Research On Brittle Fracture Prevention Design Method Of Q690qE High Strength Bridge Steel

In recent years,with the development of bridge construction and the improvement of steel strength grade in China,the problem of brittle fracture of bridge steel structures has become more prominent.For this problem,the relevant standards in China still prevent it by specifying the Charpy impact absorbed energy of materials,but the method can only qualitatively evaluate the anti brittle fracture performance of the structure,can not accurately design from a quantitative point of view.Based on the concept of stress intensity factor in linear elastic fracture mechanics,anti brittle fracture design and evaluation methods have been proposed,but these methods are only limited to the category of linear elasticity or small-scale plastic yield.With the improvement of steel quality in China,if the anti brittle fracture design of a structure is carried out only by the theory of linear elastic fracture mechanics,there will be a considerable deviation from the actual situation,and the excellent toughness of steel will not be brought into full play.The theory of elastic-plastic fracture mechanics has been studied and applied in the problem of anti brittle fracture of bridge steel structures,but the relevant research results and practical applications are still few,and there is still a lot of work to be further carried out.Aiming at the brittle fracture problem of bridge steel structures,four aspects of work are carried out,the fracture driving force of typical structures of steel bridge,the fracture toughness tests of bridge steel,the anti fracture material selection of bridge steel and the anti brittle fracture design of bridge steel structures,by methods of experimental research,finite element method and theoretical analysis.Based on the crack tip opening displacement in elastic-plastic fracture mechanics,the anti brittle fracture design method of bridge steel structures is established.The main research results and conclusions are as follows:(1)For a semi infinite plate with an edge crack,the formula for calculating the size of the plastic zone at the crack tip is derived based on the D-M model,and the formula for calculating the crck tip opening displacement is established.Through the analysis of the main load bearing and force transmitting members of steel bridges,two typical structural details of T-shaped structure and angular structure are selected,and the fracture driving force calculation formulas of these two typical structures are established based on the crack tip opening displacement.(2)Taking Q690q E high-strength bridge steel newly developed in China as the object,Charpy impact tests with small-size specimens,CTOD tests with medium-size full thickness specimens and wide plate tensile tests with large-size full thickness specimens are carried out successively.The fracture toughness of base metal and weld of Q690q E steel are evaluated from multiple perspectives.The tests provide data basis for anti brittle fracture design of bridge steel structures.(3)Take the critical crack size 2a_c=50 mm in T-shaped structure and a_c=25 mm in angular structure.According to the design stress?_D=0.3?_s?0.4?_s?0.5?_s?0.6?_s,the stress levels of bridge members are divided into four grades.The welding residual stress is considered as?_R=0.6?_s,additional stress?_c=0.3?_s.The critical fracture driving force values of T-shaped structure and angular structure under different stress levels are calculated respectively.Based on CTOD test results,the method of establishing the functional relation of fracture toughness,temperature and plate thickness is proposed,and the fracture resistance formulas of base metal and weld of Q690q E are established.According to the fracture mechanics criterion,the maximum permitted plate thickness of Q690q E steel in T-shaped structure and angular structure is calculated respectively,and the anti fracture material selection table of Q690q E steel is compiled.(4)Based on the above research results,the design and calculation process of bridge steel structures considering the anti brittle fracture performance is introduced.Taking a sea crossing bridge as an example,the anti brittle fracture design process of the truss section near the middle fulcrum of the bridge using Q690q E is described in detail,and the recommended thicknesses of key members of the bridge to meet the anti brittle fracture requirements are given.