Research On Fatigue Performance Of ASTM A572 Gr65 Steel Based On Energy Dissipation,Strain Accumulation And Microstructure Evolution

ASTM A572 Gr65 steel has excellent mechanical properties and is widely used in fields such as transmission line towers and buildings.In practical applications,these structures often subjected to complex cyclic stresses,and the traditional fatigue test methods are time-consuming and costly.The energy dissipation,strain accumulation and micro-evolution generated during the fatigue process can be used to quickly evaluate the fatigue performance of materials.Therefore,this research proposes a material fatigue performance evaluation method based on energy theory and ratchet effect,and explores the mechanism of microstructure evolution under different cyclic stresses.This subject can provide a reliable basis for the structural design of materials,fatigue limit evaluation and prediction of remaining fatigue life.The treatise analyzes the mechanical properties and energy dissipation laws of ASTM A572 Gr65 steel in combination with the results of the tensile test.According to the infrared thermal imaging technology and the ratchet effect after fatigue loading,the energy dissipation and strain accumulation of materials during the fatigue loading process are explored.Evaluate the fatigue limit of the material according to the strain accumulation in the steady state of the cycle and the stable value of the temperature rise of the sample surface.EBSD and SEM are used to characterize and analyze the microstructure and fatigue fracture under different stress states,and to explore the micro-evolution mechanism in the fatigue process.The tensile test results show that the yield strength of ASTM A572 Gr65 steel is 472 MPa,the tensile strength is 555 MPa,and the elongation is 37%.During the stretching process,the specimen produces large plastic deformation,and a large amount of energy is dissipated into the environment,which causes the surface temperature of the specimen to rise rapidly.There are three stages in the surface temperature change during the stretching process:the temperature decrease stage when the tensile stress is less than the material's tensile elastic limit.The temperature rises gradually when the tensile stress exceeds the elastic limit of the material.The temperature peak occurs when the sample breaks in tension.The specimen will deform and dissipate energy during the fatigue process,which will increase the strain accumulation and surface temperature of the specimen.The work hardening produced in the irreversible deformation process makes the strain accumulation and surface temperature of the specimen reach a stable value in a short time.The strain accumulation in the stable deformation stage of the specimen and the temperature rise value of the surface are small when the cyclic loading stress is lower than the fatigue limit of the material.At this time,the specimen will not fatigue fracture.The strain accumulation and surface temperature rise value of the specimen in the stable deformation stage will be greatly improved,but the stable state will not be maintained for a long time when the cyclic stress exceeds the fatigue limit of the material.If fatigue fracture occurs,the strain accumulation and the surface temperature rise will increase suddenly for the second time.During the fatigue process,the strain accumulation and energy dissipation of the material show the same change law.In the process of fatigue loading,the microstructure shows the change of texture,geometrically necessary dislocation density and the evolution of crystal grains.As the cyclic stress increases,the texture density in the material increases continuously.At the same time,the geometrically necessary dislocation density in the material also rises significantly,the specimen undergoes significant work hardening,and substructure grains and deformed grains are formed.The fatigue fracture presents three areas: crack initiation area,crack propagation area and fatigue transient area.The crack propagation zone exhibits typical brittle fracture characteristics,dominated by quasi-cleavage fracture characteristics,while the instantaneous fracture zone exhibits obvious plastic fracture characteristics.The fatigue limit of the material is evaluated by the strain value and surface temperature rise value during the fatigue loading process.The fatigue limit is 453 MPa and 423 MPa,respectively.Compared with the results obtained by the traditional fatigue test method,the errors are 1.1% and 7.6%,respectively.