| Weathering steel(WS)has been investigated and manufactured in developed countries since the early last century.Meanwhile,WS settled the issue of corrosion on steel in a certain degree.The uncoated WS bridge can decrease the whole life cycle cost while it also brings benefits to environment.However,environmental limits are existed in the application of uncoated WS bridges as well.With the development of steel industry,the appearance of high performance steel(HPS)with better weldability,higher corrosion resistance and higher strength can better serve the uncoated steel bridges.However,the theoretical and experimental investigation of advanced WS and HPS are still inadequate in this stage.Meanwhile,deeper and systematic research is urgent to be carried out.Based on the present situation,this article pays attention to theory,test and application.Meanwhile,the research on static tensile and fatigue performance of WS and HPS is systematically carried out under corrosion effect.The main contents of this paper include the aspects below,which are research on the corrosion model and pitting model of WS and HPS,the corrosion-fatigue model of WS and HPS,the static tensile test on corroded and uncorroded WS and HPS,the fatigue test on corroded and uncorroded WS and HPS and the fatigue design method of uncoated WS and HPS bridges.The main research contents are as follows:(1)To investigate the corrosion rule of WS and HPS,the environment categories are classified.Based on the wide collection of environment data of 129 cities in Chinese,the distribution figure of Chinese environment category is firstly proposed referring to the evaluation method of corrosive environment by ISO.Thus,the category distribution provides basis to the design of metal structure in China.Besides,combining Chinese codes for structural steel and international research on the natural corrosion achievements of WS,the uniform corrosion model of Chinese WS is firstly established.Therefore,the model provides basis to the design of uncoated WS and HPS bridges in China.Moreover,a cyclic salty accelerated corrosion test on 20 WS and 20 HPS specimens is carried out.Based on the weight measurement and surface scanning test,the uniform model and pitting model are investigated.In addition,the experimental result is compared to the effect of natural corrosion on Chinese WS.The research indicates that relative humility,temperature,concentration of Cl ion and concentration of SO2 are the main factors that can influence the uniform corrosion.Meanwhile,locally coastal areas in China belong to C4 environment.And,large part of offshore areas in China belongs to C3 environment.Gradually,most inland areas belong to C2environment.Besides,the WSs regulated by Atmospheric corrosion resisting structural steel have higher corrosion resistance and strength than that of WSs regulated by Structural steel for bridge.Thus,these WSs are more suitable to the construction of Chinese WS bridges.In the corrosion process,the rates of uniform corrosion and pitting corrosion keep decreasing along with corrosion process.As a result,the corrosion indexes of WS and HPS are 0.4013and 0.3406,respectively.Moreover,the pitting current of WS and HPS are 1.869×10-9 C/s and 1.306×10-9 C/s,respectively.Eventually,it is validated that the environment applied to the accelerated corrosion test can match typical offshore areas in China with C3 environment by comparing to the natural corrosion test data of Chinese WS.(2)To research the corrosion-fatigue process,the research status of electrochemical corrosion theory and fracture mechanics are researched and summarized.Meanwhile,the corrosion-fatigue process can take part into corrosion-fatigue crack initiation stage and propagation stage.Based on the interaction between corrosion and fatigue stress,the Faraday's Law and NASGRO equation are utilized to research and propose the corrosion-fatigue life model of WS and HPS.Meanwhile,the influence of stress range,stress ratio,pitting current and loading frequency on corrosion-fatigue life is revealed.The result indicates that the corrosion-fatigue crack initiation stage contains pitting process and competitive process.Besides,the corrosion-fatigue crack propagation stage includes crack propagation process.The corrosion-fatigue life of HPS is over 5 times higher than that of the conventional steel for bridge.Moreover,the corrosion-fatigue crack initiation life takes over60%of the corrosion-fatigue life.It is interesting to note that the increasing stress range,stress ratio and pitting current lead to the significant reduction in corrosion-fatigue life.However,the increasing of loading frequency only causes the reduction in corrosion-fatigue crack propagation life.(3)To research the influence of corrosion on the mechanical performance of WS and HPS,static tensile test on 27 base metal and welded joint specimens of Q345CNH and HPS485W is carried out.Additionally,there are 15 corroded specimens and 12 uncorroded specimens in the test.Then,the yield strength and tensile strength of the specimens are obtained combining stress-strain curve.The research result indicates that the yield strengths of Q345CNH and HPS 485W decrease by 5.02%?9.29%and 11.25%?14.31%after 60,120 and180 corrosion cycles comparing to the uncorroded ones.Meanwhile,the tensile strength of this two types of steel decrease by 0.01%?4.6%and 3.31%?4.45%after the same corrosion cycles.It is worth noting that the effect of corrosion on yield strength is greater while the effect of corrosion on tensile strength is negligible.(4)To research the influence of corrosion on the fatigue performance of WS and HPS,the corrosion fatigue test method is concluded and summarized.Based on the conclusion,in total 133 base metal and welded joint specimens of Q345CNH and HPS 485W are manufactured to carry out fatigue test.In the test,there are 75 corroded specimens and 58uncorroded specimens.Meanwhile,the failure mode,fatigue S-N curve and fatigue strength of base metal and welded joint are proposed by combining the reliability analysis on test data.Besides,the influent regulation of corrosion degree on fatigue strength is revealed.The research result indicates that the fatigue strengths of uncorroded base metal specimens of Q345CNH and HPS 485W are 204.0MPa and 180.1MP,respectively,which are much higher than FAT 160.However,the fatigue strength of Q345CNH decrease by 50.5%,52.1%and55.6%after 60,120 and 180 corrosion cycles comparing to the uncorroded ones,respectively.And,the fatigue strength of HPS 485W decrease by 41.0%,34.7%and 40.4%after 60,120and 180 corrosion cycles,respectively.Thus,the fatigue strength of the two types of steel drop below FAT 100.Moreover,the fatigue strength of welded joint specimens of Q345CNH and HPS 485W decrease by 17.3%and 16.2%after 60 corrosion cycles comparing to the uncorroded ones.As a result,the fatigue category of base metal and welded joint of uncoated WS and HPS are proposed to decrease to FAT 90 and FAT 80,respectively.It is interesting to note that NASGRO equation can predict the fatigue life of WS and HPS by choosing proper stress intensity factor range threshold for crack initiation.(5)The fatigue design method of Chinese uncoated WS and HPS bridges is proposed through the uniform corrosion model,pitting model and corrosion-fatigue life model in the previous studies.Meanwhile,the corrosion-fatigue life of uncoated WS and HPS bridges is evaluated.Additionally,the method provides theoretical support to the design,construction,application and maintaining of this type of bridge.The result indicates that the corrosion-fatigue S-N curve of WS and HPS degrade along with the increasing pitting current,stress ratio and pitting accelerated factor.Besides,the pitting accelerated factor has great effect on the corrosion-fatigue life of uncoated WS and HPS bridges.Thus,corrosion-fatigue test should be carried out to obtain a reasonable pitting accelerated factor.Moreover,the effect of uniform corrosion on corrosion-fatigue life is far less than that of pitting corrosion. |
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