Micromechanical Fracture Models And Parameter Calibrations Of Structural Steels Under Monotonic Load And Extremely Low Cycle Load

Micro-mechanism based fracture criterion can be used to predict the ductile fracture of structure steels.In order to calibrate the material parameters of the micro-mechanism based fracture criterion for Q235 B and Q345 B structural steels and ER-50 weld material,the following works were carried out in this thesis:(1)Basing on the summary for the micro-mechanism based fracture criteria and their basic assumptions,the most suitable fracture criteria for structural steels(namely the Stress Modified Critical Strain model(SMCS model),the Void Growth Model(VGM model),Degraded Significant Plastic Strain model(DSPSmodel)and the Cyclic Void Growth Model(CVGM model)),which can be used to accurately describe the ductile fracture mechanism and macro fracture behavior of the material and can be easily used for engineering application,were selected in this thesis.These criteria were then applied in the following fracture prediction analysis of steel structures in this thesis.(2)The Q235 B,Q345B structural steels and their corresponding weld material ER-50 are chosen as the research object.Initially,uniaxial tensile tests on smooth round bar specimens were conducted to obtain the basic material parameters for the three investigated materials.Subsequently,three types of notched round bar specimens,having three different notch radii,were designed to obtain different stress triaxialities.By conducting the tensile test on these notched round bar specimens,the load-displacement curves of theses specimens were obtained,which can be used to capture the fracture initiation point of the specimens.Further,nonlinear elasto-plastic finite element analyses were conducted for the investigated notched round bar specimens.The critical stress and strain at fracture initiation point were extracted and introduced in the micro-mechanism fracture models to obtain the fracture parameters of the structural steels and weld material.The affecting factors of the parameters.Finally,scanning electron microscope was used to observe the fracture surfaces of the notched specimens,and the characteristic lengths of the investigated three materials were obtained.(3)The user subroutine VUMAT,from ABAQUS software edited by the authors,was employed in the numerical simulation of the notched specimens being investigated.Using the SMCS and VGM models as fracture criteria,the post-fracture load–displacement curves of eighteen notched round bar specimens were traced by deleting the failure elements one by one.Then,the predicted results obtained from the fracture criteria were compared with the numerical results without considering the fracture criteria.The feasibility of the micro-mechanism based fracture criterion in obtaining the load-displacement curve of the steel structure is discussed.(4)Extremely low-cycle fatigue tests and corresponding finite element analyses were conducted on the smooth notched round bar specimens to calibrate the damage degradation parameters of the micro-mechanism based criteria for Q235 B,Q345B structural steels and ER-50 weld material,which provides the necessary parameters for the relevant fracture models being used in the fracture prediction analysis for steel structure.