Investigation On Fatigue Performance Of Bridge Steels And Welded Details At A Low Ambient Temperature

The welded details in steel bridges are prone to experience fatigue damage induced by the cyclic action of long-term traffic loading.Additionally,the low-temperature brittle fracture also threatens the safety of such infrastructures residing in the cold and severe cold regions.However,there are no specific codes stipulating the requirements with respect to the low-temperature fatigue problems of steel bridges so far,and relevant investigation is still inadequate.Thus,in order to get a better understanding of fatigue properties of welded components in steel bridges under the low temperature ambient,a large number of fatigue tests,together with the numerical simulation and theoretical analysis,helped to systematically study the rule and mechanism of the low temperature influencing the fatigue properties in terms of the level of materials,welded details and components.The primary research work and main conclusions lead to the four following sections.(1)Within the range of the room temperature to-60 ~oC,this study carried out a series of strain-controlled fatigue tests using 39 smooth coupons,finished numerous Charpy impact tests with 72 V-notch specimens,and performed a number of fatigue crack propagation rate tests and stress intensity factor threshold tests utilizing 91standard compact tension specimens,as all those specimens were made from the bridge steel Q345qD and Q370qE.The test programs explored the effect of low temperature on the fatigue properties of bridge steels,and collected the valuable low-temperature fatigue data of bridge steels.Results indicate that the low temperature enhances the resistance to fatigue crack initiation and propagation in bridge steels,but impairs the resistance to fatigue crack propagation in butt welds.(2)At the room temperature and-60 ~oC,this dissertation conducted the high-cycle constant-amplitufe fatigue tests of the butt-welded joints and the non-load-carrying fillet-welded cruciform joints,and the valid data points were 43 in total.Then,using the local strain method predicted the fatigue crack initiation life of those two kinds of joints,and applying the three-dimension crack propagation analysis based on linear-elastic fracture mechanics assessed the fatigue crack propagation life of the above joints.The combinated results of fatigue tests and numerical analyses revealed the variation law and mechanism of the low temperature influencing the fatigue properties of welded details.Results demonstrate that the fatigue life of the welded details at low temperatures would not be inferior to that at the room temperature,and both of the low-temperature ambient and the welding process will pose a coupled effect on fatigue properties of the welded details.But the fatigue crack initiation of the welded joints is more likely to occur at a low temperature ambient.(3)On the basis of the three-dimension crack propagation analysis,this study further validated the low-temperature fatigue properties of the flexural members with critical details from NCHRP 102 and 147.Results together with the above investigation exhibit that the reduced critical crack length has a marginal impact on the low-temperature fatigue crack propagation life of welded details,because the current low-alloy bridge steels generally possess relatively higher fracture toughness.(4)Finally,this dissertation provided the characteristic values of Paris-law parameters applicable to the fatigue damage assessment of welded details in steel bridges at the room temperature to-60 ~oC,and proposed some suggestions for the fatigue design of welded details of steel bridges depolyed in cold and sever cold areas,covering the material selection,low-temperature performance of welded details and their improvement approaches.