Fracture Research Of Q460C Steel Based On Micromechanical Damage Models

Failure of steel structures subjected to fierce earthquakes is termed of ultra-low cycle fatigue fracture which can occur in the areas with significant plastic deformation that is characteristic of ductile fracture.In recent years,researchers focus on local fracture models based on micromechanisms instead of traditional fracture mechanics gradually.Many local fracture models have proposed and proven successful at predicting ductile fracture under different conditions.However,these models are generally applicable under high stress triaxiality fatigue fracture conditions or low triaxiality tensile fracture states,and few models are available for ultra-low cycle fatigue fracture under low stress triaxiality state.Accordingly,specimens aimed at high,moderate and low stress triaxiality are designed and tested under uniaxial tensile loading and ultra-low cycle fatigue loading.Based on existing models,a new local fracture model is proposed for all relevant structural stress states.The main work of this thesis consisted of four parts:(1)Design round bar specimens,plate specimens,and shear specimens respectively aimed at axisymmetric stress state with high stress triaxiality,plane strain state with high triaxiality,and combined shear and tensile state with low triaxiality.(2)Test 11 monotonic specimens and 25 cyclic specimens.Calibrate the constitutive model for uniaxial tensile tests and the combined hardening model for ultra-low fatigue tests based on the smooth round bar tests.(3)Compile UVARM user subroutine on finite element software ABAQUS platform.Calibrate VGM model,SWDM model,and LOU model based on uniaxial tensile tests data,and contrast the prediction results derived from the three local models.(4)Develop the new CLOU model that is applicable for predicting the ductile fracture in ultra-low cycle fatigue tests.Calibrate and validate CVGM,SWDM and CLOU model using 23 cyclic specimens with different types and various loading histories,along with complementary finite element analysis.Acquire the new local fracture model for full range of stress state and provide the material parameters for Q460C steel.