Study On Fatigue Properties Of Q345 Steel Based On "single Point-Group Method" With Incomplete Data

As the number and scale of Steel structure project are growing rapidly in China,people are paying more and more attention to the corresponding safety issues of steel structures.The environmental effects will change the material properties of steel.Fatigue damage accumulates gradually in the material because of the permanent load and variable load,and the mechanical properties of steel structure system decrease gradually,damage will weaken the resistance of the structure and cause accidents.Throughing theoretical modeling and experimental analysis,fatigue properties of Q345 steel are studied in this paper.The main contents are as follows:(1)The curve equation of "single point-group method" with incomplete data Firstly,the improved single-point-group method and its implementation algorithm are introduced.On this basis,combined with the incomplete data analysis method,the three-parameter S-N curve expression of fatigue life obeying normal distribution is established.Using incomplete data "single-point-group method",the corresponding P-S-N curve and gamma-P-S-N curve expressions are established,and the corresponding implementation algorithm is given.Next,three incomplete assumptions are made for the given life data by using the published fatigue life data of helicopters.The parameters of S-N curve calculated from the complete life and incomplete life are compared.Finally,the corresponding S-N curve model is established by using the calculated S-N curve parameters.The correctness and validity of the incomplete data "single point-group method" are verified,which provide a theoretical basis for the development of the test and the establishment of S-N model.(2)Fatigue test of Q345 steel The specimens were cooled at high temperature and corroded in laboratory environment.Through the tensile test,we obtained the mechanical properties of the test piece.Then the fatigue tests of Q345 steel specimens under stress control were carried out.By comparing with the hysteretic curves of the specimens without corrosion at room temperature,the effects of high temperature cooling and corrosion were analyzed.The effect of stiffness change,damage accumulation and deformation behavior on the fatigue fracture of Q345 steel was observed by naked eye.The fatigue source,fatigue propagation zone and fatigue instantaneous fracture zone of each specimen were compared.The fatigue micro-characteristics of Q345 steel under different conditions were observed by optical electron microscopy and scanning electron microscopy.The fracture shape of Q345 steel was observed by comparing with that of non-corrosive specimens at room temperature.The effects of high temperature cooling and corrosion on the fatigue fracture of specimens were analyzed.(3)Stress-life model The fatigue life of groups of high temperature cooling and corrosion is analyzed by varianceanalysis.The changes of fatigue life length and discrete type are studied.Then the material performance parameters of Q345 steel without corrosion at room temperature,after cooling at high temperature and after corrosion are calculated with theoretical calculation method and all fatigue test life data.The corresponding S-N curve equation is given,and the corresponding P-S-N curve and gamma-P curve are established.-Based on the S-N curve model,the law of fatigue life of Q345 steel under different conditions is analyzed,and the influence of high temperature cooling and corrosion on fatigue life of Q345 steel is analyzed by comparing the fatigue life of Q345 steel under different conditions.