| In the application of traditional fracture mechanics,it is assumed that there are initial cracks and high strain constraints at the crack tip.The traditional fracture mechanics theory is no longer suitable for the study of the ductile fracture of steel structure under the action of strong earthquake,considering that the ultra-low cycle fatigue fracture of steel structure under the action of strong earthquake has the characteristics of large plastic strain and few load cycles,and the ductile fracture of steel structure with small initial macro defect under the action of low cycle repeated load.In recent years,theoretical analysis and experimental research show that the fracture theory and corresponding model based on micromechanism can effectively and accurately predict the development of ductile cracks in steel structures and the ductile fracture under the action of ground motion,but the research on the fracture model based on micromechanism is not sufficient,especially in the expression of the model,the calibration of characterization parameters and the range of values.Therefore,based on the analysis of the existing fracture model based on micro mechanism,this paper considers the influence of rod angle and stress triaxiality path,forms the improved micro fracture model,and verifies the rationality of the improved micro fracture model by theoretical analysis,numerical simulation and test.The main contents are as follows:(1)Based on the VGM model,CVGM model,and VGM-UDDF model,the existing Fracture model are improved and enriched by considering the effect of stress triaxiality and Rhode angle on the ductile microscopic fracture of structural steel.The improved model develops a subroutine that is based on ABAQUS through Fortran program that can predict the fracture life,so it can accurately analyze the ductile fracture of the material.This can be used for accurate finite analysis of the extremely low-cycle fatigue ductile fracture of steel structural parts,nodes,and structures under the seismic actions.(2)To make plate specimens,notched round bar specimens and flat notch specimens by using Q390 steel.The true stress-strain curve and elastic modulus of the material are obtained through the plate specimens tests.Then various test data required for finite element simulation and improved microscopic fracture model calibration are obtained by the notched round bar specimen test and the flat notch specimen test via unidirectional tensile test and cyclic loading test.(3)Load-displacement curve and stress-strain data of notched round bar are obtained by the finite element simulation on the unidirectional tensile test and cyclic loading test of the Q390 steel material.The calibration considers the microscopic ductile fracture model of Lode angle parameters and its parameters.The ZL-CVGM and ZL-VGM-UDDF programmed model are embedded in ABAQUS to predict fracture and further prove the applicability of the improved microscopic fracture model by comparing with the test results.(4)In order to make the improved ZL-CVGM model and ZL-VGM-UDDF micro-fracture mechanism model be applicable to other types of steel predict the actual steel structure fracture,Q235 steel,Q345 steel and Q460 steel that are researched by scholars are tested via the ABAQUS finite element analysis in the cyclic loading test.These are combined with the research group,who researched Q390 steel and Q420 steel,to obtain the fracture data of each steel.According to the fracture data of each steel,an improved micro-fracture model is fitted to the uniform stress triaxiality parameter and Lode parameter of all steels.Then the toughness fracture parameters suitable for each steel are obtained.The developed subroutine is used to predict the cracking position and the timing.By comparing with the test results,it is verified that the applicability of the improved fracture model for predicting the actual steel structure fracture. |
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