base metal and its butt weld in each temperature point,the fatigue life,fatigue limit and S-N curves with the ambient temperature were studied,and the fitting function of the ambient temperature as the independent variable was obtained.(5)The micrographs of fatigue fracture were studied by scanning electron microscopy(SEM)at room temperature and temperature at-60 ?. |
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